IMF B_Y and the seasonal dependences of the electric field in the inner magnetosphere

Abstract. It is known that the electric field pattern at high latitudes depends on the polarity of the Y component of the interplanetary magnetic field (IMF B_Y) and season. In this study, we investigate the seasonal and B_Y dependences in the inner magnetosphere using the perigee (4<L<10) Cluster data taken from low magnetic latitudes. The data consist of both components of the electric field perpendicular to the magnetic field, obtained by the electron drift instrument (EDI), which is based on a newly developed technique, well suited for measurement of the electric fields in the inner magnetosphere. These data are sorted by the polarities of IMF B_Z and B_Y, and by seasons or hemispheres. It is demonstrated from our statistics that the electric fields in the inner magnetosphere depend on these quantities. The following three points are inferred: 1) The electric fields exhibit some differences statistically between Cluster locations at the Northern and Southern Hemispheres with the same dipole L and magnetic local time (MLT) values and during the same IMF conditions. These differences in the electric fields might result from hemispherical differences in magnetic field geometry and/or those in field-aligned potential difference. 2) The IMF B_Y and seasonal dependence of the dawnside and duskside electric fields at 4<L<10 is consistent with that seen in the polar convection cell. In addition, it is possible that these dependences are affected by the ionospheric conductivity and the field-aligned current. 3) The nightside electric field in the inner magnetosphere measured by Cluster is often similar to that in the magnetotail lobe. In the future, it will be necessary to incorporate these dependencies on IMF B_Y and season into a realistic model of the inner magnetospheric convection electric field.

Keywords. Magnetospheric physics (Electric fields; Magnetosphere-ionosphere interactions; Solar windmagnetosphere interactions)