Morphological study of Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle-23 (Invited)

Details Morphological study of Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle-23 (Invited) Morphological study of Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle-23 (Invited)

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsa41b..07s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SA41B-07

Mathematics

Logic

[2415] Ionosphere / Equatorial Ionosphere, [2437] Ionosphere / Ionospheric Dynamics, [2439] Ionosphere / Ionospheric Irregularities, [2471] Ionosphere / Plasma Waves And Instabilities

Scientific paper

In this paper, we present the results of a morphological study of Equatorial Spread F (ESF) irregularities over Indian region in the descending phase of solar cycle-23 using multi-instrument observations like (a) amplitude scintillations on GPS L-band signal obtained using a network of GPS receivers, (b) amplitude scintillation observations on a VHF signal using spaced receivers at Tirunelveli (8.7°N, 77.8°E, geom. lat: 0.17°S) and (c) MST radar located at Tirupati/Gadanki (13.5°N, 79.2°E, geom. lat: 4.49°N). Occurrence of amplitude scintillations on the GPS L1 (1.575 GHz) signal recorded at several stations over India has been investigated using the strength of L-band scintillations as measured by the S4 index, which is the standard deviation of normalized intensity fluctuations. The latitudinal, longitudinal and temporal evolution of L-band scintillations suggests that scintillations occur predominantly during 20:00-23:00 IST (Indian Standard Time) over equatorial ionization anomaly (EIA) region. Further, the observations suggests that while L-band scintillations are more dominant for both equinoxes as compared to the solstice months as expected, they also reveal that there is an asymmetry in the occurrence of scintillations for these two equinoxes. We find that occurrence of L band scintillations is greater in the vernal equinox than in the autumn equinox. Further, the occurrence of scintillations is found to extend to higher latitudes in the vernal equinox than for the autumn equinox. The observations also indicate that occurrence of L-band scintillations is greater for 2004 than for 2005 with average annual mean sunspot number varying from 40 in 2004 to 30 in 2005. The new observation is that occurrence of L-band scintillations extends to hours beyond 23:00 IST only for a narrow band of 70-90 days in the vernal equinox. In order to understand this aspect, we have analyzed the occurrence characteristics of maximum cross-correlation (CI) of intensity fluctuations obtained from the VHF spaced receivers observations, which is an indictor of small scale electric field perturbation arising from RT instability. The observations suggest that the occurrence of CI less than 0.5 during 19:00-22:00 IST is also more dominant in the vernal equinox than in the autumn equinox period. It may be noted that for CI <0.5, the height of the plasma irregularities is found to be higher as CI decreases below 0.5. TIMED GUVI satellite retrieved peak electron density in the low latitude over Indian region during the same period also indicates that background electron density is higher during vernal equinox than autumn equinox suggesting the role of background electron density in the equinoctial asymmetry in the occurrence of L-band scintillations. Hence, the results suggest that both background electron density and height of the F layer play vital roles in creating the equinoctial asymmetry in the occurrence of scintillations. The scale size distribution of intermediate scales (100 m to few km), which is very important from the point of view of scintillations, is also found to depend on the height of the irregularities, and hence is different for the two equinoxes.

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