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

Abstract

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.

Identification of active fossil bubbles based on coordinated VHF radar and airglow measurements

“…The nighttime airglow emissions (OI 630.0 nm and 777.4 nm) from thermosphere altitudes (∼250 km and F region peak altitude) have been monitored using a temperature controlled narrow band (0.3 nm) optical photometer whose field of view (3 • ) was chosen to nearly coincide with radar beam width. The details of this photometer (Sekar et al, 2004) and coordinated observations (Chakrabarty et al, 2005(Chakrabarty et al, , 2006 are available in the literature. In addition to these techniques, the layer height movements of the F-region over the dip equator which correspond to zonal electric fields were obtained from the ionosonde observations located at Thumba.…”

Equatorial Spread F structures and associated airglow intensity variations observed over Gadanki

“…The detailed description of this structure in comparison with the observation on non ESF night along with the corresponding airglow signatures in the vertical direction have been discussed in a previous communication (Sekar et al, 2004). Some of the plume structures were shown to be plasma enhancements (Sekar et al, 2004). However, the airglow observations in the eastward (45 • elevation) direction were not considered earlier and are included in Fig.…”

“…It is noted that, unlike the previous two cases, this day was magnetically quiet (A P =3, F 10.7 =97). The detailed description of this structure in comparison with the observation on non ESF night along with the corresponding airglow signatures in the vertical direction have been discussed in a previous communication (Sekar et al, 2004). Some of the plume structures were shown to be plasma enhancements (Sekar et al, 2004).…”

“…The details of the multi-wavelength scanning photometer for airglow studies during nighttime have been discussed in a previous communication (Sekar et al, 2004). Temperaturetuned, narrow band (band width ∼0.3 nm) interference filters are used in this photometer.…”

“…Thus the comparative studies with various techniques are useful in obtaining physical insight of ESF. In the recent past, comparison between the narrow-band optical measurements of thermospheric airglow emissions and the VHF radar measurements revealed the presence of plasma enhancement (Sekar et al, 2004) extending beyond 350 km and an evidence for fossil bubbles turning active (Sekar et al, 2007) was obtained. In addition, simultaneous triggering of ESF at two zonally separated locations during storm time was identified (Chakrabarty et al, 2006;Sekar and Chakrabarty , 2008) using bi-directional airglow and VHF radar observations.…”