浅村 和史

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 We present the simultaneous and conjugated auroral emission and particle data obtained by a low‐altitude polar‐orbiting micro‐satellite, Reimei, for elucidating their latitudinal distributions and variations in the nightside auroral oval. Here are reported a few notable examples of the Reimei observations with high time and spatial resolutions, namely ∼120 msec. and ∼1.2 km × 1.2 km for multispectral auroral images and 40 msec. for energy‐pitch angle distributions of electrons and ions with energies of 10 eV–12 keV, respectively. The auroral images show various fine‐scale auroral activities characterized by the following types of auroral forms and variations: faint bands, streaming multiple arcs, shearing arcs, and vortices/curls, which are typical of the latitudinal properties of auroras. The particle analyzer simultaneously observed various properties of electron energy‐pitch angle and latitudinal distributions, and their temporal variations, each of which corresponds to a type of the auroral activities. Their features are summarized below. Reimei repetitively observed inverted‐V signatures of low‐energy (<1 keV) field‐aligned electrons in addition to the higher‐energy (several keV) diffuse electrons in low‐latitude auroral oval. In more active regions at higher latitudes, the dominant energy flux responsible for the multiple‐arc emissions was carried by the well‐known inverted‐V electron precipitation. The rapidly rotating vortices or so‐called curls of fine‐scale discrete auroras near the poleward boundary of the auroral oval were closely associated with the significant energy fluxes of spiky field‐aligned electron bursts with energy‐time dispersions produced by dispersive Alfvén waves.

Abstract This is the first report of significant energization (up to 7,000 eV) of low‐energy He⁺ ions, which occurred simultaneously with H‐band electromagnetic ion cyclotron (EMIC) wave activity, in a direction mostly perpendicular to the ambient magnetic field. The event was detected by the Arase satellite in the dayside plasmatrough region off the magnetic equator on 15 May 2019. The peak energy of the He⁺ flux enhancements is mostly above 1,000 eV. At some interval, the He⁺ ions are energized up to ∼7,000 eV. The H‐band waves are excited in a frequency band between the local crossover and helium gyrofrequencies and are close to a linear polarization state with weakly left‐handed or right‐handed polarization. The normal angle of the waves exhibits significant variation between 0° and 80°, indicating a non‐parallel propagation. We run a hybrid code with parameters estimated from the Arase observations to examine the He⁺ energization. The simulations show that cold He⁺ ions are energized up to more than 1,000 eV, similar to the spacecraft observations. From the analysis of the simulated wave fields and cold plasma motions, we found that the ratio of the wave frequency to He⁺ gyrofrequency is a primary factor for transverse energization of cold He⁺ ions. As a consequence of the numerical analysis, we suggest that the significant transverse energization of He⁺ ions observed by Arase is attributed to H‐band EMIC waves excited near the local helium gyrofrequency.

Abstract We present the first direct evidence of an in situ excitation of drift‐compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift‐compressional mode theory, whereas the plasma anisotropy was too small to excite the drift‐mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple‐nose structures of ring current protons.

Abstract We have developed a low-energy particle experiment that alternately measures ions and electrons in space. The ability to switch between ion and electron measurements is achieved by simply adding ultra-thin carbon foils and positive and negative outputs to a conventional top-hat electrostatic analyzer and a high-voltage power supply, respectively. The advantage of this experiment is that it can perform both ion and electron measurements using only one MCP-based detector for electrons, since it detects secondary electrons emitted from the carbon foils. For the SS520-3 sounding rocket program, we prepared two identical energy analyzers, one for ions and the other for electrons to demonstrate this technique. Laboratory tests confirmed that the performance of the two analyzers was comparable to that of conventional analyzers for ion and electrons. The SS520-3 rocket experiment in the high latitude auroral region yielded observations that captured typical features of ions and electrons, which were similar to previous observations. Graphical Abstract

Abstract We have identified for the first time an energy‐time dispersion of precipitating electron flux in a pulsating aurora patch, ranging from 6.7 to 580 keV, through simultaneous in‐situ observations of sub‐relativistic electrons of microburst precipitations and lower‐energy electrons using the Loss through Auroral Microburst Pulsation sounding rocket launched from the Poker Flat Research Range in Alaska. Our observations reveal that precipitating electrons with energies of 180–320 keV were observed first, followed by 250–580 keV electrons 0–30 ms later, and finally, after 500–1,000 ms, 6.7–14.6 keV electrons were observed. The identified energy‐time dispersion is consistent with the theoretical estimation that the relativistic electron microbursts are a high‐energy tail of pulsating aurora electrons, which are caused by chorus waves propagating along the field line.

Abstract Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H⁺), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O⁺). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.

Abstract Although many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7‐25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase at L ∼ 5. A camera at Gakona (62.39^oN, 214.78^oE), Alaska, observed a substorm auroral brightening on December 28, 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere.

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.

Abstract Mercury’s magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo’s first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.

Abstract Plasma particles and waves are important observation targets in space plasmas for understanding the mechanisms of energy and momentum transfer between waves and particles because space plasmas are essentially collisionless. Multi-point observations are crucial for understanding the spatial–temporal variations of space plasmas. To realize such observations by a large number of satellites, onboard instruments should be miniaturized to reduce their required resources. This paper proposes a small amplifier for plasma particle detectors onboard satellites. This charge-sensitive amplifier converts an electron cloud emitted from the detector, for example a microchannel plate, to a current pulse that can be handled by a time-of-flight measurement circuit to determine the particle velocity and thus mass. The amplifier is realized using application-specific integrated circuit technology to minimize size. Its dimensions are estimated to be $$2120\,\mathrm{ \mu m }\times 1680\,\mathrm{ \mu m}$$, which are much smaller than those of a conventional amplifier. The response time of the proposed amplifier has a variation of less than $$1.2\,\mathrm{ ns}$$ over the range of expected input levels. The amplifier can handle up to $$2\times {10}^{7}$$ signals per second and has a sensitivity of $$1.5\,\mathrm{ V}/\mathrm{pC}$$ at $$20\,\mathrm{^\circ{\rm C} }$$. Graphical Abstract

Abstract The second Venus flyby of the BepiColombo mission offer a unique opportunity to make a complete tour of one of the few gas-dynamics dominated interaction regions between the supersonic solar wind and a Solar System object. The spacecraft pass through the full Venusian magnetosheath following the plasma streamlines, and cross the subsolar stagnation region during very stable solar wind conditions as observed upstream by the neighboring Solar Orbiter mission. These rare multipoint synergistic observations and stable conditions experimentally confirm what was previously predicted for the barely-explored stagnation region close to solar minimum. Here, we show that this region has a large extend, up to an altitude of 1900 km, and the estimated low energy transfer near the subsolar point confirm that the atmosphere of Venus, despite being non-magnetized and less conductive due to lower ultraviolet flux at solar minimum, is capable of withstanding the solar wind under low dynamic pressure.

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.

During disturbed geomagnetic conditions, the energetic particles in the inner magnetosphere are known to undergo precipitation loss due to interaction with various plasma waves. This study, investigates the energetic particle precipitation events statistically using coordinate observations from the ground riometer network and the inner-magnetospheric satellite mission, Arase. We have compared cosmic noise absorption (CNA) data obtained from the Finnish ground riometer network located in the auroral/sub-auroral latitudes with the comprehensive data set of omnidirectional electron/proton flux and plasma waves in ELF/VLF frequency range from the Arase satellite during the overpass intervals. The study period includes one and a half years of data between March 2017 and September 2018 covering Arase conjunctions with the riometer stations from all magnetic local time sectors. The relation between the plasma flux/waves observed at the satellite with the riometer absorptions are investigated statistically for CNA (absorption >0.5 dB) and non-CNA (absorption <0.5 dB) cases separately. During CNA events, Arase observed elevated electron flux in the medium energy range (2-100 keV), and plasma wave activity in the whistler-mode frequency range (0.5-3 kHz) of the spectra. Our study provides an estimate of the statistical dependence of the electron flux and plasma wave observations at Arase with the ground reality of actual precipitation.

Flux enhancements of field-aligned low-energy O+ ion (FALEO) are simultaneously observed by Arase, Van Allen Probes A and B in the nightside inner magnetosphere during 05–07 UT on September 22, 2018. FALEOs appear after a magnetic dipolarization signature with approximately 6–20 min delay. It has the energy-dispersion signature from a few keV to ∼100 eV only in the direction parallel to the magnetic field at Arase, while it decreases its energy from a few keV down to 10 eV in both the parallel and antiparallel directions at Probes A and B. We perform a numerical simulation to trace trajectories of test O+ ions in a model magnetosphere, which are launched from above the ionosphere 3–15 min after a substorm. Flying virtual satellites that have the same orbits as the real satellites, we create virtual energy-time spectrograms of O+ ions to compare with the observed ones. Results show a very good correspondence between them, indicating that FALEOs originate from ionospheric O+ ions that are extracted from the upper ionosphere at substorm onset and flow along the magnetic field toward the geomagnetic equator. It is also revealed that 3–9 hr after their launch, test O+ ions less than 400 eV have a spatial distribution in the inner magnetosphere which is similar to those of the warm plasma cloak and the oxygen torus. We therefore conclude that FALEO is a source of those cold ion populations.

Using a novel wave-particle interaction analysis, we show observational evidence of energy transfer from fast magnetosonic waves (MSWs) to low-energy protons in the magnetosphere. The analysis clearly indicates that the transferred proton energies are further converted to excite electromagnetic ion cyclotron waves. Since MSWs are excited by hot ions, cross-energy coupling of ions occurs through MSWs. The result also suggests a new energy transfer path of exciting electromagnetic ion cyclotron waves in the magnetosphere, and a complex interplay between various wave modes and particle populations.

<title>Abstract</title>The mass spectrum analyzer (MSA) will perform in situ observations of ions and magnetic fields around Phobos as part of the Martian Moons eXploration (MMX) mission to investigate the origin of the Martian moons and physical processes in the Martian environment. MSA consists of an ion energy mass spectrometer and two magnetometers which will measure velocity distribution functions and mass/charge distributions of low-energy ions and magnetic field vectors, respectively. For the MMX scientific objectives, MSA will observe solar wind ions, those scattered at the Phobos surface, water-related ions generated in the predicted Martian gas torus, secondary ions sputtered from Phobos, and escaping ions from the Martian atmosphere, while monitoring the surrounding magnetic field. MSA will be developed from previous instruments for space plasma missions such as Kaguya, Arase, and BepiColombo/Mio to contribute to the MMX scientific objectives.

We report on the relationship between a pulsating aurora and a relativistic electron microburst using simultaneous observations of ground-based fast auroral imagers with the FIREBIRD-II CubeSat for the first time. We conducted a detailed analysis of an event on October 8, 2018 and found that the occurrence of the pulsating aurora with internal modulations corresponds to the flux enhancement of electrons with energy ranging from similar to 220 keV to >1 MeV detected with Flight Unit 4, one of FIREBIRD's CubeSat, with a time delay of similar to 585 ms. Combining of this time delay result and time of flight model, we suggest that the pulsating aurora and the microburst occur simultaneously due to the chorus waves at different latitudes along the same field-line as predicted by Miyoshi et al. (2020, ).

Using Exploration of energization and Radiation in Geospace (ERG or Arase) spacecraft data, we studied low-frequency magnetic field and energetic particle flux oscillations and high-frequency waves deep in the inner magnetosphere at a radial distance of ~4–5 (Formula presented.) during substorm dipolarization. The magnetic field oscillated alternately between dipole-like and taillike configuration at a period of (Formula presented.) 1 min during dipolarization. When the magnetic field was dipole-like, the parallel magnetic component of the Pi2 waves was at trough. Both energetic ion and electron fluxes with a few to tens of kiloelectronvolts enhanced out of phase, indicating that magnetosonic waves were in slow mode. Field-aligned currents also oscillated. These observations are consistent with signatures of ballooning instability. In addition, we found that broadband waves from the Pi1 range to above the electron cyclotron frequency tended to appear intermittently in the central plasma sheet near dipole-like configuration.

<title>Abstract</title>A study using Arase data gives the first observational evidence that the frequency drift of electromagnetic ion cyclotron (EMIC) waves is caused by cyclotron trapping. EMIC emissions play an important role in planetary magnetospheres, causing scattering loss of radiation belt relativistic electrons and energetic protons. EMIC waves frequently show nonlinear signatures that include frequency drift and amplitude enhancements. While nonlinear growth theory has suggested that the frequency change is caused by nonlinear resonant currents owing to cyclotron trapping of the particles, observational evidence for this has been elusive. We survey the wave data observed by Arase from March, 2017 to September 2019, and find the best falling tone emission event, one detected on 11th November, 2017, for the wave particle interaction analysis. Here, we show for the first time direct evidence of the formation of a proton hill in phase space indicating cyclotron trapping. The associated resonance currents and the wave growth of a falling tone EMIC wave are observed coincident with the hill, as theoretically predicted.

© 2021, The Author(s). Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

©2020. The Authors. In this study, by simulating the wave-particle interactions, we show that subrelativistic/relativistic electron microbursts form the high-energy tail of pulsating aurora (PsA). Whistler-mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few kiloelectron volts (PsA) to several megaelectron volts (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of subrelativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few hertz. The chorus bursts for a few seconds cause the microburst trains of subrelativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave-particle interactions along the field line.

JAXA’s ERG (Exploration of Energization and Radiation in Geospace) Spacecraft, which is nicknamed Arase, was launched on 20 December 2016. Arase is a spin-stabilized and Sun-oriented spacecraft. Its mission is to explore how relativistic electrons in the radiation belts are generated during space storms. Two different on-ground attitude determination algorithms are designed for the mission: A TRIAD-based algorithm that inherits from old missions and a filtering-based new algorithm. This paper, first, discusses the design of the filtering-based attitude determination algorithm, which is mainly based on an Unscented Kalman Filter (UKF), specifically designed for spinning spacecraft (SpinUKF). The SpinUKF uses a newly introduced set of attitude parameters (i.e., spin-parameters) to represent the three-axis attitude of the spacecraft and runs UKF for attitude estimation. The paper then presents the preliminary attitude estimation results for the spacecraft that are obtained after the launch. The results are presented along with the encountered challenges and suggested solutions for them. These preliminary attitude estimation results show that the expected accuracy of the fine attitude estimation for the spacecraft is less than 0.5°.

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.].

Low-energy ion experiments–ion mass analyzer (LEPi) is one of the particle instruments onboard the ERG satellite. LEPi is an ion energy-mass spectrometer which covers the range of particle energies from &lt 0.01 to 25 keV/q. Species of incoming ions are discriminated by a combination of electrostatic energy-per-charge analysis and the time-of-flight technique. The sensor has a planar field-of-view, which provides 4π steradian coverage by using the spin motion of the satellite. LEPi started its nominal observation after the initial checkout and commissioning phase in space. [Figure not available: see fulltext.].