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

During Parker Solar Probe’s first close encounter with the Sun in early 2018 November, a large number of impulsive rotations in the magnetic field were detected within 50 R s ; these also occurred 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 timescales of less than ∼10 s; these thus appear 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 dropout suggests there is a diamagnetic current present at the boundary.