三谷 烈史

Abstract Near‐equatorial measurements of energetic electron fluxes, in combination with numerical simulation, are widely used for monitoring of the radiation belt dynamics. However, the long orbital periods of near‐equatorial spacecraft constrain the cadence of observations to once per several hours or greater, that is, much longer than the mesoscale injections and rapid local acceleration and losses of energetic electrons of interest. An alternative approach for radiation belt monitoring is to use measurements of low‐altitude spacecraft, which cover, once per hour or faster, the latitudinal range of the entire radiation belt within a few minutes. Such an approach requires, however, a procedure for mapping the flux from low equatorial pitch angles (near the loss cone) as measured at low altitude, to high equatorial pitch angles (far from the loss cone), as necessitated by equatorial flux models. Here we do this using the high energy resolution ELFIN measurements of energetic electrons. Combining those with GPS measurements we develop a model for the electron anisotropy coefficient, , that describes electron flux dependence on equatorial pitch‐angle, , . We then validate this model by comparing its equatorial predictions from ELFIN with in‐situ near‐equatorial measurements from Arase (ERG) in the outer radiation belt.

Abstract We made observations of magnetic field variations in association with pulsating auroras with the magneto‐impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all‐sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin‐layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one‐pair field‐aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated.

Abstract Variations of relativistic electron fluxes (E ≥ 1 MeV) and wave activity in the Earth magnetosphere are studied to determine the contribution of different acceleration mechanisms of the outer radiation belt electrons: ULF mechanism, VLF mechanism, and adiabatic acceleration. The electron fluxes were measured by Arase satellite and geostationary GOES satellites. The ULF power index is used to characterize the magnetospheric wave activity in the Pc5 range. To characterize the VLF wave activity in the magnetosphere, we use data from PWE instrument of Arase satellite. We consider some of the most powerful magnetic storms during the Arase era: May 27–29, 2017; September 7–10, 2017; and August 25–28, 2018. Also, non-storm intervals with a high solar wind speed before and after these storms for comparison are analyzed. Magnitudes of relativistic electron fluxes during these magnetic storms are found to be greater than that during non-storm intervals with high solar wind streams. During magnetic storms, the flux intensity maximum shifts to lower L-shells compared to intervals without magnetic storms. For the considered events, the substorm activity, as characterized by AE index, is found to be a necessary condition for the increase of relativistic electron fluxes, whereas a high solar wind speed alone is not sufficient for the relativistic electron growth. The enhancement of relativistic electron fluxes by 1.5–2 orders of magnitude is observed 1–3 days after the growth of the ULF index and VLF emission power. The growth of VLF and ULF wave powers coincides with the growth of substorm activity and occurs approximately at the same time. Both mechanisms operate at the first phase of electron acceleration. At the second phase of electron acceleration, the mechanism associated with the injection of electrons into the region of the magnetic field weakened by the ring current and their subsequent betatron acceleration during the magnetic field restoration can work effectively. Graphical Abstract

Abstract Using Arase observations of the inner magnetosphere during 26 CIR‐driven geomagnetic storms with minimum Sym‐H between ‐33 and ‐86 nT, we investigated ring current pressure development of ions (H⁺, He⁺, O⁺) and electron during prestorm, main, early recovery and late recovery phases as a function of L‐shell and magnetic local time. It is found that during the main and early recovery phase of the storms the ion pressure is asymmetric in the inner magnetosphere, leading to a strong partial ring current. The ion pressure becomes symmetric during the late recovery phase. H⁺ ions with energies of ∼20‐50 keV and ∼50‐100 keV contribute more to the ring current pressure during the main phase and early/late recovery phase, respectively. O⁺ ions with energies of ∼10‐20 keV contribute significantly during main and early recovery phase. These are consistent with previous studies. The electron pressure was found to be asymmetric during the main, early recovery and late recovery phase. The electron pressure peaks from midnight to the dawn sector. Electrons with energy of <50 keV contribute to the ring current pressure during the main and early recovery phase of the storms. Overall, the electron contribution to the total ring current is found to be ∼11% during the main and early recovery phases. However, the electron contribution is found to be significant (∼22%) in the 03‐09 MLT sector during the main and early recovery phase. The results indicate an important role of electrons in the ring current build up. This article is protected by copyright. All rights reserved.

During July to October of 2019, a sequence of isolated Corotating Interaction Regions (CIRs) impacted the magnetosphere, for four consecutive solar rotations, without any interposed Interplanetary Coronal Mass Ejections. Even though the series of CIRs resulted in relatively weak geomagnetic storms, the net effect of the outer radiation belt during each disturbance was different, depending on the electron energy. During the August-September CIR group, significant multi-MeV electron enhancements occurred, up to ultra-relativistic energies of 9.9 MeV in the heart of the outer Van Allen radiation belt. These characteristics deemed this time period a fine case for studying the different electron acceleration mechanisms. In order to do this, we exploited coordinated data from the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms Mission (THEMIS), Arase and Galileo satellites, covering seed, relativistic and ultra-relativistic electron populations, investigating their Phase Space Density (PSD) profile dependence on the values of the second adiabatic invariant K, ranging from near-equatorial to off equatorial mirroring populations. Our results indicate that different acceleration mechanisms took place for different electron energies. The PSD profiles were dependent not only on the mu value, but also on the K value, with higher K values corresponding to more pronounced local acceleration by chorus waves. The 9.9 MeV electrons were enhanced prior to the 7.7 MeV, indicating that different mechanisms took effect on different populations. Finally, all ultra-relativistic enhancements took place below geosynchronous orbit, emphasizing the need for more Medium Earth Orbit (MEO) missions.

Abstract This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017–2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher $L$-shells up to $L \sim 10$. After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5° in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.

<title>Abstract</title>Pulsating aurorae (PsA) are caused by the intermittent precipitations of magnetospheric electrons (energies of a few keV to a few tens of keV) through wave-particle interactions, thereby depositing most of their energy at altitudes ~ 100 km. However, the maximum energy of precipitated electrons and its impacts on the atmosphere are unknown. Herein, we report unique observations by the European Incoherent Scatter (EISCAT) radar showing electron precipitations ranging from a few hundred keV to a few MeV during a PsA associated with a weak geomagnetic storm. Simultaneously, the Arase spacecraft has observed intense whistler-mode chorus waves at the conjugate location along magnetic field lines. A computer simulation based on the EISCAT observations shows immediate catalytic ozone depletion at the mesospheric altitudes. Since PsA occurs frequently, often in daily basis, and extends its impact over large MLT areas, we anticipate that the PsA possesses a significant forcing to the mesospheric ozone chemistry in high latitudes through high energy electron precipitations. Therefore, the generation of PsA results in the depletion of mesospheric ozone through high-energy electron precipitations caused by whistler-mode chorus waves, which are similar to the well-known effect due to solar energetic protons triggered by solar flares.

This study investigates the characterization and calibration of the high-energy electron experiments (HEP) instrument onboard the exploration of energization and radiation in geospace (ERG). Two detector modules, HEP-L and HEP-H, which employ stacks of multichannel silicon strip detectors, detect electrons in the energy ranges of 70 keV–1 MeV and 700 keV–2 MeV, respectively. The detector response to electron irradiation needs to be assessed to obtain accurate electron fluxes from these detectors. In this study, we perform Monte Carlo simulations using the Geant4 particle simulation tool to reconstruct incident electron fluxes from detected count rates. Based on the simulation results, we investigate the response characteristics of the detectors when electrons with a certain range of energy are irradiated onto them. A response function is constructed by combining the simulation results for different incident energies. A response matrix is calculated by binning the response function according to the energy channels of the detector, and an inverse matrix derived from the response matrix is used to calibrate the observational data. Compared with the data obtained from another electron instrument onboard the Arase satellite (MEP-e), whose energy range overlaps with that of the HEP, the differential flux data for the overlapping energy range (85–95 keV) are consistent with each other. The basic characteristics of the HEP detectors are thus confirmed to provide well-calibrated data.

GEO-X (GEOspace X-ray imager) is a 50 kg-class small satellite to image the global Earth's magnetosphere in X-rays via solar wind charge exchange emission. A 12U CubeSat will be injected into an elliptical orbit with an apogee distance of similar to 40 Earth radii. In order to observe the diffuse soft X-ray emission in 0.3-2 keV and to verify X-ray imaging of the dayside structures of the magnetosphere such as cusps, magnetosheaths and magnetopauses which are identified statistically by in-situ satellite observations, an original light-weight X-ray imaging spectrometer (similar to 10 kg, similar to 10 W, similar to 10x10x30 cm) will be carried. The payload is composed of a ultra light-weight MEMS Wolter type-I telescope (similar to 4x4 deg(2) FOV, <10 arcmin resolution) and a high speed CMOS sensor with a thin optical blocking filter (similar to 2x2 cm(2), frame rate similar to 20 ms, energy resolution <80 eV FWHM at 0.6 keV). An aimed launch year is 2023-25 corresponding to the 25th solar maximum.

The high-energy electron experiments (HEP) instrument on board the Arase satellite employs two sensors, HEP-L and HEP-H, and was designed to measure electrons with energies from 70 keV to 2 MeV. The recent Van Allen Probes observations indicate that MeV electron flux is very small in the inner radiation belt, while the HEP has detected significant counts at MeV energy channels in the inner radiation belt. Counts in the inner radiation belt are registered similarly at different energy channels of HEP-H and higher energy channels of HEP-L, and show no clear energy dependence. Their properties suggest contamination of high-energy protons that populate densely the inner radiation belt. In order to identify the energy of the penetrating protons we compare the spatial distribution of the HEP counts with NASA's AP9 mean model. We find that the primary peak of the count distribution measured with HEP in MeV energy range is located at L = 1.5 at the magnetic equator, which in in agreement of > 60 MeV inner belt protons of AP9 mean model. The secondary distribution is also found at higher L values, which can be attributed to MeV protons. We have been conducting Geant4 simulation for penetrating protons into the HEP. Our result of the simulation is consistent with suggestions of analysis on the spatial distribution.

©2020. American Geophysical Union. All Rights Reserved. Plasmaspheric hiss is an important whistler-mode emission shaping the Van Allen radiation belt environment. How the plasmaspheric hiss waves are generated, propagate, and dissipate remains under intense debate. With the five spacecraft of Van Allen Probes, Exploration of energization and Radiation in Geospace (Arase), and Geostationary Operational Environmental Satellites missions at widely spaced locations, we present here the first comprehensive observations of hiss waves growing from the substorm-injected electron instability, spreading within the plasmasphere, and dissipating over a large spatial scale. During substorms, hot electrons were injected energy-dispersively into the plasmasphere near the dawnside and, probably through a combination of linear and nonlinear cyclotron resonances, generated whistler-mode waves with globally drifting frequencies. These waves were able to propagate from the dawnside to the noonside, with the frequency-drifting feature retained. Approximately 5 hr of magnetic local time away from the source region in the dayside sector, the wave power was dissipated to (Formula presented.) of its original level.

Transient mesospheric echo in the VHF range was detected at an altitude of 65-70km during the auroral breakup that occurred from 2220 to 2226 UT on June 30, 2017. During this event, the footprint of the Arase satellite was located within the field of view of the all-sky imagers at Syowa Station in the Antarctic. Auroral observations at Syowa Station revealed the dominant precipitation of relatively soft electrons during the auroral breakup. A corresponding spike in cosmic noise absorption was also observed at Syowa Station, while the Arase satellite observed a flux enhancement of >100keV electrons and a broadband noise without detecting chorus waves or electromagnetic ion cyclotron waves. A general-purpose Monte Carlo particle transport simulation code was used to quantitatively evaluate the ionization in the middle atmosphere. Results of this study indicate that the precipitation of energetic electrons of >100keV, rather than X-rays from the auroral electrons, played a dominant role in the transient and deep (65-70km) mesospheric ionization during the observed auroral breakup.

Substorm-associated electron injection, starting on Apr. 5, 2017, was observed by the ERG (Arase), GOES-15 and GOES-13 spacecraft. ERG successfully observed a clear and sufficient extent of manifestations of the dispersionless injection and the successive drift echoes at radial distances shorter than geosynchronous orbit (GEO) during a unique period of the satellite mission. The GOES-15 and GOES-13 measured the drift echoes of the event as well. The observations provided constraints to study the event and opportunities to make adjustments to the previous substorm injection models. Models built on an impulsive earthward-propagating electromagnetic field have been proposed to simulate substorm injections. So far such models showed good results of dispersionless features compared to spacecraft observations, but could only produce drift echoes with periods somewhat different from geosynchronous observations. To study the substorm injection event and produce drift echoes with better periods, we modify an existing model in the literature. ERG and GOES spacecraft measured tens to a few hundred keV electrons injected during the substorm, providing important seed population for ring current and radiation belts. Since the electron energies of interest are comparable to the rest mass energy, our work further provides the relativistic form of the previous model and employs a semiempirical model as background field instead of a dipole-based one in the previous study. Our work shows that the main features of the substorm injection event are successfully reproduced with the drift echoes periods showing a better fit to the observations of this event when relativistic effects are considered. Despite possible deviation of the model magnetic fields from reality, the relativistic computations still show dominant effect on the drift echoes periods. The substorm injection expanding earthward farther than GEO was observed by ERG, and the event can be better simulated by the further-developed model shown in this work.

© 2018, The Author(s). The software-type wave–particle interaction analyzer (S-WPIA) is an instrument package onboard the Arase satellite, which studies the magnetosphere. The S-WPIA represents a new method for directly observing wave–particle interactions onboard a spacecraft in a space plasma environment. The main objective of the S-WPIA is to quantitatively detect wave–particle interactions associated with whistler-mode chorus emissions and electrons over a wide energy range (from several keV to several MeV). The quantity of energy exchanges between waves and particles can be represented as the inner product of the wave electric-field vector and the particle velocity vector. The S-WPIA requires accurate measurement of the phase difference between wave and particle gyration. The leading edge of the S-WPIA system allows us to collect comprehensive information, including the detection time, energy, and incoming direction of individual particles and instantaneous-wave electric and magnetic fields, at a high sampling rate. All the collected particle and waveform data are stored in the onboard large-volume data storage. The S-WPIA executes calculations asynchronously using the collected electric and magnetic wave data, data acquired from multiple particle instruments, and ambient magnetic-field data. The S-WPIA has the role of handling large amounts of raw data that are dedicated to calculations of the S-WPIA. Then, the results are transferred to the ground station. This paper describes the design of the S-WPIA and its calculations in detail, as implemented onboard Arase.[Figure not available: see fulltext.].

We describe the principles of the Wave–Particle Interaction Analyzer (WPIA) and the implementation of the Software-type WPIA (S-WPIA) on the Arase satellite. The WPIA is a new type of instrument for the direct and quantitative measurement of wave–particle interactions. The S-WPIA is installed on the Arase satellite as a software function running on the mission data processor. The S-WPIA on board the Arase satellite uses an electromagnetic field waveform that is measured by the waveform capture receiver of the plasma wave experiment (PWE), and the velocity vectors of electrons detected by the medium-energy particle experiment–electron analyzer (MEP-e), the high-energy electron experiment (HEP), and the extremely high-energy electron experiment (XEP). The prime objective of the S-WPIA is to measure the energy exchange between whistler-mode chorus emissions and energetic electrons in the inner magnetosphere. It is essential for the S-WPIA to synchronize instruments to a relative time accuracy better than the time period of the plasma wave oscillations. Since the typical frequency of chorus emissions in the inner magnetosphere is a few kHz, a relative time accuracy of better than 10 μs is required in order to measure the relative phase angle between the wave and velocity vectors. In the Arase satellite, a dedicated system has been developed to realize the time resolution required for inter-instrument communication. Here, both the time index distributed over all instruments through the satellite system and an S-WPIA clock signal are used, that are distributed from the PWE to the MEP-e, HEP, and XEP through a direct line, for the synchronization of instruments within a relative time accuracy of a few μs. We also estimate the number of particles required to obtain statistically significant results with the S-WPIA and the expected accumulation time by referring to the specifications of the MEP-e and assuming a count rate for each detector.[Figure not available: see fulltext.].

© 2018, The Author(s). The medium-energy particle experiments—electron analyzer onboard the exploration of energization and radiation in geospace spacecraft measures the energy and direction of each incoming electron in the energy range of 7–87 keV. The sensor covers a 2π-radian disklike field of view with 16 detectors, and the full solid angle coverage is achieved through the spacecraft’s spin motion. The electron energy is independently measured by both an electrostatic analyzer and avalanche photodiodes, enabling significant background reduction. We describe the technical approach, data output, and examples of initial observations.[Figure not available: see fulltext.].

© 2018, The Author(s). We describe the principles of the Wave–Particle Interaction Analyzer (WPIA) and the implementation of the Software-type WPIA (S-WPIA) on the Arase satellite. The WPIA is a new type of instrument for the direct and quantitative measurement of wave–particle interactions. The S-WPIA is installed on the Arase satellite as a software function running on the mission data processor. The S-WPIA on board the Arase satellite uses an electromagnetic field waveform that is measured by the waveform capture receiver of the plasma wave experiment (PWE), and the velocity vectors of electrons detected by the medium-energy particle experiment–electron analyzer (MEP-e), the high-energy electron experiment (HEP), and the extremely high-energy electron experiment (XEP). The prime objective of the S-WPIA is to measure the energy exchange between whistler-mode chorus emissions and energetic electrons in the inner magnetosphere. It is essential for the S-WPIA to synchronize instruments to a relative time accuracy better than the time period of the plasma wave oscillations. Since the typical frequency of chorus emissions in the inner magnetosphere is a few kHz, a relative time accuracy of better than 10 μs is required in order to measure the relative phase angle between the wave and velocity vectors. In the Arase satellite, a dedicated system has been developed to realize the time resolution required for inter-instrument communication. Here, both the time index distributed over all instruments through the satellite system and an S-WPIA clock signal are used, that are distributed from the PWE to the MEP-e, HEP, and XEP through a direct line, for the synchronization of instruments within a relative time accuracy of a few μs. We also estimate the number of particles required to obtain statistically significant results with the S-WPIA and the expected accumulation time by referring to the specifications of the MEP-e and assuming a count rate for each detector.[Figure not available: see fulltext.].

The software-type wave–particle interaction analyzer (S-WPIA) is an instrument package onboard the Arase satellite, which studies the magnetosphere. The S-WPIA represents a new method for directly observing wave–particle interactions onboard a spacecraft in a space plasma environment. The main objective of the S-WPIA is to quantitatively detect wave–particle interactions associated with whistler-mode chorus emissions and electrons over a wide energy range (from several keV to several MeV). The quantity of energy exchanges between waves and particles can be represented as the inner product of the wave electric-field vector and the particle velocity vector. The S-WPIA requires accurate measurement of the phase difference between wave and particle gyration. The leading edge of the S-WPIA system allows us to collect comprehensive information, including the detection time, energy, and incoming direction of individual particles and instantaneous-wave electric and magnetic fields, at a high sampling rate. All the collected particle and waveform data are stored in the onboard large-volume data storage. The S-WPIA executes calculations asynchronously using the collected electric and magnetic wave data, data acquired from multiple particle instruments, and ambient magnetic-field data. The S-WPIA has the role of handling large amounts of raw data that are dedicated to calculations of the S-WPIA. Then, the results are transferred to the ground station. This paper describes the design of the S-WPIA and its calculations in detail, as implemented onboard Arase.[Figure not available: see fulltext.].

This paper reports the design, calibration, and operation of high-energy electron experiments (HEP) aboard the exploration of energization and radiation in geospace (ERG) satellite. HEP detects 70 keV-2 MeV electrons and generates a three-dimensional velocity distribution for these electrons in every period of the satellite's rotation. Electrons are detected by two instruments, namely HEP-L and HEP-H, which differ in their geometric factor (G-factor) and range of energies they detect. HEP-L detects 70 keV-1 MeV electrons and its G-factor is 9.3 x 10(-4) cm(2) sr at maximum, while HEP-H observes 0.7-2 MeV electrons and its G-factor is 9.3 x 10(-3) cm(2) sr at maximum. The instruments utilize silicon strip detectors and application-specific integrated circuits to readout the incident charge signal from each strip. Before the launch, we calibrated the detectors by measuring the energy spectra of all strips using gamma-ray sources. To evaluate the overall performance of the HEP instruments, we measured the energy spectra and angular responses with electron beams. After HEP was first put into operation, on February 2, 2017, it was demonstrated that the instruments performed normally. HEP began its exploratory observations with regard to energization and radiation in geospace in late March 2017. The initial results of the in-orbit observations are introduced briefly in this paper.

The Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron generation/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which different plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite.

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

©2018. American Geophysical Union. All Rights Reserved. Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of millihertz or lower-frequency range with horizontal velocities of a few hundred meters per second appeared in the subauroral to midlatitude region during a magnetic storm on 27 March 2017. A set of the radars have provided the first ever observations that the fluctuations propagate azimuthally both westward and eastward simultaneously, showing bifurcated phase propagation associated with substorm expansion. Concurrent observations near the conjugate site in the inner magnetosphere made by the Arase satellite provide evidence that multiple drifting clouds of electrons in the near-Earth equatorial plane were associated with the electric field fluctuations propagating eastward in the ionosphere. We interpret this event in terms of mesoscale pressure gradients carried by drifting ring current electrons that distort field lines one after another as they drift through the inner magnetosphere, causing eastward propagating ionospheric electric field fluctuations.

©2018. American Geophysical Union. All Rights Reserved. There has been increasing evidence for pitch angle scattering of relativistic electrons by electromagnetic ion cyclotron (EMIC) waves. Theoretical studies have predicted that the loss time scale of MeV electrons by EMIC waves can be very fast, suggesting that MeV electron fluxes rapidly decrease in association with the EMIC wave activity. This study reports on a unique event of MeV electron loss induced by EMIC waves based on Arase, Van Allen Probes, and ground-based network observations. Arase observed a signature of MeV electron loss by EMIC waves, and the satellite and ground-based observations constrained spatial-temporal variations of the EMIC wave activity during the loss event. Multisatellite observation of MeV electron fluxes showed that ~2.5-MeV electron fluxes substantially decreased within a few tens of minutes where the EMIC waves were present. The present study provides an observational estimate of the loss time scale of MeV electrons by EMIC waves.

© 2017, The Author(s). The medium-energy particle experiments–ion mass analyzer (MEP-i) was developed for the exploration of energization and radiation in geospace (ERG) mission (Arase), in order to measure the three-dimensional distribution functions of the inner-magnetospheric ions in the medium energy range between 10 and 180 keV/q. The energy, mass, and charge state of each ion are determined by a combination of an electrostatic energy/charge analyzer, a time-of-flight mass/charge analyzer, and energy-sensitive solid-state detectors. This paper describes the instrumentation of the MEP-i, data products, and observation results during a magnetic storm.

月周回探査衛星SELENE に搭載されたα線検出器(ARD : Alpha-Ray Detector)は月面から放出されるRn-222 およびその崩壊過程で生じるPo-210 から放射されるα線を観測し,月表層下のウラニウムの分布および断層などの地殻構造,月の希薄大気の動態等に関する情報を得ることを目的とする.本論文では,ARD のデータ処理の流れと,角度応答関数を考慮した月面のα線強度分布の導出方法について紹介する.この方法は月ラドンα線観測データの解析では始めて用いられるもので,α線強度分布マップの解像度を向上し,月地形等の詳細な比較を可能とする.