Magnetic Reconnection Within the Boundary Layer of a Magnetic Cloud in the Solar Wind

The twisted local magnetic field at the front or rear regions of the magnetic clouds (MCs) associated with interplanetary coronal mass ejections (ICMEs) is often nearly opposite to the direction of the ambient interplanetary magnetic field. There is also observational evidence for magnetic reconnection (MR) outflows occurring within the boundary layers of MCs. In this study, a MR event located at the western flank of the MC occurring on October 3, 2000 is studied in detail. Both the large-scale geometry of the helical MC and the MR outflow structure are scrutinized in a detailed multipoint study. The ICME sheath is of hybrid propagation-expansion type. Here, the freshly reconnected open field lines are expected to slip slowly over the MC resulting in plasma mixing at the same time. As for MR, the current sheet geometry and the vertical motion of the outflow channel between ACE-Geotail-WIND spacecraft were carefully studied and tested. The main findings on MR include (a) first-time observation of non-Petschek-type slow-shock-like discontinuities in the inflow regions; (b) observation of turbulent Hall magnetic field associated with a Lorentz-force-deflected electron jet; (c) acceleration of protons by reconnection electric field and their back-scatter from the slow-shock-like discontinuity; (d) observation of relativistic electron near the MC inflow boundary/separatrix; these electron populations can presumably appear as a result of nonadiabatic acceleration, gradient B drift, and via acceleration in the electrostatic potential well associated with the Hall current system; and (e) observation of Doppler-shifted ion-acoustic and Langmuir waves in the MC inflow region.

Key Points
Magnetic reconnection at the western flank of a left-handed helical magnetic cloud is studied
The reconnection reorganizes the large-scale magnetic field and generates a plasma mixing layer in the sheath
The reconnection outflow strongly interacts with the ambient plasma at magnetohydrodynamic and kinetic scales

Plain Language Summary
Energetic eruptions on the Sun send fast eruptive plasmas containing a helical magnetic field (a magnetic cloud, MC) into interplanetary space where they interact with the ambient solar wind and magnetic field. In this study, the focus is on identification of large-scale characteristics (motion and geometry) of an MC followed by a detailed study of magnetic reconnection (MR) outflow occurring within the boundary layer of the MC. The new results on reconnection outflow interactions with the ambient plasma and magnetic field indicate that MR in the solar wind might be associated with rather dynamical processes such as flow shears, generation of flow related discontinuities, particle acceleration, and wave activity. These interrelated processes occur at the same time, leading to massive energy conversions and reorganizations of the large-scale magnetic field.