Effects of Geomagnetic Storm Activity on the O+/H+ Transition Height

Abstract:

The O+/H+ transition height (H_T) is an important parameter indicative of the underlying physical processes impacting plasma dynamics along a field line. Using whistler mode (WM) radio sounding experiments from the IMAGE satellite, we report first measurements of H_T variations as a function of geomagnetic storm activity during the 09-24 Sep 2005 period. This period included three successive storms: a moderate storm on 10 Sep (Dst,min=-70 nT), a major storm on 11 Sep (Dst,min=-147 nT), and another moderate storm on 15 Sep (Dst,min=-86 nT). Upper hybrid emissions observed on IMAGE showed that the plasmapause (Lpp) location before the storms was at L~4.5. Lpp decreased to L~3 and L~2, respectively, after the main phases of the first moderate storm and major storm. Then Lpp increased to L~3 after the main phase of the second moderate storm. We used WM sounding measurements of ion densities (H+, He+, O+) in the L=1.7 to L=2.7 range to study the variation in H_T as a function of geomagnetic storm activity. WM sounding results were complemented by in situ ion density measurements from the DMSP satellite (850 km). Our results show: (1) H_T increases with L-shell. (2) During the major and moderate storm activity, H_T increased at all L-shells relative to pre-storm values, from 1000 km to ~1300-1400 km at L~1.7, and from ~1400 km to ~1800 km at L~2.7. (3) The increase in H_T started during a storm’s main phase and reached the peak on the first or second day of the recovery phase. For all three storms, our results based on the measurements of H_T and H+ and O+ densities indicate that different physical mechanisms were responsible for the increase in H_T during a storm’s main and recovery phases. During the main phase, storm time neutral winds led to an increase in the upflow of plasma along field lines leading to an increase in both H+ and O+ densities. The relative increase in O+ was more than H+ causing an increase in H_T. During the recovery phase, neutral H density decreased, leading to a reduced rate of O+-H charge exchange, which in turn resulted in decreased H+ and increased O+ densities, causing an increase in H_T. We also discuss the storm time variations in upper transition height (O⁺/(H⁺+He⁺) and effective ion mass. We explore the implications of our results for the thermosphere-ionosphere-plasmasphere coupling.