Parker Solar Probe Observations of a Highly Energetic and Asymmetric Reconnecting Heliospheric Current Sheet during Encounter 13
T. D. Phan, Orlando Romeo, J. F. Drake, D. E. Larson, M. Øieroset, S. Eriksson, B. Lavraud, S. D. Bale, A. Rahmati, R. Livi, P. L. Whittlesey, M. Pulupa, Jia Huang, J. S. Halekas, M. L. Stevens, Á. Szabó, N. E. Raouafi
Abstract
Abstract We report Parker Solar Probe observations of reconnection during an unusual heliospheric current sheet (HCS) crossing at 14.8 solar radii that occurred ∼15 hr after the passage of a CME. The HCS was surrounded by unusually low solar wind density. The hybrid Alfvén speed ( V A ∼ 515 km s −1 ) and available magnetic energy per particle ( m i V A 2 ∼ 3 keV) were the highest observed to date surrounding an HCS. Inside the HCS, the peak outflow speed (∼525 km s −1 ) and proton and electron bulk heating, at ∼400 eV and ∼25 eV, respectively, were also the highest observed to date associated with HCS reconnection. Unusual for an HCS, the density on the two sides of the HCS differed by a factor of ∼13. Consequently, the spatial plasma profiles across the HCS resembled those of planetary magnetopauses, with the fastest outflow occurring near the low-density edge of the HCS. A striking feature of this HCS is the asymmetric nature of the superthermal protons escaping the HCS along separatrix field lines. On the low solar wind density side, the low-velocity cutoff of escaping protons was substantially higher than on the high-density side. The cutoffs were close to the bulk outflow speeds measured near the two edges of the asymmetric HCS, suggesting that only particles faster than the outflow speeds can escape the HCS. Electrons were also energized and leaked out of the HCS, forming a layer of field-aligned superthermal electrons. These escaping electrons are distinguishable from solar strahl electrons observed further away from the HCS, which had lower energies.