Storm‐substorm relationship: Variations of the hydrogen and oxygen energetic neutral atom intensities during storm‐time substorms

The present study observationally addresses the role of the magnetospheric substorm in the storm‐time ring current intensification. The intensity of energetic neutral atom (ENA) emission, which is measured by the high‐energy neutral atom (HENA) imager onboard the IMAGE satellite, is carefully used as a guide for inferring the change of the ring current intensity. First, a storm event of August 2000 is examined in detail with a focus on a substorm that occurred at the start of the storm recovery phase (as defined by Sym‐H). During the expansion phase of this substorm, the Sym‐H index recovered (increased) as the geosynchronous magnetic field dipolarized. At the same time the low‐energy (27–60 keV) hydrogen, high‐energy (60–119 keV) hydrogen, and total oxygen (<160 keV for this event) ENA intensities increased, suggesting that the ring current intensified. The apparent recovery of Sym‐H can therefore be attributed to the reduction of the tail current rather than the decay of the ring current. The substorm‐related change of the ENA intensity is examined statistically by conducting a superposed epoch analysis, for which the onset of geosynchronous dipolarization is adopted as a reference time. The result reveals that the ENA intensity tends to decrease before substorm onsets and to increase after onsets, and so does the Sym‐H index although its pre‐onset decrease is less clear than the post‐onset increase. It is therefore suggested that in the course of substorms, the change of the ring current intensity is opposite to what is expected from the change of the Sym‐H index. The decay and intensification of the ring current can be attributed to substorm‐related changes of the near‐Earth magnetic field and convection. Another important result is that the response of the ENA intensity to substorms strongly depends on species and energy range. The variation of the low‐energy hydrogen ENA intensity is not clearly organized by the substorm onset, and its relative change is less than 10%. The high‐energy hydrogen intensity decreases by about 20% during the growth phase and then recovers to the initial level leaving no significant net increase. In contrast, the increase in the oxygen ENA intensity during the expansion phase overcompensates for the preceding reduction, resulting in a net increase of 20%. The net enhancement of the oxygen ENA intensity suggests the importance of non‐adiabatic acceleration associated with near‐Earth dipolarization. The timescale of dipolarization is comparable to the oxygen gyroperiod, and therefore the oxygen ions may be accelerated preferably by the associated inductive electric field. It is inferred that the substorm‐related energization of the oxygen ions makes an important contribution to the storm‐time ring current intensification.