Solar Wind Influence on Dual-Lobe Reconnection and Horse-Collar Aurora

During periods of low clock angle interplanetary magnetic field (IMF), where θ = atan(B_Y/B_Z) ≈ 0, dual-lobe magnetic reconnection (DLR) closes the open magnetospheric flux at the magnetotail lobes, tailward of the cusps.  This process results in the reversal of the ionospheric twin-cell convection system and a contraction of the open/closed field line boundary, which, if sustained for a prolonged period, can lead to a fully closed magnetosphere. DLR is also the proposed generation mechanism for Horse-Collar Aurora (HCA), an auroral formation consisting of two cusp aligned arcs that close across the polar cap, accompanied by a 'web' of smaller cusp-aligned arcs (CAAs) equatorward of the main two. We study how different IMF parameters influence the rate at which the open polar cap flux closes, using this as a proxy for the DLR rate, and compare this to MHD models.

We measured the HCA arc velocity and polar cap flux depletion rates using observations from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on board the Defense Meteorological Satellite Program (DMSP), which provides auroral spectral and positional data. HCA arc velocity was found to increase with higher IMF B_Z magnitudes, with no correlation found for solar wind flow speed or density. The open flux depletion rate was also found to increase with increasing IMF B_YZ. Coupling functions were also fitted to the arc velocity and open flux depletion rate data, with Pearson r values of 0.58 and 0.52 respectively.

For comparison, 27 magnetohydrodynamic (MHD) models were also run on the NASA Community Coordinated Modeling Center using a range of idealized solar wind conditions. In the models, both IMF B_Z and clock angle have a linear correlation with the open flux depletion rate. Solar wind speed also resulted in an increased flux closure rate, contrary to our observational results. No dependence on solar wind density was found. A coupling function was also fitted to the model’s data, resulting in a V_SW^1.6 solar wind speed dependence, a B_YZ^0.52 IMF dependence, and a cos^3.98(θ/2) clock angle dependence. A number of the MHD simulations also showed extended magnetotails during NBZ, with some extending to over -200R_E down-tail.