Magnetosphere‐Ionosphere Connection of Storm‐Time Region‐2 Field‐Aligned Current and Ring Current: Arase and AMPERE Observations

Storm‐time region‐2 field‐aligned currents (R2 FACs) are believed to be connected between the ring current region and the ionosphere, but this connection has not been clarified by simultaneous in situ observations. We confirmed the connection of upward R2 FACs during 16 July and 18 June 2017 storm events using coordinated magnetic observations by the Arase satellite in medium‐Earth orbit and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). The upward FACs were determined by drastic changes in the azimuthal magnetic field at Arase in the off‐equatorial (3‐ to 4‐RE radial distance and 1–2 RE above the magnetic equator) postmidnight inner magnetosphere. The magnetic latitude of the FAC observed by Arase projected onto the ionosphere was consistent with that of the ionospheric FAC observed by the AMPERE. Using the conservation of the ratio between the current density and the total magnetic field along the field line, we showed that the current between Arase and AMPERE was almost conserved, meaning that a large portion of the R2 FAC was generated in the low‐latitude inner magnetosphere. We also calculated the plasma pressures of H+ and O+ ions and pressure‐driven currents to examine their relationship for the first event. The O+ pressure contributed significantly to the inner part of the total azimuthal current. The peaks of combined pressure of H+ and O+, and pressure‐driven currents were located inside and outside the FAC, respectively. A simple model calculation indicated that this spatial relationship is controlled by the day‐night asymmetry of magnetic field. Key Points Connection of FACs in the inner magnetosphere and in the ionosphere was quantitatively confirmed by Arase and AMPERE observations We present simultaneous in situ observations of storm‐time region‐2 FACs and the ring current derived from plasma pressures The region‐2 FAC was located at the inner part of the ring current that was significantly contributed to by O+ ions