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Recent studies have shown that the ion precipitation induced by EMIC waves can contribute significantly to the total energy flux deposited into the ionosphere and severely affect the magnetosphere-ionosphere coupling. During the geomagnetic storm of 27-28 May 2017, the ARASE and the RBSPa satellites observed typical signatures of EMIC waves in the inner magnetosphere. The DMSP and MetOp satellites observed enhanced proton precipitation during the main phase of the storm. In order to understand the evolution of proton precipitation into the ionosphere, its correspondence to the time and location of excitation of the EMIC waves and its relation to the source and distribution of proton temperature anisotropy, we conducted two simulations of the BATSRUS+RAMSCBE model with and without EMIC waves included. Simulation results suggest that the H- and He-band EMIC waves are excited within regions of strong temperature anisotropy of protons in the vicinity of the plasmapause. In regions where the Arase/RBSPa satellite measurements recorded EMIC wave activity, an increase in the simulated growth rates of H- and He-band EMIC waves is observed indicating that the model is able to capture the EMIC wave activity. The RAM-SCBE simulation with EMIC waves reproduces the precipitating fluxes in the premidnight sector fairly well, and is found to be in good agreement with the DMSP and MetOp satellite observations. The results suggest that the EMIC wave scattering of ring current ions gives rise to the proton precipitation in the premidnight sector at subauroral latitudes during the main phase of the 27 May 2017 storm.

Geomagnetic storms are the main component of space weather. Enhancement of the ring current is a typical feature of the geomagnetic storm and a global decrease in the H component of the geomagnetic field is observed during the main phase of the geomagnetic storm. The ring current represents a diamagnetic current driven by the plasma pressure in the inner magnetosphere. The plasma pressure is mainly dominated by protons in an energy range of a few to a few hundred keVs during quiet times. The O+ contribution is also important, and sometimes dominates more than H+ during intense geomagnetic storms. However, electron contribution to the ring current is not studied well. Recently, we showed that the electron pressure also contributes to the depression of ground magnetic field during the November 2017 CIR-driven storm by comparing Ring current Atmosphere interactions Model with Self Consistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. Arase satellite observed 26 geomagnetic storms driven by Corotating Interaction Regions (CIR) during 2017-2021. In this study, we examine statistically the spatial and temporal distribution of ions (H+, He+, O+) and electrons pressure as a function of magnetic local time, L shell and wide range of energies during prestorm, main phase, early recovery and late recovery phase for 26 CIR storms using in situ plasma/particle data obtained by Arase. The results indicate that the electrons (20-50 keV) contribution to the ring current pressure is non-negligible.

We have identified for the first time an energy-time dispersion of precipitating electron flux in a pulsating aurora patch, ranging from 6.7 keV to 580 keV, through simultaneous in-situ observations of sub-relativistic electrons of microburst precipitations and lower-energy electrons using the LAMP 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 500-1000 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.

The 13th Symposium on Polar Science/Ordinary sessions [OS] Space and upper atmospheric sciences, Wed. 16 Nov.

The 13th Symposium on Polar Science/Ordinary sessions [OS] Space and upper atmospheric sciences, Wed. 16 Nov.

The 13th Symposium on Polar Science/Ordinary sessions [OS] Space and upper atmospheric sciences, Wed. 16 Nov.

1Swedish Institute of Space Physics, Solar System Physics and Space Technology, Kiruna, Sweden (shimoyama@irf.se) 2Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kanagawa, Japan 3University of Bern, Bern, Switzerland Hide Summary Jovian Neutrals Analyzer (JNA) is one of six sensors in the Particle Environment Package (PEP) onboard the JUICE mission to Jovian system. The JNA provides the low-energy (10 eV – 3.3 keV) energetic neutral atom (ENA) images originating from the Jovian magnetospheric plasma interaction with the surface/magnetosphere of the Galilean icy moons, and Io torus images through ENA emissions generated from charge-exchange between the corotating plasma and the neutral torus for the first time. While the design is inherited from successful predecessors, the JNA is highly-optimized for a harsh radiation environment in Jupiter. In this paper, we discuss the calibrated performance of JNA flight and flight spare models as well as possible science cases that JNA will address. ENA imaging technique A technique for space plasma imaging using Energetic Neutral Atoms (ENAs) is a powerful tool to remotely study global plasma phenomena [Wieser, 2010]. ENAs are primarily generated by ion neutralization through charge-exchange processes or ion interaction with various space objects, such as planets, moons, comets or asteroid, via backscattering, sputtering or recoiling processes. Since ENAs as neutrals are not affected to electromagnetic field, we can remotely observe ion properties where they are born by detecting ENAs. Icy moon observation Jovian Neutrals Analyzer (JNA) is designed to detect low-energy ENAs with energy from 10 eV to 3.3 keV produced in the Jovian system. ENA images of the moons directly display plasma precipitation patterns on the moon’s surface and make it possible to visualize the magnetic field structure of Ganymede’s magnetosphere and to investigate how Callisto affects magnetic environment in Jupiter’s magnetosphere. The plasma precipitation maps also give clues to understanding surface weathering processes of Ganymede and Callisto. JNA also aims to detect LENAs from charge-exchange of the co-rotating hot plasma and neutral tori of Io and Europa in the inner magnetosphere. For Io torus imaging, the Io torus is not visible in high-energy ENAs due to low energetic ion fluxes in these regions (Futaana, 2015). Instrument The JNA detects ENAs by converting neutrals to ions on a charge conversion surface. Ionized neutrals, namely ions, are subsequently guided through the electrostatic energy analyzer and subjected to time-of-flight (TOF) analysis (Figure 1). Combination of energy- and TOF-analysis provides information on mass discrimination. Whereas the design is based on successful predecessors, i.e. Chandrayaan/CENA and BepiColombo/ENA instruments, a TOF system is heavily optimized to mitigate extremely harsh radiation environment in the Jovian system. JNA uses 11 Ceramic Channel Electron Multipliers (CCEMs) for start signal detection and 11 CCEMs for stop signal detection for the TOF measurement, which allow us to substantially suppress background noises originating in radiation by a single coincidence scheme. Figure 1: JNA measurement principle Calibration JNA flight and flight spare models were calibrated at the calibration facility of the Swedish Institute of Space Physics. The JNA was mounted on the 4-axis turntable in the vacuum chamber with the particle beam source. Species used for calibrations were H, H2, N and Ar. The beam energies were varied from 100 eV to 15 keV. Based on the whole set of calibration data, instrument performance such as angular response, energy response, mass response and sensitivity is verified. The summary of the performance is listed in the table 1. Performance of the charged particle deflector was also verified with high energy ion beams. For JNA flight spare model, advanced calibrations were carried out at University of Bern with focus on the response to relevant species (O and S) and low energy beams (< 300 eV). Figure 2: JNA flight model Figure 3: Calibration setup Table 1: JNA performance Parameters Values Energy range 10 eV - 3.3 keV Energy resolution 80 - 100 % Mass range 1 - 40 amu Mass resolved 1, heavy Angular resolution 15-25° (azimuth), 7° (elevation) Acknowledgements The project is supported by Swedish National Space Agency. References Futaana, Y., et al. (2015), Low-energy energetic neutral atom imaging of Io plasma and neutral tori, Planet. Space Sci., https://doi.org/10.1016/j.pss.2014.12.022. Wieser, M., et al. (2010), First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms, Geophys. Res. Lett., doi:10.1029/2009GL041721. Kazama, Y., et al. (2006), Energetic neutral atom imaging mass spectroscopy of the Moon and Mercury environments, Adv. Space Res., https://doi.org/10.1016/j.asr.2005.05.047.

Mercury does not possess an atmosphere. The magnetospheric plasma is thus interacting with the surface directly. The solar wind may directly access the surface when its dynamic pressure is high. The existence of the surface governs the plasma dynamics at Mercury. For example, the surface behaves as the sink of the space plasma (magnetospheric ions, electrons, and solar wind plasma). In addition, the surface is also the source of plasma (sputtered ions, reflected ions, photoelectrons) and neutral atoms (sputtered constituents, reflected atoms at various energies). The ongoing physics is a complex, interdisciplinary solar wind–magnetosphere–surface–exosphere coupling. The energetic neutral atom (ENA) imaging, a powerful diagnostic tool to image the plasma dynamics in a remote-sensing way, is highly suitable for characterizing the coupling. The surface is the source of ENAs via the sputtering and scattering (reflecting) processes. Therefore, the ENA measurements provide information on the plasma characteristics at the surface (at 0 km altitude) on a global scale, which is very difficult to obtain via in-situ plasma measurements. The information is highly unique by itself to characterize the plasma–surface interaction at Mercury. In addition, the ENA data includes information about physics operated on the precipitating ions between the spacecraft and the surface. This will allow complementary investigation of the plasma dynamics by combining with the in-situ plasma measurements. The other relevant source of ENAs is charge-exchange of energetic ions with the Mercury’s exosphere. The exosphere is tenuous, but still sufficiently dense to make charge-exchange collisions happen and then generate ENAs. Due to the small size of the Mercury’s magnetosphere, the dayside magnetosphere is easily filled with ions injected from the tail, which makes magnetosphere more variable than Earth’s. ENA images on the day side expect to visualize the shape of the magnetosphere. In this presentation, we will give a short review of sciences that ENA imaging at Mercury can address. The major science cases include: Precipitating ions at the surface. Imaging of open/close field regions: How much are the ions precipitating on the surface? What is the difference in flux, energy, and constituents between the open and close field lines? What are the temporal and spatial variations of ion precipitation? Physics below the spacecraft altitude. Measuring the potential drops between the spacecraft and the surface: What are the characteristic energies of ions at the surface? How much does the field-aligned potential exist? What are the temporal and spatial variations of the potential drop? Shape and variability of the magnetosphere: What is the shape of the dayside magnetosphere? How variable is the magnetosphere? During the cruise phase of BepiColombo, the field-of-view of the MPPE/ENA instrument is partially blocked by the MOSIF, the sunshield structure that will be discarded upon MPO and Mio separation, and a few side pixels are looking towards space. The instrument has been operated during the Earth, Venus and Mercury flybys as well as interplanetary cruising phase. We will present the instrument status as well as preliminary flight data taken during flybys.

Whistler chorus waves cause energetic electron precipitations into the Earth’s atmosphere, and signatures of precipitations are observed as diffuse,pulsating aurora, and microbursts of energetic electrons. In this talk, we present our model that propagating chorus waves cause wide energy electron precipitations and relativistic electron microbursts are a high-energy tail of the pulsating aurora electrons. The chorus waves can resonate with tens keV electrons near the magnetic equator, which causes the pulsating aurora emissions, while the chorus waves can resonate with sub-relativistic/relativistic electrons at the off-equator, which cause the microbursts of energetic electrons. Moreover, we discuss that relativistic electrons cause a significant ozone destruction at the middle atmosphere by conjugate observations of Arase, ground-based observations, SIC ion chemistry simulation.

Geomagnetic storms are the main component of space weather and are driven by coronal mass ejections (CMEs) or corotating interaction regions (CIRs). During the main phase of geomagnetic storms, the ring current enhances and a global decrease in the H component of the geomagnetic field is observed. The storm time distribution of ring current ions and electrons in the inner magnetosphere depend strongly on their transport in evolutions of electric and magnetic fields along with acceleration and loss. Recently, we showed that the electron pressure contributes to the depression of ground magnetic field during the storm time by comparing Ring current Atmosphere interactions Model with Self Consistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. In this study, we compare the contribution of electron pressure to the ring current during selected CIR and CME geomagnetic storms using ground observations and the self-consistent inner magnetosphere model: RAM-SCB. The previous results show that the ions are the major contributor (~ 90 %) to the total ring current and the electron contributes ~10 % to the ring current pressure in the post-midnight to dawn sector where electrons flux is higher compared to ions flux. As CIR and CME storms have different origins, we will discuss expected differences in the contribution of electron pressure to the ring current.

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

We examined the growth and propagation of fine-structured electromagnetic ion cyclotron (EMIC) waves related to time-varying density irregularities using multipoint measurement data observed by Arase, Van Allen Probe A, and two ground-based induction magnetometers (Gakona and Dawson) during a field line conjunction event on April 18, 2019. We analyzed the wave data obtained by the aforementioned spacecraft and stations, and found that the appearance of fine structures in the observed EMIC waves clearly coincided with the ambient electron density irregularities in the magnetosphere, which can cause periodic wave growth and waveguiding on their propagation. Furthermore, we found that the latitudinal widths of the EMIC wave activity region and the wave propagation duct were ∼185 km and less than 80 km at an auroral altitude of 100 km, respectively. We also found thermal ion heating (urn:x-wiley:00948276:media:grl63318:grl63318-math-0001 eV/q) during the EMIC wave activity.

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.

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 during substorm dipolarization. The magnetic field oscillated alternately between dipole-like and taillike configuration at a period of 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.

International audience; BepiColombo Mio (previously called MMO: Mercury Magnetospheric Orbiter) was successfully launched by Ariane 5 from Kourou, French Guiana on October 20, 2018. The Mercury Plasma/Particle Experiment (MPPE) is a comprehensive instrument package onboard Mio spacecraft used for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors including two Mercury Electron Analyzers (MEA1 and MEA2), Mercury Ion Analyzer (MIA), Mass Spectrum Analyzer (MSA), High Energy Particle instrument for electron (HEP-ele), High Energy Particle instrument for ion (HEP-ion), and Energetic Neutrals Analyzer (ENA). Significant efforts were made pre-flight to calibrate all of the MPPE sensors at the appropriate facilities on the ground. High voltage commissioning of MPPE analyzers was successfully performed between June and August 2019 and in February 2020 following the completion of the low voltage commissioning in November 2018. Although all of the MPPE analyzers are now ready to begin observation, the full service performance has been delayed until Mio’s arrival at Mercury. Most of the fields of view (FOVs) of the MPPE analyzers are blocked by the thermal shield surrounding the Mio spacecraft during the cruising phase. Together with other instruments on Mio including Magnetic Field Investigation (MGF) and Plasma Wave Investigation (PWI) that measure plasma field parameters, MPPE will contribute to the comprehensive understanding of the plasma environment around Mercury when BepiColombo/Mio begins observation after arriving at the planet Mercury in December 2025.

The present study examines the low-energy ion flux variations observed by the Arase satellite in the inner magnetosphere. From the magnetic field and ion flux data obtained by the fluxgate magnetometer and the low-energy particle experiments–ion mass analyzer onboard Arase, we find 55 events of the low-energy O+ flux enhancement accompanied with magnetic field dipolarization in the periods of April 1–October 31, 2017 and July 1, 2018–January 31, 2019. The low-energy O+ flux enhancements (a) start a few minutes after the dipolarization onset, (b) have energy-dispersed signatures with decreasing energy from a few keV down to ∼10 eV, (c) are observed in both storm and non-storm periods, (d) have a field-aligned distribution (α ∼ 0° in the southern hemisphere and α ∼ 180° in the northern hemisphere), (e) are accompanied by the low-energy H+ flux enhancements that have lower energies than O+ by a factor of 3–10, and (f) increase the O+ density and the O+/H+ density ratio by ∼10 times and 4–5 times, respectively. We perform a numerical simulation to trace ion trajectories forward in time from the Arase positions. It is revealed that both H+ and O+ ions drift eastward and reach the dawn-to-morning sector without being lost in the ionosphere, if the pitch angle scattering effect is considered near the equatorial plane. This result suggests that these low-energy field-aligned ions can contribute to formation of the warm plasma cloak.

This study reports a relation between electron flux modulations and chorus emissions by using correlation analysis. On April 18, 2017, Arase observed an enhancement of electron fluxes and intensification of banded chorus emissions at the same time. A result of the analysis shows that both the upper-band and lower-band chorus emissions have good correlations with field-aligned electron fluxes that satisfy their resonance conditions. This indicates simultaneous interactions with both the emission bands and electrons, resulting in electron pitch-angle scattering toward the magnetic field direction. In addition, low-energy electron fluxes in the perpendicular direction also show positive correlations with the chorus intensities, probably because the chorus emissions are modulated by a fluctuation of perpendicular low-energy electron fluxes.

1. Introduction The Jovian Neutrals Analyzer (JNA) was developed as part of the Particle Environment Package (PEP), a suite of particle instruments onboard the JUpiter ICy moon Explorer (JUICE) spacecraft. JUICE will visit the Jovian system in the early 2020s to perform detailed observations of Jupiter’s icy moons and their complex interaction as a whole [1]. Part of this complex interaction can be probed through the remote observations of Energetic Neutrals Atoms (ENA). The Jovian Neutrals Analyzer was designed to detect low-energy ENAs produced in the Jovian system by charge exchange in the Io plasma torus, as well as when co-rotating plasma impacts the icy surfaces of Jupiter’s moons. JNA was designed, developed and calibrated at the Swedish Institute of Space Physics in Kiruna. Here we report on the performance of JNA based on the results from the calibration campaign of the JNA Flight Model. 2. Instrument design JNA’s design takes heritage from ENA instruments such as CENA on Chandrayaaan-1 and ENA on Bepi-Colombo [2, 3] and uses the same measurement principle: (1) ENAs enter the instrument through a set of collimator blades and are converted to ions upon impact on a conversion surface (polished Si surface coated with Al2O3). (2) ions are energy-analyzed using a wave energy analyzer, which also guides the ions into the Time-of-Flight (TOF) cell where (3) the speed of ions is measured. The TOF cell comprises of a start surface on which the converted ion hits: upon impact, a secondary electron is generated and detected by a Channel Electron Multiplier (CEM). The original particle is detected by a second CEM, and the time difference between those two detections is used to derive the speed of the original particle. Figure 1. Longitudinal cut of a 3D model of JNA. Numbers in white indicate the 3 different steps of an ENA measurement. JNA’s design improves on CENA and ENA by providing a better angular resolution, with 11 pixels instead of 7. Those 11 pixels are arranged in a fan-like fashion (see Fig. 2), giving JNA a geometrical field-of-view of 150° in azimuth, and are each comprised of two CEMs. The use of CEMs (instead of MCPs as on CENA and ENA) provides a better mitigation of radiation-induced noise and was motivated by the harsh radiation environment expected at Jupiter. Figure 2. Schematic of JNA 3. Calibration procedure JNA was calibrated at the calibration facility of the Swedish Institute of Space Physics in Kiruna, Sweden. The calibration facility includes a vacuum chamber equipped with an ion beam line a 4-axis turntable. Figure 3. JNA Flight Model in the calibration facility at IRF-Kiruna. During the main part of the calibration campaign, JNA was placed in the centre of the vacuum chamber on the 4-axis turntable and exposed to a low-energy neutral beam produced by charge-exchange between the ion beam and the residual gas present in the chamber. The ion beam species used included H, H2, H2O, N and Ar. Beam energies were varied from 150 eV to 3.3 keV. At the end of the campaign, the voltage applied to JNA’s ion deflector was progressively lowered to verify the efficiency of the deflector. Finally, JNA was exposed to a radiation source to investigate the noise produced by penetrating radiation, expected to be significant at Jupiter. 4. Initial results Energy resolution (ΔE/E) 100% Mass resolution H, Heavy Mass range 1-40 amu Angular resolution 15°-22° Table 1. JNA performance based on preliminary analysis of JNA Flight Model calibration data Table 1 shows JNA performance specifications derived from a preliminary analysis of calibration results. JNA was found capable to resolve H from heavier neutrals with an energy resolution of 100%. Calibration measurements with Ar showed that JNA can measure up to at least 40 amu. Angular scans of the neutral beam showed that JNA’s mean angular resolution in azimuth ranges from 15° for center pixels to 21° depending on the pixel. JNA was delivered to the European Space Agency in 2020 and is currently integrated on the JUICE spacecraft, planned for launch in September 2022. Figure 4. JNA Flight Model before delivery Acknowledgements Angèle Pontoni acknowledges support from the Swedish National Space Agency, grant 189/16. References [1] Grasset et al., 2013, JUpiter ICy moons Explorer (JUICE): An ESA mission to orbit Ganymede and to characterise the Jupiter system [2] Barabash et al., 2009, Investigation of the Solar Wind–Moon Interaction Onboard Chandrayaan-1 Mission with the SARA Experiment [3] Kazama et al., 2007, Development of an LENA instrument for planetary missions by numerical simulations

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.

We present the results of a multi-point and multi-instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP-A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP-B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low-orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch-angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low-Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity.

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., 宇宙空間で電波を生み出す陽子の集団を発見 --JAXAの人工衛星「あらせ」の観測と解析から--. 京都大学プレスリリース. 2021-07-12.