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
11/15 Thu Hiroki Ito Flux decrease of outer radiation belt electrons associated with solar wind pressure pulse: A Code coupling simulation of GEMSIS-RB and GEMSIS-GM
Abstract : Relativistic electron flux of the outer radiation belt dynamically changes in response to solar wind variations. There exist several conditions to cause the flux drop-out of the outer belt electrons. The magnetopause shadowing (MPS) is one of the processes to cause the rapid loss of outer belt electrons (e.g.,Kim et al.,2008). In this study, we have done a code-coupling simulation using GEMSIS-RB test particle simulation code (Saito et al., 2010) and GEMSIS-GM global MHD magnetosphere simulation code (Matsumoto et al., 2010) to demonstrate how radiation belt electrons are lost through the MPS process, by focusing pm the equatorial pitch angle and local time dependence. We calculate trajectories of electrons in electromagnetic fields calculated from GEMSIS-GM with initial L-shells from 9 to 11, initial energies from 100 keV to 10 MeV, and initial pitch angles between 85 degrees and 90 degrees, using the guiding-center equation. The simulation consists of the following three phases associated with variations of the solar wind dynamic pressure; [i] The standoff distance of magnetopause at the subsolar point is 12 Re with the initial dynamic pressure of 1.0 nPa. [ii] The solar wind dynamic pressure becomes 2.5 nPa, and the magnetopause moves to 9 Re. [iii] The solar wind dynamic pressure decreases, so that the inflation of the magnetopause takes place and the standoff distance of the magnetopause is 10 Re. The simulation shows that electrons move to the open field line due to the earthward movement of the magnetopause and then electrons escape to the interplanetary along the field line at the dayside in the phase [ⅱ]. On the other hand, electrons are lost at the night side along the open field lines when the dynamic pressure decreases in phase [ⅲ] due to the outward motion of trapped electrons caused by dusk-to-dawn electric fields associated with the expansion of the magnetosphere. Almost all lost electrons (~90%) have strong pitch angle scattering by the drift shell bifurcation which breaks the second adiabatic invariant.
11/8 Thu Katsuya Kondo Identification of Active Longitude from the solar magnetograms and Debrecen Photoheliografic Data (DPD)
Abstract : It is well known that the sun has the cycle of 11 years for its activity. In the active time, we can see lots of sunspots on the solar surface and sometime it cause large flares which largely affect the Earth’s environments. For example, the release of large-scale plasma called Coronal Mass Ejection (CME) hits the Earth’s magnetosphere and causes some effects, e.g. the Earth’s magnetic field fluctuation or trouble of the satellite. The appearance of sunspots has several characteristics. For example, sunspots appear around in the higher latitudes at the beginning of the cycle and in the lower latitudes at the end of the cycle on the sun. However, recent studies have shown that the appearances of sunspots also depend on longitudinal direction. The solar longitude where the sunspots are frequently observed is called Active Longitude (AL). The presence of active longitude has been discussed long time. But it is still not clear whether there is an AL or not. In this study, we researched about AL using magnetic field observation on the solar surface and Debrecen Photoheliographic Data (DPD). As a result, we can study the tendency of flux emergences on Active Longitude and the 180 degree opposite longitude is similar. Also we found the probability of sunspots reappearance is little different between the real data and the random emergence data. We think the region where is emerged a bigger sunspot is likely to emerge sunspots again but we cannot show this hypothesis.
11/1 Thu Yuki Kobayashi Application of two-fluid equations to MMS data: A possibility of anomalous resistivity near the magnetic neutral line
Abstract : Magnetic reconnection is a basic physical process by which energy of magnetic field is converted into the kinetic energy of plasmas. In recent years, MMS mission consisting of four spacecraft has been conducted aiming at elucidating the physical mechanism of merging the magnetic fields in the vicinity of the magnetic neutral line that exists in the central part of the structure. In this paper, we examine the magnetic field frozen-in relation near the magnetic neutral line as well as the causal relationship between electron and ion dynamics in the frame of two fluid equations. It is thought that the electron dissipation region with the thickness of about the electron inertial length surrounds the magnetic neutral line, and the ion dissipation region with the thickness of about the ion inertia length further surrounds them. It is said that an anomalous resistivity exists in the dissipation region, but its existence has not been demonstrated from the observation. In the initial report of MMS, Torbert et al. [2016] evaluated the anomalous resistivity from observational data based on generalized Ohm's law. However, the verification what kind of wave is responsible for the anomalous resistivity was left as an open question. Focusing on the well-known Burch event occurred around 2015-10-16 / 13: 07 UT, and adopting two-fluid equations to this event, we found that the collision terms in the equations of motion for both electrons and ions should be invoked. The vectors consisted of three components of the electron and ion collision terms direct almost oppositely with each other. Further, the absolute value of electron collision term and the intensity of the lower hybrid waves (LHWs) were found to be highly correlated. Synthesizing these results, we conclude that the excited LHWs were responsible for generating the anomalous resistivity in the diffusion region.
10/24 Thu Inchun Park Energetic electron flux variations of the outer radiation belt during magnetic storms observed by Arase/HEP - Calibration of the instrument using Geant4 and superposed epoch analysis
Abstract : The Arase satellite has been observing high-energy radiation-belt electrons since March 2017 with the high-energy electron experiments (HEP) instrument which measures electrons from 70 keV to 2 MeV. Since the beginning of the operation, the HEP instrument observed more than 15 magnetic storms with Dst values under −30 nT. Using these data, we investigated the variations of energetic electron flux with a focus on the dropout of the outer radiation belt during the storms. In order to enable the quantitative analysis of HEP data, we calibrated the instrument using the Geant4 simulations. Because high-energy particles interact with matter, incident electrons lose their energy probabilistically when they come into the detector. The HEP instrument is also affected by the background, such as high-energy protons coming from SEP and inner belt trapped protons. We simulated particle interactions with the detectors and surrounding materials by the Monte-Carlo method. These results give us the “response function”, which is the statistical distribution of energy values for incident electrons measured by the instrument. We reconstructed the incident differential fluxes of electrons using the inverse matrix of the response function. The HEP data after the calibration showed good agreement with the flux data from MEP-e, for the energy range common to the two instruments. We conducted a proton simulation to examine penetration of protons into the instrument at L<2. The simulation result suggests the possibility of contamination by proton energies of 900 keV and higher. Using the calibrated data, we studied the spectral characteristics of electrons by performing a superposed epoch analysis and found energy dependency of flux dropouts and subsequent recovery of the radiation belt electron. The energy spectrum tends to harden between L ~ 3 to 7 during storm recovery phases. We also examined simultaneous observations of electrons by Van Allen probes to compare them with the Arase data for the same magnetic storm.
10/18 Thu Johan Muhamad Study on the Magnetic Field Characteristics of Flaring Active Regions Based on Nonlinear Force-free Field
Abstract : Solar flare can unleash huge amount of energy from the Sun to the interplanetary space and may severely disturb the Earth’s magnetosphere and ionosphere, especially when the flare is accompanied by large coronal mass ejection (CME). Although studies of solar flare have been conducted extensively for many years, it is still not clear what condition of active region (AR) in the Sun that will produce flare and how big the flare can be produced by the AR. However, it is generally believed that a solar flare is produced in the region containing high magnetic energy, where the magnetic field is strongly sheared. Recently, Ishiguro and Kusano (2017) proposed the double-arc instability (DAI) theory explaining that a solar eruption can happen when a double-arc flux rope, formed through the magnetic reconnection of sheared magnetic loops, is unstable. This instability is determined by the kappa (K) parameter, which is a product of the magnetic twist and the ratio of the reconnected flux and the overlying flux. In order to study the applicability of the DAI analysis for the real flare events, we have developed a method to extract the information of magnetic twist in AR 11158 and use it to measure the proxy of kappa parameter before two large flares on February 13 and 15, 2011. Instead of using the reconnected flux to measure the kappa, we used the highly-twisted flux to calculate the proxy parameter, so called kappa*. By using the extrapolated coronal fields derived from the nonlinear force-free field (NLFFF) method, we investigated the evolution of magnetic fields of the AR. We found that the kappa* evolution during the three days of the AR flaring period shows a clear enhancement before the two flares. We also found that the kappa* reaches a certain value before the two large flares, which is consistent with the theoretical kappa. Recently, we extent our analysis of the DAI to the AR 12673, which is more complex than the AR 11158. To overcome the problem of finding the relevant flux in the complex structure, we determine the flux rope region based on the quasi separatrix layer (QSL) map, where we can see how the magnetic field lines diverts from one to another region. Our study shows that the DAI analysis can be applied to the real flare events and may have prospect to be applied for assessing the occurrence of the impending solar flare.
10/11 Thu Kei Kamiya Study of ULF waves and its effect on radial transport of relativistic electrons in the inner magnetosphere based on model coupling simulations
Abstract : Radiation belts are regions of enhanced population of relativistic electrons with energies over MeV in the Earth’s inner magnetosphere. There are two candidates of the acceleration mechanism: radial transport from external source and local acceleration within the inner magnetosphere. Pc5 Ultra-Low Frequency (ULF) waves in the inner magnetosphere are observed as electromagnetic fluctuations with frequencies of 1.67-6.67 mHz and considered as a driver of radial transport of electrons in the outer radiation belt. Previous observations have used the gradient in the radial profile of phase space density to distinguish the acceleration mechanisms. Radial transport of electrons is considered as a diffusive process and smooth out the gradient in phase space density, however, under certain conditions, the radial transport of electrons can produce localized peaks in the phase space density, which is considered as the evidence of local acceleration of the outer radiation belt electrons. Therefore, it is necessary to understand the fundamental process of the relativistic electrons in the outer radiation belt. We recently showed that electron interactions with a continuous monochromatic Pc5 wave can produce butterfly pitch angle distributions in the outer radiation belt. The mechanism depends on the latitudinal profile of the wave power and spectrum of the Pc5 waves. In order to model more realistic spatial distributions and spectra of ULF waves, we need to improve the outer boundary setup of the global ring current simulation. In this study, we have developed a one-way model coupling method between two models, i.e., Global MHD-based simulation of the magnetosphere, BATS-R-US+CIMI, and a 5-D drift-kinetic ring current simulation, GEMSIS-RC. The BATS-R-US+CIMI solves the ideal MHD equations for the regions from the sunward boundary to the inner magnetosphere including the ring current pressure gradient. The GEMSIS-RC describes the distribution function of ring current ion together with time evolutions of the electric and magnetic fields self-consistently, which include the ULF waves in the inner magnetosphere. We developed a method to adopt the time-dependent outer boundary condition for GEMSIS-RC based on ion density, temperature, bulk velocity, and magnetic field obtained from BATS-R-US+CIMI simulation to solve the propagation of ULF waves in the inner magnetosphere. The results show that the fast mode waves imposed at the outer boundary propagates in GEMSIS-RC simulation domain as expected from the theoretical fast mode speed. The mode conversion from fast mode to shear Alfven wave along the field line are also seen as the wave propagates inward. We will discuss the validity of the model coupling method and the ULF wave interactions with radiation belt electrons using a guiding center test particle simulation: GEMSIS-RB.
10/4 Thu Takuya Shibayama Plasmoid Chain Reconnection with Petschek-type Diffusion Region
Abstract : Sweet-Parker theory predicts that fast reconnection doesn’t realize in highly conductive plasma such as the solar corona. Petschek proposed another reconnection theory, in which localized magnetic diffusion region realizes efficient reconnection. However, recent studies suggest that steady Petschek reconnection doesn’t realize in a system with spatially uniform resistivity because of the lack of localization mechanism. Current understanding of MHD fast reconnection in systems with high Lundquist number (>104) is based on so-called plasmoid chain reconnection, in which global current sheet is divided by plasmoids. As a result, small Sweet-Parker type current sheets form between plasmoids. The local Lundquist number of these current sheets will be around the critical Lundquist number of tearing instability, that is ~104. Consequently, fast reconnection with reconnection rate of 0.01 realizes. However, plasmoid chain fast reconnection in even higher Lundquist number (>106) becomes quite time dependent because of the repeated formation and coalescence of numerous plasmoids. Our 2D resistive MHD simulation with uniform resistivity suggests that diffusion region in this regime has characteristics of Petschek model rather than Sweet-Parker model. In this case, reconnection rate is determined by the Petschek-type diffusion region, not by the Sweet-Parker scaling. This Petschek-type diffusion region is always located adjacent to a plasmoid. The localization mechanism of this Pestchek-type diffusion region is provided by the plasmoid and surrounding plasma flow. The relation between reconnection point and flow stagnation point also plays an important role on the localization. On the other hand, there are difficulties to apply these 2D theories to 3D system. We will also introduce our recent study related to next-generation satellite, EUVST. EUVST will conduct spectroscopic observation of the solar corona with high spatial, temporal resolution in wide temperature range. In such phenomena, ionization processes of ions should be taken into account. We developed a numerical solver of ionization of Fe ions along with MHD quantities.
9/27 Thu Kenji Mitani Statistical Study of Selective Oxygen Increase in High-Energy Ring Current Ions during Magnetic Storms
Abstract : The ion transport from the plasma sheet to the ring current is the main cause of the development of the ring current. Energetic (>150 keV) ring current ions are known to be transported diffusively in several days. A recent study suggested that energetic oxygen ions are transported closer to the Earth than protons due to the diffusive transport caused by a combination of the drift and drift-bounce resonances with Pc3-5 ULF waves during the April 24, 2013 magnetic storm. To understand the occurrence conditions of such selective oxygen increase (SOI), we investigate the phase space densities (PSDs) between protons and oxygen ions with the first adiabatic invariants (µ) of 0.1 - 2.0 keV/nT observed by Van Allen Probes at L~3-6 during 90 magnetic storms in 2013-2017. We identified the SOI events in which oxygen PSDs increase while proton PSDs do not increase during a period of ~9 hours (one orbital period). Among the 90 magnetic storms, 33% were accompanied by the SOI events. Global enhancements of Pc4 and Pc5 waves observed by ground magnetometers during the SOI events suggest that radial transport due to combination of the drift-bounce resonance with Pc4 oscillations and the drift resonance with Pc5 oscillations can be the cause of SOIs. The contribution of the SOI events to the magnetic storm intensity is roughly estimated to be ~9 % on average.
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