Complex dayside particle precipitation observed during the passage of solar wind discontinuities

Most studies of the geoeffectiveness of the solar wind tend to focus on coronal mass ejections (CMEs) and Corotating Interactive Regions (CIRs). By comparison, fewer studies focus on the geoeffectiveness of the solar wind directional discontinuities (DD), either tangential discontinuity (TD) or rotational discontinuity (RD).  We present two examples of solar wind DD interaction with the magnetosphere that lead to complex dayside particle precipitation structures even though the geomagnetic activities remain quiet in those two events.  In the first example, the DD leads to the formations of unusual boundary layer, overlapping mantle, and double cusp as observed by the DMSP spacecraft. The double cusp signature is consistent with simultaneous magnetic reconnection occurring at both low and high latitudes due to the dominant IMF By as confirmed in a global MHD simulation for the event. The existence of a high-latitude reconnection in this even is also supported by Cluster C2, which observes velocity fluctuations and reversals with peak-to-peak amplitudes >800 km s^–1 as C2 crosses the magnetopause.  Guided by the MHD simulation, the Cluster observation can be interpreted as the spacecraft crossing reconnection outflows while moving from one side of the X-line to the other.  In the second example, the DD leads to the unusual particle precipitation structure where three distinct ion populations can be found on the same magnetic field line.  These three populations have energies of a few hundreds eVs, a few keVs, and a few tens of keVs, suggesting that ions originating from the magnetosphere, solar wind, and ionosphere, respectively, can coexist on the same field line unthermalized.  Moreover, this unusual particle precipitation region has a spatio-temporal scale of about 1 min or 500 km in the ionosphere.