Dual Sounding Rocket Observations of Plasma Convection and Shear, Field-aligned Currents, and Alfven waves and Irregularities in the Cusp during IMF Bz Negative Conditions

On December 7, 2018, two NASA Black Brant X sounding rockets were launched simultaneously from Ny Ålesund, Spitzbergen (79˚ N) into the dayside cusp and subsequently cut across the open/closed field line boundary, reaching apogees of 601 km and 807 km. The launches occurred during Bz negative conditions. We provide an overview of the electrodynamic observations and focus on the DC electric field results, available from the lower altitude rocket, and field-aligned currents, observed on both rockets. The in situ DC electric fields reveal highly dynamic, structured flows within the cusp with steady flow towards the north and afternoon sector poleward of the cusp. Just equatorward of the cusp, strong meridional electric fields associated with westward plasma flow is observed in conjunction with zonal magnetic field perturbations, indicative of strong field-aligned currents. We discuss the large scale electrodynamics, including plasma shears and currents, as signatures of the open/closed character of the cusp/low latitude boundary layer as a function of the IMF. The electric field, magnetic field, and plasma density data also reveal strong ULF/ELF plasma waves and irregularities that exist within the open magnetic field line cusp region yet disappear when the payload was equatorward of the cusp on closed magnetic field lines. We present detailed wave characteristics and discuss them in terms of Alfvén waves and static irregularities that pervade the cusp region yet change character as a function of altitude.