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Title: Equatorial F-region irregularities during low and high solar activity conditions
Authors: Joshi, L M
Patra, A K
Rao, S V B
Keywords: Equatorial F-region irregularities;Field-aligned irregularities (FAI);High altitude plume structures;Equatorial plasma bubbles;E-F region coupling
Issue Date: Apr-2012
Publisher: NISCAIR-CSIR, India
PACS No.:; 94.20.dt; 94.20.Vv; 96.60.Q-
Abstract: In this paper a comparative study on equatorial F-region irregularities observed during the extremely low solar activity period of 2007-2009 and high solar activity period of 2002-2003 based on the Gadanki radar and ionosonde observations has been made. Observations show that the occurrence probability of the F-region field-aligned irregularities (FAI) on night-to-night basis was 65% in the high solar activity and 30-40% in the low solar activity. During the high solar activity condition, F-region FAI extended to higher altitude (600 km) than those in the low solar activity condition (400-500 km). Intriguingly, the altitude extent of the FAI during 2008 was 100 km higher than that of 2009. The lowest altitude at which the F-region FAI were observed during the high solar activity period was 250 km while it was 200 km during the low solar activity period of 2008-2009. During the high solar activity condition, the high altitude plume structures were observed earlier than those observed in the low solar activity period. Also, in the high solar activity period, the FAI were mostly confined to pre-midnight hours while during the low solar activity condition, the FAI extended well beyond post-midnight and to dawn hours. While the duration of the ionosonde observed range-spread echoes was generally consistent with that of the radar observed F-region FAI echoes, on a few occasions, range-spread echoes were observed during the post-midnight hours when radar echoes were completely absent. Concurrent observations of Es layer displayed descending behaviour reminiscent of tidal/gravity wave associated winds. Importantly, the descending property led to disruption of Es during the evening hours when equatorial plasma bubbles are formed. Interestingly, some of the high altitude plumes appear to be linked with such Es disruption. Examination of the growth perspective of the Rayleigh-Taylor instability suggests that even though the height of F-layer in the post sunset hour was lower during the low solar activity period than that of the high solar activity period. The chance of bubble formation is near equal owing to the advantage of lower ion-neutral collision frequency in the low solar activity than in the high solar activity. Detailed observational and theoretical results, including the E-F region coupling, are presented and discussed in the light of the current understanding of the equatorial plasma bubble phenomenon.
Page(s): 208-219
ISSN: 0975-105X (Online); 0367-8393 (Print)
Appears in Collections:IJRSP Vol.41(2) [April 2012]

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