Exospheric Neutral Hydrogen Density at the Nominal 10 R E Subsolar Point Deduced From XMM-Newton X-Ray Observations

Abstract

We investigate the exospheric neutral density near the subsolar magnetopause, assuming Xgse = 10 R E as its typical location, to support the upcoming Solar wind-Magnetosphere-Ionosphere Link Explorer that will visualize the Earth's magnetosheath and cusps in soft X-rays. Neutral hydrogen density is a key parameter that controls soft X-ray emission and can be inversely extracted from the soft X-ray observations. We introduce a unique method to estimate the dayside neutral density by using X-ray Multi-Mirror Mission (XMM)-Newton astrophysics X-ray observations and Open Geospace Global Circulation Model (OpenGGCM) global magnetosphere-ionosphere magnetohydrodynamics (MHD) model. On 4 May 2003 and 16 October 2001, the XMM-Newton line of sight traversed the dayside of the Earth's magnetosheath and observed strong near-Earth soft X-ray emission. We simulate these two events using the OpenGGCM model. Although the model tends to produce a spatially thicker magnetosheath than measured, modeled magnetosheath plasma fluxes match well with the in situ observations of Cluster and Geotail. We calculate the neutral densities every thousand seconds by comparing the modeled count rates with the XMM-Newton rates. The densities are averaged at 39.9 ± 8.0 and 57.6 ± 8.0 cm −3 for the 4 May 2003 and 16 October 2001 events, respectively. Since our MHD tends to underestimate neutral densities due to its thicker magnetosheath, the true neutral density is likely to lie within the upper half of these error bars.