Description: We had common technical requirements for the magnetic field measurement in the ARASE(ERG) mission to study the the Earth's radiation belt and the BepiColombo mission to study the Mercury environment. We had to build very precise magnetometers which are small, light and low power-consumption as well as being tolerant of the severe radiation in the Earth's radiation belt and around Mecury. We first investigated and developed MGF-I, one of two magnetometers for the BepiColombo MIO(MMO) satellite. For MGF-I, the tolerance to the high-temperature environment was also required. Another magnetometer, MGF, with almost the same design as MGF-I, was built and installed on the ARASE satellite. ARASE was launched in 2016 and has been continuously observing the radiation belt for six years since it started regular observations in March 2017. Thereafter, BepiColombo was launched in October 2018. It has performed one Earth flyby, two Venus flybys, and two Mercury flybys until 2022. The observations by MIO are mainly performed at flyby events until the Mercury orbit insertion in 2025. Due to the strict limitations and restrictions of the data, it is not easy to evaluate precisely the in-flight performance of MGF-I on BepiColombo MIO. We are investigating the performance of MGF-I based on our knowledge and analysis of data from ARASE MGF. We report the results and discuss the perspective of the observation by BepiColombo MIO MGF-I after the insertion to the orbit around Mercury.

Description: Using the plasma instruments (LEPi, LEPe, MEPi and MEPe) onboard the Arase satellite, we can comprehensively investigate the fluxes of electrons and ions with different energies. Also, the in-situ electron density can be estimated by using the upper hybrid resonance frequencies detected from the plasma wave spectrum. The geomagnetic storm that commenced on 8 September 2017 has a Dst minimum of -147nT and appears to be a medium-sized magnetic storm at first glance, but it is known that it was a peculiar magnetic storm in various ways. For example, it is an extreme erosion of the plasmasphere to around L=1.5, and a strong SAPS generation. Our previous research suggested that this extreme erosion of the plasmasphere was the result of a convective electric field that continued to penetrate into the magnetic equatorial region for more than 6 hours, however, the reason for the continuation of the convection field was unclear. We therefore compare the evolution of ring current particles during this peculiar magnetic storm to the magnetic storm which commenced on 25 August 2018, which has a similar Dst minimum (-175nT) and erode plasmasphere to around L=2.5.

Description: ULF waves in Pc4 and Pc5 bands are often correspond to Alfvén waves that are eigenoscillations of terrestrial magnetic field lines. These waves can be generated from both external and internal sources. The former are thought to be a source of toroidal waves, while the latter ones are related to poloidal wave generation. Compressional waves are observed at the same frequency band as well. In the present study we used 46 months (March 2017–December 2020) of Arase satellite data to analyze spatial distribution of Pc4-5 waves. During this time interval Arase made three observations of all MLTs. A high inclination of Arase orbit was used to show meridional distribution in addition to equatorial one. We discuss the obtained distributions for poloidal, toroidal and compressional waves taking into account case studies. A clear difference between distributions of all 3 polarizations suggests a prominent separation in polarization analysis, but we found just a single cluster on 〈b||〉/〈ba〉 vs. 〈b||〉/〈br〉 diagram. This fact shows that polarization transformation from poloidal to toroidal wave and vice versa, and coupling of Alfvén and compressional waves play an important role in the magnetosphere. We used SYM-H index to characterize global magnetic disturbances and SME index to pay attention to substorm activity. Poloidal and toroidal wave distributions step back from the Earth during quiet geomagnetic conditions. Plasmapause location may be responsible for this behavior. We analyzed solar wind parameters and interplanetary magnetic field vector during wave observations as well.

Description: Compressional Pc 5 waves are important for the energy dynamics in the terrestrial inner magnetosphere because they can accelerate the relativistic electrons (Ukhorskiy et al., 2009). One of the origins of compressional Pc 5 waves is local plasma instability, but the generation mechanism of the compressional Pc 5 waves is still controversial. Recently, drift compressional instability has been focused on because it can generate compressional waves even if plasma beta is less than unity (Rubtsov et al., 2018; Mager et al., 2019; Takahashi et al., 2022). However, the wave property and excitation mechanism of the drift compressional mode are not fully understood due to the lack of observation. This study reports the role of drift resonance in the excitation of drift compressional waves observed by the Arase satellite on 19th November 2018. From the theoretical calculations of the wave frequency (Mager et al., 2013) and pressure perturbation (Takahashi et al., 2022), we concluded that the observed wave is the drift compressional mode. We estimated the azimuthal wave number (m number) from the finite Larmor radius effect and obtained m ~ -140. This is consistent with the drift resonance with protons at 20-30 keV, whose fluxes showed the most coherent oscillations. In addition to a positive radial gradient of ion temperature, we found that an inward phase space density gradient and positive df/dW sometimes appeared, which indicates that the drift resonance gives free energy to the wave, where f and W are phase space density and energy, respectively.

Description: Chorus waves play an important role in the dynamic evolution of energetic electrons in the Earth’s radiation belts and ring current. Due to the orbit limitation of Van Allen Probes, our previous chorus wave model developed using Van Allen Probe data is limited to low latitude. In this study, we extend the chorus wave model to higher latitudes by combining measurements from the Van Allen Probes and Arase satellite. As a first step, we intercalibrate chorus wave measurements by comparing statistical features of chorus wave observations from Van Allen Probes and Arase missions. We first investigate the measurements in the same latitude range during the two years of overlap between the Van Allen Probe data and the Arase data. We find that the statistical intensity of chorus waves from Van Allen Probes is stronger than those from Arase observations. After the intercalibration, we combine the chorus wave measurements from the two satellite missions and develop an analytical chorus wave model which covers all magnetic local time and extends to higher latitudes. This chorus wave model will be further used in radiation belt and ring current simulations.