Fraunhofer-Institut für Physikalische Messtechnik, Heidenhofstr. 8 D - 79110 Freiburg, Germany
"Solar Variability in Ionizing Radiation (UV, X-rays)"
This topic refers to the interaction of the solar XUV radiation with the Earth's upper atmosphere: The highly variable photon flux from the Sun is absorbed in the terrestrial thermospheric-ionospheric (T/I) region thus controlling most of the processes in the altitude region from about 90 km to 600 km. In this context two aspects are to be discussed.
1. The nature of the relevant solar XUV spectrum from about 16 nm to 110 nm It is generated in the lower chromosphere through the corona at temperature levels from some 10.000 K up to about 100.000.000 K with corresponding spectral emissions from neutral atoms (H I, He I, C I, O I,...) and moderate to highly ionized atoms (He II, C II, C III, O II, OIII, O IV,..., Ne VII, Ne VIII,..., Mg IX,...,Si XI,..., Fe X, Fe XI, FeXII,..., Fe XVI,...). With the exception of the hydrogen Lyman-continuum (91 nm – 70 nm) spectral line emissions dominate the XUV spectrum.
2. The nature of the terrestrial T/I region From lower to higher T/I altitude regions molecular nitrogen plus molecular oxygen and atomic oxygen are the primary constituents. Absorbing the XUV solar photons below 102.7 nm (photoionization limit of molecular oxygen) the daytime ionosphere is generated. The interaction of the photoelectrons with the T/I constituents (neutral particles, lectrons, and ions) is also responsible for its heating. Temperatures are lowest at the thermopause at about 90 km and highest at the exosphere at about 1000 km.
In view of a very active Sun with flares, solar rotations and cycles these aspects lead to a vivid picture of our fascinating terrestrial atmospheric T/I altitude region. It is still under investigation also in the frame of the international TIGER (Thermsopheric-Ionospheric Geospheric Research) Program of ISCS/SCOSTEP.