Changes in the Magnetic Field Topology and the Dayside/Nightside Reconnection Rates in Response to a Solar Wind Dynamic Pressure Front: A Case Study.

One of the most significant observations associated with a sharp enhancement in solar wind dynamic pressure, PSW, is the poleward expansion of the auroral oval and the closing of the polar cap. The polar cap shrinking over a wide range of magnetic local times (MLTs), in connection with an observed increase in ionospheric convection and the transpolar potential, led to the conclusion that the nightside reconnection rate is significantly enhanced after a pressure front impact. However, this enhanced tail reconnection has never been directly measured. We demonstrate the effect of a solar wind dynamic pressure front on the polar cap closure, and for the first time, measure the enhanced reconnection rate in the magnetotail, for a case occurring during southward background Interplanetary Magnetic Field (IMF) conditions. We use Polar Ultra‐Violet Imager (UVI) measurements to detect the location of the open‐closed field line boundary, and combine them with Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials to calculate the ionospheric electric field along the polar cap boundary, and thus evaluate the variation of the dayside/nightside reconnection rates. We find a strong response of the polar cap boundary at all available MLTs, exhibiting a significant reduction of the open flux content. We also observe an immediate response of the dayside reconnection rate, plus a phased response, delayed by ∼15–20 min, of the nightside reconnection rate. Finally, we provide comparison of the observations with the results of the Open Geospace General Circulation Model (OpenGGCM), elucidating significant agreements and disagreements. Plain Language Summary: This study provides valuable information on how the Earth's magnetosphere (the magnetized protective bubble around the Earth) is eroded by powerful explosions at the Sun. The response of the polar cap size and the reconnection rates in the magnetosphere to a solar wind high density front are investigated for an event with Interplanetary Magnetic Field orientation anti‐parallel to the Earth's magnetic field. We make a data‐based assessment, using Polar spacecraft ultraviolet images and assimilative model‐generated potentials, plus a model‐based comparison using a global magnetospheric model. An immediate response is observed at the dayside ionosphere for both the polar cap boundary and the reconnection rate mapped to the ionosphere. We also observe about 15–20 min delayed effect on various sectors of the nightside ionosphere. The comparison with the model reveals considerable discrepancies on the dayside ionosphere and significant agreements at the nightside ionosphere. Key Points: Effect of a solar wind dynamic pressure front on the open‐closed polar cap boundaryDetermination of the magnetic reconnection rate for a southward interplanetary magnetic field caseResponse of the dayside/nightside reconnection rate to a solar wind dynamic pressure front [ABSTRACT FROM AUTHOR]