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We study the effect of the mirror force on the collision rate due to the energetic electron precipitation into the ionosphere. We develop a simulation code for the motion of energetic electrons with the mirror force to solve the variation of the pitch angle of electrons during their precipitation. In this code, a module computing the collision between precipitating energetic electrons and neutral gas using the Monte Carlo method is employed. By combining the developed modules, altitude profiles of the collision rate due to energetic electron precipitation in the keV energy range are investigated, and the effect of the mirror force acting on precipitating electrons is examined. The simulation results show that the influence of the mirror force on the altitude profile of the collision rate is significant for electrons with a high initial pitch angle, corresponding to the pitch angle close to the loss cone. The effect of the mirror force results in the broadening of the altitude profile of the collision upward due to the reflection of mirroring electrons. Simulation results for energetic electrons with kinetic energies above 100 keV show that a secondary peak near the mirror point is formed in the altitude profile of the collision rate. The formation of the secondary peak can be explained by the stagnation of electrons around the mirror point. The relatively long duration staying in neutral gas results in the increase of the collision rate around the mirror point, against the smaller collision cross-section in the higher energy range. Simulation results reveal that the maximum collision rate in the altitude range lower than 100 km becomes one order of magnitude smaller if electrons in the kinetic energy range larger than tens of keV precipitate with the pitch angle close to the loss cone. The results of the present study emphasize the importance of the mirror force for the precise modeling of ionospheric response due to the energetic electron precipitation.

There are several plasma and particle populations in the Geospace. The plasmasphere consists of cold/dense plasma populations, and the ionosphere is the source of the plasmaspheric plasma. The ring current and plasma sheet energies range from a few keV to about 100 keV, and these populations contribute to the ambient plasma pressure. The energies of the radiation belt particles range from a few hundred keV to more than 10 MeV, which are the highest energy population in geospace. Thus, plasma and particle populations with a wide range of energies from a few eV to more than 10 MeV coexist in the inner magnetosphere. Besides plasma/particle populations, there are different kinds of plasma waves at the wide frequency range from ULF to HF frequency range in geospace. Geospace is often referred as "zoo of plasma waves", i.e., there exist MHD waves in the low-frequency range, ion cyclotron waves, whistler mode waves, free space mode waves, and several kinds of electrostatic waves. These waves work essential roles for dynamical evolutions of plasma/particles in geospace via the cross-energy coupling. The geospace exploration satellite Arase (ERG) has been launched in 2016 and started the prime mission from the end of March 2017 [ Miyoshi et al., 2018 ]. The Arase satellite observes 3-dimenational distribution function and/or pitch angle distribution for wide energy electrons (~20 eV – a few MeV) and ions (10 eV/q – 180 keV/q with mass discriminations. The satellite also measures electric fields from DC to 10 MHz with three different receivers and magnetic fields from DC to 100 kHz with fluxgate and search coil magnetometer [ Kasahara et al., 2018 , Matsuda et al., 2018 , Matsuoka et al., 2018 ]. As an example, heating and acceleration of plasma and energetic electrons and ions are identified by Arase. Arase also observed direct evidence to show scattering of energetic electrons by whistler mode waves, which cause the aurora emissions in the ionosphere. In this presentation, we will show some highlight observations from Arase and discuss future directions of measurement of plasma waves in geospace.

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

We consider the equatorial region of the magnetosphere, where the magnetic field is perpendicular (or near to perpendicular) to the density gradient and mode conversion process from UH to LO-mode waves or reverse process are expected. We review and study the mode conversion from UH (upper hybrid) to LO (left hand polarized ordinary) mode waves by a spatially two dimensional plasma fluid code. Several simulations with different initial wave vectors under the same background plasma condition have been performed. We focus on the conversion efficiency from the UH-mode waves with purely perpendicular wave normal angle to the LO-mode waves, since one of the source of generation UH wave can be Bernstein mode as the purely perpendicular electrostatic waves. For this special case, the UH wave normal is kept in perpendicular direction with respect to the magnetic field, and difficult to be in matching direction for conversion to LO mode. Simulation results show that the mode conversion efficiency in this particular case is very weak, since two branches of Z-mode wave and LO mode wave in the dispersion relation are disconnected. We present a discussion to show that for this case (purely perpendicular propagation) a special angle (except 90°), between the magnetic field and the density gradient is necessary for occurrence of efficient mode conversion. For the case (purely perpendicular propagation and the magnetic field perpendicular to the density gradient), the mode conversion just occurred via the tunneling effect, where a steepness of the inhomogeneity plays an essential role.

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror-mode type structures. We apply the Wave-Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst-mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time.

This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

The present study is a parallel in-situ visualization for geodynamo simulations using a open-source dynamo code Calypso. Calypso solves magnetohydrodynamic (MHD) simulations in a rotating shell on a massively parallel computing environment. We develop a parallel volume rendering modules for scalar component and line integral convolution (LIC) modules for vector fields on an unstructured grid, and performed these visualization modules and numerical simulation concurrently.

著者人数: 30名, Accepted: 2018-11-09, 資料番号: SA1180263000

We carry out a series of self-consistent electron hybrid code simulations for the dependence of chorus generation process on the temperature anisotropy and density of energetic electrons in the Earth's inner magnetosphere. We use the same magnetic field gradient in the simulation system and different temperature anisotropy AT for the initial distribution of energetic electrons at the magnetic equator. We conduct 6 sets of simulations for different AT from 4 to 9, changing the initial number density N-h of energetic electrons at the equator in each set of simulations. By analyzing the spectra obtained in the simulation results, we identify chorus elements with rising tones in the results for higher Nh but no distinct chorus in smaller Nh. We compare the simulation results with estimations of the threshold and optimum amplitude proposed by the nonlinear wave growth theory. We find that the chorus generation processes reproduced in the simulation results are consistently explained by the theoretical estimates. We also compare the simulation results with linear growth rates for all simulation runs. We find clear disagreement between the spectral characteristics of reproduced chorus and the predictions by the linear theory. The present study clarifies that the spectra of chorus are essentially different from those predicted by the linear theory and are determined fully by nonlinear processes of wave-particle interactions in the chorus generation region.

In this study, we have introduced the Code-to-Code Adapter (CoToCoA) library to couple the magnetohydrodynamic (MHD) simulation and the Electron Hybrid (EH) simulation of planetary magnetospheres. CoToCoA has been developed newly to connect the different codes easily. The concept of CoToCoA is that we do not add modifications to each code as possible without data transfer functions, and we do not need to know the referred code without data format. With CoToCoA, we have been developing the cross-reference simulation of macro (MHD) and micro (EH) scales in the magnetosphere. Then, we have evaluated the performance of cross-reference simulation using CoToCoA on the massively parallel computer system.

Whistler-mode chorus emissions play crucial roles in the evolution of radiation belt electrons. Chorus emissions are coherent electromagnetic plasma waves with varying frequencies in the typical frequency range of 0.2 to 0.8 $\mathrm { f } _ { \mathrm { ce } 0 } , \text { where } \mathrm { f } _ { \mathrm { ce } 0 }$ is the electron gyrofrequency at the magnetic equator. They often have a gap at half the local cyclotron frequency. The generation process of chorus has been explained by the nonlinear wave growth theory [see review by Omura et al., 2012] and has been reproduced by self-consistent numerical experiments [e.g., Katoh and Omura, 2007, 2011, 2013, 2016; Katoh et al., 2018].In the present study, we investigate dependencies of the chorus generation process on properties of energetic electrons, the background magnetic field, and the thermal plasma condition. First, we conduct a series of electron hybrid simulations for different temperature anisotropy $(\mathrm{A}_{{\mathrm {T}}})$ of the initial velocity distribution function of energetic electrons. We vary $\mathrm{A}_{{\mathrm {T}}}$ in the range from 3 to 9 with changing the number density of energetic electrons $(\mathrm{N}_{{\mathrm {h}}})$ so as to study whether distinct rising-tone chorus emissions are reproduced or not in the assumed initial condition. Simulation results reveal that $\mathrm{N}_{{\mathrm {h}}}$ required for the chorus generation decreases as the temperature anisotropy of energetic electrons increases. We also find that reproduced spectra become hiss-like for large $\mathrm{N}_{{\mathrm {h}}}$ cases. Next, we carry out simulations by changing the spatial gradient of the background magnetic field intensity along a field line. Simulation results clarify that the small magnetic field gradient lowers the threshold amplitude for the chorus generation. These simulation results demonstrate the validity of the nonlinear wave growth theory and suggest that the coherent nonlinear wave-particle interaction is essential for generation of whistler-mode chorus emissions in the magnetosphere.

著者人数: 28名, Accepted: 2018-12-05, 資料番号: SA1180259000

We report rapid motion of pulsating auroras associated with so called ​3 ± 1 Hz modulations embedded in the main pulsations. During the pulsation ON phase, repetitive expansions are often observed around the edges of pulsating patches. Some events show a few detached expansions traveling away from the main deformed pulsating patch. Approximately 80% of all expansion speeds were found to be less than 70 km s −1 at ionospheric altitudes, which is less than the projected Alfven speed from the magnetospheric equator to the ionosphere. The rapid motions with speeds of tens of km s −1 are unlikely to be explained by obliquely propagating chorus elements, which are known to cause the 3 ± 1 Hz modulation, because the perpendicular speed of the oblique chorus waves is higher than the Alfven speed. We discuss the slow-mode Alfven wave as a candidate modulation source to generate the rapid motions. A few non-repetitive expansion events with a speed of more than 150 km s −1 also appear at the onset of the ON phase. These non-repetitive expanding motions are characterized by a long displacement compared to the repetitive expanding motions. The differences in the expansion speeds indicate different formation mechanisms of the patch motions.

The Wave-Particle Interaction Analyzer (WPIA), a new instrument proposed by Fukuhara et al. (2009), measures the relative phase angle between the wave magnetic field vector and the velocity vector of each particle and calculates the energy exchange from waves to particles. In this study, we expand its applicability by proposing a method of using the WPIA to directly detect pitch angle scattering of resonant particles by plasma waves by calculating the g values. The g value is defined as the accumulation value of the Lorentz force acting on each particle and indicates the lost momentum of waves. We apply the proposed method to the results of a one-dimensional electron hybrid simulation reproducing the generation of whistler mode chorus emissions around the magnetic equator. Using the wave and particle data obtained at fixed observation points assumed in the simulation system, we conduct a pseudo-observation of the simulation result using the WPIA and analyze the g values. Our analysis yielded significant values indicating the strong pitch angle scattering for electrons in the kinetic energy and pitch angle ranges satisfying the cyclotron resonance condition with the reproduced chorus emissions. The results of this study demonstrate that the proposed method enables us to directly and quantitatively identify the location at which pitch angle scattering occurs in the simulation system and that the method can be applied to the results of space-based observations by the forthcoming Exploration of energization and Radiation in Geospace (ERG) satellite.

We report on the first simultaneous measurements of medium-high frequency (MF/HF) auroral radio emissions (above 1 MHz) by ground- and satellite-based instruments. Observational data were obtained by the ground-based passive receivers in Iceland and Svalbard, and by the Plasma Waves and Sounder experiment (PWS) mounted on the Akebono satellite. We observed two simultaneous appearance events, during which the frequencies of the auroral roar and MF bursts detected at ground level were different from those of the terrestrial hectometric radiation (THR) observed by the Akebono satellite passing over the ground-based stations. This frequency difference confirms that auroral roar and THR are generated at different altitudes across the F peak. We did not observe any simultaneous observations that indicated an identical generation region of auroral roar and THR. In most cases, MF/HF auroral radio emissions were observed only by the ground-based detector, or by the satellite-based detector, even when the satellite was passing directly over the ground-based stations. A higher detection rate was observed from space than from ground level. This can primarily be explained in terms of the idea that the Akebono satellite can detect THR emissions coming from a wider region, and because a considerable portion of auroral radio emissions generated in the bottomside F region are masked by ionospheric absorption and screening in the D/E regions associated with ionization which results from auroral electrons and solar UV radiation.

In the beaming angle theory, the magnetic field direction is assumed perpendicular to the normal boundary, and the prediction of this theory, from beaming angle is base on the Jones' formula. We investigate the effect of deviation the magnetic field direction respect to normal boundary direction. In this study, we present new conditions that under these conditions two modes, extraordinary and ordinary modes waves can match. Also, we show for these cases the beaming angle does not correspond to Jones' formula. This effect leads to the angles larger and smaller than the angle estimated by Jones' formula. This effect on the mode conversion process becomes important in a case where local fluctuations in the direction of the density gradient vector or the magnetic field direction are observed. By comparing the beaming angle from observations with the beaming angles resulting from different ∆ Φ , we showed a ∆ Φ about 3 to 5° are necessary in consistence with observation.

We observed a mesoscale aurora (100 km × 100 km) with patchy structure and equatorward propagation at Poker Flat Research Range on 1 December 2011. Fast Fourier transform (FFT) analysis revealed that this pulsating patch clearly exhibited temporal variations that can be categorized into two types: on-off pulsation (7.8–10 s) with large amplitudes and luminosity modulations excited during on phase with a frequency of about 3.0 Hz. In addition, we applied principal component analysis (PCA) to time series image data of the pulsating aurora for the first time. Time coefficients were estimated by PCA for the whole patch and the substructures were consistent with those obtained from the FFT analysis, and therefore, we concluded that PCA is capable of decomposing several structures that have different coherent spatiotemporal characteristics. Another new insight in this study is that the rapid variations were highly localized; they were excited in only the substructures embedded in the whole structure. Moreover, the whole patch propagated equatorward because of E × B drift of cold plasma, while the substructures did not show such systematic propagation but rather forward-backward oscillations. The horizontal scale of the substructures was estimated to be no smaller than 410 km at the magnetic equator, which is comparable to that of the wave packet structure of a whistler mode chorus perpendicular to the field line. We suggest that the apparent horizontal oscillation of the substructures is associated with field-aligned propagations of the whistler mode chorus in a duct.