SHORT-TERM AND LONG-TERM VARIATIONS IN IEC OVER WALTAIR

The Faraday rotation measurements made at a low latitude station, Waltair (17.7-degrees-N, 83.3-degrees-E), during the period 1978-90 have been used to study the various features observed in the ionospheric electron content (IEC), which is an important parameter to sky-wave communication. The IEC shows large day-to-day and seasonal variations. The diurnal variation of IEC over Waltair through a solar cycle is characterized by a pre-dawn minimum, forenoon increase, an afternoon maximum and a nighttime decay. During moderate and high solar activity periods, frequent (for about 50% of the nights) and significant nighttime enhancements of IEC are observed. The IEC day-maximum is found to be positively correlated with 10.7-cm solar flux in all the three different seasons, while the strength of the equatorial electrojet is found to be positively correlated only in summer when the magnetic indices (A(p)) did not show any significant relation. The day-to-day variability, described by the standard deviation of the average daily IEC values, are found to vary from 10 to 45%. The seasonal variability in the day-maximum values is relatively small in low sunspot activity periods. The winter anomaly observed in the day-maximum values and their occurrence timings are explained on the basis of the thermospheric neutral wind velocities deduced from the SLIM model. The sunspot activity is found to show very good positive correlation (0.84) with the IEC day-maximum values. The peak electron density (N(m)F2) variations, obtained at Waltair, during 1990 are observed to follow the IEC variations, while the noontime bite-out is more prominent in N(m)F2. The ratio between IEC and N(m)F2 known as the slab thickness of the ionosphere, shows a sharp pre-dawn increase and fall followed by a gradual increase up to noontime.