The ionosphere is part of Earth’s upper atmosphere stretching between heights of 60 km to more than 1000 km. A certain fraction of the gas particles in that region is ionized by solar extreme ultra violet radiation. Since radio waves are influences and sometimes strongly disturbed 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 atmosphere. Subsequently, they are received by low-Earth orbiting satellites containing now information on electron density at altitudes above approximately 80 km and on atmospheric pressure, temperature, density and water vapor content in stratosphere and troposphere. This measurement method is termed radio occultation technique and it allows receiving a global picture on the mentioned atmospheric and ionospheric parameters.
Here, we focus on the detection and analysis of sporadic E layers from GPS radio occultation measurements aboard the low-Earth orbiting satellites CHAMP, GRACE and FORMOSAT-3/COSMIC. Sporadic E layers are localized patches of enhanced electron density appearing at the lower boundary of the E layer of the ionosphere. They are represented in GPS signals as intense fluctuations.
We will demonstrate that global sporadic E occurrence rates underlie variations on various time scales. The sporadic E formation process depends on several geophysical parameters and it is subject to coupling processes between the neutral atmosphere and ionosphere. For example, the global sporadic E distribution is oriented along the Earth’s magnetic field. It is shown additionally that sporadic E altitudes are subject to solar tidal winds and it’s annual cycle varies with meteor influx.