Global sounding of sporadic E layers by the GPS/MET radio occultation experiment

The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earth's limb. These structures cause strong fluctuations of the GPS L1 and L2 phase paths which have been measured by a GPS receiver onboard of Microlab-1 satellite in 730 km orbit during the GPS/Meteorology experiment (GPS/MET of UCAR, Boulder). By means of GPS/MET radio occultation data, profiles of electron density fluctuations are derived for the mesosphere/lower thermosphere region with a height resolution of around 1 km. Data analysis of 1900 radio occultation events in June/July 1995, 1540 events in October 1995, and 2690 events in February 1997 confirms seasonal dependence of sporadic E layers. The meridian slices of average sporadic E activity show a dominance of plasma irregularities in the summer hemisphere. The irregularities mainly occur at heights 90–110 km. Auroral and equatorial sporadic E, electron density depletions, and multiple ionization layers are also present in the high resolution GPS/MET data. The multiple layers often have a distance of around 5–10 km in height, and appear up to a height of 140 km (upper height limit for 50 Hz sampling rate of GPS receiver). For February and June, the GPS/MET observations are compared to ground-based observations of the Asia/Australia ionosonde chain.