The Earth's outer exosphere densities during solar maximum and solar minimum, estimated from XMM-Newton soft X-ray observations and OpenGGCM global MHD model

Abstract:

The Earth’s exosphere is the outermost atmosphere which is composed of mostly hydrogen atoms. These exospheric neutrals are used to remotely sense our magnetospheric system. For example, the TWINS ENA instruments have and SMILE soft X-ray imager will visualize the charge exchange processes between exospheric neutrals and charged particles. However, exospheric density is not steady and varies depending on the Sun – solar wind – magnetosphere – upper atmosphere interaction. Understanding the exospheric density and its response to space weather is key to advance our knowledge of the Sun-Earth interaction and to improve our analysis of remote-sensing data. This presentation estimates exospheric neutral densities near the subsolar magnetopause during solar maximum and solar minimum using XMM-Newton X-ray observations and the OpenGGCM global MHD model. Since its launch in 1999, the XMM-Newton astrophysics mission observes near-Earth X-ray signals when its look direction passes through the dayside magnetosheath. The OpenGGCM global MHD model is used to calculate the plasma contribution to the observed soft X-ray signals and thus to inversely extract an exospheric density from the XMM-Newton observations. Our case studies show that the exosphere density at the subsolar location is higher during solar minimum than solar maximum. This suggests that photoionization plays an important role in this outer exospheric region. Weak solar irradiance during solar minimum leads to less ionization of the neutrals, resulting in higher exospheric densities.