We analyzed time-of-flight (TOF) data from the Arase satellite to investigate temporal variations of O2+, NO+, and N2+ at 19.2 keV/q in the inner magnetosphere for 6.5 years from the solar declining to rising phases. Molecular ion counts were estimated by subtracting the background contamination of oxygen counts. While the number of clear molecular events was small, the estimated molecular ion counts exhibited good correlation with the solar wind dynamic pressure and SYM-H index. Long-term variations of molecular ions were different from that of oxygen ions. Additionally, we discuss the importance of the solar wind dynamic pressure in causing the escape of molecular ions into the magnetosphere through an increase in the convection electric field, which causes different evolutions of oxygen ions and molecular ions.

We performed a statistical study of electromagnetic ion cyclotron (EMIC) wave distributions and their coupling with energetic protons in the inner magnetosphere using the Arase satellite data from May 2017 to December 2020. We investigated the energetic proton pitch-angle distributions and partial thermal pressures associated with EMIC waves using inter-calibrated proton data in the energy range of 30 eV/q-187 keV/q. With a cold plasma approximation, we computed the proton minimum resonance energies using the observed EMIC wave frequency and plasma density values. We found that the EMIC waves had left-handed polarization near the magnetic equator close to the threshold of proton cyclotron instability, and propagated to higher latitudes along the field line with polarization reversal. H-EMIC waves showed two peak occurrence regions in the morning and noon sectors at L=7.5-9 outside the plasmasphere. The flux enhancements associated with morning side H-EMIC waves appeared at E<1 keV/q among all pitch angles, while H-EMIC waves in the noon sector exhibited flux enhancement in field-aligned directions at E=1-100 keV/q. He-EMIC waves showed a broad occurrence region from 12 to 20 magnetic local time at L=5.5-8.5 inside the plasmasphere with strong flux enhancements at all pitch-angle ranges at E=1-100 keV/q. The proton minimum resonance energy using the obtained central frequency was consistent with the observed flux enhancements at different peak occurrence regions. We conclude that the free energy sources of EMIC waves in different geomagnetic environments drive the two different types of EMIC waves, and they interact with energetic protons at different energy ranges.

Microbursts are impulsive (<1s) injections of electrons (few keV to >MeV) from the outer radiation belt into the atmosphere, primarily caused by nonlinear scattering by chorus waves. Although attempts have been made to quantify their contribution to outer belt electron loss, the uncertainty in the overall size and duration of the microburst region is typically large, so that their contribution to outer belt loss is uncertain. We combine datasets that measure chorus waves (Van Allen Probes (RBSP), Arase, ground-based VLF stations) and microburst (>30 keV) precipitation (FIREBIRD II and AC6 CubeSats, POES/MetOp) to determine the size of the microburst-producing chorus source region beginning on 5 December 2017. We show that the lower/upper limits on the long-lasting (~30 hours) microburst precipitation region is 4 to 8 MLT and 2 to 8.5 L. We conclude that microbursts likely represent a major loss source of outer radiation belt electrons for this event.

A specialized ground-based system has been developed for simultaneous observations of pulsating aurora (PsA) and related magnetospheric phenomena with the Arase satellite. The instrument suite is composed of 1) six 100-Hz sampling high-speed all-sky imagers (ASIs), 2) two 10-Hz sampling monochromatic ASIs observing 427.8 and 844.6 nm auroral emissions, 3) Watec Monochromatic Imagers, 4) a 20-Hz sampling magnetometer and 5) a 5-wavelength photometer. The 100-Hz ASIs were deployed in four stations in Scandinavia and two stations in Alaska, which have been used for capturing the main pulsations and quasi 3 Hz internal modulations of PsA at the same time. The 10-Hz sampling monochromatic ASIs have been operative in Tromsø, Norway with the 20-Hz magnetometer and the 5-wavelength photometer. Combination of these multiple instruments with the European Incoherent SCATter (EISCAT) radar enables us to reveal the energetics/electrodynamics behind PsA and further to detect the low-altitude ionization due to energetic electron precipitation during PsA. In particular, we intend to derive the characteristic energy of precipitating electrons during PsA by comparing the 427.8 and 844.6 nm emissions from the two monochromatic ASIs. Since the launch of the Arase satellite, the data from these instruments have been examined in comparison with the wave and particle data from the satellite in the magnetosphere. In the future, the system will be utilized not only for studies of PsA but also for other categories of aurora in close collaboration with the planned EISCAT_3D project.

An energy spectrum of electrons from 180 keV to 550 keV precipitating into the dayside polar ionosphere is observed for the first time by the HEP instrument onboard the RockSat-XN sounding rocket under geomagnetically quiet condition (AE ≤100 nT) at Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of -4.86 and the electron flux does not vary much over the observation period (~274.4 seconds). A few minutes before the RockSat-XN observation, POES18 / MEPED observed precipitating electrons, which suggest chorus wave activities at the location close to the rocket trajectory. A ground-based VLF receiver observation at Lovozero, Russia also supports the presence of chorus waves during the rocket observation. A test-particle simulation for wave-particle interactions based on the Arase satellite data shows a similar energy spectrum of precipitating electrons, consistent with the RockSat-XN observation. These results suggest that the precipitation observed by RockSat-XN is likely to be caused by the wave-particle interactions between chorus waves and sub-relativistic electrons.

Electrostatic electron cyclotron harmonic (ECH) waves are generally excited in the magnetic equator region, in the midnight and the morning sectors during geomagnetically active conditions, and cause the pitch angle scattering by cyclotron resonance. The scattered electrons precipitate into the Earth’s atmosphere and cause auroral emission. However, there is no observational evidence that ECH waves actually scatter electrons into the loss cone in the magnetosphere. In this study, from simultaneous wave and particle observation data obtained by the Arase satellite equipped with a high-pitch angular resolution electron analyzer, we present evidence that the ECH wave intensity near the magnetic equator is correlated with an electron flux inside the loss cone with energy of about 5 keV. The simulation suggests that this electron flux contributes to auroral emission at 557.7 nm with intensity of about 200 R.

The temporal variation of the energetic electron flux distribution caused by whistler mode chorus waves through the cyclotron resonant interaction provides crucial information on how electrons are accelerated in the Earth’s inner magnetosphere. This study employing a data-driven test-particle simulation demonstrates that the rapid deformation of energetic electron distribution observed by the Arase satellite is not simply explained by a quasi-linear diffusion mechanism, but is essentially caused by nonlinear scattering: the phase trapping and the phase dislocation. In response to upper-band whistler chorus bursts, multiple nonlinear interactions finally achieve an efficient flux enhancement of electrons on a time scale of the chorus burst. A quasi-linear diffusion model tends to underestimate the flux enhancement of energetic electrons as compared with a model based on the realistic dynamic frequency spectrum of whistler waves. It is concluded that the nonlinear phase trapping plays an important role in the rapid flux enhancement of energetic electrons observed by Arase.

Accurate measurements of trapped energetic electron fluxes are of major importance for the studies of the complex nature of radiation belts and the characterization of space radiation environment. The harmonization of measurements between different instruments increase the accuracy of scientific studies and the reliability of data-driven models that treat the specification of space radiation environment. An inter-calibration analysis of the energetic electron flux measurements of the Magnetic Electron Ion Spectrometer (MagEIS) and the Relativistic Electron-Proton Telescope (REPT) instruments on-board the Radiation Belt Storm Probes (RBSP) versus the measurements of the Extremely High Energy Electron Experiment (XEP) unit on-board Arase (ERG) mission is presented. The performed analysis demonstrates a remarkable agreement between the majority of MagEIS and XEP measurements and suggests the re-scaling of MagEIS HIGH unit and of REPT measurements for the treatment of flux spectra discontinuities. The proposed adjustments were validated successfully using measurements from ESA Environmental Monitoring Unit (EMU) on-board GSAT0207 and the Standard Radiation Monitor (SREM) on-board INTEGRAL. The derived results lead to the harmonization of science-class experiments on-board RBSP (2012-2019) and Arase (2017- ) and propose the use of the datasets as reference in a series of space weather and space radiation environment developments.

Inner magnetospheric electrons are precipitated into the ionosphere via pitch-angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic (ECH) waves at different magnetic latitudes. The PA scattering efficiency of low-energy electrons (0.1–10 keV) is yet to be investigated via in situ observations because of difficulties in flux measurements inside loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energies using the Arase (ERG) satellite observation data and successively detected the loss cone filling, i.e., strong diffusion. For LBC waves, strong diffusion occurred at energies above ~1 keV, whereas it occurred below ~1 keV for UBC and ECH waves. The occurrence rate of the strong diffusion by high-amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ~70%, 20%, and 40% higher than that without simultaneous wave activity. The energy range where the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory.

The auroral acceleration region plays an important role in the magnetosphere-ionosphere coupling system. In this study, signatures of an auroral U-shaped potential structure were found for the first time in the near-equatorial inner magnetosphere by the Arase satellite at ~6.0 RE geocentric distance and 11˚ magnetic latitude. The observed magnetic and electric field variations corresponded to the equatorward motion of the upward field-aligned current and converging perpendicular electric field. Examining the three-dimensional velocity distribution function of H+ and O+ ions, we demonstrate that upflowing ion beams were significantly deflected in an east-west direction with a perpendicular velocity up to ~80 km/s, which is consistent with the ExB drift velocity. A simple particle drift model with the inferred auroral perpendicular potential presents a new kink-like drift path of ions from the magnetotail, implying that the auroral potential structure has a great impact on particle dynamics in the near-earth plasma sheet.

We performed a comprehensive statistical study of electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes and Exploration of energization and Radiation in Geospace satellite (ERG/Arase). From 2017 to 2018, we identified and categorized EMIC wave events with respect to wavebands (H+ and He+ EMIC waves) and relative locations from the plasmasphere (inside and outside the plasmasphere). We found that H-band EMIC waves in the morning sector at L>8 are predominantly observed with a mixture of linear and right-handed polarity and higher wave normal angles during quiet geomagnetic conditions. Both H+ and He+ EMIC waves observed in the noon sector at L~4-6 have left-handed polarity and lower wave normal angles at |MLAT|< 20˚ during the recovery phase of a storm with moderate solar wind pressure. In the afternoon sector (12-18 MLT), He-band EMIC waves are dominantly observed with strongly enhanced wave power at L~6-8 during the storm main phase, while in the dusk sector (17-21 MLT) they have lower wave normal angles with linear polarity at L>8 during geomagnetic quiet conditions. Based on distinct characteristics at different EMIC wave occurrence regions, we suggest that EMIC waves in the magnetosphere can be generated by different free energy sources. Possible sources include the freshly injected particles from the plasma sheet, adiabatic heating by dayside magnetospheric compressions, suprathermal proton heating by magnetosonic waves, and off-equatorial sources.

ERG/Arase satellite observations show a simultaneous enhancement of whistler-mode chorus emissions and electron flux associated with toroidal mode ultra-low frequency (ULF) waves. The satellite observed the intensification of both chorus emissions and electron flux in the energy range satisfying the cyclotron resonance condition during the westward oscillation phase of the toroidal mode ULF waves. A model for the observed periodic variations is proposed. We consider the modulation of the drift speed of energetic electrons to be the drift due to the radial component of the wave electric field and the ambient magnetic field . Assuming a wave phase variation of the toroidal mode ULF oscillation in the azimuthal direction, we expect the accumulation of energetic electrons at locations corresponding to a specific phase angle, consistent with the observed phase relationship.