In situ measurements of winds, electric fields and electron densities at the onset of equatorial spread-F

Raghavarao, R.; Gupta, S.P.; Sekar, R.; Narayanan, R.; Desai, J. N.; Sridharan, R.; Babu, V. V.; Sudhakar, Vasudhaven

doi:10.1016/0021-9169(87)90042-0

Cited by 64 publications

(21 citation statements)

References 19 publications

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“…Eastward tilts seen in our data suggest that tilts might be associated with the variation of plasma drifts with altitude, thus supporting the mechanism proposed by Anderson and Mendillo (1983). This is further strengthened by the evidence of the existence of positive shears in the zonal winds with altitude which have been obtained using barium vapor release method during the onset time of ESF (Raghavarao et al, 1987;Sridharan et al, 1997). Ionization hole campaign (Sridharan et al, 1997) conducted from SHAR on 19 February, 1993, revealed that zonal winds exhibit a positive shear of 2.2 ms −1 km −1 .…”

Section: Tiltssupporting

confidence: 89%

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

Sinha

Raizada

2014

Earth Planet Sp

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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.

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Section: Tiltssupporting

confidence: 89%

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

Sinha

Raizada

2014

Earth Planet Sp

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“…The magnetic dip angle at Gadanki is finite (13.2 • ) and therefore the wind shear effects are expected to be significant (Sridharan et al, 1989). On the other hand, vertical winds if present, could have a considerable impact over the dip equator (Raghavarao et al, 1987). In their analysis, Raghavarao et al (1987) ascribed the vertical winds to be of gravity wave origin; they could also be part of a larger scale circulation cell that could have been set up during eclipse due to the sudden changes brought out in the overall circulation pattern.…”

Section: Discussionmentioning

confidence: 99%

“…On the other hand, vertical winds if present, could have a considerable impact over the dip equator (Raghavarao et al, 1987). In their analysis, Raghavarao et al (1987) ascribed the vertical winds to be of gravity wave origin; they could also be part of a larger scale circulation cell that could have been set up during eclipse due to the sudden changes brought out in the overall circulation pattern. This is a conjecture and could not be confirmed for want of neutral atmospheric data.…”

Section: Discussionmentioning

confidence: 99%

Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India

et al. 2002

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Abstract. The effects on the electrodynamics of the equatorial E-and F-regions of the ionosphere, due to the occurrence of the solar eclipse during sunset hours on 11 August 1999, were investigated in a unique observational campaign involving ground based ionosondes, VHF and HF radars from the equatorial location of Trivandrum (8.5 • N; 77 • E; dip lat. 0.5 • N), India. The study revealed the nature of changes brought about by the eclipse in the evening time E-and Fregions in terms of (i) the sudden intensification of a weak blanketing E S -layer and the associated large enhancement of the VHF backscattered returns, (ii) significant increase in h F immediately following the eclipse and (iii) distinctly different spatial and temporal structures in the spread-F irregularity drift velocities as observed by the HF radar. The significantly large enhancement of the backscattered returns from the E-region coincident with the onset of the eclipse is attributed to the generation of steep electron density gradients associated with the blanketing E S , possibly triggered by the eclipse phenomena. The increase in F-region base height immediately after the eclipse is explained as due to the reduction in the conductivity of the conjugate E-region in the path of totality connected to the F-region over the equator along the magnetic field lines, and this, with the peculiar local and regional conditions, seems to have reduced the Eregion loading of the F-region dynamo, resulting in a larger post sunset F-region height (h F ) rise. These aspects of Eand F-region behaviour on the eclipse day are discussed in relation to those observed on the control day.

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“…Many observations from various longitudinal sectors (Tsunoda, 1980;Patra et al, 1995) have been reported ever since the first backscatter radar observation was reported by Woodman and La Hoz (1976). In order to understand different aspects of ESF, a number of co-ordinated measurements have been carried out by different workers (Szuszezewicz et al, 1980;Kelley et al, 1986;Raghavarao et al, 1987;Sridharan et al, 1997). Plasma irregularities associated with equatorial Spread F manifest themselves in a variety of forms on VHF radar maps ranging from rising plumes and multiple plumes to ESF structures confined to the bottomside of the ionosphere.…”

Section: Introductionmentioning

confidence: 99%

Characterization of VHF radar observations associated with equatorial Spread F by narrow-band optical measurements

et al. 2004

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Abstract. The VHF radars have been extensively used to investigate the structures and dynamics of equatorial Spread F (ESF) irregularities. However, unambiguous identification of the nature of the structures in terms of plasma depletion or enhancement requires another technique, as the return echo measured by VHF radar is proportional to the square of the electron density fluctuations. In order to address this issue, co-ordinated radar backscatter and thermospheric airglow intensity measurements were carried out during March 2003 from the MST radar site at Gadanki. Temporal variations of 630.0-nm and 777.4-nm emission intensities reveal smallscale ("micro") and large-scale ("macro") variations during the period of observation. The micro variations are absent on non-ESF nights while the macro variations are present on both ESF and non-ESF nights. In addition to the well-known anti-correlation between the base height of the F-region and the nocturnal variation of thermospheric airglow intensities, the variation of the base height of the F-layer, on occasion, is found to manifest as a bottomside wave-like structure, as seen by VHF radar on an ESF night. The micro variations in the airglow intensities are associated with large-scale irregular plasma structures and found to be in correspondence with the "plume" structures obtained by VHF radar. In addition to the commonly observed depletions with upward movement, the observation unequivocally reveals the presence of plasma enhancements which move downwards. The observation of enhancement in 777.4-nm airglow intensity, which is characterized as plasma enhancement, provides an experimental verification of the earlier prediction based on numerical modeling studies.

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In situ measurements of winds, electric fields and electron densities at the onset of equatorial spread-F

Cited by 64 publications

References 19 publications

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

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

Effects of solar eclipse on the electrodynamical processes of the equatorial ionosphere: a case study during 11 August 1999 dusk time total solar eclipse over India

Characterization of VHF radar observations associated with equatorial Spread F by narrow-band optical measurements

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