Satellite Beacon and Ionosonde Measurements of Ionospheric Irregularities over India During the Equinoctial Month of September 1991

Abstract

The spread-F and scintillation observations made at ten Indian stations covering 8 to 25°N geographic latitudes during the equinoctial month of September 1991 are used to study the occurrence characteristics of ionospheric irregularities and their association with the changes in the virtual height of the F layer at an equatorial station, Trivandrum (0.3° S dip). No event of scintillation occurrence is observed at off-equatorial latitudes (within the anomaly crest region) without their prior occurrence at the equatorial station (Tiruchendur, 2.3°N dip which is close to Trivandrum). However, the duration of scintillations at far equatorial stations (10 to 20°N dip) are, sometimes, longer than the durations at the near equatorial stations (0 to 10°N dip). On such occasions, the onset of scintillations and spread-F is prior to the post-sunset reversal of the height of the F layer (h′F) from upward to downward direction at the equatorial station, Trivandrum. But, if the duration of the irregularities is longer at equatorial latitudes and decreases towards higher latitudes, the onset of irregularities is observed to coincide with the time of post-sunset reversal in h′F at Trivandrum. These two types of irregularities are interpreted to be generated by the two different instability mechanisms, namely E × B drift and RT instability respectively.