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Indian Journal of Radio & Space Physics (IJRSP) >
IJRSP Vol.41 [2012] >
IJRSP Vol.41(2) [April 2012] >
| 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.dj; 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 |
| CC License: |  CC Attribution-Noncommercial-No Derivative Works 2.5 India |
| ISSN: | 0975-105X (Online); 0367-8393 (Print) |
| Source: | IJRSP Vol.41(2) [April 2012]
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