Abstract
Abstract. Equatorial Spread F (ESF) is a manifestation of ionospheric interchange instabilities in the nighttime equatorial F region. These instabilities generate plasma density irregularities with scale sizes ranging from centimetres to thousands of kilometres. The irregularities can be detected from a variety of instruments such as digisonde, coherent and incoherent scatter radars, in situ space probes, and airglow photometers. In the present study, occurrence statistics of the ESF, based on various parameters are presented using data obtained from the VHF radars located at three longitudinally separated equatorial stations: Christmas Island (2° N, 202.6° E, 2.9° N dip latitude), São Luís (2.59° S, 315.8° E, 0.5° S dip latitude) and Jicamarca (12° S, 283.1° E, 0.6° N dip latitude). The ESF parameters presented here are the onset altitude, onset time (onset refers to first appearance of signal in the radar field of view) of the bottom-type and plume, and the peak altitude of the plume. Recent studies have used these parameters to classify the spread F occurrence characteristics. The present study reveals novel features namely, the dependence of ESF parameters on the seasonal, solar flux, declination angle and longitudinal dependence from the three radar sites. In addition, we also present an empirical model to determine the nature of these ESF parameters as a function of the solar flux which may enable us to forecast (with 30 min to 1 h tolerance) the plume occurrence at any longitude located in between São Luís and Christmas Island.
Highlights
Upward developing plumes from upwelling of the bottomside F layer associated with equatorial spread F (ESF) events have been studied with VHF/UHF/L-band backscatter radars located near the geomagnetic equator (e.g. Woodman and La Hoz, 1976; Tsunoda et al, 1979; Tsunoda 1980a, b; Hysell and Burcham, 1998; de Paula et al, 2004; Kudeki et al, 2007)
Coherent scatter radar observations of the ionospheric plasma irregularities are usually displayed in range time intensity (RTI) format, in which the backscatter power is plotted against altitude and time
From CXI, we note the following characteristics: (1) the % occurrence of ESF is maximum during summer solstice month of August and secondary maximum is in equinoctial September month, (2) the % occurrence is minimum during winter-solstice month of February and equinoctial March– April, (3) the % occurrence of the plume follows the seasonal trend of % occurrence of ESF, (4) the % occurrence www.ann-geophys.net/31/2137/2013/
Summary
Upward developing plumes from upwelling of the bottomside F layer associated with equatorial spread F (ESF) events have been studied with VHF/UHF/L-band backscatter radars located near the geomagnetic equator (e.g. Woodman and La Hoz, 1976; Tsunoda et al, 1979; Tsunoda 1980a, b; Hysell and Burcham, 1998; de Paula et al, 2004; Kudeki et al, 2007). Related to initiation of ESF, the eastward electric field in the dayside equatorial ionosphere is sharply intensified just before it reverses to the nightside westward direction. This intensification gives origin to the so-called evening prereversal enhancement
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