Some new features of ionospheric plasma depletions over the Indian zone using all sky optical imaging

An all sky optical imaging system was operated from Sriharikota rocket range (SHAR) (14 degrees N, 80 degrees E, 5.5 degrees N dip latitude) during January-March, 1993 to observe ionospheric plasma depletions through 630 nm and 777.4 nm night glow emissions. Strong plasma depletions were observed only on four nights viz., 14, 17, 19 and 21 February, 1993. Except the 17 February, which was a magnetically disturbed day, all the other nights pertained to magnetically quiet period. A number of plasma depletion parameters such as, degree of depletion, east-west extent, tilt with respect to the geomagnetic field, inter-depletion distance, drift velocity and plasma enhancements or brightness patterns were estimated. Some of the important results are: (a) It was found that the east-west extent of plasma depletions varied with the degree of depletion; for the 630 nm images the degree of depletion ranged between 6-9% per 100 km east-west extent and for 777.4 nm images it was 3% per 100 km east-west extent, (b) The average inter-depletion distance (IDD) was in the range of 1500 +/- 100 km during the magnetically disturbed period and 740 +/- 60 km during quiet periods. This is suggestive of gravity wave modulation of the bottom side of the F-region. While the large scale gravity waves (1500 +/- 100 km) of auroral origin could be responsible during magnetically disturbed period, smaller scale gravity waves (740 +/- 60 km) having their origin in the lower atmosphere could produce initial perturbation in the bottom side of the F-region, (c) Plasma depletions are observed to have an eastward tilt in the range of 10-15 degrees with respect to the geomagnetic field. It has been suggested here that these tilts are associated with the variation of plasma drift with altitude, (d) plasma depletions are observed to be moving eastwards with drift velocities in the range of 40-190 ms(-1), and (e) Strong plasma enhancements or brightness patterns were observed on three nights. The degree of enhancement was by a factor of 1.4-3.8. These enhancements lasted for more than 15 minutes. Although, prima facie, these observations look similar to the transient brightness wave reported by Mendillo ei al. (1997a), the high degree of enhancement and an extended duration of more than 15 minutes, observed in the present case, need to be understood.