Evolution of low altitude and ring current ENA emissions from moderate magnetospheric storms: Continuous and simultaneous TWINS observations

The TWINS mission measures energetic neutral atoms (ENAs) using sensors mounted on two separate spacecraft. Because the two spacecraft's orbital planes and phases are significantly offset, the pair provides a nearly optimal combination of continuous magnetospheric observations from at least one of the TWINS platforms with several hours of simultaneous, dual-platform viewing over each orbit. Two types of ENA signals are observed during storms, corresponding to two fundamental magnetospheric ion processes. The first signal is the high-altitude ring current emission (RCE) due to a parent population of trapped ions in the inner magnetosphere, undergoing charge exchange with the extended neutral hydrogen exosphere. The second is the low altitude emission (LAE) signal that results from precipitating ions which undergo charge exchange with the oxygen exosphere at a few hundred km altitude. Valek et al. (2010) investigated the temporal evolution of the moderate storm of 22 July 2009 and showed the LAE began earlier and was the brightest emission seen during the main phase, while later, during the recovery, the LAE was only as bright as the bulk ring current emissions. Using this same methodology applied to a larger number of storms, we determine the general relative temporal evolution of the RCE and LAE as seen by TWINS. Valek, P., P. C. Brandt, N. Buzulukova, M.-C. Fok, J. Goldstein, D.J. McComas, J. D. Perez, E. C. Roelof, and R. Skoug (2010), Evolution of low altitude and ring current ENA emissions from a moderate magnetospheric storm: Continuous and simultaneous TWINS observations,J. Geophys. Res., doi:10.1029/2010JA015429, in press.