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

Abstract

An all sky optical imaging system was operated from Sriharikota rocket range (SHAR) (14• N, 80• E, 5.5• 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• 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 et 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.

Multi-wavelength imaging observations of plasma depletions over Kavalur, India

“…These observations are commensurate with the normal behaviour of the F-region dynamo and are in agreement with values reported earlier (Mendillo et al, 1997a;Fejer, 1991;Sinha and Raizada, 2000). Some of the plasma depletions had very small to practically zero eastward drift velocity.…”

“…Images grabbed by the CCD camera were processed for flat field correction, image intensifier noise, background illuminations and stationary features, as described by Sinha and Raizada (2000). Calculations of plasma depletion parameters were made by using the assumption that 630 nm, 777.4 nm and 557.7 nm emissions come from layers which have their centroids located at 250 km, 350 km and 250 km, respectively.…”

“…Plasma depletions are large-scale structures of plasma density associated with the phenomenon of the equatorial spread F. Ionospheric plasma depletions, first observed by Hanson and Sanatani (1973) using a satellite-borne experiment, have been observed subsequently by a number of in situ (Kelley et al, 1976;McClure et al, 1977;Benson and Briton, 1983;Jahn et al, 1997;Singh et al, 1997a) and groundbased (Weber et al, 1978;Mendillo and Baumgardner, 1982;Rohrbaugh et al, 1989;Sinha et al 1991;Mukherjee et al, 1993;Sahai et al, 1994;Sinha, et al, 1996;Mendillo et al, 1997a, b;Taylor et al, 1997;Tinsley et al, 1997;Bittencourt et al, 1997;Fagundes et al, 1997Fagundes et al, , 1999Sinha and Raizada, 2000) techniques.…”

“…It has been generally accepted that plasma depletions are generated in the bottom side of the F-region through the Generalized Rayleigh-Taylor instability mechanism. Various theoretical and experimental studies have identified gravity waves as the seed for the R-T instability to produce irregularities (Kelley et al, 1981(Kelley et al, , 1986Huang and Kelley 1996;Singh et al, 1997a;Sinha and Raizada, 2000). Sinha and Raizada (2000) Fig.…”

Latitudinal and temporal variation of equatorial ionospheric irregularities determined from GPS scintillation observations

“…Singh et al, 1997), and optical observations of F region night-glow emissions (e.g. Fagundes et al, 1999;Sinha and Raizada, 2000) have increasingly strengthened the hypothesis that gravity waves are a seed mechanism. Recently Nicolls and Kelley (2005) observed wave activity several hours prior to the onset of the conditions suitable for plasma instabilities.…”