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

The Faraday rotation measurements made at a low latitude station, Walt air (17.7-degrees-N, 83.3-degrees-E), during the period 1978-90 have b een used to study the various features observed in the ionospheric ele ctron content (IEC), which is an important parameter to sky-wave commu nication. The IEC shows large day-to-day and seasonal variations. The diurnal variation of IEC over Waltair through a solar cycle is charact erized 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 enh ancements of IEC are observed. The IEC day-maximum is found to be posi tively correlated with 10.7-cm solar flux in all the three different s easons, 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, de scribed by the standard deviation of the average daily IEC values, are found to vary from 10 to 45%. The seasonal variability in the day-max imum values is relatively small in low sunspot activity periods. The w inter anomaly observed in the day-maximum values and their occurrence timings are explained on the basis of the thermospheric neutral wind v elocities deduced from the SLIM model. The sunspot activity is found t o show very good positive correlation (0.84) with the IEC day-maximum values. The peak electron density (N(m)F2) variations, obtained at Wal tair, 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 shar p pre-dawn increase and fall followed by a gradual increase up to noon time.