場所は研究所共同館 3階 講義室です。
|6/28 Thu||Shinji Saito||The influence of intermittent nature of magnetosonic-whistler turbulence on ion dynamics|
Solar wind turbulence contains fluctuations with a broad range of frequency and wavenumber. Nonlinear cascade process in the solar wind turbulence transports fluctuation energy from MHD to ion/electron kinetic scales. At the kinetic scales, kinetic Alfven and whistler mode waves are observed at kinetic scales where the MHD approximation is broken. Turbulent fluctuations at kinetic scales can interact with ions and electrons. The wave-particle interactions cause particle acceleration and heating through the dissipation of the kinetic turbulence.
In this study we demonstrate the influence of magnetosonic-whistler turbulence on ion acceleration and heating by using a two-dimensional particle-in-cell simulation of plasma turbulence. It is shown that probability distribution functions of the magnetosonic-whistsler turbulence indicate intermittent nature of compressible magnetic fluctuations propagating to the quasi-perpendicular direction of the mean magnetic field. The intermittent magnetic fluctuations make steep magnetic gradients, whose electrostatic potential field is expected to be a source of the perpendicular ion heating and acceleration. The fully kinetic particle-in-cell simulation suggests that ions and electrons in magnetosonic-whistler mode turbulence are accelerated/heated by not only Landau and cyclotron resonance by linear modes at electron kinetic scales but also nonlinear scattering related to ion kinetic scales.
|6/21 Thu||Shinsuke Imada||Effect of collisionality and partial ionization on magnetic reconnection|
|Abstract : One of the most famous rapid energy conversion mechanism in space is a magnetic reconnection. The general concept of a magnetic reconnection is that the rapid energy conversion from magnetic field energy to thermal energy, kinetic energy or non-thermal particle energy. The understanding of rapid energy conversion rates from magnetic field energy to other energy is the fundamental and essential problem in the space physics. One of the important goal for studying magnetic reconnection is to answer what plasma condition/parameter controls the energy conversion rates. Recently, solar atmosphere has been focused as a space laboratory for magnetic reconnection because of its variety in plasma condition. So far considerable effort has been devoted toward understanding the energy conversion rates of magnetic reconnection, and various typical features associated with magnetic reconnection have been observed in the solar atmosphere with the modern spacecraft/ground base telescopes. In this talk, we first introduce the variety of plasma condition/parameter in solar atmosphere. Later, we discuss the effect of collisionality and partial ionization on magnetic reconnection.|
|6/14 Thu||Takayuki Umeda||Does the "cyclic self-reformation of perpendicular shocks" really exist in the real space plasma?|
Large-scale two-dimensional (2D) full particle-in-cell (PIC) simulations are carried out
for studying periodic self-reformation of a supercritical collisionless perpendicular shock
with an Alfven Mach number ~6. |
Previous self-consistent one-dimensional (1D) hybrid and full PIC simulations have demonstrated that the periodic reflection of upstream ions at the shock front is responsible for the formation and vanishing of the shock foot region
on a timescale of the local ion cyclotron period, which was defined as the reformation of (quasi-)perpendicular shocks.
The present 2D full PIC simulations with different ion-to-electron mass ratios show that the dynamics at the shock front is strongly modified by large-amplitude ion-scale fluctuations at the shock overshoot, which are known as ripples.
In the run with a small mass ratio, the simultaneous enhancement of the shock magnetic field and the reflected ions take place quasi periodically, which is identified as the reformation. In the runs with large mass ratios, the simultaneous enhancement of the shock magnetic field and the reflected ions occur randomly in time, and the shock magnetic field is enhanced on a timescale much shorter than the ion cyclotron period.
These results indicate a coupling between the shock-front ripples and electromagnetic microinstabilities at the foot region in the runs with large mass ratios. Some comments on the spacecraft in-situ observations of the reformation are also given.
|6/6 wed||takefumi Kaneko||Numerical modeling of solar prominence eruption|
|Abstract : A MMS satellite and EISCAT radar case study We reproduced solar prominence eruption using 2.5-dimensional magnetohydrodynamic (MHD) simulations including nonlinear anisotropic thermal conduction and optically thin radiative cooling. Solar prominences are cool dense plasma clouds in the hot tenuous corona. Some prominences erupt into space and cause coronal mass ejection. Prominence eruption is driven by explosive release of magnetic energy stored in the corona. The physical role of prominence for such an explosive event is unclear and under debates for decades in solar physics. Numerical modeling is one way to understand the physical mechanism of prominence eruption. Only a few numerical simulations have been carried out for prominence eruption due to difficulty in reproducing the cool dense plasmas of prominence. In this study, we succeeded in modeling prominence eruption by combination of a prominence formation model (Kaneko & Yokoyama, 2017) and a flare trigger model (Kusano et al, 2012). In the present simulations, a magnetic bipole is injected into the coronal arcade fields. The structure of the coronal magnetic field is changed into a flux rope after reconnection with the bipole. The prominence is formed inside the flux rope by radiative condensation (thermal instability), and the flux rope hosting prominence is ejected by torus instability. We carried out a parameter survey on magnetic flux injected by the bipole. As a result, when the injected flux is smaller than the coronal magnetic flux, magnetic free energy increases during mass condensation of prominence, resulting in fast eruption of dense prominence. We conclude that even when the amount of the injected magnetic energy is small, the prominence condensation supplies extra magnetic free energy for the coronal magnetic field and facilitates eruptions.|
|5/31 Thu||Akimasa Ieda||Dayside magnetic reconnection and subauroral ionospheric flow bursts:|
|Abstract : A MMS satellite and EISCAT radar case study An impulsive ion flow was found in the dayside subauroral ionosphere when magnetic reconnection was impulsively enhanced. Magnetospheric multiscale (MMS) satellites were located near the subsolar magnetopause on 15 December 2015. MMS satellites observed a southward turning of the interplanetary magnetic field (IMF) at 1030 UT, followed by a magnetic flux transfer event (FTE) 18 minutes later at 1048 UT. Simultaneously, a poleward moving auroral form (PMAF) was observed by a 630 nm all-sky imager at Longyearbyen (75 degrees in magnetic latitude, MLAT) in Norway. However, a number of other PMAFs were also observed, making it difficult to conclude an association between the FTE and the PMAF. An ionospheric response of the FTE was rather uniquely identified in the subauroral latitudes by the European incoherent scatter (EISCAT) VHF radar at Tromso (Norway). The radar was pointed to geographic north, with an elevation angle of 30 degrees, and was monitoring the ionospheric F region near 13 hour in magnetic local time between 68 and 75 MLAT. The Tromso radar did not observe an impulsive ionospheric flow below 72 MLAT at the time of the IMF southward turning but instead at the time of the FTE. We infer that a large-scale FTE removes significant magnetic flux and leads to a subauroral impulsive flow as a rarefaction inflow. Because small-scale FTEs may be missed by satellite in-situ observations these results do not exclude a possibility of a one-to-one association between FTEs and PMAFs.|
|5/10 Thu||Satoshi Masuda||Collaborative researches on solar flares between Nobeyama Radioheliograph and New Chinese Solar Radio Telescope (MUSER)|
|Abstract : Recently, a new solar radio telescope (MUSER) has been built in China and soon it will start observations regularly. MUSER observes the sun in the different frequencies from Nobeyama Radioheliograph (NoRH). Combining these two telescopes, we can derive full-sun radio images in the wide frequency range from 0.4 GHz to 34 GHz. This is a quite new dataset in the history of solar radio observations in the world. Using this dataset, we would like to study mainly the following topics, 'particle acceleration process in solar flares' and 'Quasi-periodic pulsation bserved during a solar flare'. The operation of NoRH will be finished at the end of March in 2020, so the two years from now is very important to realize simultaneous observation between MUSER and NoRH.|
|4/26 Thu||Mariko Teramoto||Periodic modulations of energetic electron flux and the spatial distributions of Pc5 pulsations observed by the ERG satellite.|
|Abstract : One widely accepted scenario for the dynamic process of the radiation belt is that ULF oscillations in the Pc5 frequency band drive radial transport. When the electron drift period matches the wave period ULF waves cause violation of the third adiabatic invariant with the first and second invariants conserved. In previous studies, Radiation Belt Storm Probes (RBSP) observations show the evidence for drift-resonant interaction between energetic electron and ULF waves in the inner magnetosphere [Claudepierre et al., 2013, Hao et al.,2014 ].However, spatial variations of energetic electrons affected by ULF waves via drift resonance in the inner magnetosphere have not been reported. To understand spatial properties of energetic electron flux modulations, we compared energetic electron flux modulations observed by RBSP and Exploration of energization and Radiation in Geospace (ERG) satellites separated in longitude. In this presentation, we show that the energetic electron flux modulations over an energy range from 500keV to 2.5 MeV were simultaneously observed in the dawn and dusk sectors by the ERG and RBSP, respectively. While the RBSP satellite observed compressional Pc5 pulsations with the small amplitude in the magnetic field data, no Pc5 pulsations appeared in the magnetic field data obtained by the ERG satellite. We concluded that energetic electron flux were modulated by Pc5 pulsations, which was localized in the noon-dusk sector, via drift-resonance interaction. In this present, we will also present the spatial distributions of Pc5 pulsations observed by the ERG satellite from March 2017 to March 2018.|
|4/17 Tue||Shinnosuke Ishikawa||Investigations of high energy plasma in the Sun by high sensitivity X-ray imaging and spectroscopic observations|
|Abstract : X-ray observations of the Sun is a unique and important tool to investigate energy release processes by detecting high energy plasma including super hot (>10 MK) plasma and accelerated non-thermal particles. The Yohkoh satellite successfully performed X-ray observations of the Sun and contributed to understanding of physical processes of solar flares. For further investigations of the energy release processes in the Sun, high sensitivity imaging and spectroscopic observations are desired to investigate detailed physical processes of energy releases and detect smaller events to evaluate contributions to heating of the corona. However, we had two technical difficulties; sensitivity in hard X-rays, and capability of imaging and spectroscopic observations in soft X-ray range. The sensitivity limitation was caused by an “indirect” imaging technique of past and current instruments, such as modulation collimators. To overcome this difficulty, we performed a sounding rocket experiment Focusing Optics Solar X-ray Imager (FOXSI). FOXSI provides the first focusing observations in hard X-ray energy range, and achieves superior sensitivity to detect faint sources. We performed two successful launches and found tiny energy releases which were not detectable before FOXSI. We also work on soft X-ray imaging spectroscopy of the Sun with high-speed CMOS sensors. Sun was too bright in soft X-rays and time scales of energy release events are too short, it was difficult to perform imaging and spectroscopic observations of the Sun with CCD cameras used in past and current instruments. We will achieve the first photon counting imaging spectroscopy of the Sun by the next launch of the FOXSI sounding rocket experiment this summer. Based on those technologies, we have proposed a satellite mission Physics of Energetic and Non-thermal plasmas in X-region (PhoENiX) to perform high sensitivity soft and hard X-ray imaging and spectroscopic observations. The PhoENiX proposal is now under review for the JAXA satellite to be launched in mid 2020s.|