Predictions of magnetosheath merging between IMF field lines of opposite polarity

Abstract

Magnetohydrodynamic (MHD) simulations using the Integrated Space Weather Prediction Model (ISM) show merging in the magnetosheath between interplanetary magnetic field (IMF) field lines on opposite sides of a directional discontinuity. As the discontinuity passes the bow shock and traverses the magnetosheath, the magnetic field gradients on either side become steeper, and the geometry is distorted by the nonuniform flow speed in the magnetosheath. Associated current densities are intensified. In a location isolated from the magnetopause, we use the appearance of an X magnetic field configuration, associated dissipation electric fields, and increases in plasma velocity in the exhaust direction from the X as evidence for merging in the magnetosheath. We suggest that these signatures might be observable by Polar, as it now traverses the magnetosheath near the nose or by the Cluster spacecraft when the discontinuity is tilted away from the YZ plane. Merging signatures are seen in simulations with both 90° and 180° rotations of the field across the discontinuity. Magnetosheath merging creates a hole in the directional discontinuity, allowing the magnetosphere to penetrate through the structure as the discontinuity passes downstream. Magnetosheath merging has the potential to affect ionospheric convection pattern changes, open–closed boundaries, and magnetotail dynamics. A possible association with hot flow anomalies (HFA) is also indicated.