|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|
|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.