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.

We examined soft X-ray emission by the solar wind charge-exchange process around the Earth’s magnetosphere using a global magnetohydrodynamic simulation model. The dayside magnetopause reconnection heats and accelerates the plasma whereby the X-ray emission becomes as bright as $\sim 6 \times 10^{-6} {\rm\ eV}\ {\rm cm}^{-3}\ {\rm s}^{-1}$ under the southward interplanetary magnetic field conditions. In particular, under low plasma-$\beta$ solar wind conditions, we found that the X-ray intensity reflects the bulk motion of outflows from the reconnection region. We propose that this particular solar wind condition would allow visualization of the mesoscale magnetopause reconnection site, as observed in the solar corona.

We report results from a test particle simulation to reveal that electron scattering driven by lower band whistler chorus waves propagating along a magnetic field line plays an important role to produce the butterfly distribution of relativistic electrons. The results show that two nonlinear scattering processes, which are the phase trapping and the dislocation process, contribute to the formation of the butterfly distribution within a minute. We confirm that the quasilinear diffusion estimated from the whistler chorus waves are too slow to reproduce the butterfly distribution within a minute. The simulation results also show that there is the upper limit of rapid electron acceleration. We expect that the upper limit of the rapid flux enhancement is an evidence that the phase trapping process contributes to relativistic electron acceleration in the heart of the outer radiation belt.

We propose a novel and accurate calibration method for short-time waveform signals passed through a linear time-invariant (LTI) system that has a non-negligible group delay. Typically, the calibration process of waveform data is expressed by the Fourier transform and is performed in the frequency domain. If the short-time Fourier transform is applied to the waveform data in the calibration process, multiplying the data by a window function is highly recommended to reduce side-lobe effects. However, the multiplied window function is also modified in the calibration process. We analyzed the modification mathematically and derived a novel method to eliminate the modification of the multiplied window function. In the novel method, calibrated data in the frequency domain are inverse-transformed into waveform data at each frequency, divided by a modified window function at each frequency, and accumulated over the frequencies. The principle of this method derived quantitatively indicates that the calibration accuracy depends on the transfer function of the system, frequency resolution of the Fourier transform, type of the window function, and typical frequency of the waveform data. Compared with conventional calibration methods, the proposed method provides more accurate results in various cases. This method is useful for calibration of general radio wave signals through passed LTI systems as well as for calibration of plasma waves observed in space.

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.

We report on the relationship between pulsating aurora and 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 a pulsating aurora with internal modulations corresponds to the flux enhancement of electrons with energy ranging from 219.7 to 984.95 keV detected with Flight Unit 4, one of FIREBIRD’s CubeSat, with a time delay of 525 ms. Assuming that the pulsating aurora was produced by 10-keV electrons, we suggest that this time difference of 525 ms is consistent with the theory by Miyoshi et al. (2020) that a pulsating aurora and microburst occur due to the chorus waves at different latitudes along the same field line.

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.