Field‐Aligned Electron Density Distribution of the Inner Magnetosphere Inferred From Coordinated Observations of Arase and Van Allen Probes. Issue 10 (5th October 2021) - Obana, Yuki
Abstract: The Radiation Belt Storm Probes (RBSP) and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP during the three conjunction events during and after the September 7, 2017 storm. The electron number density at the satellite positions was estimated from frequencies of the Upper Hybrid Resonance emissions obtained by the High Frequency Analyzer of the Plasma Wave Experiment onboard the Arase and the Waves instrument onboard the RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between the main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially refilled plasmasphere. The power‐law index m for the inner trough and plume was inferred to be 4–7 and ∼0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was relatively near diffusive equilibrium. In the plasmatrough with the varying density, both the high‐density and low‐density regions had m ∼ 0. The low‐density portion of this region may have a different origin from the inner trough, because of the different m indices. For the partially refilled plasmasphere in the storm recovery phase, the power‐law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes. Plain Language Summary: The plasmasphere is a region filled with cold, dense ionized gas in geospace. The ionized gas mainly consists of protons, helium ions, oxygen ions, and electrons, which come from Earth's ionosphere and fill in magnetic flux tubes. The density distribution of the ionized gas along the flux tube provides important information to understand how the ions and electrons have been supplied from the ionosphere. Many satellites fly in the equatorial plane; hence, they do not provide information on the electron density along the field. The Radiation Belt Storm Probes and the Arase satellites have different inclinations and sometimes they simultaneously fly near the equator and off the equator on the same magnetic field line. Using electron densities observed by these satellites during the September 7, 2017 storm, we successfully estimated the electron density distribution along the field lines inside the partially refilled plasmasphere, outside of the plasmasphere, and in the tail‐like structure called a plume. Key Points: Using in situ measurements of the electron density from Arase and Radiation Belt Storm Probes, we estimated the density distribution along the magnetic field lines The power‐law index of the electron density distribution was 4∼7, ∼0, and −2∼−1 for the trough, plume, and partially refilled plasmasphere This is the first estimation of the power‐law index using the data from different spacecraft projects