Effects in the near-Magnetopause Magnetosheath Elicited by Large-Amplitude Alfvénic Fluctuations Terminating in a Field and Flow Discontinuity

In this work we report on a sequence of large-amplitude Alfvénic fluctuations terminating in a field and flow discontinuity and their effects on electromagnetic fields and plasmas in the near-magnetopause magnetosheath. An arc-polarized structure in the magnetic field was observed by the Time History of Events and Macroscale Interactions during substorms (THEMIS-C) in the solar wind, indicative of nonlinear Alfvén waves. It ends with a combined tangential discontinuity/vortex sheet (TD/VS), which is strongly inclined to the ecliptic plane and at which there is a sharp rise in the density and a drop in temperature. Several effects resulting from this structure were observed by the Magnetospheric Multiscale (MMS) spacecraft in the magnetosheath close to the subsolar point (11:30 MLT) and somewhat south of the geomagnetic equator (-33 deg MLAT): (i) kinetic Alfven waves; (ii) a peaking of the electric and magnetic field strengths where E.J becomes strong and negative (-1 nW/m^3 just prior to an abrupt drop-out of the fields; (iii) evolution in the pitch angle distribution of energetic (a few tens of keV) ions (H+, He^(n+), O^(n+) and electrons inside a high-density region, which we attribute to gyrosounding of the TD/VS structure passing by the spacecraft; (iv) field - aligned acceleration of ions and electrons that could be associated with localized magnetosheath reconnection inside the high-density region; (v) variable and strong flow changes, which we argue to be unrelated to reconnection at partial magnetopause crossings and likely result from deflections of magnetosheath flow by a locally-deformed, oscillating magnetopause.