Observations of ionospheric plasma depletions were made over Kavalur (12.56°N, 78.8°E, Mag. Lat 4.6°N). India during March-April 1998 using an all sky optical imaging system operating at 630 nm, 777.4 nm and 557.7 nm. Out of 14 nights of observations, plasma depletions were seen only on 9 nights. Except for 21 March 1998, which was a magnetically disturbed period, all other nights belonged to a magnetically quiet period. Some of the important results obtained from these observations are: (a) After the onset of the equatorial spread F (ESF), plasma depletions take typically about 2 hrs 40 min to come to a fully developed state, (b) There are three distinct types of plasma depletions: type 1 have an east-west (e-w) extent of 250-350 km with an inter-depletion distance (IDD) of 125-300km; Type 2 have an e-w extent of 100-150 km and IDD of 50-150 km; Type 3 have smallest the e-w extent (40-100km) and IDD of 20-60 km, (c) Most of the observed plasma depletions (> 82%) had their eastward velocity in the range of 25-125 ms-1. Almost stationary plasma depletions (0-25 ms-1 ) were observed on one night, which was magnetically disturbed. These very slow moving depletions appear to be the result of a modification of the F-region dynamo field due to direct penetration of the electric field and/or changes in the neutral winds induced by the magnetic disturbance, (d) On the night of 21/22 March 1998, which was a magnetically disturbed period, plasma depletions could be seen simultaneously in all three observing wavelengths, i.e. in 630nm, 777.4 nm and 557.7 nm. It is believed that this simultaneous occurrence was due to neutral density modifications as a result of enhanced magnetic activity, (e) Well developed brightness patterns were observed for the first time in 777.4 nm images. Earlier, such brightness patterns were observed only in 630 nm and 557.7 nm images. These brightness patterns initially appear as very small regions in the northern part of the image and then in about 90 min time, they attain their peak brightness and encompass the entire field-of-view in about 2 hrs 30 min. In some cases, brightness patterns contain one or two well developed plasma depletions within them, (f) The brightness patterns reported here differ from the earlier observations in that they do not show any differential behaviour in the direction of movement before and after the midnight, and that they are present for extended periods of time as large as 6 hrs.
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