GNSS-Fernerkundung, Navigation und Positionierung

A Global Survey of Sporadic E Layers based on GPS Radio Occultations by CHAMP, GRACE and FORMOSAT–3 / COSMIC

Christina Arras (Erfolgreiche Fertigstellung April 2010)

Fakultät für Physik und Geowissenschaften, Universität Leipzig

The ionosphere is a part of the upper atmosphere stretching from a height of about 60 km to more than 1 000 km. A certain fraction of the gas particles in that region is ionised by solar extreme ultra violet radiation. Since electromagnetic waves are influenced and significantly modified by ionospheric free charge carriers, the altitude range is of great scientific interest.

GPS satellites emit electromagnetic waves on L – band frequencies travelling through the ionosphere and lower neutral atmosphere. Subsequently, they are received by low–Earth orbiting satellites. Consequently, the signals are affected by strong electron density gradients at altitudes above approximately 80 km and by atmospheric density, pressure and water vapour content in the troposphere and stratosphere. This measurement method is termed radio occultation technique and it allows to receive a global picture of ionospheric and lower neutral atmospheric conditions.

This study focusses on the detection and analysis of sporadic E layers from GPS radio occultation measurements on a global scale. Sporadic E layers are localised patches of relatively high electron density appearing in the E region of the ionosphere. They are represented in GPS signals as intense fluctuations.

This work reveals that global sporadic E occurrence rates underlie variations on different time scales. It is demonstrated that the sporadic E occurrence depends on several geophysical parameters and it is subject to coupling processes between the neutral atmosphere and ionosphere. For example, the global sporadic E occurrence is oriented along Earth’s magnetic field. It is shown additionally that sporadic E altitudes are subject to tidal winds and that its annual cycle varies with meteor influx.