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)
|Effects of ion-ion collisions on vertical distribution of CO2+ in Martian ionosphere based on multi-fluid MHD simulation|
Comparison of the mass fraction of CO2 and N2 with regard to the total mass of each terrestrial planet suggests importance of the atmospheric escape to space in Martian atmospheric evolution [Chassefiere et al., 2006]. It has been considered that heavy CO2+ ions are difficult to escape based on known atmospheric escape processes. Observations of a large amount of CO2+ ion escape by the Mars Express thus challenged the existing escape processes. Vertical distribution of CO2+ density in the ionosphere is one of important factors that determine the rate of CO2+ escape. Chemical reactions in ionosphere have been implemented in previous studies using multi-species MHD simulations [e.g., Ma et al., 2004; Terada et al., 2009]. The velocity difference between ion fluid cannot be reproduced by the multi-species MHD approximation. On one hand, the importance of vertical transport in the upper ionosphere (>300km altitude) was pointed out by some ionospheric models [Fox and Hac, 2009].|
Multi-fluid MHD code [e.g., Najib et al., 2011] can solve such ion-species dependent velocity. In this study, we developed a multi-fluid MHD simulation code. Our code includes ion-ion collisions and electron-neutral collisions in order to investigate their effects on the vertical distribution of CO2+ density in the Martian ionosphere. We run five cases of the simulation in different collison setting. We compared the results after each simulation run reached to a quasi-steady state. The results suggest that collisions are important to reproduce the realistic CO2+ transport from lower to upper ionosphere.
|9/1||堀 智昭 (特任准教授)
|Intermediate to high-m ULF-like fluctuations of ionospheric flow observed by SuperDARN during 2015 St. Patrick's day magnetic storm|
|Abstract: We study an interesting wavy structure of ionosphere flow at sub-auroral latitudes observed by the Super Dual Auroral Radar Network (SuperDARN) during a magnetic storm on March 17-18, 2015. The wavy modulation of ionospheric flow actually occurs during the relatively stagnant period between the two major substorm activities during the storm main phase. At sub-auroral latitude, the fast eastward flow prevailing from midnight to early morning during the first substorm ceases and subsequently the mid- latitude SuperDARN radars start to see a series of flow reversals of toward-radar flows and away-from- radar flows. Each of the flow reversal structures has a longitudinal wave length of roughly ~1h magnetic local time (MLT) and fairly large peak-to- peak amplitude of several hundreds of m/s. Interestingly, those flow structures pass by the fields of view of the radars one after another, showing a clear westward propagation over a wide MLT range from early morning all the way to midnight. We infer that the westward-traveling modulation of ionospheric electric field could be the footprint of westward-propagating ULF waves, which are driven by drifting ring current ions with energies of tens of keV. Further comparison with ground magnetic field data and in situ observations by inner magnetospheric satellites will be made to test this hypothesis as well as to examine how these propagating structure of ionospheric electric field is generated.|
|Causal Relationship between Relativistic Electron Acceleration and Microbursts associated with Scattering by Whistler Chorus Elements|
Whistler chorus waves in the Earth’s magnetosphere are considered to
have important roles to change relativistic electron flux in the outer
radiation belt. Kurita et al. (GRL2015) found that whistler chorus
waves responsible for flux enhancement of relativistic electrons can
be a cause of precipitation of relativistic electrons.|
The precipitation appears as relativistic electron microbursts which consist of multiple intermittent bursts. They concluded that the microbursts are a good proxy to indicate that whistler chorus activity actually causes significant variations of relativistic electrons. In order to understand the causal relationship between relativistic electron flux enhancements and the microbursts, we investigate flux enhancement and atmospheric precipitation of relativistic electrons associated with whistler chorus elements propagating along a magnetic field line, by using GEMSIS-RBW simulation code (RBW). The RBW is a test-particle code solving bounce motion of electrons along a field line, parallel propagating whistler chorus, and wave-particle interactions by using the equation of motion. The RBW simulation can calculate scattering processes, not only diffusion process but also nonlinear scattering processes such as phase bunching and phase trapping in coherent whistler chorus [Saito et al. JGR 2016].
Especially the phase trapping process accelerates electrons from a few hundred keV to a few MeV in a short time scale which is of the order of a second. Our simulations demonstrate that both the relativistic electron flux and relativistic electron precipitation into the atmosphere are more enhanced as the whistler chorus waves propagate more away from the equator. We will discuss dependency of latitude of the whistler chorus on the flux enhancement and precipitation of relativistic electrons.
|9/19||川嶋 貴大 (M2)
|Analysis of fast plasma flows by magnetic reconnection in the magnetotail during Substorm|
|Abstract: A substorm is a large-scale disturbance including auroral breakup in the ionosphere and magnetic reconnection in the magnetotail. Two predominant models of the substorm time history have been proposed: the near-Earth neutral line (NENL) model and the current disruption model. The former is of outside-in type with tailward propagation of the disturbance, whereas the latter is of inside-out type with earthward propagation of the disturbance. To determine such time histories of such substorms using aurora all-sky and magnetotail multi-satellite observations, the National Aeronautics and Space Administration (NASA) is conducting a mission named the “Time History of Events and Macroscale Interactions during Substorms (THEMIS).” The time history of a substorm is expected to be best clarified when satellites are aligned along the tail axis. A substorm occurred under such a satellite distribution on 0743:42 UT February 27, 2009, and we investigated the auroral breakup and fast plasma flows produced by the magnetic reconnection in this substorm. The THEMIS satellites observed that a northward magnetic field variation propagated earthward. Because this earthward propagation is consistent with the NENL model, observation of a substorm onset after the magnetic reconnection was expected. However, the substorm onset was observed in the all-sky images before the magnetic reconnection, as noted in a previous study. In this study, we report that another earthward fast plasma flow occurred before the substorm onset, indicating that another magnetic reconnection occurred before the substorm onset. In addition, we confirm that the above mentioned post-onset magnetic reconnection occurred simultaneously with auroral poleward expansion, within a 1-min period. These results support the NENL model and further suggest that the two-step development of magnetic reconnection is a key component of the substorm time history.|
|9/21||旭 友希 (M2)
|The statistical analysis of correlation between solar flares and photospheric magnetic field|
A solar flare is caused by the explosive release of magnetic energy and sometimes greatly disturbs the Earth’s electromagnetic environment and may impact socio-economic system. For that reason, the prediction of flare occurrence is important for space weather forecast. However, the accurate prediction of flare occurrence has not been realized yet, because the mechanism of flare trigger is not well understood yet. According to previous studies, some parameters, such as the area of active region, total magnetic flux, the magnetic shear on PIL and the magnetic gradient have certain relation with the flare activities. Recently, Kusano et al. (2012) indicated that the magnetic reconnection in the small magnetic structures that appear in the strong-sheared magnetic field near the magnetic polarity inversion lines can trigger solar flares. In addition, Ishiguro & Kusano (M25a for ASJ meeting 2016 in March) found a possibility that the magnetic twist causes instability which is able to cause flares and CMEs.|
The object of this study is to give a new parameter related to flare activity on the basis of these previous studies. In order to achieve it, we have performed the statistical analysis of magnetic field data on photosphere surface. For 294 ARs which sunspot area is relatively large in 2012 to 2016, we took the correlation analysis on the total flux and various magnetic parameter in each AR using the magnetic field data of SDO/HMI. As a result, photospheric free energy has good correlation with flare activity. Especially, ARs in which the distribution of photospheric free energy is long and narrow along PIL tend to be flare-active. The results suggest that not only magnetic free energy but also the shape of distribution of magnetic free energy correlates with the flare activity. In addition, we analyzed new parameter showing the relationship between free energy and PIL. We report the results of the analyses and discuss about the application to space weather forecast.
|Solar Surface Velocity in the Large Scale estimated by Megnetic Element Tracking Method|
The 11 years variation in the solar activity is one of the important sources of decadal variation in the solar-terrestrial environment. Therefore, predicting the solar cycle activity is crucial for the space weather. To build the prediction schemes for the next solar cycle is a key for the long-term space weather study. Recently, the relationship between polar magnetic field at the solar minimum and next solar cycle activity is intensively discussed. Nowadays, many people believe that the polar magnetic field at the solar minimum is one of the best predictor for the next solar cycle. To estimate polar magnetic field, Surface Flux Transport (SFT) model have been often used. On the other hand, SFT model needs several parameters, for example Meridional circulation, differential rotation, turbulent diffusion etc.. So far, those parameters have not been fully understood, and their uncertainties may affect the accuracy of the prediction.
In this study, we try to discuss the parameters which are used in SFT model. We focus on two kinds of the solar surface motions, Differential rotation and Meridional circulation. First, we have developed Magnetic Element Tracking (MET) module, which is able to obtain the surface velocity by using the magnetic field data. We have used SOHO/MDI and SDO/HMI for the magnetic field data. By using MET, we study the solar surface motion over 2 cycle (nearly 24 years), and we found that the velocity variation is related to the active region belt. This result is consistent with [Hathaway et al., 2011]. Further, we discuss the relationship between the surface motion and the magnetic field strength.
|10/5||三浦 翼 (M2)
|Association between substorm onsets in auroral all-sky images and geomagnetic Pi2 pulsations|
Substorms are explosive disturbances in the magnetosphere and ionosphere of Earth. Substorm onsets are often identified using sudden auroral brightenings (auroral breakup) or geomagnetic Pi2 pulsations. These auroral brightenings and Pi2 pulsations are supposed to occur simultaneously within approximately 1 min of each other. However, as auroral brightenings typically include a two-stage development, this simultaneity is not straightforward. |
In this study, we clarify the correspondence between Pi2 pulsations and auroral brightenings, including the two-stage development. The first stage of the development is the sudden brightening of an auroral arc near the midnight (initial brightening) and the second stage is the poleward expansion of the auroral arc. We compared all-sky images (3 s resolution) in Canada and geomagnetic observations (0.5-1 s resolution) in North and Central America, using data from the THEMIS project. In this study, we examined three substorms events that exhibit evidence of the two-stage auroral development. In the first event (4 March 2008), an auroral initial brightening occurred at 0533:57 UT and a poleward expansion was observed at 0538:12 UT (4 min after the initial brightening) in Gillam (magnetic latitude:66.0 °, longitude:333 °, MLT:22.9). In contract, the Pi2 pulsation started at 0539:30 UT, which is closer to the time of the poleward expansion, in Carson City (magnetic latitude: 45.0 °, longitude:304 °). and San Juan (magnetic latitude:27.9 °, longitude:6.53 °). Thus, we consider this Pi2 pulsation as corresponding to the poleward expansion rather than the initial brightening. This correspondence was also seen in the other two events, suggesting that it is not exceptional. We interpret that the Pi2 pulsation corresponds to the poleward expansion because both are caused by the magnetic field dipolarization, which is a drastic change that propagates from low- to high-latitude field lines.
|10/12||堀 智昭 (特任准教授)
Tomoaki Hori (CIDAS)
|SECS reconstruction of flow fluctuations with SuperDARN data|
|Abstract: We analyze detailed properties of two-dimensional (2-D) structures of the ULF-like, ionospheric flow fluctuations during a short (~2 hours) break of the main phase of the March 2015 storm. Line-of-sight (LOS) Doppler velocities observed by two SuperDARN radars in the early morning sector were used to deduce the 2-D horizontal flows by means of the spherical elementary current system (SECS) expansion. Similar to results deduced by the conventional map potential technique, the SECS reconstruction shows that ionospheric plasma in the subauroral region flows primarily in the geomagnetically eastward direction before and after the period of the ULF-like fluctuations. The reconstructed flow pattern shows that, during the first half of the ULF-like event interval, background convection subsides and circular/elliptically polarized flow fluctuations pass over the field-of-view of the radars as they propagate westward. Multiple flow bursts likely associated with small injections occur concurrently during the second half period, while the westward-propagating flow fluctuations still continue regardless of the bursts until a major substorm activity starts later on. A new finding from the reconstructed flow is that the eastward-propagating structures are also dominated by a poloidal component. The common feature of poloidal-dominant fluctuations implies that the westward and subsequent eastward-propagating fluctuations are both caused by a similar mechanism.|
|10/26||淺野 貴紀 (M2)
|MLT dependencies of the precipitating electron energy responsible for pulsating auroras based on the multi-wavelength observation|
Pulsating aurora (PsA) is a kind of diffuse aurora and shows quasi-periodic intensity modulation with 2 s to 30 s intervals. PsA is mainly observed from the post-midnight to the morning sectors during the recovery phase of substorms. Based on the EISCAT observations, Hosokawa and Ogawa  showed that energy of precipitating electrons responsible for pulsating aurora tends to be higher on the late MLT sector. On the other hand, applying the triangulation method on pulsating auroras, Partamies et al.  reported that there are no significant MLT dependence of the emission height of pulsating auroras. This result suggests that precipitating electron energies for pulsating aurora do not depend on MLT.|
In order to investigate MLT dependencies of the characteristic energy of precipitating electrons responsible for pulsating aurora,we apply the method proposed by Ono et al. to estimate the characteristic energy by choosing the two optical wavelengths of 427.8 nm and 844.6 nm. Auroral images at these emission lines were obtained by monochromatic EMCCD cameras installed in Tromso, Norway, and the emission intensity along the magnetic field line is used for the estimation. We analyzed 13 nights (about 900 minutes) of pulsating aurora events from February 2017 to April 2017. The average of characteristic energy of precipitating electrons are 3.6 keV at pre-midnight and 4.4 keV at post-midnight, so that it is expected that the characteristic energy of precipitating electron at post-midnight tends to be larger than that at pre-midnight. Besides the MLT dependence of precipitating electron energy, we will discuss the spatial distribution of precipitating elelectron energy in the pulsating patches.
|11/2||上村 亮弥 (M2)
|Statistical analysis of active-region micro/nanoflares observed with Hinode/XRT|
|Abstract: The nanoflare-heating model is one of possible coronal heating mechanisms. The frequency distribution of small explosive events as a function of their energy is important for evaluation of this model. It was reported as a result of Yohkoh/SXT observations that the distribution in the energy range of 10^27 - 10^29 erg is represented by a single power-law with a power-law index of -1.5 or -1.6 (Shimizu 1995). It concluded that the total energy supplied by microflares and flares is not enough to maintain the active-region corona even if this distribution is extended to the lower energy range. To investigate the frequency distribution below 10^27 erg, we analyzed high-cadence (3-6 sec) and high-resolution (1.03 arcsec/pixel) soft X-ray images of the active-region NOAA10923 taken with Hinode/XRT using four filters (Al poly, C poly, Ti poly, and thin Be) on November 13-16, 2006. We detected more than 1000 enhancements in the light curve of 4x4 pixel-binned XRT images in the observation period (45-90 min) of each filter, and then found that the frequency distribution is represented by a power-law in the range of about 10^25.6 - 10^27.8 erg, steeper than with a power-law index of -2. We also investigated the energy balance of active-region corona, and found that the total energy supplied by the detected events in this analysis is smaller than the active-region energy loss. However, there is a possibility that smaller events can supply sufficient energy required for the active-region coronal heating if the same power-law distribution continues to the energy of about 10^22.0 - 10^23.4 erg.|
|Fast Magnetic Reconnection by Quadrupole Field|
Magnetic reconnection is energy conversion process in plasma. Magnetic energy is converted to thermal and kinetic energy. In a reconnection model of resistive MHD regime, reconnection proceed too slow compared to reconnection in actual solar atmosphere. This is known as 'Fast reconnection problem'.
In Hall-MHD regime, it is found that reconnection proceed fast as solar observation. However, the mechanisms of this acceleration is not well understood. There is a hypothesis to try to explain this acceleration. In this hypothesis, quadrupole magnetic field in reconnection site cause this acceleration.
In this study, we try to verify this hypothesis. We reconstruct quadrupole magnetic field in resistive MHD regime in order to eliminate other effects of Hall-MHD regime. Constant out of plain velocity field is in reconnection site to reconstruct this quadrupole magnetic field.
As a result, we found that quadrupole structure strength became grater as speed of velocity became large. We also found that reconnection proceed faster in strong quadrupole field compared to no quadrupole field structure.
|11/9||林 昌広 (M2)
|Rapid acceleration of outer radiation belt electrons associated with solar wind pressure pulse: Simulation and Arase and Van Allen Probes observations|
|Abstract: Relativistic electron fluxes of the outer radiation belt rapidly change in response to solar wind variations. One of the shortest acceleration processes of electrons in the outer radiation belt is wave-particle interactions between drifting electrons and fast-mode waves induced by compression of the dayside magnetopause associated with interplanetary shocks. Recently, the Arase (ERG) satellite and Van Allen Probes observed the rapid enhancement of electron fluxes for hundreds of keV to MeV associated with storm sudden commencement (SSC) on July 16, 2017. During the period, the sharp enhancement of the solar wind dynamic pressure from 1 nPa to 15 nPa was observed. In this event, drift echoes after the flux enhancement and magnetic field variations due to the fast mode wave propagation were observed. In order to investigate this process by a solar wind pressure pulse, we perform a code-coupling simulation using the GEMSIS-RB test particle simulation (Saito et al., 2010) and the GEMSIS-GM global MHD magnetosphere simulation (Matsumoto et al., 2010). We have derived effective acceleration model and condition : The electrons whose drift velocities are accelerated efficiently. In this study, we perform a multi-particle simulation. As a result, the fluxes of electrons whose energy > 4.5 MeV are enhanced strongly. The results suggest the existence of critical energy into effective acceleration. In this presentation, we will compare the energy spectrum of our simulation results with that of observations from Arase and Van Allen Probes, and investigate the acceleration condition of relativistic electrons associated with the SSC.|
|11/13||水野 雄太 (M2)
|Study of the occurrence condition of eruptive flares and CMEs based on non-linear force-free field model|
The solar flares and CMEs sometimes largely disturb the Earth's electromagnetic environment and may impact various social systems. In particular, large magnetic storms (Dst ≦ -100nT) are caused by CMEs. Therefore, the prediction of CMEs occurrence is an important issue for space weather forecast. The SOHO observations indicated that, although solar flares often occurred under CMEs, CMEs do not always occurred associated with all solar flares (Yashiro et al, 2006). Solar flares accompanying CMEs are called “eruptive flare”, other solar flares are called “non-eruptive flares”. The occurrence condition of eruptive/non-eruptive has not been yet well-understood.
Recently, Toriumi et al. (2017) pointed out that the ratio of flare ribbons flux to the total flux of active region tends to be larger for the eruptive flare compared to the non-eruptive flares. On the other hand, Inoue et al. (2016) suggested that there is a correlation between the shape of the ribbon and the region of high magnetic twist using the non-linear force free field (NLFFF) model. In this study, we investigated the relationship between the area fraction of high magnetic twist region and the property of whether flare is eruptive or not using the SDO/HMI data and the NLFFF model. The result suggests that the area fraction of highly twisted flux of the active region producing non-eruptive flares tends to be lower than that of the regions producing eruptive flares, although the number of sample is not yet enough to make a clear conclusion.
|A statistical study of near-Earth magnetotail evolution during substorms and pseudosubstorms with THEMIS data|
|Abstract: Substorms and pseudosubstorms (pseudobreakups) are very similar phenomena. In terms of auroral morphology, pseudosubstorms are generally more localized and more short-lived, compared with substorms, and are not accompanied by poleward expansion. We examined auroral development for events from November 2007 through April 2010, using data from THEMIS all-sky imagers. We defined events accompanied and not accompanied by poleward expansion as substorms and pseudosubstorms, respectively. To understand the cause of auroral development, we investigated temporal and spatial development of the near-Earth magnetotail during substorms and pseudosubstorms, based on superposed epoch analysis of THEMIS data. We find that Vx begins to increase at -9.5 >X(GSM)>-11.5 Re around onset for both substorms and pseudosubstorms. This seems to be due to earthward flows caused by magnetic reconnection. The northward Bz also increases around onset at -6.5 >X>-9.5 Re both substorms and pseudosubstorms. The amount and rate of Bz change are larger for substorms than for pseudosubstorms. In the tailward (-10.5 >X>-12.5 Re) regions, Bz increases substantially for substorms, whereas it does not increase very much for pseudosubstorms. These results indicate that dipolarization is weaker for pseudosubstorms than for substorms, and the dipolarization region does not spread extensively for pseudosubstorms. We, therefore, suggest that current disruption related to dipolarization does not develop tailward and hence auroral poleward expansion does not occur for pseudosubstorms. Meanwhile, the plasma pressure prior to the onset is larger at -8.5 >X>-10.5 Re for substorms than for pseudosubstorms. At -6.5 >X>-8.5 Re the magnetic pressure is larger for substorms than for pseudosubstorms before onset. Thus we conclude that the amount of the plasma and magnetic pressure is a key that determines whether the current disruption takes place, that is, whether initial activation develops into a substorm or into a subsiding pseudosubstorm.|
|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)