Low-latitude TEC variability studied from magnetically conjugate locations along 73°E longitude

A comparative study on the variation of total electron content (TEC) was performed by using the observations from two GPS receivers located at magnetically conjugate stations near the equatorial ionization anomaly crests: Udaipur, India in the nortnern hemisphere and Diego Garcia in the southern hemisphere, located at approximately same geographic longitude. Our study reveals that the TECs at both stations shows the semi-annual variability being higher in equinoxes than in solstices. An important finding of the work is that in the northern hemisphere, during the declining phase of solar cycle 23, the amplitude of the winter crest is smaller than that in the summer whereas it is found to be comparable/higher (which is normally termed as “winter anomaly” in the mid latitudes) during the rising phase of solar cycle 24. Thus it is concluded that the so called “winter anomaly” at low latitudes is a phenomenon totally different from the one observed at mid latitudes. In fact its manifestation at low latitudes can be accounted for in terms of variation of the solar flux and/or the strength of the equatorial electrojet during different seasons.