The fluxgate magnetometer for the Arase (ERG) spacecraft mission was built to investigate particle acceleration processes in the inner magnetosphere. Precise measurements of the field intensity and direction are essential in studying the motion of particles, the properties of waves interacting with the particles, and magnetic field variations induced by electric currents. By observing temporal field variations, we will more deeply understand magnetohydrodynamic and electromagnetic ion-cyclotron waves in the ultra-low-frequency range, which can cause production and loss of relativistic electrons and ring-current particles. The hardware and software designs of the Magnetic Field Experiment (MGF) were optimized to meet the requirements for studying these phenomena. The MGF makes measurements at a sampling rate of 256 vectors/s, and the data are averaged onboard to fit the telemetry budget. The magnetometer switches the dynamic range between ± 8000 and ± 60,000 nT, depending on the local magnetic field intensity. The experiment is calibrated by preflight tests and through analysis of in-orbit data. MGF data are edited into files with a common data file format, archived on a data server, and made available to the science community. Magnetic field observation by the MGF will significantly improve our knowledge of the growth and decay of radiation belts and ring currents, as well as the dynamics of geospace storms.
Simultaneous observations of the magnetic field and plasma waves made by the Arase and Van Allen Probe A satellites at different magnetic local time (MLT) enable us to deduce the longitudinal structure of an oxygen torus for the first time. During 04:00-07:10 UT on 24 April 2017, Arase flew from L = 6.2 to 2.0 in the morning sector and detected an enhancement of the average plasma mass up to~3.5 amu around L = 4.9-5.2 and MLT = 5.0 hr, implying that the plasma consists of approximately 15% O + ions. Probe A moved outbound from L = 2.0 to 6.2 in the afternoon sector during 04:10-07:30 UT and observed no clear enhancements in the average plasma mass. For this event, the O + density enhancement in the inner magnetosphere (i.e., oxygen torus) does not extend over all MLT but is skewed toward the dawn, being described more precisely as a crescent-shaped torus or a pinched torus.Plain Language Summary In the early 1980s, it was discovered that the O + ion density is sometimes enhanced in a limited range of altitude in the deep inner magnetosphere (approximately 10,000-km to 30,000-km altitude). This O + density enhancement was originally named the oxygen torus, which implies azimuthal symmetry of the density enhancement. However, its longitudinal structure remains poorly known. This study investigates the longitudinal structure of the oxygen torus for the first time using simultaneous observations from the Arase and Van Allen Probe A satellites. We find that the oxygen torus does not extend over all longitudes but is localized to the dawn sector, indicating a crescent-shaped torus.
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