Dayside reconnection, which is particularly effective for a southward interplanetary magnetic field (IMF), allows magnetosheath particle to enter the magnetosphere where they form the plasma mantle. The motions of the reconnected flux tubes produce convective flows in the ionosphere. It is known that the convection patterns in the polar cap are skewed to the dawnside for a positive IMF By (or duskside for a negative IMF By) in the northern polar cap. Correspondingly, one would expect to find asymmetric distributions of mantle particle precipitation, but previous results h ave been unclear. In this paper the correlation between By and the distribution of mantle particle precipitation is studied for steady IMF conditions with southward IMF. Ion and electron data from the DMSP F6 and F7 satellites are used to identify the mantle region and IMP 8 is used as a solar wind monitor to characterize the IMF. We study the local time extension of mantle precipitation in the prenoon and postnoon regions.

We find that, in accordance with theoretical expectations for a positive (negative) IMF By, mantle particle precipitation mainly appears in the prenoon region of the northern (southern) hemisphere. The mantle particle precipitation can extend to as early as 0600 magnetic local time (MLT) in the prenoon region but extends over a smaller local time region in the postnoon sector (we did not find mantle plasma beyond 1600 MLT in our data set although coverage is scant in this area). Magnetometer data from F7 are used to determine whether part of the region 1 current flows on open field lines. We find that at times part of the region 1 sense current extends into the region of mantle particle precipitation, and is therefore on open field lines. In other cases, region 1 currents are absent on open field lines. Most of the observed features can be readily interpreted in terms of the open magnetosphere model. ¿American Geophysical Union 1995