場所は研究所共同館 3階 講義室です。
|4/13||草野 完也 (教授)
|Overview of the Integrated Studies Division and the Project for Solar-Terrestrial Environment Prediction (PSTEP)|
|Abstract : The solar-terrestrial environment is a complex system that consists of nonlinear, non-equilibrium, and multi-scale interacting processes. The research in the Integrated Studies Division aims at improving our understanding of the dynamics of various phenomena in the solar-terrestrial environment through data analyses and modeling studies. The Integrated Studies Division is now playing the leading role for the Project for Solar-Terrestrial Environment Prediction (PSTEP) that is a Japanese-wide research collaborative project aiming to develop a synergistic interaction between predictive and scientific studies of the solar-terrestrial environment and to establish the basis for next-generation space weather forecasting. In this talk, we will talk about the overview of the Integrated Studies Division and the PSTEP, and also discuss about what kind of strategy we should take in order to develop our reseach in future. In particular, I would like to emphasize how important is the interdisciplinary aspect to open up new research based on my personal experience.|
|4/20||町田 忍 (教授)
|Frontier of Substorm Study|
Since Prof. Akasofu first proposed the concept of substorm in 1964 based
on the all-sky auroral network observations on the ground, the study on
substorm has been extended to understand the nature and causal relationship
of various variations and disturbances appear on the ground, ionosphere
and magnetosphere associated with the enhancement of auroral activity.
At the same time, the study of the triggering and driving mechanisms of substorm has also progressed, and numbers of substorm models have been proposed so far, but no final conclusion has been obtained yet. Under such circumstances, the recent work done by Fukui et al.  in which the data from THEMIS probes were analyzed, and they found that the spatial gradient of the plasma pressure around X ~ -7.5 Re is nearly 1.4 times higher in substorms compared to the pseudo-substorms around their onsets although the magnetic reconnection occurs in both cases. This result seems to have a primary importance.
Also recently, Ebihara and Tanaka  performed a large scale 3D global MHD simulation, and succeeded in reproducing the development of aurora expansion during the substorm in their calculation. Interestingly, their results do not show the occurrence of turbulent oscillations corresponding to the current disruption at the time of auroral expansion. In this sense, the assessment of their model comparing with actual observational data is so significant. In this talk, the status of recent substorm study including those works will be reported.
|4/27||松本 琢磨 (研究員)
|Connecting the Sun and the solar wind: the self-consistent transition of heating mechanisms|
|Abstract : We have performed a 2.5-dimensional magnetohydrodynamic simulation that resolves the propagation and dissipation of Alfv´en waves in the solar atmosphere. Alfv´enic fluctuations are introduced on the bottom boundary of the extremely large simulation box that ranges from the photosphere to far above the solar wind acceleration region. Our model is ab initio in the sense that no corona and no wind are assumed initially. The numerical experiment reveals the quasi-steady solution that has the transition from the cool to the hot atmosphere and the emergence of the high speed wind. The global structure of the resulting hot wind solution fairly well agrees with the coronal and the solar wind structure inferred from observations. The purpose of this study is to complement the previous paper by Matsumoto & Suzuki and describe the more detailed results and the analysis method. These results include the dynamics of the transition region and the more precisely measured heating rate in the atmosphere. Particularly, the spatial distribution of the heating rate helps us to interpret the actual heating mechanisms in the numerical simulation. Our estimation method of heating rate turned out to be a good measure for dissipation of Alfv´en waves and low beta fast waves.|
|5/9||金子 岳史 (研究員)
|Numerical study of solar prominence formation|
|Abstract: We propose a model in which magnetic reconnection triggers radiative condensation for solar prominence formation and demonstrate it by three-dimensional magnetohydrodynamic (MHD) simulations including anisotropic nonlinear thermal conduction and optically thin radiative cooling. Solar prominences are cool dense plasma clouds in the hot tenuous corona. Prominences have potential to give an impact on the solar-terrestrial plasma environment because they suddenly erupt to interplanetary space and evolve into coronal mass ejections. Not only the mechanism of eruptions, the formation mechanism of prominence is still unclear. We propose a reconnection–condensation model in which the topological change of a coronal magnetic field via reconnection triggers radiative condensation for prominence formation. Previous observational studies suggested that reconnection at a polarity inversion line of a coronal arcade field creates a flux rope sustaining a prominence, however, the origin of the cool dense plasmas of a prominence was not clear. Using three-dimensional MHD simulations including anisotropic nonlinear thermal conduction and optically thin radiative cooling, we demonstrate that reconnection leads not only to flux rope formation but also to radiative condensation under a certain condition. This critical condition in our model is described by the Field length (introduced by George B. Field, 1965), which is defined as the scale length for thermal balance between radiative cooling and thermal conduction. The multi-wavelength extreme ultraviolet emissions synthesized with our simulation results have good agreement with observational signatures in prominence formation events.|
|5/11||寺本 万里子 (特任助教)
|ULF waves interaction with energetic electron during a relativistic electron loss event|
|Abstract: During the main phase of a geomagnetic storm, flux decrease in the outer radiation belt. Radial transport of the electron via drift resonance of ULF waves is consider as one important process for this radiation belt losses. We focus on a moderate geomagnetic storm during 13-22 September 2014 which were associated with high speed solar wind (~600 km/s) and strongly northward IMF. The electron flux level in the outer zone with energy raging from 1 MeV to 5.6 MeV decreased on stating 12 September 2014. We compared the disturbance in the flux of the electron with ULF waves in the magnetic filed data from the Van Allen Probes and estimated the electron resonant energies, which indicate strong drift resonant interaction occurring between the energetic electron and the ULF waves. We found that the resonant energy is much lower than the relativistic electron associated with the losses in the outer belt. We conclude that ULF waves might not play a key role of outward diffusion of electron to help drive radiation belt losses in this event.|
|5/18||井上 諭 (特任助教)
|Formation and Dynamics of Solar Eruptive Flux Tube.|
|Abstract: We perform a magnetohydrodynamic simulation to reveal the dynamics of the solar eruption accompanied with M6.6 class flare observed in the solar active region 11158. In order to shorten a distance between the theoretical models and observations, our simulation takes into account the observed photospheric magnetic field from which a non-linear force-free field is reconstructed to employ the initial state. In our simulation we confirmed that the tether-cutting reconnection occurring locally above the polarity inversion line makes twisted flux tube which destroys the equilibrium magnetic field and drives the eruption. Then the eruption, surprisingly, can be driven even in the stable area of the torus instability. We suggested that the reconnection between the twisted lines during the eruption is essential to make more highly twisted flux tube which further drives away from the equilibrium state. Then the dynamics is no longer controlled by the decay index, rather by its nonlinear process.|
|6/1||家田 章正 (助教)
|Auroral breakup (substorm onset) in satellite-based and ground-based images|
Auroral breakup (or substorm onset) is explosive brightening of aurora, physically similar to solar flare.
Solar flare is triggered by magnetic reconnection.
In contrast, it is controversy whether reconnection triggers substorm or not.
It is not widely appreciated that this controversy stems from the
diversity in substorm onset definitions.
In this talk, I will discuss differences in substorm onsets between
satellite images and ground images as follows.
Substorm onsets have originally been defined as longitudinally extended sudden auroral brightening ("Akasofu initial brightening"), which is followed a few minutes later by auroral poleward expansion in ground-based all-sky images. In satellite global images, in contrast, such a clearly marked two-stage development has not been observed, and instead substorm onsets have been identified as localized sudden brightening, which immediately expands poleward. To resolve these differences, optical substorm onset signatures in global images and all-sky images were compared for a substorm that occurred on 7 December 1999. We have used the Polar satellite ultraviolet global images with a fixed filter (170 nm), enabling a high time resolution (37 s), and have used the 20~s resolution green line (557.7 nm) all-sky images at Muonio in Finland for comparison. As a result, the substorm onset in the all-sky images consisted of the Akasofu initial brightening (2124:50 UT) and the poleward expansion (2127:50 UT). In contrast, this two-stage development was not evident in the global images, where the onset brightening started at 2127:49 UT. Thus, the onset in global images was not simultaneous with the Akasofu initial brightening but rather with the poleward expansion in the all-sky images. The Akasofu initial brightening was not observed in the global images, which may possibly be attributed to the limited sensitivity of global images for thin auroral arc brightenings. These comparisons suggest that substorm onset identified in global images does not corresponds to the Akasofu substorm onset, but rather to the poleward expansion.
|6/6||伊集 朝哉 (研究員)
|Correlation between solar flares and magnetic properties for active regions in 2010 - 2016|
|Abstract: In our Mag-twist research, we found 271 active regions which observed by SDO/HMI in 2010 - 2016 and calculated their coronal magnetic field using a non-linear force free field (NLFFF) model (Inoue et al., 2014; Inoue, 2016). Magnetic parameters such as the magnetic twist, kappa-value, and highly twist area were extracted from the NLFFF calculation and compared with the flare activity and magnetic free energy for each active region. From a statistical study, we found a good correlation (C.C. = 0.627) between the highly twisted area and total flare flux. In this talk, we report a statistical analysis of magnetic parameters for the active regions.|
|6/8||神谷 慶 (D2)
|Characteristics of pitch angle distributions of relativistic electrons in the outer radiation belt with a monochromatic Pc5 wave|
Radial transport of relativistic electrons in the inner magnetosphere can be driven by drift resonance with Pc5 Ultra Low Frequency (ULF) waves. The radial transport due to the drift resonance has been considered as one of important acceleration mechanisms of the outer radiation belt electrons.
In the course of the radial transport, the energy and equatorial pitch angle of electron change under conservation of the first and second adiabatic invariants. The change in the drift period in the course of radial transport thus depends on the adiabatic process, and it can affect the radial transport rate of the electrons. In other words, the radial transport rate due to the drift resonance depends on the equatorial pitch angle and can form the characteristic pitch angle distributions (PADs). In this study, we investigate the radial transport of relativistic electrons due to the drift resonance with a monochromatic Pc5 wave and focus on formation of PADs of the outer radiation belt electrons.
We use two simulation models of the inner magnetosphere: GEMSIS-Ring Current (RC) and GEMSIS-Radiation Belt (RB). The RC simulation, which is a self-consistent and kinetic numerical simulation code, solves the five-dimensional Boltzmann equation for the ring-current ions coupled with Maxwell equations. The RB simulation calculates trajectories of guiding center of test-particles in arbitrary magnetic and electric field. We used electric and magnetic fields of a monochromatic Pc5 wave in the inner magnetosphere obtained from the RC simulation as background fields in the RB simulations. We traced an order of 107 of radiation belt electrons to calculate phase space density of the electrons at each position in the equatorial plane.
Simulation results show formation of characteristic PADs depending on the energy and location (L value), which can be explicable of the pitch angle dependence of resonance conditions. At some fixed location and energy range, the PADs can change from pancake-like to butterfly-like distributions, as the transport by the monochromatic Pc5 wave progresses. These butterfly distributions can be seen when electrons with lower pitch angles satisfy the resonance condition.
|6/15||Jie Ren (研究員)||Phase relationship between ULF waves and drift-bounce resonant ions: a statistical study.|
|Abstract: We explore the phase relationship between the poloidal mode ULF wave electric field oscillations and drift-bounce resonant oxygen ions under the resonant condition of N=2 at the magnetic equator. Using Cluster data from 2001 to 2004, we identify 55 fundamental poloidal mode wave events, among which 42 show negative slope pitch angle dispersion signatures in the southern hemisphere, 11 show positive slope dispersion in the northern hemisphere, and 2 near-equatorial events are associated with in-phase field-aligned signatures. For each event, the off-equatorial resonant ions are traced along their bouncing trajectories to determine the last time they moved across the equator. The resulting time-series of the resonant oxygen ion fluxes at the equator are found to be statistically in anti-phase with the wave electric fields. The resonant ion flux variation depends on both ion energy change and radial transportation. This anti-phase relationship in statistics suggests two possibilities: (1) the fundamental poloidal mode wave electric fields are generally characterized by electric field intensity peaking near the magnetic equator if the flux variation is mainly caused by energy change; (2) the radial gradient of phase space density is positive and ions are accelerated if the flux variation caused by radial transportation is dominant .|
|6/22||Johan Muhamad (D2)||Trigger Mechanism and Magnetic Field Structure Leading to Solar Flare|
Solar flare unleashes large amount of energy from the Sun to the interplanetary space. It is widely believed that the energy of a flare is produced by the conversion of the magnetic energy to the plasma and thermal energies. Therefore, it is important to understand the characteristics of the magnetic field in the Sun, especially in the active region (AR), which have large free energy (non-potential). Since the eruption of magnetic field in a solar flare process is related with the stability of the magnetic field, we also need to understand the magnetic stability of an AR prior to a flare. Both MHD instability and reconnection are believed to work during the flare process. Accordingly, in order to be able to predict a flare one need to analyze how the magnetic field in an AR evolve, investigate its stability, and find the possibility of structure that can trigger flare reconnection.
In this talk, I will discuss my studies about the trigger mechanism of a solar flare and how to extract the information of non-potentiality in an AR that can be used for predicting a flare. One of the representations of large non-potentiality in an AR is the highly magnetic twisted field. We analyze magnetic twist properties of several ARs by using nonlinear force-free field (NLFFF) approach. The NLFFF method extrapolates coronal field of the AR from the given photospheric magnetic field. As case studies, we analyze AR 11158 and 12371 and the flares occurred from these ARs. I will present the results of magnetic twist evolution of AR 11158 and 12371 before and after the flares. Here, we propose and investigate a parameter, so called kappa*, to represent the stability of coronal magnetic field in an AR based on the ideal MHD instability and reconnected flux. I will show and discuss how this parameter evolved during a flare with the value decreased significantly after a flare. It implies that kappa* parameter can represent the stability of a magnetic field in an AR and may be useful for solar flare prediction.
|6/29||石黒 直行 (D2)
|Numerical calculation of formation of flux rope and Double Arc Instability|
|Abstract: Before and after the solar explosive phenomena, flux rope is often observed, which is herical magnetic tube. In previous studies, the flux rope is modeled by electric current loop and discussed the stability and the equilibrium condition. Recently, we modeled the double arc shaped loop that is observed before the eruptive phenomena like tether-cutting reconnection, and suggested Double Arc Instability (DAI) as a result of analysis (Ishiguro & Kusano 2017, accepted ApJ). We proposed the new parameter "kappa" for the necessary condition of destabilizing DAI in the study. The "kappa" is defined as the product of the magnetic twist and normalized flux of tether-cutting reconnection. For next step, we should confirm whether DAI can apply to the numerical simulation and real solar eruption. In this study, we verify the model of DAI under the 3-D zero-beta MHD simulation. We make the magnetic field like a sunspot accumulated strong shear and twist, and cause magnetic reconnection following the tether-cutting scenario. As a result, the double arc shaped loop is formed and the structure of it is like sigmoid which is often observed in solar flare. This flux rope do not be unstable and not erupt, and the "kappa" is smaller than the critical kappa for DAI. To make the eruption in numerical simulation, we should make the condition accumulated more twist and free energy.|
|7/6||柴山 拓也 (D2)
|Plasmoids in 2D and 3D MHD magnetic reconnection process|
Magnetic reconnection is a process of changing the connectivity of magnetic field lines, and thought to play a core role in explosive energy conversion of magnetospheric substorm, solar flare, and tokamak disruption. According to the Sweet-Parker theory, it is, however, difficult to conduct magnetic reconnection efficiently in highly conductive plasma such as in the solar corona. Petschek proposed another reconnection theory, in which small magnetic diffusion region realizes efficient reconnection with the energy conversion occurring in slow mode MHD shocks. However, recent numerical simulations suggest that Petschek reconnection is not stable in a system with spatially uniform resistivity. Some mechanism such as anomalous resistivity or kinetic physics is needed to sustain the localized diffusion region. It is, therefor, not clear yet how fast reconnection realizes in the reality.|
In order to answer to this question, we have performed resistive 2D MHD simulation in a large system with a high spatial resolution, and find that small-scale slow mode MHD shocks predicted by Petschek spontaneously form even under a uniform resistivity. In this process, large plasmoids in the current sheet play a role of localizing the diffusion region, and slow mode shocks form in front of the moving plasmoids. These plasmoids enhance magnetic reconnection intermittently and repeatedly. As a result, the reconnection rate increases up to 0.02, which is high enough to explain the time-scale of solar flares. The mechanism of the formation of Petschek-type structure, however, is still not clear. In the previous simulation, Petschek-type structure is always attached to plasmoid. We speculate that existence newly forming plasmoid is important for formation of Petschek-type structure. We performed 2D MHD simulation of evolution of newly forming plasmoid, in other words, local simulation of diffusion scale. As a result, Petschek-type structure is formed even without motion of plasmoid.
Plasmoid formation in a reconnection region has been mostly studied using 2D numerical simulation or 2D theory. In the actual 3D system, however, there are instable modes that are not taken into account in the 2D theory. Oblique tearing mode, whose wave vector is parallel to the reconnecting magnetic field, is one of 3D instability modes related to the formation of plasmoid. There are cases where the oblique tearing mode have higher growth rate than 2D mode (Baarlad et al. 2012). Oblique plasmoid will dominate the reconnection in the case. It is, therefore, not clear if 2D theory of plasmoid formation is directory applicable also to the actual 3D system. On the other hand, observed flux tube or plasmoid usually have coherent 2D-like structure.
We conducted large scale numerical MHD simulation to study plasmoid formation and interaction in a 3D system. We observe growth of oblique plasmoid and the oblique structure dominate the system. They change their structure through coalescence and reconnection. The resultant middle-scale structure is somewhat 2D-like and we observed formation process of 2D-like structure in larger scale through interaction of 3D structure in small scale.
In the seminar talk, I will talk also about new satellite project, PhoENiX. This satellite will observe X-ray photon-by-photon. I simulate DEM from my simulation.
|7/13||Inchun Park (D2)||Calibration of HEP instrument on board ARASE via particle simulation|
Since the Arase satellite was launched to orbit last year, new observation data has been obtained. In order to precise scientific data, the data should be calibrated and then converted to the physical unit.|
The HEP instrument on board ARASE is designed to observe 70 keV- 2 MeV high-energy electrons. In this energy range, the background radiation caused by protons interferes with observations.
Our research aims to discriminate background radiation events through data calibration. Using Geant4 simulation tool, we develop the geometry of the HEP detector, and conduct the particle simulation to calculate more realistic G-factor of the HEP instrument. Moreover, in order to assess the energy dependency of G-factor, we are planning a Monte Carlo simulation. Note that a similar detector, MagEIS onboard Van Allen Probes, was calibrated with the background correction algorithm (Claudepierre, S.G., et al., 2015), which identified that inner zone protons and bremsstrahlung x-rays are the main sources of radiation contamination. We believe that a similar method might be used for HEP data calibration. Besides the topics on the calibration, we would like to briefly talk about HEP data analysis for the storm event on March 27.
|7/20||三谷 憲司 (D2)
|Radial transport of high-energy oxygen ions into the deep inner magnetosphere observed by Van Allen Probes|
|Abstract: It is observationally known that proton is main component of the ring current and oxygen ions also can be main component expected to contribute to the development of the late main phase of the magnetic storm. At the same time, we also found enhancement of poloidal magnetic fluctuations that can resonate with drift and bounce motions of high-energy oxygen ions when azimuthal mode number (m number) is low. The combination of the drift and drift-bounce resonances is suggested to cause radial transport of high-energy oxygen ion into the deep inner magnetosphere.|
|7/27||小山 響平 (D3)
|9/7||三好 由純 (准教授)
|9/14||堀 智昭 (特任准教授)
|9/19||川嶋 貴大 (M2)
|9/21||旭 友希 (M2)
|9/28||藤山 雅士 (M2)
|10/3||水野 雄太 (M2)
|10/5||三浦 翼 (M2)
|10/12||與那覇 公泰 (M2)
|10/26||淺野 貴紀 (M2)
|11/2||上村 亮弥 (M2)
|11/9||林 昌広 (M2)
|11/21||増田 智 (准教授)
|11/30||梅田 隆行 (講師)
|12/5||近藤 克哉 (M1)
|12/7||箕浦 桜子 (M1)
|12/21||小林 勇貴 (M1)
|1/11||小路 真史 (特任助教)
|1/25||松田 昇也 (研究員)
|2/1||齊藤 慎司 (特任助教)
|2/8||栗田 怜 (研究員)
|2/15||今田 晋亮 (助教)