Evolution of remotely measured inner magnetospheric ion temperatures during a geomagnetic storm

摘要

Previous studies have demonstrated that with the Medium Energy Neutral Atom instrument aboard the IMAGE spacecraft it is possible to remotely measure the ion temperature of the terrestrial magnetosphere during periods of strong geomagnetic activity. However, neutral atom imaging of the magnetosphere during quiet intervals is more difficult. In this work we show that by mapping neutral atom fluxes obtained over many days of observation to an equatorial plane fixed in geocentric solar magnetospheric coordinates, it is possible to construct neutral atom images of the quiet time magnetosphere. Enhanced neutral fluxes in the energy range of 1–70 keV/nucleon are observed in the quiet time premidnight region. A superposed-epoch analysis of multiple storm intervals also permits imaging of the ion temperature structure as a function of time through a geomagnetic storm. Using nearly 40 geomagnetic storms to produce average ion temperature maps as a function of storm phase, we find that there are significant differences between the spatial distribution of neutral fluxes and ion heating during the main phase of storms. Pronounced ion heating (up to 12 keV) is observed on the dayside during storm main phase from 5 to 8 Earth radii. During the early recovery phase, the ion temperature on the dayside drops to approximately 9 keV, and a colder region of approximately 6.5 keV persists near predawn. In the late recovery phase of the storm the ion temperature throughout the inner magnetosphere appears to relax to a nearly uniform 8 keV.