Additional acceleration of solar wind particles in the heliosphere and diagnostics from space observations

This research will explore solar wind parameters and present evidences of additional acceleration of particles in 3D reconnecting current sheet formed in the interplanetary magnetic field. The observational results will be compared with simulations of particle acceleration in 3D reconnecting current sheet using particle-in-cell approach. We also show the variations of electron pitch-angle distribution (PAD) during spacecraft crossing reconnecting current sheets (RCSs) with magnetic islands. The energy gains and the PADs of particles would change depending on the specific topology of magnetic fields. Besides, the observed PADs also depend on the crossing paths of the spacecraft. When the guiding field is weak, the bi-directional electron beams (strahls) are mainly present inside the islands and located closely above/below the X-nullpoints in the inflow regions. The magnetic field relaxation near X-nullpoint converts the PADs towards 90◦. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of RCSs. Mono-directional strahls are quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature drift acceleration.   Our results link the electron PADs to local magnetic structures and directions of spacecraft crossings derived from in-situ observations by WIND, ACE and Parker Probe.