Disturbance time variation of geomagnetic vertical field in the Indian equatorial electrojet

The disturbance time variations of the vertical component (Z) of the geomag netic field at stations in the Indian chain, following a sudden commencemen t storm show a spectacularly large decrease at only the electrojet stations , Thiruvananthapuram, Etaiyapuram, Kodaikanal and Annamalainagar. This decr ease is not synchronous in time with the decrease of the horizontal (H) fie ld. The peak depression of the Z field occurs about three hours before the time of peak decrease of H, i.e. during the middle of the main phase of the storm, when the ring current is developing at the fastest rate. This first clear description of Dst(Z) at Indian electrojet stations is further confi rmation of the sub-surface conducting channel in Palk Strait, where current s are induced by the ionospheric source current, producing large decrease o f Z field at ground level. Similar analysis of Dst(Z) at different equatori al stations around the world is recommended to understand the abnormal feat ures of Indian equatorial electrojet.