Equatorial plasma depletions observed by the DMSP F13 satellite near dawn during geomagnetic disturbances in a solar minimum.

We report on in situ measurements of equatorial plasma depletions in the dawn sector from the DMSP F13 satellite during magnetic disturbances which occurred within the solar minimum period of 1995–1996. Examples are given when DMSP F13 observed prominent pre-sunrise depletions associated with weak magnetic disturbances, while the most severe magnetic storms of the period showed only small amplitude depletions of plasma density. Our results indicate that the depletions were observed within about two third of the magnetically perturbed days with the negative maximum excursions of SYM-H less than −40 nT at any season and at various longitudes. Commonly, the depletions appear following the negative SYM-H peaks in the recovery phase of the magnetic disturbances. We discuss the roles of corotating evening depletions and near-dawn effects of the prompt penetration overshielding and the disturbance dynamo eastward electric fields as sources for the reported depletions. [ABSTRACT FROM AUTHOR]