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In this paper, we present simultaneous observations of temporal and spatialvariability of total electron content (TEC) and GPS amplitude scintillationson L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF)while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E,Dip latitude 6.3° N), a low latitude station, made simultaneousobservations. In particular, the latitudinal and longitudinal extent of TECand L-band scintillations was studied in the Indian region for differenttypes of ESF structures observed using the MST radar during the low solaractivity period of 2004 and 2005. Simultaneous radar and GPS observationsduring severe ESF events in the pre-midnight hour reveal that significantGPS L band scintillations, depletions in TEC, and the double derivative ofthe TEC index (DROTI), which is a measure of fluctuations in TEC, obtainedat low latitudes coincide with the appearance of radar echoes at Gadanki. Asexpected, when the irregularities reach higher altitudes as seen in theradar map during pre-midnight periods, strong scintillations on an L-bandsignal are observed at higher latitudes. Conversely, when radar echoes areconfined to only lower altitudes, weak scintillations are found and theirlatitudinal extent is small. During magnetically quiet periods, we haverecorded plume type radar echoes during a post-midnight period that isdevoid of L-band scintillations. Using spectral slopes and cross-correlationindex of the VHF scintillation observations, we suggest that theseirregularities could be "dead" or "fossil" bubbles which are just driftingin from west. This scenario is consistent with the observations wheresuppression of pre-reversal enhancement (PRE) in the eastward electric fieldis indicated by ionosonde observations of the height of equatorial F layerand also occurrence of low spectral width in the radar observations relativeto pre-midnight period. However, absence of L-band scintillations duringpost-midnight event, when radar observed plume like structures andscintillations were recorded on VHF band, raises questions about the processof evolution of the irregularities. A possible explanation is that whereassmall scale (~3 m) irregularities are generated through secondarywaves that grow on the walls of km scale size irregularities, in this caseevolution of the Rayleigh-Taylor instability itself did not extend toirregularities of scale sizes of a few hundred meters that producescintillation on a L-band signal

Simultaneous observations of ESF irregularities over Indian region using radar and GPS