Abstract { The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around hundred kilometers and with vertical scales of a few hundred meters or more at the Earth's limb. These structures cause strong uctuations of the GPS L1 and L2 phase paths which have been measured by a GPS receiver on- board 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, pro- les of electron density uctuations 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 conrms 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 sporadicE, 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. (Keywords: sporadic E distribution, radio occultation technique, ionospheric irregulari- ties, GPS atmosphere sounding, ionosonde chain, MLT region)

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Hocke, K.; Igarashi, K.; Nakamura, M.; Wilkinson, P.; Wu, J.; Pavelyev, A. G.; Wickert, J. (2001): Global sounding of sporadic E layers by the GPS/MET radio occultation experiment. Journal of Atmospheric and Solar-Terrestrial Physics, 63, 18, 1973-1980.