# 太陽地球系物理コロキウム

## 2018年度 太陽地球系物理コロキウム

概要 宇宙空間・太陽・磁気圏・電離圏・大気圏の物理学・化学に関する新しい話題の紹介・研究発表により、この分野の学問の基礎的・包括的な理解を目的とする。 毎週金曜日15:00〜 理学部1号館807Aなど 星野真弘、 関華奈子、横山央明、 天野孝伸 、笠原慧、大平豊、桂華邦裕 桂華邦裕

## 第22回

Date 日時 November 30, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Makoto Suzuki Analysis of Pc5 Wave Characteristics observed by MMS spacecraft

Abstract 要旨を見る

Ultra-Low-Frequency (ULF) waves having periods between 2 to 10 min called Pc5 waves, particularly, high-m number Pc5 waves (Storm-time Pc5s) that appear during geomagnetic storms are believed to play a role for the acceleration of radiation belt electrons (Ukhorskiy et al., 2009) and modulation of chorus wave intensity (Xia et al.,2016). Such storm-time Pc5s may be excited by plasma instabilities caused by an enhanced particle transport from the plasma sheet to the inner magnetosphere during a geomagnetic storm. However, it isn’t understood plasma instabilities excite storm-time Pc5s. In order to understand which plasma instability excites storm-time Pc5s, it needs to estimate the characteristics of Pc5 waves.
Such ULF waves likely to be generated by the drift-mirror instability which is one of the most plausible candidates (Cheng & Lin, 1987) are more frequently observed at L > 8 where the MMS (Magnetospheric Multiscale) spacecraft observations are available (Takahashi et al., 1990). With the MMS observations, we can perform both single-spacecraft and multi-spacecraft analysis methods independently. Comparisons between these methods enable us to verify the accuracy of the method based only on single-spacecraft measurement. However, since observations in the inner magnetosphere are usually made with a single satellite, it is difficult to obtain reliable estimates for the characteristics of Pc5 waves. The MMS observations are thus useful to test the applicability of single-spacecraft analysis methods in the inner magnetosphere.
In this study, we present results of such analysis for Pc5 waves observed by the MMS satellite on the dusk side in the magnetosphere on September 1, 2015. We applied the multi-spacecraft timing method for selected time intervals and obtained a wavelength of several thousands of km, westward propagation with a propagation speed of 30-40 km/s in the spacecraft frame. We also performed Minimum Variance Analysis to estimate the wave propagation directions. We will discuss the accuracy by comparing these results.

## 第21回

Date 日時 November 16, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Ryuta Asami Characteristics of hot/cold components of plasma sheet derived from two-component fits of velocity distribution functions

Abstract 要旨を見る

Heating and acceleration of magnetospheric plasma have been studied using in-situ plasma and field observations. A large number of observations reported two distinct plasma populations with different temperatures in the Earth’s magnetosphere [e.g., Seki et al 2003]. However, the dominant heating/acceleration mechanisms and regions are not well understood. Moreover, it remains unclear whether the heating/acceleration mechanisms depend on mass. Only a few satellite missions have been able to observe the thermal component of magnetospheric plasma with mass determination. Therefore, a small number of studies focused on mass-dependent processes in the typical energy range (<1-10 keV) of magnetospheric ions. In this study, we first separate the plasma into hot and cold populations, and then perform statistical analysis for each population. Hot Plasma Composition Analyzer (HPCA) instrument on board the MMS satellite, which is in a low-inclination elliptical orbit with an apogee of about 24 Re measures the distribution function in velocity space every 10 seconds in an energy range of a few eV to 40 keV. Using data for a period of 2017-05-01 to 2017-08-31, we examined density and temperature spatial distributions for hot and cold plasma populations. Specifically, we performed two-Maxwellian fitting to the observed distribution functions. Two different populations were clearly identified in the plasma sheet. Next, we investigate temperature and density profile. We found that the cold ion (< 100 eV) coexist with hot ion (~ keV ) at the near the Earth (X < -10 [RE]). We will discuss the acceleration and mixing processes of cold plasma.

## 第20回

Date 日時 November 9, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Shogo Inui Heavy ion outflows from Mars observed in the Martian induced magnetotail by MAVEN

Abstract 要旨を見る

Geological studies have suggested that Mars had a warm climate and liquid water on surface about 4 billion years ago. Now, Mars has a cold surface temperature and little water on surface. Escape of greenhouse gases such as CO2 to space is considered as the plausible reason to cause the drastic climate change. On the other hand, mechanisms enabling the large amount of the CO2 loss is far from understood. The planetary ion escape through interaction between the solar wind and the Martian upper atmosphere is one of the candidate mechanisms to achieve the atmospheric escape. To understand atmospheric loss from Mars, MAVEN has observed the ion escape from Mars as well as space environment around Mars since November 2014.
In this talk, we present two studies: event study and statistical study. In the event study, we investigated detailed characteristics of a dense cold ion outflow event observed in the Martian induced magnetotail by MAVEN. We suggested that the combination of the mini-magnetosphere and the -E hemisphere facilitates the cold ion escape from low altitude Martian ionosphere. In the statistical study, we report on a statistical analysis of heavy ion outflows from Mars to investigate influences of the crustal magnetic fields and the direction of solar wind convection electric field on the ion outflows by using the data from MAVEN. Results show heavy ion outflows in the Martian magnetotail are affected by the direction of solar wind convection electric field. Results also show although crustal magnetic fields affect heavy ion outflows, effects of the location of crustal magnetic fields on heavy ion outflows are small. We also discuss acceleration/escape mechanisms of heavy ion outflows in the Martian magnetotail.

## 第19回

Date 日時 November 2, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Shotaro Sakai Low electron temperatures observed at Mars by MAVEN on dayside crustal magnetic field lines

Abstract 要旨を見る

The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies temperatures in the dayside for two regions where: (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal-field regions, that is, the strong magnetic field, which is due to a transport along magnetic field lines between cold electrons at lower altitudes and the upper atmosphere. Electron heat conduction in the crustal-field regions is altered due to the magnetic mirror force and/or the ambipolar electric field above 250 km altitude because the electron mean free path exceeds the relevant length scale. These effects should be considered for future simulations. The temperature is also greater under the high solar extreme ultraviolet.

## 第18回

Date 日時 October 26, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takuma Kato Stochastic shock drift acceleration in the shock transition region

Abstract 要旨を見る

The acceleration of non-thermal electrons is an important subject in space physics. The shock accelerated non-thermal electrons have been observed at the Earth bow shock. These electrons are supra-thermal (with energies from 1keV to 100keV), and their distribution is power-law. However, acceleration models proposed in the past could not reproduce such observed spectra. Recent in-situ satellite observations of the Earth bow shock found wave-particle resonances between whistler waves and electrons at the shock transition region. These results indicate that whistler waves play an important role for an acceleration of supra-thermal electrons. We propose a new acceleration mechanism that takes into account the effect of stochastic pitch-angle scatterings by whistler waves during the course of the Shock Drift Accelerations (SDA), which is an adiabatic acceleration process for supra-thermal electrons at the shock transition region. By introducing pitch-angle scatterings, the acceleration efficiency may be improved. We theoretically analyzed the energy spectrum of electrons by using a box model which considers the dependence of an electron distribution only on energy and pitch-angle. We showed that the electron energy spectrum becomes a power-law and its spectral index depends only on the magnetic field gradient in the limit of strong scattering. We also found the maximum energy attainable through the proposed model, and it scales linearly with the pitch-angle scattering coefficient. We have confirmed these results by performing Monte Carlo simulations. We also demonstrated that the proposed model is qualitatively consistent with in-situ observations.

## 第17回

Date 日時 October 19, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Taku Namekawa Development of high-energy electron detector for observation of microburst precipitation over pulsating aurora

Abstract 要旨を見る

The Pulsating Aurora (PsA) is one of the auroral phenomena whose emission intensities are modulated quasiperiodically in a few to tens of seconds. This quasiperiodicity is accounted for periodic precipitation of a few to tens of keV electrons which is thought to be generated by pitch angle scatterings due to whistler mode chorus waves in the magnetosphere. On the other hand, microburst precipitations of relativistic electrons are often observed by low-altitude satellites. Recently, EISCAT radar observation suggested that >200keV electron from radiation belt is precipitating during PsA. Miyoshi et al. [2010,2015a] proposed a model to explain this precipitation: First, whistler mode chorus waves that are generated at the magnetic equator cause the pitch angle scattering of ~10 keV electron leading to the PsA. Then, the waves propagate to higher latitudes, the wave-particle resonant energy increase and pitch angle scattering with ~few hundreds of keV/~MeV takes place. Recent numerical simulations successfully reproduce this model. However, wide energy spectrum during PsA has not been observed which is essential to observational verification of this model.
I have developed a high-energy electron detector (HEP) in order to observe precipitating high energy electron during PsA. HEP is designed to measure electrons with energies ranging from 300 keV to 2 MeV with high-time resolution. Minimum energy resolution of HEP is less than 5 %, and 1 event processing time is less than 5 µs based on the laboratory experiments using 1 MeV electron beam. HEP is a part of PARM (Pulsating AuroRa and Microburst) package to be onboard the RockSat-XN sounding rocket which will be launched from Andøya, Norway in winter season of early 2019. PARM consists of four instruments, High Energy Particle detector: HEP, which is above-mentioned, Medium Energy particle Detector: MED, Auroral Imaging Camera: AIC, and ASIC FluxGate magnetometer: AFG. Energy range of electron measurement provided by PARM is extended by MED down to 20 keV.

## 第16回

Date 日時 October 12, 16:30 Room 807, Science Bldg. 1 理学部1号館807号室 Yota Ishigaki N-body simulation of planetary formation with planetesimal fragmentation

Abstract 要旨を見る

In standard theory of planetary system formation, it is believed that planets are formed through runaway and oligarchic growth of planetesimals in protoplanetary disk. The process of runaway and oligarchic growth stage have been mainly discussed by N-body simulation of planetesimal system.
It is important to discuss the migration of protoplanets in process of planetary system formation in order to understand the various features of exoplanets. Recently, a mechanism of planetesimal-driven migration (PDM) has been proposed, in which a protoplanet migrate by interaction with a surrounding small planetesimals. These recent progress in planetary system formation theory urges us to re-discuss how migration of protoplanets during the phases of the formation process that has been studied using N-body simulations. This new situation requires N-body simulations to involve a wider range of radial distance from a central star and to include effects of planetesimal fragmentations upon their collisions.
To this goal, I have developed a new code utilizing the Particle-Particle Particle-Tree (P3T) method, which is so efficient as to enable us to perform simulations of the kind requested above. Here we report the results from preliminary simulation runs of a well-studied setting but now involving planetesimal fragmentation upon collision (improvement from the perfect accretion approximation) and study how these newly added elements affect the outcomes.

## 第15回

Date 日時 October 12, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takeshi Abe Simultaneous Hinode-IRIS-ALMA observations of a solar plage region

Abstract 要旨を見る

The solar corona is much hotter than the surface and the chromosphere exists as an interface between them. The non-thermal energy transfer, such as waves and tiny magnetic reconnection (nanoflare), is believed to create the corona and chromosphere, but we are still lack of observational knowledge to address which mechanism plays a vital role in heating. Time variations in physical conditions, such as temperature, are one of important hints to address this question. ALMA micro-wave observations can be used to probe the chromosphere and the time variations in ALMA signals are mainly caused by changes in chromospheric temperature. On 19 March 2017, an ALMA observation (Band 3 – 100GHz, 3.7 arcsec beam size) was successfully carried out in coordination with Hinode and IRIS observations. The observed region is a plage region located at the north-east hemisphere on the solar disk. We established the co-alignment among ALMA synthesized images, Hinode SOT/SP, and IRIS images with the accuracy better than a few arcsec. Following Shimojo et al. (2017), we evaluated the noise level of synthesized images to be 25 K (1σ) by using the difference between two orthogonal linear polarization signals. In this study, we extracted temperature increases exceeding 3σ level (75K) from the time profile of ALMA, and we classified them to two types (‘slow’ and ‘fast’ events) according to the slope of the increase. We found slow events distributed globally, and fast events are localized near a polarity inversion line. This result may indicate that the fast events are a signature of frequent occurrence of sudden energy releases occurring near the polarity inversion line.

## 第14回

Date 日時 October 5, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Nana Higashio Radial profiles of phase space density of relativistic electrons in the outer radiation belt during 6 magnetic storms in 2017: Arase/XEP observations

Abstract 要旨を見る

Radiation belt electrons drastically vary during geomagnetic storms. However, the relative significance of physical mechanisms of the acceleration and loss remains unclear. Two types of the electron acceleration processes have been proposed. One is the adiabatic acceleration due to radial transport of electrons from the plasma sheet to the inner magnetosphere (adiabatic transportation). Another process is the non-adiabatic acceleration of sub-relativistic electrons to the relativistic energies in heart of the radiation belt. Radial profiles of phase space density (PSD), i.e., PSD profile along L* (McIlwain L-parameter), have used to distinguish the two acceleration processes [e.g., Reeves et al., Science, 2003]. The peak in a PSD profile is considered as an evidence of the non-adiabatic acceleration, while the diffusion-type radial transport will result in the monotonic decrease of PSD from high to low L*.
In order to investigate the electron acceleration processes, we examine variations of radial profiles of relativistic electrons PSD based on observations by the eXtremely high-energy Electron exPeriment (XEP) onboard the Arase satellite, which measures electrons from 400 keV to 20 MeV. Six geomagnetic storms that commenced from (a) March 27, (b) May 27, (c) September 7, (d) September 27, (e) November 7 and (f) December 4, 2017 are studied. The minimum Dst of the storm events (a)-(f) was -74, -125, -142, -76, -70, and -50 nT, respectively. The events (b) and (c) are related to CMEs, and others are associated with CIRs. In order to minimize the error from the assumption in pitch angle distributions (PADs), we only use data obtained in low MLAT regions and do not conduct interpolation of PADs, while the energy spectrum is interpolated to obtain PSD profiles of the fixed magnetic moment (m). The time variations of PSD profiles as a function of L* are investigated. In all events, PSD of relativistic electrons decreases in the main phase and then increase in the recovery phase. We observed a clear peak in the PSD profiles during the early phase of the electron PSD increase. Locations of the PSD increase tend to become closer to Earth (lower L*) with increasing minimum Dst. We will also report on the m dependence of the timing and location of the PSD increase in details.

## 第13回

Date 日時 September 28, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Reina Hikida A study on supply mechanism of hot electrons to the Io plasma torus with the spectral image obtained by Hisaki satellite

Abstract 要旨を見る

In the Jovian inner magnetosphere, plasmas originating from the gas released from Io’s volcanos are picked up by the magnetic field and co-rotated to form the Io plasma torus (IPT). The IPT emits the extreme ultraviolet spectral lines by the electron impact excitation. Through the in-situ observations, it is known that the electrons inside the IPT are composed of core components and trace amounts of hot components (~100 eV). While the main origin of the IPT emission is the pickup energy, energy of hot electrons is reported to be responsible for ~60% of the emission at most. However, unified understanding about the supply mechanism of hot electrons has not been made yet. The radial transport from the outside of the IPT where abundant hot electrons exist is considered as the candidate for the supply system. In the Jovian inner magnetosphere, the plasma flow in the co-rotation direction is dominant, and interchange instability has been suggested to play an important role in the inward transport. However, the observational evidence is inadequate. In this research, I aim to obtain clues about the supply mechanism hot electrons by focusing on the response of density and temperature of plasma in the IPT to the change of the amount of volcanic gas supply. In this presentation, I will introduce the time variation in radial profile of density and temperature of plasma in the IPT, obtained by using a method called spectral diagnosis for the spectral image of the IPT acquired by the Hisaki satellite.

## 第12回

Date 日時 July 20, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Masahiro Hoshino Thermodynamics of Collisionless Magnetic Reconnection

Abstract 要旨を見る

The plasma heating during collisionless magnetic reconnection is investigated using particle-in-cell simulations. By analyzing the time evolution of the plasma temperature associated with the motion of the reconnecting flux tube, we show that the plasma heating during magnetic reconnection can be separated into two distinct stages: the nonadiabatic heating stage, in which the magnetic field lines are just reconnecting in the X-type diffusion region, and the adiabatic heating stage, in which the flux tube is shrinking after two flux tubes merge. During the adiabatic heating stage, the plasma temperature $T$ can be approximated by $TV^{\gamma-1}=const.$, where $\gamma=5/3$ is the specific heat, and $V$ is the volume of the flux tube. In the nonadiabatic heating stage, we found numerically that the ratio of the increment of the ion temperature to that of the electron temperature can be approximated by $\Delta T_i/\Delta T_e \approx (m_i/m_e )^{1/4}$, where $m_i$ and $m_e$ are the ion and electron masses, respectively. We also present a theoretical model based on a magnetic-diffusion-dominated reconnection to explain the simulation result.

## 第11回

Date 日時 July 13, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Yikang Wang MHD simulation of Alfvén wave propagation in magnetized solar chromosphere: effect of mode coupling on chromospheric heating

Abstract 要旨を見る

We perform magnetohydrodynamic simulation to investigate propagation of Alfvén wave in solar chromosphere. We use 1.5-dimensional expanding flux tube geometry setting and transverse perturbation at the bottom to generate Alfvén wave following Matsumoto & Shibata (2010) and Kudoh & Shibata (1999). Alfvén wave undergoes non-linear mode coupling which generate longitudinal wave that contribution to shock formation and chromospheric heating. Comparing with previous studies, our expansion is that we include radiative loss term introduced by Carlsson & Leenaarts (2012). When an observational-based transverse wave generator is applied, we find that the spatial distribution of the time-averaged radiative loss profile in the middle and higher chromosphere in our simulation is consistent with classical VALC (Vernazza et al. 1981) model. While the temperature profile is different from the classical model mainly due to shocks. In addition, wave flux in the corona is larger than 3 × 105 erg s−1 cm−2 , which is the averaged required value for quiet sun coronal heating. Our study shows that Alfvén wave driven model has the potential to explain chromospheric heating and transport enough energy to the corona simultaneously. We also discuss the potential observational signature of non-linear mode coupling by “synthesizing” observational features from our simulation data. We find it is likely very difficult to make an objective determination to identify mode coupling from observational data.

## 第10回

Date 日時 July 6, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takanobu Amano Temperature anisotropy instability near mirror instability threshold

Abstract 要旨を見る

The plasma instability driven by a finite temperature anisotropy is
important for the transport of collisionless space plasmas, such as
the solar wind or planetary magnetospheres. Both parallel and
perpendicular anisotropy drive instabilities of at least two different
kinds: One propagates nearly parallel and another in an oblique
direction with respect to the ambient magnetic field. For the case of
perpendicular anisotropy, the parallel mode is called the
Electro-Magnetic-Ion-Cyclotron (EMIC) instability, and the oblique
mode is called the mirror instability. In general, the EMIC
instability has larger growth rates than the mirror instability at low
to moderate plasma beta. However, nonlinear mirror-mode-like
structures have been observed frequently in conditions where naively
the EMIC instability should be the dominant mode. Therefore, the
competition between the EMIC and mirror instability has been a topic
of substantial debate over the decades.
In this talk, we discuss hybrid simulation results for a homogeneous
magnetized plasma initialized by a bi-Maxwellian distribution
function. We found that the mirror-mode wave may be generated even if
the simulation was initialized with a marginal stability condition for
the mirror instability, but was unstable against the EMIC instability.
We attribute this generation of the mirror mode to be a nonlinear
outcome of pitch-angle scattering of particles via EMIC generated
Alfvenic fluctuations. Although the generated mirror-mode amplitude is
much smaller than the level often observed in space plasmas, this
might give us a hint to solve the problem.

## 第9回

Date 日時 June 29, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Munehito Shoda Local MHD simulations of the solar wind acceleration

Abstract 要旨を見る

The solar wind is the only directly-observable stellar wind. Stellar wind controls the evolution of a star by regulating the mass and angular momentum loss. Once a star loses mass and momentum, the stellar interior changes its property, and thus, the dynamo activity and global magnetic field regulate back the stellar wind. This wind-dynamo feedback system has a critical role in the spin evolution of a star.
The important parameters of a stellar wind is the mass loss rate and (termination) velocity, and these are determined by the amount of heating in the subsonic region. In many cases, however, most studies do not solve the precise heating processes but instead assume a given temperature profile or consider a specific process that is easy to incorporate. The mass loss rate and wind velocities predicted by these models are therefore unacceptable in quantitative sense, although some of them give physically meaningful results quantitatively.
To overcome this problem, I have conducted MHD simulations with realistic heating(cascading) process. Although the actual thermalization process is the wave-particle interaction, MHD simulation is helpful for my purpose because the cascading rate approximately the same as the heating rate in a quasi-steady state. 1D simulations are performed for parameter survey and 3D simulations for realistic modeling. Both are limited to localized magnetic flux tube region. Our simulation results suggest that the solar wind is heated by compressible MHD turbulence. The compressibility and resultant density fluctuation plays a crucial role in exciting the classical Alfvén-wave turbulence. Our simulation is worth testing by forthcoming spacecraft mission Parker Solar Probe.

## 第8回

Date 日時 June 22, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Kenji Mitani Statistical study of selective transport of higher-energy oxygen ions into the inner magnetosphere during geomagnetic storms observed by Van Allen Probes

Abstract 要旨を見る

The transport mechanism of the ring current ions differs among ion energies. Lower-energy (<~150 keV) ions are well known to be transported convectively. Higher-energy (>~150 keV) protons are reported to be transported diffusively, while there are few reports about transport of higher-energy oxygen ions. Selective transport of higher-energy ring current oxygen ions is detected during a storm main phase on 24 April 2013 by Van Allen Probes spacecraft. An enhancement of 1-100 mHz magnetic fluctuations is simultaneously observed. Observations of 3 and 30 mHz geomagnetic pulsations indicate the azimuthal mode number is ≤10. The fluctuations can resonate with the drift and bounce motions of the oxygen ions. The results suggest that the combination of the drift and drift-bounce resonances is responsible for the radial transport of higher-energy oxygen ions. To understand occurrence and condition of the selective transport of the higher-energy oxygen ions, we investigate all magnetic storms observed by Van Allen Probes from February 2013 to September 2017.
In order to investigate the ion supply to the ring current during magnetic storms, we study the properties of energetic proton and oxygen ion phase space densities (PSDs). We calculated ion PSDs for various first adiabatic invariants (mu=0.1-2.0 keV/nT) and for the local pitch angles near 90 degrees. Comparing neighbor mu-L diagrams of PSDs regarding each orbit of Van Allen Probes during a storm, we detect >3 times enhancement of oxygen ion PSDs without no enhancement of proton PSDs in >0.5 RE and in the higher energy part, mu>0.4 keV/nT is corresponding to the energy. We regard the selective enhancement of higher-energy oxygen ions as selective transport of higher-energy oxygen ions. We found similar events during the 28 magnetic storms over 89 magnetic storms from February 2013 to September 2017. The selective transport of higher-energy oxygen ions occurs in the dusk-night sector. We also investigate global distributions of magnetic fluctuations during magnetic storms observed by ground magnetometers. The selective transport of higher-energy oxygen ions occurs when Pc5 and Pc4 magnetic fluctuations are enhanced globally and partially, respectively. Our results suggest that the higher-energy oxygen ions are transported selectively to the inner magnetosphere by combination of the drift and the drift-bounce resonances with Pc5 and Pc4 ULF waves.

## 第7回

Date 日時 June 15, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Masanori Iwamoto Wakefield acceleration in two-dimensional relativistic shocks

Abstract 要旨を見る

The origin of high energy cosmic rays has not been fully understood, and the acceleration mechanism is still controversial. Recently Chen et al. (2002) proposed the particle acceleration by the large-amplitude Alfvén waves at gamma-ray bursts as a model of the generation of ultra-high energy cosmic rays, based on the wakefield acceleration (WFA) mechanism which was initially proposed by Tajima and Dowson (PRL, 1979) in the context of laser-plasma interactions in the laboratory. The WFA in laboratory is induced by an intense laser pulse (or transverse electromagnetic waves) propagating in a plasma. The mechanism may also operate in relativistic shocks in nature because it is known that large-amplitude electromagnetic precursor waves are excited by synchrotron maser instability (SMI) driven by the particles reflected off the shock-compressed magnetic field in relativistic shocks (Hoshino and Arons, 1991). In fact, Hoshino (2008) demonstrated the generation of the non-thermal electrons by the wakefield induced by the ponderomotive force of the electromagnetic precursor waves in relativistic magnetized shocks by means of 1D PIC simulation.
In multidimensional systems, it is well known that Weibel instability (WI) develops in the transition region of weakly magnetized shocks. Previous PIC simulation studies in multiple dimensions indeed showed that the shock transition is dominated by the WI at low magnetization. Since both WI and SMI are excited from the same free energy source in the same region and the growth rate of the WI is larger than that of the SMI at low magnetization, it was believed that the WI dominates over the SMI and the precursor wave emission could be shut off in multidimensional shocks.
Recently, by using 2D PIC simulations, we have shown that the SMI can coexist with the WI and that the precursor wave emission continues to persist to the Weibel-dominated regime (Iwamoto et al. 2017). We also showed that the wave power is sufficient enough to induce wakefield for a wide range of magnetization parameter. However, the WFA did not operate in our previous simulation in pair plasmas because the finite mass ration between positive and negative charges is essential for the WFA. To investigate the feasibility of the WFA in relativistic shocks, we carried out 2D simulations in ion-electron plasmas. We found that the wakefield is indeed induced in the upstream. In this presentation, we discuss the acceleration mechanism.

## 第6回

Date 日時 June 1, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Walia Nehpreet A statistical investigation of slow-mode shocks observed in the dayside magnetopause by MMS

Abstract 要旨を見る

The existence of slow-mode shocks in the magnetic reconnection region has been proposed since 1964 [e.g., Petschek, 1964; Levy et al., 1964]. While, there have been many reports on the observation of slow-mode shocks in the magnetotail region [e.g., Feldman et al., 1987; Saito et al., 1995; Eriksson et al., 2004], there are only two event studies which have reported the presence of slow-mode shocks in the magnetopause reconnection [Walthour et al., 1994; Sonnerup et al., 2016]. Many MHD simulations of magnetopause reconnection [e.g., Hoshino and Nishida, 1983; Biernat et al., 1989, Hau and Wang, 2016] have reported the presence of slow-mode shocks and their dependence on the local magnetosphere and magnetosheath parameters. This talk presents the results of statistical investigation of characteristics of slow-mode shocks in the dayside magnetopause of Earth based on MMS observations from September 2015 to February 2017.

## 第5回

Date 日時 May 25, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Naoko Takahashi Global distribution of ULF waves during magnetic storms: Comparison of Arase, ground observations and BATSRUS+CRCM modeling

Abstract 要旨を見る

The relativistic electron population in the Earth’s outer radiation belt is drastically variable, especially during the active condition of the magnetosphere such as magnetic storms. One of the candidate mechanisms to cause the increase or decrease of relativistic electrons is the radial diffusion of the electrons driven by ultra-low-frequency (ULF) waves in Pc5 frequency range (1.6-6.7 mHz). However, when, where, and how relativistic electrons accelerate during individual magnetic storms are still open issue. To understand the temporal variations of the global distribution of ULF waves in the inner magnetosphere, the comprehensive study using a combination of in situ observations and numerical simulations plays an important role. Recently, the Arase satellite successfully detected the ULF wave activity and acceleration of relativistic electrons. We simulate the global distribution of observed ULF waves using the CRCM with BATSRUS model. This talk introduces the acceleration mechanisms of relativistic electrons, and then present recent results and future perspectives.

## 第4回

Date 日時 May 11, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Sara Tomita The Process of Magnetic Field Amplification in Relativistic Collisionless Shock Propagating into the Inhomogeneous Media

Abstract 要旨を見る

The Weibel instability generates magnetic fields in a collisionless plasma with anisotropic temperature, which is thought to be a crucial role for particle acceleration and magnetic field generation in relativistic collisionless shocks. According to observations of afterglows of Gamma-ray Busts (GRBs), magnetic fields are strongly amplified to about 100 times the shock-compressed value in the large downstream region of the relativistic shock. However, recent simulations of collisionless shocks in homogeneous plasmas suggest that the magnetic field generated by the Weibel instability decays rapidly, which cannot explain observed properties of afterglows of GRBs. In reality, there must be density fluctuations in the interstellar medium. In this talk, I’d like to talk about Particle-In-Cell (PIC) simulations of relativistic unmagnetized collisionless shocks propagating into the inhomogeneous plasma. I’ll start from the introduction of GRBs.

## 第3回

Date 日時 April 27, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Naritoshi Kitamura Direct Measurements of Two-Way Wave-Particle Energy Transfer in a Collisionless Space Plasma

Abstract 要旨を見る

Particle acceleration by plasma waves and spontaneous wave generation are fundamental energy and momentum exchange processes in collisionless plasmas. Such wave-particle interactions occur ubiquitously in space. We present ultra-fast measurements in Earth’s magnetosphere by the Magnetospheric Multiscale (MMS) spacecraft that enabled quantitative evaluation of energy transfer in interactions associated with electromagnetic ion cyclotron (EMIC) waves. The observed ion distributions are not symmetric around the magnetic field direction but are in phase with the plasma wave fields. The wave-ion phase relations demonstrate that a cyclotron resonance transferred energy from hot protons to waves, which in turn non-resonantly accelerated cold He+ to energies up to ~2 kilo-electron volts. These observations provide direct quantitative evidence for collisionless energy transfer in plasmas between distinct particle populations via wave-particle interactions.

## 第2回

Date 日時 April 20, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Yutaka Ohira Particle accelerations, plasma instabilities, and collisionless shocks in partially ionized plasmas

Abstract 要旨を見る

The interstellar medium in our galaxy is not always completely ionized. Cosmic rays are thought to be accelerated by shocks propagating to the interstellar medium, which is driven by a supernova. In fact, the existence of neutral hydrogen atoms around supernova remnants has been identified by observations of H-alpha emission. In this talk, I’d like to talk about particle accelerations, plasma instabilities, and collisionless shocks in partially ionized plasmas. I introduce the standard model of cosmic ray acceleration and its problems, then I show results of hybrid plasma simulation with ionization of hydrogen atoms.

## 第1回

Date 日時 April 13, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Kunihiro Keika Ion mass and energy dependence of the contribution to plasma pressure in the Earth’s inner magnetosphere

Abstract 要旨を見る

Singly-charged oxygen ions, O+, which are of Earth’s atmospheric origin, are accelerated up to >100 keV in the magnetosphere. The energetic O+ population makes a significant contribution to the plasma pressure in the Earth’s inner magnetosphere in response to the arrival of solar wind structures such as coronal mass ejections and corotating interacting regions. The O+ pressure frequently becomes comparable to the H+ pressure. Previous in-situ and remote-sensing observations and numerical simulations suggest non-adiabatic and/or selective acceleration of O+ in the near-Earth plasma sheet during substorm activity. O+ can be accelerated by the electric fields generated during the course of magnetic reconnection, at depolarization fronts (reconnection jet fronts) with a steep gradient of the magnetic field, and near the flow-braking regions where the magnetic field dipolarizes and piles up. Characteristics and mechanisms of O+ transport following the outflow from the ionosphere are also keys to understanding the overall O+ pressure enhancements. This talk introduces important science topics and previous studies related to O+ dynamics in the Earth’s magnetosphere, and then presents recent studies using data from the Arase spacecraft.

## 2017年度 太陽地球系物理コロキウム

概要 宇宙空間・太陽・磁気圏・電離圏・大気圏の物理学・化学に関する新しい話題の紹介・研究発表により、この分野の学問の基礎的・包括的な理解を目的とする。 毎週金曜日15:00〜 理学部1号館807Aなど 星野真弘、 関華奈子、横山央明、 天野孝伸 、笠原慧、大平豊、桂華邦裕 桂華邦裕

## 第34回

Date 日時 February 9, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Kanako Seki Solar energetic electron penetration into the Martian upper atmosphere observed by MAVEN

Abstract 要旨を見る

Discovery of diffuse aurora at Mars caused by the SEP (solar energetic particle) electrons [Schneider et al., Science, 2015] sheds a new light on the high-energy particle environment at Mars. Since Mars has no global intrinsic magnetic field, direct interaction between the solar wind and Martian upper atmosphere results in the draping of the interplanetary magnetic field (IMF) around Mars and forms the induced magnetosphere. The diffuse aurora observation in the northern hemisphere, where the crustal field is absent, indicates penetration of the high-energy electrons of ~100 keV down to the altitudes around 70 km most likely along the draped IMF around the planet. However, to what extent the draped magnetic field configuration around Mars controls the SEP electron penetration to the atmosphere is far from understood.
In this study, we investigated three SEP events observed by MAVEN in order to investigate relation between the SEP penetration and magnetic field configuration. The results support the scenario that the solar energetic electrons penetrate into the ionosphere along the draped magnetic fields. It also suggests that global diffuse aurora observations can give us clues to understand interaction between solar wind and Mars, especially the dynamics of the nightside ionosphere.

## 第33回

Date 日時 February 2, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Makoto Suzuki MMS observation of Pc5 waves

Abstract 要旨を見る

Low-frequency MHD waves having periods between 2 to 10 min called Pc5 waves are believed to play a role for acceleration of radiation belt electrons. In particular, the waves which have low azimuthal wavenumber (m number) are capable of accelerating particles via the drift resonance. However, it is suggested that high-m number Pc5 waves generated during storm times may also be important. Such storm-time Pc5 waves may be caused by enhanced particle injections into the inner magnetosphere that excite plasma instabilities. Similar low-frequency waves considered to be generated by plasma instabilities are more frequently observed at L > 9 by the MMS spacecraft. The advantage of MMS is the capability of separating spatial and temporal variations because the MMS consists of four spacecraft.
Here, we focus on such Pc5 waves events observed by the MMS and investigate the characteristic of Pc5 waves by using a multi-spacecraft timing method. We will report results of this analysis.

## 第32回

Date 日時 February 2, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Kasumi Kamizato A study on ultraviolet spectra of organic compounds

Abstract 要旨を見る

In the universe, there is the spectral structure commonly observed at various places in the UV region. It is called “the 2175 Å bump”. According to one of the hypothesis, the bump is thought to be the spectral feature of PAH (Polycyclic Aromatic Hydrocarbon). In our solar system, spectral feature of the surface of Phobos is similar to the 2175 Å bump, suggesting the presence of PAH on Phobos. However, the PAH hypothesis has some problems. One of the problems is stated that the spectral feature of PAH does not perfectly coincide with the 2175 Å bump. In the previous study, the spectrum of another organic compound changed due to be exposed to outer space. Therefore, in my study, I would like to compare the spectral feature of PAH that had been exposed to outer space with the bump.

## 第31回

Date 日時 January 26, 15:45 Room 807, Science Bldg. 1 理学部1号館807号室 Takuma Kato An electron pre-acceleration process in the shock transition region

Abstract 要旨を見る

The acceleration of non-thermal electrons is one of the most important process in space physics and astrophysics. Non-thermal electrons with energies below 1PeV are believed to be accelerated by the DSA (Diffusive Shock Acceleration) [Blandford & Eichler 1987, Drury, 1983]. However, the DSA is not efficient for non-relativistic electrons. Observations imply that there exists a pre-acceleration mechanism that injects non-relativistic electrons to the DSA. This is called the injection problem and has been one of the unsolved issues in the shock acceleration theory.
We here propose a new acceleration mechanism in the shock transition region that is combined by the SDA (Shock Drift Acceleration) [Wu 1984, Leroy and Mangeney, 1984] and pitch angle scatterings via wave-particle interactions. To simplify the analysis, we employ the box model [Drury, 1999], and use the Monte Carlo simulation for modeling pitch angle scatterings. The Monte-Carlo simulations have been performed to investigate the dependence on model parameters such as Mach numbers, shock angles, and pitch-angle scattering coefficients. We find that, for a limiting case where an analytic estimate on the accelerated particle spectra is possible, the simulation results agree with the theoretical prediction. Based on the simulation results, we discuss the properties of the proposed acceleration mechanism in this presentation.

## 第30回

Date 日時 January 26, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Shogo Inui Heavy ion escape from Mars observed by MAVEN

Abstract 要旨を見る

Geological studies have suggested that Mars had a warm climate and liquid water on surface about 4 billion years ago. Now, Mars has a cold surface temperature and little water on surface. Escape of greenhouse gases such as CO2 to space is considered as the plausible reason to cause the drastic climate change. On one hand, mechanisms enabling the large amount of the CO2 loss is far from understood. The planetary ion escape through interaction between the solar wind and the Martian upper atmosphere is one of the candidate mechanisms to achieve the atmospheric escape. To understand atmospheric loss from Mars, MAVEN has observed the ion escape from Mars as well as space environment around Mars since November 2014.
In this talk, we present two studies: event study and statistical study. In the event study, we investigated detailed characteristics of a dense cold ion outflow event observed in the Martian induced magnetotail by MAVEN. We suggested that the combination of the mini-magnetosphere and the -E hemisphere facilitates the cold ion escape from low altitude Martian ionosphere. In the statistical study, we report on a statistical analysis of heavy ion outflows from Mars to investigate influences of the crustal magnetic fields and the direction of solar wind electric field on the ion outflows by using the data of MAVEN. The results show that there are asymmetry in density and velocity between +E and -E hemispheres.

## 第29回

Date 日時 January 19, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Ryuta Asami Mass dependence of ion temperature spatial distributions in the magnetosphere

Abstract 要旨を見る

Heating and acceleration of magnetospheric plasma have been studied using in-situ plasma and field observations. However, dominant heating/acceleration mechanisms and areas are not well understood. Moreover, it remains unclear whether the heating/acceleration mechanism depends on mass. It is thus necessary to investigate ion velocity distributions for several major species in a wide energy range including typical ion energies in the magnetosphere. However, there were a few satellite missions that can observe the thermal component of magnetospheric plasma with mass determination. There have been therefore a small number of studies that focus on mass dependent processes in the typical energy range ( In this study, we investigate heating and acceleration mechanisms of magnetospheric ions by comparing spatial variations of H+, He+, and O+. We use the data obtained by the HPCA instrument on board the MMS satellite which is in a low-inclination elliptical orbit with an apogee of about 12 Re and a perigee of about 1000km. The HPCA instrument can measure H+, He+, and O+ in a few eV to 40 keV energy range with a time resolution of about 10 seconds. We will report on the results of statistical analysis about the average image of H+, O+, He+ temperatures and density spatial distributions in the magnetosphere (6RE – 12RE) by calculating the moment amount using the MMS satellite data for one year.

## 第28回

Date 日時 January 12, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Shotaro Sakai Effects of a weak intrinsic magnetic field on the atmospheric escape from Mars

Abstract 要旨を見る

The magnetization of the planet significantly changes the face of the atmospheric escape. The ion
escape processes of Mars are investigated under only the interplanetary magnetic field (IMF) and
both a weak intrinsic magnetic field and IMF using a magnetohydrodynamics modeling. Existence
of the weak dipole field results in enhancement of the tailward flux of heavy ions through the four
escape channels. Two of them are associated with the open field lines from the cusps, and the others
originate from the open field lines yielded by a reconnection between the dipole field and IMF around
the magnetosheath. The escape rate of O2+ is about a factor of 5 than with the IMF only. It is
suggested that the intrinsic magnetic field encourages escape of heavy ions that are mostly present
in the lower ionosphere.

## 第27回

Date 日時 December 22, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Reina Hikida A study on generation mechanisms for hot electrons in the Jovian inner magnetosphere

Abstract 要旨を見る

In the Jovian inner magnetosphere, the plasma originating from the gas released from Io’s volcano is picked up by the Jovian magnetic field and co-rotated to form the Io plasma torus (IPT). Through in-situ observations in the past, it is known that the electrons inside the IPT are composed of low temperature components (several eV) and trace amounts of high temperature components (several hundred eV). However, unified understanding about the mechanism of generation of hot electrons has not been made yet. Three theories have been proposed as a mechanism for generation of hot electrons. The first is supply by radial transport from the outside of the IPT where abundant hot electrons exist. In the Jovian inner magnetosphere, the plasma flow in the co-rotation direction is dominant, and interchange instability has been suggested to play an important role in the inward transport. However, the study on growth rate of interchange instability and observational evidence are inadequate. The second is heating by the wave particle interaction between ion cyclotron waves and electrons. The third is heating by the field-aligned electron beam in the Io flux tube. In this research, I aim to obtain clues about the mechanism of generation of hot electrons by focusing on the response of density and temperature of plasma in the IPT to the change of the amount of volcanic gas supply. In this presentation, I will introduce the time variation of density and temperature of plasma in the IPT, obtained by using a method called spectral diagnosis for spectral image of IPT acquired by the HISAKI satellite.

## 第26回

Date 日時 December 8, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Nana Higashio Energy dependence of relativistic electron variations in the outer radiation belt during the recovery phase of magnetic storms: Arase/XEP observations

Abstract 要旨を見る

The Arase satellite was launched in December 2016. The extremely high-energy electron experiments(XEP) onboard Arase measures electrons in the energy range of 400 keV – 20 MeV. After the launch, the XEP has observed variations of the relativistic electrons successfully in the inner magnetosphere. There are roughly two candidate processes of electron acceleration. The first one is the adiabatic acceleration due to the radial transport of electrons from the plasma sheet to the inner magnetosphere. Interaction with ultra-low frequency (ULF) waves are a plausible candidate to drive the radial transport. Another acceleration process is the non-adiabatic acceleration of sub-relativistic electrons to the relativistic energies in the heart of the radiation belt. The interaction with very-low frequency (VLF) waves is considered to play an important role in the internal acceleration. One of the science goals of the XEP instrument is to understand the acceleration mechanisms of the relativistic electrons.
In order to investigate the electron acceleration processes, we here focus on three geomagnetic storms occurred on March 27, April 4, and May 28, 2017, respectively. In these events, relativistic electrons in the outer belt showed a typical time variation, i.e., decrease in the main phase and then increase in the recovery phase. On one hand, the increase rates of the electrons are different between the storms. The March 27 storm, which is caused by the arrival of the high-speed coronal hole stream, accompanies a large increase of the relativistic electrons. The April 4 storm, which has a rapid Dst development and recovery, shows less acceleration and does not recover to the pre-storm level. The May 28 storm is caused by a CME and with moderate increase of the relativistic electrons especially in the small L region (L=[3,4]) . We will report on energy dependence of the increase rate and location of the relativistic electrons during the recovery phase, and their comparison between the three geomagnetic storms.

## 第24回

Date 日時 November 24, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Yusuke Imai Development of reflectron mass spectrometer for future planetary exploration

Abstract 要旨を見る

In-situ material measurement in planetary exploration is quite important in understanding origin and evolution of the planets. For the purpose of performing in-situ elemental analysis, mass spectrometers are installed, for example, on NASA’s Curiosity rover and the ESA’s Rosetta spacecraft. However, we still do not have a mass spectrometer that is suitable for the future planetary exploration. Therefore, we have decided to develop a Time Of Flight Mass Spectrometer (TOF-MS) aiming at using for the future planetary exploration. The mass spectrometer that we are developing can also be used for in-situ Potassium-Argon (K-Ar) isochron dating. The instrument for Potassium-Argon (K-Ar) isochron dating is the combination of a laser-induced breakdown spectroscopy (LIBS) for the K concentration measurement and a mass spectrometer for the Ar isotopic measurement. Considering that the instrument should be installed on a planetary lander, there exists limitation on the weight, size and power etc, it is necessary to design a small size mass spectrometer which has a mass resolution capable of the Ar isotopic measurement. In order to minimize the variation of the initial position and initial energy of the ionized ions for maximizing the mass resolution, we have decided to adopt a single-stage reflectron with two-stage acceleration part. We have analytically optimized the design parameters of the reflectron. By using SIMION charged particle simulation software we have confirmed that mass resolution of our TOF-MS is high enough for Ar isotopic measurement. We are aiming to develop the multi-reflector type TOF-MS which has the potential to increase mass resolution under the size constraint. Compared to the single-reflector TOF-MS, the flight path becomes about three times longer which makes the mass resolution of the TOF-MS improved. However, as the flight length increases, variations in the flight path of the ions increase and the detection rate decreases. In order to solve this problem, we have decided to use electron lens for reducing ion dispersion. Under the same conditions as the single-reflector TOF-MS, we confirmed improvement of mass resolution about three times as compared with the single-reflector TOF-MS.

## 第23回

Date 日時 November 17, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takahiro Hasegawa 長谷川 隆祥 Observational study of a solar flare focusing on magnetic helicity

Abstract 要旨を見る

A solar flare is the most energetic phenomenon in our solar system. The origin of energy released by flares is non-potential part of magnetic field in the solar corona. Therefore magnetic helicity, which represents complexity of magnetic field, is regarded as one of important properties of active regions and long term magnetic helicity evolution has been studied well to investigate the process of solar flares (Chae et al. 2001; Kusano et al. 2002). Some of these studies reported that magnetic helicity injection whose polarity is opposite to the global magnetic helicity has an important role on occurrences of energetic solar flares (Park et al. 2010, 2012). However, it isn’t discussed well which characteristics of magnetic field evolution the opposite magnetic helicity injection correspond to. To attack this problem, we studied the magnetic field evolution in the active region NOAA 12297 before and after an X2.1 flare. In the initial stage, the main sunspot of this region rotated in the clockwise direction. However, due to the eastward motion of the emerging flux between the sunspot and another emerging region, the sunspot started to rotate counterclockwise. This rotational motion injected the magnetic helicity opposite to the global magnetic helicity of the active region. On the other hand, average force-free α (i.e. current helicity) in the sunspot didn’t change its sign, but increased. This means the twist of the sunspot magnetic field was enhanced. Our result implies that a reversed rotation of a sunspot on the photosphere and helicity injection opposite to that of global structure have an important role for destabilization of magnetic field and an onset of solar flares.

## 第22回

Date 日時 November 10, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Sooman Han 韓秀萬 A simulation study on time variability of Jupiter’s synchrotron radiation associated with solar-wind-driven electric field inferred from HISAKI

Abstract 要旨を見る

Radiation belt is a layer of energetic particles ( ̃few tens MeV) held by geomagnetic fields, ranging up to several planetary Radii in distance. Jovian Radiation Belt, where in-situ measurement is limited, Jupiter’s synchrotron radiation (JSR) observation is a main tool for determining physical process therein, and various diffusion models have been proposed to account for the observed JSR’s short-term (the total JSR flux density varies by a few % over a few days or weeks) and long-term (10-20% over a few years) variations observed in the past. In this talk, I review the the previous JSR studies and present my ongoing master thesis work based on the recent HISAKI’s finding – conclusive evidence of dawn-to-dusk electric field associated with solar wind ram pressure [Murakami et al. 2016]. From the simulation study, I suggest that a puzzling nature of long-term variation which has time lag of 2-3 years with solar wind ram pressure can quantitatively be addressed.

## 第20回

Date 日時 October 20, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takafumi Doi 土井崇史 Observational study of the formation of coronal sigmoid in the simple solar active region

Abstract 要旨を見る

The origin of solar flare has been identified as free magnetic energy accumulated in solar atmosphere. Sigmoid, which is S-shaped coronal loop seen in soft X-ray, indicates sheared and twisted coronal magnetic structure and likely to carry current and thus free magnetic energy. Thus, in order to understand formation of flare-productive active region, it is important to understand sigmoid formation. It has been shown that sigmoid formation is driven by flux convergence and cancellation between sheared arcades. This process should likely to occur in a complex active region (AR), composed of many pairs of positive and negative sunspot and statistically this is a major trend. Unlike that, there are small number of examples that sigmoid was formed even in simple AR, composed of a pair of sunspot. In the 10-year Hinode satellite observation, NOAA AR 11692 is the unique AR that is simple but sigmoidal and occurred a large scale flare. In order to comprehend the sigmoid formation in the simple AR, we have analyzed the observational data before the flare. The main results should suggest that flux cancellation occurred near the polarity inversion line until a day before the flare and there was mainly two flux ropes related to the flare.

## 第19回

Date 日時 October 13, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Yuto Bekki 戸次宥人 Convective velocity suppression via the enhancement of subadiabatic layer: Role of the effective Prandtl number

Abstract 要旨を見る

It has recently been recognized that the convective velocities achieved in the current solar convection simulations might be over-estimated. The newly-revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this talk, we propose and numerically confirm a possible suppression mechanism of convective velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global convective amplitude in the high-Prandtl thermal convection is thus reduced especially in the lower part of the convection zone via the change in the mean entropy profile which becomes more subadiabatic near the base and less superadiabatic in the bulk.

## 第18回

Date 日時 October 6, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Takehiko Kitahara 北原岳彦 Stationary features at Venusian cloud top observed by Akatsuki UV Imager

Abstract 要旨を見る

Using the cloud image obtained with the Ultraviolet imager (UVI) on board the Venus orbiter Akatsuki, we detect stationary features and investigate their origin. Huge bow-shaped structures extending from northern to southern high latitudes have been discovered by the Longwave infrared camera (LIR), which is also installed in Akatsuki, and such structures have been observed several times. Since they appear above certain highlands and continue to be there against the zonal wind, they are attributed to topographic gravity waves. This study shows that there exist similar features also in other wavelengths,and we estimate the wave parameters by comparing the observed brightness variation and the result of model.

## 第17回

Date 日時 September 29, 15:00 Room 807, Science Bldg. 1 理学部1号館807号室 Lynn Kistler Contributions of Oxygen to the Storm-Time Ring Current

Abstract 要旨を見る

Observations of the storm-time ring current have shown that the O+ contribution to the plasma pressure increases significantly during storms, in some cases becoming dominant during the storm main phase. This is surprising because the direct source of the ring current is the near-earth plasma sheet, and O+ is only rarely dominant there, in terms of either density or pressure. This talk will address three aspects of the oxygen in the ring current to understand how the O+ dominance occurs. First it will address whether the O+ that contributes to the ring current comes predominantly from the cusp outflow or from the nightside auroral outflow. Second, it will address how the O+ can become dominant in the inner magnetosphere, when it is usually not dominant in the plasma sheet. Finally it will address whether the high charge state oxygen from the solar wind can play a role in providing oxygen to the inner magnetosphere.

## 第16回

日時 9/22 15:00 理学部1号館807号室 Walia Nehpreet A statistical study of slow-mode shocks observed by MMS in the dayside magnetopause

Petschek’s reconnection theory [1964] provides a means for faster reconnection by creating X-line geometry with two pairs of slow- mode shocks. Earth’s magnetosphere acts as a natural laboratory to investigate the presence and role of these slow-mode shocks. Considerable amount of studies have reported the presence of the slow- mode shocks in the magnetotail [e.g. Feldman et al., 1987; Saito et al., 1995; Erikson et al., 2004] but only a few have reported the slow-shocks in the magnetopause [Walthour et al., 1994; Sonnerup et al., 2016]. The slow-shocks are observed in the magnetosheath side and/or magnetosphere side of the magnetopause. These studies suggest that strong pressure anisotropy and presence of cold ions could play an important role in determining the structure of the slow-mode shocks in the magnetopause. These studies also report the presence of rotational discontinuity and theoretical studies have also indicated that the magnetopause consists of multiple MHD discontinuities [e.g., Hau and Wang, 2016]. The solar wind conditions as well as the local conditions in the magnetosphere can affect the structure of the magnetopause. One of the reasons of the small number of the slow-shock events reported for the magnetopause is the lack of the high time resolution data to separate multiple discontinuities before MMS. Thus, an exhaustive study with many events is needed to understand the underlying physics of the slow- shocks in the magnetopause.
Here we present a statistical study of slow-mode shocks in the dayside magnetopause crossings observed by MMS (Magnetospheric Multiscale). For this study, we used the data from FGM and FPI instruments onboard the MMS satellites. Fast survey data were analyzed from 1st Sept, 2015 to 31st Jan, 2017. For event selection, we checked the southward IMF magnetopause crossings with jet (|Vgsm|  200 km/s). We ensured the presence of the magnetosheath side in our events by using Plasma Beta > 1 and MA < 1 conditions. The events obtained by using this criterion were then checked by using burst mode data and incomplete magnetopause crossings were removed to get a set of 71 full crossings from the magnetosheath to the magnetosphere. Rankine-Hugoniot analysis was applied on these crossings after determining two separate deHoffmann- Teller frames for each side. Out of these 71 crossings, 23 magnetopause crossings were identified as the slow-mode shocks. Among the 23 events, 13 events contained slow-shock on the magnetosheath side whereas 10 contained slow-shock on magnetosphere side. We will report on the relation of these slow-mode shocks with solar wind conditions and the local parameters.

## 第15回

日時 7/21 15:00 理学部1号館807号室 高田雅康 Differences among species in the magnetosphere and the interaction with ULF waves

There are many differences among species which compose the plasma in the magnetosphere. Especially when it comes to the interaction between particles and ULF waves, they play important roles in order to understand mechanisms such as the particle acceleration process, the particle diffusion process and so on. I will talk about what have been discussed so far by introducing some basic papers on the differences after brief self-introduction and show the results of ERG data I’m focusing on now.

## 第14回

日時 7/14 15:00 理学部1号館807号室 天野孝伸 In-situ signatures of whistler-electron interactions in the Earth’s bow shock and the implication for the electron injection

The acceleration of nonthermal particles is one of the most important problems in space physics and astrophysics. The standard first-order Fermi mechanism needs a seed population. It must be provided by some microscopic plasma processes occurring in the collisionless shock transition layer, which is
known as the injection problem. Recently, NASA’s MMS spacecraft with its unprecedented temporal resolution revealed the electron scale dynamics in the shock for the first time. I will discuss how the observed signatures fit into proposed theories of electron acceleration that may ultimately lead to the injection. Similarities and dissimilarities between the observations and fully kinetic particle-in-cell simulation results will also be discussed.

## 第12回

日時 6/30 15:00 理学部1号館701号室 岩本昌倫 Particle Acceleration in Relativistic Magnetized Pair Shocks

The origin of ultra-high-energy cosmic rays (UHECRs) is not well-understood. UHECRs are believed to be generated in active galactic nuclei (AGNs) and gamma-ray bursts (GRBs).Since the shock wave associated with the relativistic outflow form the compact central object is the common feature in these environment, Fermi acceleration is one of the most commonly proposed mechanisms for producing UHECRs. However, it is well known that the Fermi acceleration becomes less efficient in a relativistic shock propagating in a magnetized plasma. Therefore, other particle acceleration mechanism is required to explain the origin of UHECRs.
In this talk, we will show the non-thermal particle acceleration for relativistic shocks propagating in magnetized pair plasmas by using two-dimensional PIC simulation. The particle acceleration efficiency was measured as a function of the magnetization parameter. Based on this results, we discuss the non-thermal particle acceleration for astrophysical application.

## 第11回

日時 6/23 15:00 理学部1号館807号室 星野真弘 Ion and Electron Acceleration during Magnetic Reconnection

Magnetic reconnection receives a great deal of interest for its role in explosive magnetic energy release in many space and astrophysical phenomena such as earth’s substorms and solar flares. Not only the hot plasma production but the nonthermal high energy particle acceleration has been reported by modern satellite observations, but the strong ion acceleration events are not necessarily observed compared to the electron acceleration. PIC simulation studies have also demonstrated the high efficient electron acceleration in several key regions such as the diffusion region, the magnetic field pile-up region, and the plasma sheet boundary and so on, but the ion acceleration have not necessarily clearly demonstrated in the simulation, and is still controversial whether or not the efficient ion acceleration can occur during reconnection. We study both electron and ion accelerations in a driven reconnection system by using PIC simulations, and elucidate the high energy ion acceleration. We show in two-dimensional PIC simulation that not only the electron acceleration but also the ion acceleration can happen by injecting a finite Poynting flux from the upstream boundary. However, in three-dimensional system, the lower-hybrid-drift instability can be excited in the plasma sheet boundary, and the hot electron plasma with a flat-top velocity distribution function are generated. Therefore, the energy transfer to the electron heat occurs faster than ion, and the ion acceleration becomes less efficient compared to the electron acceleration.

## 第10回

日時 6/16 15:00 理学部1号館807号室 河野隼也 MHD wave propagation and heating in the solar chromosphere

The chromospheric heating is a major problem in solar physics. In the chromosphere, the dominant mechanism of energy loss is radiation. The mechanism that heats the chromosphere and maintains the temperature profile remains unclear. MHD waves, which are generated by convective motion in the photosphere, have been suggested to carry sufficient energy to the upper solar atmosphere and cause heating through wave dissipation.
In this talk, we report on our numerical works of MHD wave propagation from the convection zone to the corona. In one-dimensional MHD simulations, we calculated the transmission rate of Alfven waves from the boundaries of the chromosphere, and estimated the energy used for the chromospheric heating. We are going to extend this work to two-dimensional study and investigate the propagation of magnetoacoustic and Alfven wave modes in the solar atmosphere.

## 第9回

日時 6/9 15:00 理学部1号館807号室 三谷憲司 Deeper penetrations of oxygen ions than protons into the inner magnetosphere observed by Van Allen Probes

It is observationally known that protons and oxygen ions are the main components of the ring current during magnetic storms and are considered to have different source and supply mechanisms. In order to characterize the ion supply to the ring current during magnetic storms, we study the properties of energetic proton and oxygen ion phase space densities (PSDs) during the 23-25 April 2013 geomagnetic storm observed by the Van Allen Probes spacecraft. We calculated ion PSDs for specific first adiabatic invariants ( for proton; for oxygen ion) and the local pitch angles near 90 degrees. The PSD profiles as a function of L show that both proton and oxygen ions penetrated to L < 5 during the main phase of the magnetic storm. The timing of oxygen ion penetration was approximately the same for all values. The observations also show that oxygen ions penetrated more deeply in L and earlier in time than protons for the same value. We discuss the possibility that the interaction between >200 keV oxygen ions and Pc3 or Pc5 ULF waves in the inner magnetosphere causes selective transport of oxygen ions. Our results imply the importance of the contribution from >200 keV oxygen ions to the storm-time ring current.

## 第8回

日時 6/2 15:00 理学部1号館807号室 Yikang Wang Effect of radiative loss in chromosphere to spicule formation and wave propagation

Radiation is one of the major tools for observing the sun. In the chromosphere, it is even more important since it is a main source of energy loss, that has a significant influence on chromosphere dynamics. While previous studies are likely to ignore radiative loss due to its difficulty, we perform 1D radiative MHD simulation and shown that the height of transition region and temperature distribution is affected by radiative loss obviously. The effect on temperature distribution also changes the Mg II line profiles. On the other hand, the energy flux transported to corona, which is crucial to coronal heating, however, is not affected decidedly.

## 第7回

日時 5/26 15:00 理学部1号館807号室 横山央明 Solar chromospheric dynamics by ALMA observations

From Cycle-4 of the ALMA proposal period, solar observation capability became open to the community. In the millimeter and sub-millimeter range, the solar chromosphere is a main target for studies. In this talk, we would like to describe the content of our proposal for the solar observations by using ALMA. Our team proposed an observation of chromospheric spicules; the needle-like jets ubiquitously found in the chromosphere, though their driving mechanism is still under debate. The observation has finished on late-April but, unfortunately the data is not yet delivered. So we describe what we are expecting to see based on our numerical simulations.

## 第6回

日時 5/19 15:00 理学部1号館807号室 庄田宗人 High-frequency chromospheric Alfven waves generated via mode conversion

Alfvenic waves propagating along spicules have high-frequency components whose typical period is around 40-50 sec. Because the typical period of photospheric oscillations is a few minutes, the origin of this high-frequency component is not trivial. Using one-dimensional numerical simulation, we show that these high-frequency waves come from longitudinal-to-transverse mode conversion occurring around the equipartition layer. Our calculation is performed in a self-consistent manner, except an additional heating that maintains coronal temperature. We show that (1) mode conversion efficiently excites high-frequency transverse waves; (2) the typical period of the high-frequency waves is explained as the sound-crossing time of the mode conversion region; (3) simulated root-mean-square velocity of high-frequency component is consistent with the observed value, respectively. Our result indicates that the observed oscillation shows high enough amplitudes if we take into account the low-frequency components.

## 第5回

日時 5/12 16:00 理学部1号館807号室 Shuoyang Wang Reconnection pattern in 3D MHD reconnection

Magnetic reconnection is one of the most important fundamental processes in plasma physics. The fast magnetic energy conversion is always an interesting issue since it helps to understand many eruptive astronomical events, such as solar flares. As the classical models fail to explain the fast reconnection as observations suggested, advanced models come into view. Presently, the three-dimensional (3D) turbulence reconnection becomes a hot topic. Many studies prove that reconnection rate becomes independent on diffusivity when turbulence is highly developed, but the details for energy transfer or how the reconnection pattern changes are still not clear. In our study, we implement eigenfunctions of tearing instability to initiate reconnection in a sheared current sheet. By doing so, we could track the energy cascade in a clear way by observing the coupling between two tearing layers. We notice that the cascade primarily is by the coupling of the initial perturbed tearing layers. Then a transfer of energy is initiated along the guide field direction near the current sheet boundary on the newly convected in magnetic field. They activate new tearing instability and create new diffusion regions. On the other hand, the mode with highest energy is along the anti-parallel field at the sheet center. New diffusion regions couple with each other, which further enhances reconnection. We believe that this secondary coupling is the key to understand the final state of turbulence reconnection.

## 第4回

日時 4/28 15:00 理学部1号館807号室 高橋直子 Evolution and propagation of electric fields during magnetospheric disturbances based on multiple spacecraft and ground-based observations

The Earth’s magnetosphere is affected by the solar wind. The input from the solar wind leads to the global variation of the particle and electromagnetic field, which triggers magnetospheric disturbances. Among these processes, there is global evolution of electric fields, which involve the energy transmission from the solar wind and the development of large-scale convection and current systems. Thus, the evolution and propagation of electric fields is essential to understand the electromagnetic energy transmission in the magnetosphere and the magnetosphere-ionosphere (M-I) coupling system. The studies on the electromagnetic energy transmission have been performed for more than half of a century. However, there are few papers that have focused on the electric field variation due to the lack of simultaneous and multi-point in-situ measurements in the magnetosphere and ionosphere.
For the last decade, many satellites have been launched and widely distributed in the whole M-I coupling system. It can provide us a great opportunity to investigate the spatial and temporal evolution of electric fields in the magnetosphere and ionosphere. Taking advantage of this opportunity, we focus on electric field as a key parameter to clarify the electromagnetic energy transmission, and investigate the evolution and propagation processes of electric fields in the M-I coupling system. First, we deal sudden commencements (SCs) known as the phenomenon associated with the compression of the magnetosphere by the enhancement of the solar wind dynamic pressure. Unlike magnetic storms and substorms, which involve complex plasma physical processes, SCs can be identified as distinct magnetic variations that sharply change on a global scale. Those magnetic variations are caused by shock waves and discontinuity in the solar wind and propagate through the magnetosphere. From the three-dimensional evolution and propagation of electric and magnetic fields in the M-I coupling system, SCs contributes to the identification of the magnetospheric response to the outer disturbances, which is the basis to understand the whole magnetospheric reaction processes. Next, we verify whether the established evolution and propagation processes are applicable to more complex plasma physical process, such as substorms.
We investigated the following three topics:
1. Response of ionospheric electric field at mid-low latitude based on ROCSAT-1 satellite
2. Evolution and propagation of electric fields in the M-I coupled system based on multiple spacecraft (THEMIS, RBSP, GOES, and C/NOFS) and ground-based observations (SuperDARN and magnetometers)
3. Propagation of Pi2 pulsations in the M-I coupled system based on spacecraft (THEMIS and RBSP) and ground-based observations (THEMIS-ASI and magnetometers).
In December 2016, Japan Aerospace Exploration Agency (JAXA) successfully launched the Exploration of energization and Radiation in Geospace (ERG) satellite, which recently renamed ‘ARASE’, into the inner magnetosphere. Our results can contribute to one of the ARASE’s science goal of understanding how the electric field in M-I coupling system evolves and propagates during the geospace storms.

## 第3回

日時 4/21 15:00 理学部1号館807号室 成影典之 Next satellite mission for the investigation of magnetic reconnection and particle acceleration

We plan to realize the satellite mission for the detailed investigation of magnetic reconnection and particle acceleration. The observation target is the sun (with 5 degree offset to observe other heavenly bodies, namely, Crab Nebula). The observed wavelength is soft X-rays (0.5 – 10 keV), hard X-rays (5 – 20 keV) and soft gamma rays (up to 600 keV). For the X-ray observations, we use new observation technology as follows: In the soft X-ray observation, the imaging spectroscopic observation will be realized for the first time in the solar coronal observation using a high-speed CMOS camera. In the hard X-ray observation, we will use focusing mirrors for the higher dynamic range than the existing (modulation collimator type) hard X-ray telescope. These instruments are designed to investigate the region around the X-point, where key phenomena related to reconnection, i.e., shocks, particle acceleration, etc.,are predicted. Now, we are organizing a working group for this mission as an consortium among solar, magnetosphere, astronomy, and laboratory groups.

## 第2回

日時 4/14 15:00 理学部1号館807号室 桂華邦裕 Contribution of ionospheric oxygen ions to plasma pressure in the Earth’s inner magnetosphere: Relative importance of enhanced outflow and local acceleration

Singly-charged oxygen ions, O+, which are of Earth’s atmospheric origin, are accelerated up to >100 keV in the magnetosphere. The energetic O+ population makes a significant contribution to the plasma pressure in the Earth’s inner magnetosphere in response to the arrival of solar wind structures such as coronal mass ejections and corotating interacting regions. The pressure enhancements are caused by adiabatic heating through earthward transport of source population in the plasma sheet, local acceleration in the inner magnetosphere and near-Earth plasma sheet, and/or enhanced ion supply from the topside ionosphere. Although several acceleration mechanisms and O+ supply processes have been proposed, it remains an open question what mechanism(s)/process(es) play the dominant role in O+ pressure enhancements.
In this talk, I introduce previous important observational studies and ongoing/future research on heavy ion transport and acceleration. Examples are remote-sensing observations that indicate oxygen non-adiabatic acceleration; in-situ observations that suggest adiabatic transport of pre-existing warm (100 eV to 10 keV) oxygen ions. Another study to be presented is about the long-term evolution of energetic ion energy spectra during an unexpectedly intense magnetic storm.

## 第1回

日時 4/7 15:00 理学部1号館807号室 顔合わせと趣旨説明

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