Irregularities of the topside ionosphere observed by GPS/MET radio occultation

Large-scale fluctuations (vertical scales ∼40–60 km) of total electron content (TEC) are retrieved from phase path fluctuations of GPS radio occultation links of the GPS/MET experiment. The large-scale TEC fluctuations at observation heights 400–600 km are maximal at local times from 20:00 to 24:00 h. The fluctuations mainly occur on the summer hemisphere at low magnetic latitudes 0–30°N and seem to be concentrated over Africa and Arabia. The derived global distribution of TEC fluctuations around solar minimum (June/July 1995) is assumed to be closely related to the occurrence of spread F and radio scintillations. The meridional distribution of the background electron density (averaged for 19:00–23:00 LT) has a maximum in the topside ionosphere at around 10°N, beyond an anomaly structure of the F2 peak layer. The meridional location of the anomaly structure agrees with the location of enhanced TEC fluctuations. Both phenomena are possibly related to the enhancement of upward ion velocity in the postsunset ionosphere at equatorial latitudes. Large TEC depletions of 1–20 TEC-units are present at observation heights 200–400 km in many TEC profiles of the equatorial ionosphere after sunset. The TEC depletions possibly indicate plasma bubbles on their way from the lower ionosphere to the topside ionosphere. GPS navigation signal loss due to spread F or high-latitude irregularities is not found in the GPS/MET data of June/July 1995.