総合解析セミナー

総合解析セミナーは基本的に、毎週木曜日の16:30から行われます。
場所は研究所共同館 3階 講義室です。

date Speaker title
9/13 Thu Naoyuki Ishiguro Simulation study of the MHD instabilities for solar flares and coronal mass ejections
Abstract : Solar explosive phenomena, such as solar flares and coronal mass ejections, may disturb the whole heliosphere including the magnetosphere of the Earth. Many previous observations suggested that the eruption of twisted flux rope accompanies the solar explosive phenomena. Recently, we theoretically proposed the Double Arc Instability (DAI, Ishiguro & Kusano 2017) as an initial driver of the solar eruptions. This theory suggests that solar flare can be triggered by the DAI which destabilizes the concaved double arc flux rope with strong electric current which is formed through the magnetic reconnection of sheared magnetic loops. The critical condition of the DAI is determined by the parameter , which is defined as the product of the magnetic twist of a double arc loop and the ratio between magnetic flux contained in the double arc and the overlying flux. Since the DAI can stimulate the torus instability (TI) which can cause the full eruption of flux rope, the DAI might be responsible also for the formation of coronal mass ejection (CME). To exam how the DAI and the TI can work for the onset of flares and CMEs, we developed 3-dimensional zero-beta magnetohydrodynamics (MHD) simulation by parameterizing the length scales of magnetic gradient around the polarity inversion line (PIL) and the external magnetic field. We numerically form a sheared magnetic field around the PIL by imposing slow twisting motion and trigger the tether cutting reconnection by injecting a small magnetic bi-pole field on the PIL. As a consequence, we confirm the twisted flux rope of concaved structure is formed through magnetic reconnection of the sheared field. In this presentation, we report and discuss the relationship between the dynamics of flux rope and the instabilities.
9/6 Thu Yoshizumi Miyoshi Pulsating aurora as an element of electron microbursts
Abstract : The short-bursts of electron precipitations are commonly observed by the low-altitude satellites. The precipitation of electrons with a few keV - tens keV causes the pulsating aurora. The precipitation of sub-relativistic and relativistic electrons has been known as the microbursts of the radiation belt electrons. Here, we show that the electron precipitations from keV to more than MeV energy range are same phenomenon. Based on a model of Miyoshi+[2010,2015], we conduct a computer simulation of GEMSIS-RBW [Saito+, 2012] on the wave-particle interactions between the whistler mode chorus waves and the trapped electrons along the field line. The resonance energy becomes high, when the whistler mode waves propagate to higher latitude, causing wide energy electron precipitations from keV to more than MeV. The time variation of precipitating electrons depends on their energy. The main precipitations with a period of a few second and the internal modulations within a few hundred msec can be observed at the lower-energy electrons, and these modulations are caused by the whistler mode chorus bursts and the rising tone elements, respectively. On the other hand, the discrete precipitations within a few hundred msec can be found in the energy above a few hundred keV. These precipitations are microbursts of relativistic electrons, caused by rising tone elements of whistler mode chorus. From this study, we propose a unified model for both the pulsating aurora electrons and energetic electron microbursts. The pulsating aurora electrons are a part of electron microbursts, and both tens keV electrons and sub-/relativistic electron precipitations are commonly caused by the propagation of whistler mode chorus. In this seminar, we also introduce our new sounding rocket experiment to observe the wide energy electron spectrum from a few tens keV to more than MeV to confirm this model.
7/26 Thu Takuma Matsumoto Thermal responses in a coronal loop maintained by wave heating mechanisms
Abstract : A full 3-dimensional compressible magnetohydrodynamic (MHD) simulation is conducted to investigate the thermal responses of a coronal loop to the dynamic dissipation processes of MHD waves. When the foot points of the loop are randomly and continuously forced, the MHD waves become excited and propagate upward. Then, 1-MK temperature corona is produced naturally as the wave energy dissipates. The excited wave packets become nonlinear just above the magnetic canopy, and the wave energy cascades into smaller spatial scales. Moreover, collisions between counter-propagating Alfv´en wave packets increase the heating rate, resulting in impulsive temperature increases. Our model demonstrates that the heating events in thewave-heated loops can be nanoflare-like in the sense that they are spatially localized and temporally intermittent.
7/19 Thu Satoshi Inoue Magnetohydrodynamic Modeling of a Solar Eruption Associated with X9.3 Flare Observed in Active Region 12673
Abstract : On SOL2017-09-06 solar active region 12673 produced an X9.3 flare which is regarded as largest to occur in solar cycle 24. In this work we have preformed a magnetohydrodynamic (MHD) simulation in order to reveal a three-dimensional (3D) dynamics of the magnetic fields associated with the X9.3 solar flare. We first reconstructed 3D magnetic fields based on the observed photospheric magnetic field prior to the flare and then inserted them into the MHD simulation as the initial state. Consequently, the simulation showed a dramatic eruption. In particular, we first found that a large coherent flux rope composed of highly twisted magnetic field lines is formed during the eruption. Although the small flux ropes are found prior to the flare, a reconnection occurring among them plays an important role to make the large and highly twisted flux rope. Furthermore, we found the writhing motion of the eruptive flux rope. The understanding of these dynamics is important in increasing the accuracy of the space weather forecast. We will present the detailed dynamics of the 3D eruptive flux rope and discuss the possible mechanisms of the writhing motion.
7/12 Thu Tomoaki Hori Azimuthally propagating ionospheric flow fluctuations during storm times as seen from satellite-SuperDARN conjunctions
Abstract : The recent Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of the mHz or lower frequency range appear even in the subauroral to mid-latitude region during magnetic storm times. An intriguing feature of the flow fluctuations is that they appear to propagate azimuthally either westward or eastward, and occasionally bifurcate toward the both directions. Taking a closer look with high spatial resolution measurements provided by the radars reveals that those flow fluctuations consist of meso-scale patchy structures of ionospheric convection with a significant latitudinal flow component and a longitudinal scale of ~1h MLT. The azimuthal propagation properties strongly suggest that westward-drifting ions and eastward-drifting electrons of tens of keV in the inner magnetosphere can be the moving sources responsible for excitation of the flow fluctuations seen at the ionospheric height. Recent observations in the inner magnetosphere by the Arase satellite and the Van Allen Probes have provided excellent evidence for it as well as a good opportunity to examine their magnetospheric counterpart in further detail. The close conjugate observations of the radars and the satellites reveal that multiple drifting clouds of ions and electrons can be mapped to the electric field fluctuations propagating westward and eastward, respectively, in the ionosphere. The most likely interpretation for it would be that meso-scale pressure gradients carried by drifting ring current ions and electrons distort field lines one after another as they drift through the inner magnetosphere.
6/28 Thu Shinji Saito The influence of intermittent nature of magnetosonic-whistler turbulence on ion dynamics
Abstract : 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.
6/21 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?
Abstract : 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.
2017年度のセミナー