Magnetic storm effects at equatorial electrojet stations

It is shown that there are distinctly two different mechanisms for the interaction of solar wind with the earth’s magnetosphere to produce electric field in the ionospheric heights at low and middle latitudes. A persistent or slowly varying southward Interplanetary magnetic field i.e. negative IMF-Bz generates dawn to dusk electric field at polar latitudes and a westward (dusk to dawn) electric field at dayside electrojet region which is anti-Sq in direction. A rapid increase of IMF-Bz (northward turning) imposes a westward (dusk to dawn) electric field at all latitudes of the dayside hemisphere. The process involved is the imposition of -V × Bz electric field at the magnetosphere transmitted instantaneously to the magnetic equator through the polar latitudes. The changes in the H field at ground are largest at stations close to the magnetic equator and the midday longitudes. During magnetic storms, associated with the southward IMF-Bz, there are definite enhancements in the decrease of H field at the dayside equatorial electrojet stations. These changes may be coincident with the development of auroral or magnetospheric ring currents. These enhancements may be observed during the main phase as well as during the recovery phase of the magnetic storm, but the essential condition is that the IMF-Bz has to be steadily significant southward. Sometimes during the storm period abnormally large changes in H field at day side equatorial electrojet regions are observed associated with large sudden changes in IMF-Bz following the second mechanism of solar wind magnetospheric interaction. Thus the equatorial magnetic storm effects are due to combined effects of disturbance ring current, slow and steady IMF-Bz as well as due to sudden large changes in IMF-Bz.