Response of the Earths thermosphere during extreme solar events. A contribution of satellite observations to atmospheric evolution studies

In this interdisciplinary study in the field of astronomy and geodesy the thermospheric response during extreme solar flares that irradiated the Earths upper atmosphere is analysed. It is suggested that such events can serve as proxies for the intense electromagnetic and corpuscular radiation environment of the Sun and other stars during their early phases of evolution. Accelerometer measurements aboard the GRACE spacecraft are used too investigate the atmospheric density and its temporal and spatial variation. The measurements comprise the non-gravitational forces acting on the satellite, which is orbiting at an altitude of about 450 km. At that height level the atmospheric drag force, which depends on the atmospheric density is of particular interest. For this reason, the observations must be reduced by non-gravitational forces originating from the solar radiation pressure and Earth albedo. The density variations in the thermosphere are analysed over 7.5 years with a special focus on extreme solar events like the X17.2 flare during the so-called Halloween period in 2003 or the X2.0 flare in November 2004. Furthermore, a comparison with empirical thermosphere models like the NRLMSISE-00 and the Jacchia-Bowman 2008 is performed. Based on the additional analysis of EUV measurements from the TIMED satellite during these solar flares a connection between the acceleration measurements and theoretical studies related to thermospheric heating and expansion caused by the solar EUV flux is established.