FIELD-ALIGNED CURRENTS AT EQUATORIAL LATITUDES DURING GEOMAGNETIC DISTURBANCES

The yearly average daily variation of the vertical component (Z) of geomagnetic field at the equatorial electrojet station, Kodaikanal, has been found to differ very little on international quite (Q) and disturbed (D) days. However, the values of westerly declination (D) and of the horizontal component (H) for any hour of the day or night have been found to be smaller on the disturbed days than on the quiet days. The decrease in (D-degrees-W) on disturbed days has been found to be present on other Indian stations, viz. Trivandrum, Annamalainagar and Alibag. The daily mean values of D and H decrease systematically with increasing geomagnetic character index C(i). The monthly mean (D-Q) value of easterly component (Y) varies linearly with the corresponding value of (D-Q) of H component. Geomagnetic storm-time variations of Y are remarkably similar to those of H field. It is concluded that with the development of the disturbance-associated ring current causing the decrease of the H field at low latitudes, there is simultaneous increase of the eastward component of the geomagnetic field at low latitudes. This phenomenon is attributed to the development of equatorward field-aligned current over Indian latitudes during geomagnetic disturbances.