Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006ihy..worke.106c&link_type=abstract
2nd UN/NASA Workshop on International Heliophysical Year and Basic Space Science. Proceedings of the conference held 27 Novembe
Astronomy and Astrophysics
Astrophysics
Scientific paper
Ionospheric total electron content (TEC) has been measured at Calcutta (22.580 N, 88.380 E geographic, dip: 320 N), which is situated virtually below the northern crest of equatorial anomaly over a period 1979-90. This paper presents possibly for the first time a long term study on the control of solar activity over the total electron content of the ionosphere covering both the increasing (1986-90) and decreasing (1979-85) phases of solar activity. The TEC at this location is controlled by production due to solar EUV fluxes, the so-called equatorial fountain effect due to electrodynamics drift and neutral wind. 10.7 cm (2800 MHz) solar flux is considered as a surrogate index of solar EUV flux. Starting from the early morning hours, TEC increases with solar flux. A good correspondence between the two is reflected throughout the increasing and decreasing solar activity periods. Moreover no saturation effect is observed in the TEC variation at high levels of S10.7 flux. The correlation coefficient, R2 maximizes mostly in the 08-10 IST time slot, indicating a greater solar control and sometimes more or less the same value persists for several hours later. R2 values in the declining phase is somewhat lower compared to that in the increasing phase but minima in the R2 appear in both the phases at about 10-14 IST primarily. The minima suggest less solar control over TEC compared to other geophysical parameters. In the equinoctial months minima are observed at the afternoon hours and this suggests that during the equinoctial months other than solar flux, fountain effect caused by electrodynamic drift is more prominent. A pronounced hysteresis effect with the solar activity is observed in various months during the time intervals (6-8 IST), (12-15 IST) and (18-20 IST) and the effect is most sensitive in moderate to high solar activity period. One remarkable feature of TEC variation with 10.7cm solar flux is that for few months TEC of the declining phase (1979-85) revert positions with respect to that of the increasing phase (1986-90). It occurs for more or less same values of solar flux in the time interval (10-14 IST) of moderate to high solar activity periods. The same feature is occasionally repeated around the post midnight period also though it is not under direct solar control. Seasonal variation of TEC, considered as a result of fountain effect modulated by neutral wind, both zonal and meridional components, is also observed to be influenced by solar activity with asymmetrical nature in increasing and decreasing phases. Long term studies on solar radiation and TEC from Calcutta reveal that ionization around the equatorial anomaly crest is greatly influenced by the sun with asymmetrical variations in the increasing and decreasing phases. The study will be useful in predicting the ionospheric group delay of trans-ionospheric signal as used in GPS and other systems.
Chakraborty S. K.
Hajra R.
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