Ion Energies Dominating Energy Density in the Inner Magnetosphere: Spatial Distributions and Composition, Observed by Arase/MEP‐i

• Published in: Geophysical research letters 2018-11, Vol.45 (22), p.12,153-12,162
• Main Authors: Keika, K., Kasahara, S., Yokota, S., Hoshino, M., Seki, K., Nosé, M., Amano, T., Miyoshi, Y., Shinohara, I.
• Format: Article
• Language: English
• Subjects: Acceleration; Arase; Composition; Convection; Coronal mass ejection; Data processing; Earth; Earth magnetosphere; Energy; energy density; Flux density; Geomagnetic storms; Geomagnetism; Ions; Magnetic storms; Magnetism; Magnetosphere; Mass; Oxygen; Oxygen ions; Plasma; Protons; ring current ions; Ring currents; Satellites; Solar corona; Solar energy; Solar wind; Spacecraft; Spatial distribution; Storms; substorms; transport and acceleration
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL080047

We investigate the spatial distributions and composition of contributing energies, which we term an energy range that makes the dominant contribution to energy density, in the inner magnetosphere during the main phase of magnetic storms. We analyze data from the medium‐energy particle experiments‐ion mass analyzer (MEP‐i) on board the Arase satellite during six magnetic storms in year 2017 with the SYM‐H minimum smaller than −50 nT. The results show that the inner part (L ≤ 5) is dominated by relatively low‐energy ions adiabatically transported from the plasma sheet by enhanced convection. The contributing energies are higher for O+ than for H+ at higher L shells (L > 5), particularly during the storms driven by coronal mass ejections. The results provide in situ evidence of the contribution from mass‐dependent/selective acceleration processes associated with substorm activity to the buildup of the outer part the ring current. Plain Language Summary The inner magnetosphere, at altitudes up to ~50,000 km, is filled with plasma of upper atmospheric and solar wind origins. The plasma energy stored in this region is dominated by energetic ions with energies of a few to a few hundreds of kilo electron volts. This study presents how energies of ions contributing to total plasma energy change as a function of altitudes and how the contributing energies depend on ion species. The results are based on in situ observations made during geomagnetic storms in year 2017 by a recently launched spacecraft, Arase. The contributing energies decrease with decreasing altitudes and differ between protons and singly charged oxygen ions of atmospheric origin at higher altitudes. We propose that the near‐Earth magnetosphere can be separated into two regions in terms of characteristics of energetic ion populations. It is likely that the plasma energy in the outer part is supplied by mass‐dependent/selective acceleration processes associated with substorms. Key Points Arase/MEP‐i measured continuous energy spectra of H+ and O+ in an energy range that fully covers energies dominating energy density Contributing energies do not depend on mass at low L, while they are higher for O+ than H+ at high L particularly for CME‐driven storms The difference provides in situ evidence of the contribution from mass‐dependent/selective acceleration to the ring current buildup