The Influence of IMF B_y on the Bow Shock: Observation Result

Abstract

In this study we use the bow shock crossings contained in the Space Physics Data Facility database, collected by four spacecraft (IMP 8, Geotail, Magion-4, and Cluster1) to analyze the effect of the interplanetary magnetic field (IMF) B_y component on the bow shock position and shape. Although the IMF B_z component is usually considered much more geoeffective than B_y, we find that the dayside bow shock is more responsive to the eastward component of the IMF than the north-south one. We believe that the explanation lies in the changes that the B_z component induces on the magnetopause location and shape, which largely compensate the corresponding changes in the dayside bow shock location. In the tail, we find that the bow shock cross section is elongated roughly in the direction perpendicular to the IMF direction, which agrees with earlier modeling studies.