Magnetic Field Dropouts at near-Sun Switchback Boundaries: A Superposed Epoch Analysis

Abstract:

During Parker Solar probe’s first close encounter with the Sun in early November 2018, a large number of impulsive rotations in the magnetic field were detected within 50 R_s, these also occurring in association with short-lived impulsive solar wind bursts in speed. These impulsive features are now called ‘switchback’ events. We examined a set of these switchbacks where the boundary transition into and out of the switchback was abrupt, with fast B rotations and simultaneous solar wind speed changes occurring on time scales of less than ~ 10 seconds – these thus appearing as step-function like changes in the radial component of B and V. Our objective was to search for any diamagnetic effects that might occur especially if the boundaries are associated with quick changes in density (i.e., a steep spatial density gradient at the switchback boundary). We identified 25 switchback entries where the radial component of B, B_r, quickly transitioned from large negative to positive values and V_r simultaneously abruptly increased (i.e., step-up transitions) and 28 switchback exits where B_r quickly transitioned from large positive to negative values and V_r simultaneously abruptly decreased (i.e., step-down transitions). We then performed a superposed epoch analysis on each of these sets of events. We found these fast-transitioning events typically had a clear and distinct decrease in the magnetic field magnitude by 7-8% detected exactly at the boundary. The presence of the |B| dropout suggests there is a diamagnetic current present at the boundary. We also present detailed analysis of this current layer and surface normal directions consistent with the change in B-field and the current.