Electromagnetic ion cyclotron (EMIC) waves are believed to cause the loss of relativistic electrons from the outer radiation belt into the atmosphere due to pitch angle scattering. However, it is still unclear whether all EMIC waves can scatter relativistic electrons or which conditions are favorable for pitch angle scattering by EMIC waves. In this study, we performed a two-year data analysis of EMIC waves and relativistic electron precipitation (REP) caused by EMIC waves, from 1 November 2016 to 31 October 2018. EMIC waves were observed using a ground-based magnetometer installed at Athabasca (ATH), Canada. REP events were identified from very low-frequency radio waves propagated from the transmitters at the NDK and NLK stations (North Dakota and Seattle, USA, respectively) to the receiver installed at ATH. The magnetic local time dependence of EMIC waves showed higher occurrence rates in the dawn sector. In contrast, EMIC waves accompanied by REP were localized in the dusk sector and were likely to occur during geomagnetic substorms. We found that EMIC waves accompanied by REP were associated with the main phase of geomagnetic storms and occurred inside the plasmapause. These results suggest that the EMIC waves that cause REP occur in the overlap region between the ring current and dense cold plasma during the main phase of geomagnetic storms. This is consistent with previous studies describing that the electron resonant energy with EMIC waves is lower in regions with high plasma density.

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 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.

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.

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.

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.