Equatorial enhancement of Pc5-6 magnetic storm time geomagnetic pulsations

Abstract

Geomagnetic Pc5-6 (1-4 mHz) pulsations during three magnetic storms have been analyzed using ground observations at an array of 5 Indian stations ranging from 0.8° to 25° dip latitudes and two pairs of low-latitudes (~20°)--near equator (~4-5°) stations, located at ~4 hours to the West and East from this meridian. The pulsations are found to occur as individual wave packets that are characterized by similar spectra at all stations with two dominate bands of frequencies: ~1.5-2.0 mHz and ~2.5-4.0 mHz. Very strong daytime equatorial enhancement of the wave intensity has been noted. The higher enhancement is observed near noon while it is reduced towards the morning and evening sectors. The polarization of both frequency bands at the equator is found to be almost linear, mainly along the H-direction. It has been found that the amplitude of the D-component decreased with decrease in the latitude towards the equator. One of the spectral peaks is observed exclusively at the dip equator station and it has been attributed to the generation/amplification of MHD waves by the equatorial electrojet instabilities. The mechanism of day time equatorial enhancement of long period geomagnetic pulsations and the propagation characteristics are discussed.