Abstract:
Partial ring currents during the main phase of a geomagnetic storm cause a longitudinal asymmetry in the magnetic field at the earth’s surface at low and mid latitudes. The horizontal component of the magnetic field obtained by ground magnetometers (H), for the magnetic storms on May 14-15, 1997, September 24-25, 1998 and October 18-19, 1998, was studied by means of UT LT diagrams at six magnetic observatories, with approximately uniform longitudinal distribution. Following the same steps used to generate lyemori’s symmetric index (Dst), local time minute by minute profiles of the H-component were obtained for each magnetic storm. An asymmetric disturbance associated with the presence of a partial ring current is identified in the UT-LT maps as contours of depleted magnitude in the H-component, centered around dusk. Increased H-magnitude around midnight is attributed to the substorm expansion current. The UT-LT maps are compared to geomagnetic indices, solar wind parameters and interplanetary magnetic fields.
