Short‐term relationship of total electron content with geomagnetic activity in equatorial regions

The short‐term relationship between equatorial ionosphere and geomagnetic activity is examined. Hourly averages of the total electron content (TEC) and critical frequency of the F 2 layer ( f o F 2 ) are compared with the Dst index, a proxy for equatorial geomagnetic activity, at three local times (0700–0800, 1200–1300, and 1600–1700 LT) from March 1998 to August 1999. Owing to the geomagnetic latitude and local times used, positive storms, almost exclusively, are observed (cf. Prölss, 1995). While f o F 2 measurements over an extended period (∼10 years) have been studied (Matsushita, 1959) and TEC and f o F 2 are coupled, TEC measurements can provide a significantly better signal‐to‐noise ratio. At timescales of 2–3, 3–5, 5–9, and 9–11 days, there are significant correlations (∼0.4 at local noon, when all the data are included) between TEC and Dst . These correlations increase from morning to afternoon. By comparison, correlations between f o F 2 and Dst are significantly smaller, ∼0.2 (near the noise level) at local noon. Even during geomagnetically quiet times ( Dst > −20), a clear correlation (0.21, which exceeds the 95% confidence level by 0.05) is seen between TEC and Dst at the shortest timescale examined. As geomagnetic activity increases, the correlations increase rapidly. For example, when moderate levels of geomagnetic activity ( Dst > −50) are included for observations at local noon, distinct correlations (∼0.3) are seen and persist for all but the longest timescale; with higher levels of geomagnetic activity included, there are distinct correlations at all the timescales examined. The presence of a significant correlation at quiet conditions and persistence of the correlation at moderate levels of activity are both unexpected.