|参加教員||星野真弘、 関華奈子、横山央明、 天野孝伸 、笠原慧、桂華邦裕|
|Date 日時||October 20, 15:00|
|Place 場所||Room 807, Science Bldg. 1 理学部1号館807号室|
|Speaker 話者||Takafumi Doi 土井崇史|
|Title 題目||Observational study of the formation of coronal sigmoid in the simple solar active region|
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.
|Date 日時||October 13, 15:00|
|Place 場所||Room 807, Science Bldg. 1 理学部1号館807号室|
|Speaker 話者||Yuto Bekki 戸次宥人|
|Title 題目||Convective velocity suppression via the enhancement of subadiabatic layer: Role of the effective Prandtl number|
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.
|Date 日時||October 6, 15:00|
|Place 場所||Room 807, Science Bldg. 1 理学部1号館807号室|
|Speaker 話者||Takehiko Kitahara 北原岳彦|
|Title 題目||Stationary features at Venusian cloud top observed by Akatsuki UV Imager|
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.
|Date 日時||September 29, 15:00|
|Place 場所||Room 807, Science Bldg. 1 理学部1号館807号室|
|Speaker 話者||Lynn Kistler|
|Title 題目||Contributions of Oxygen to the Storm-Time Ring Current|
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.
|題目||A statistical study of slow-mode shocks observed by MMS in the dayside magnetopause|
Petschek’s reconnection theory  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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.
|題目||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.