Localized magnetic field structures and their boundaries in the near-Sun solar wind from Parker Solar Probe measurements
2020
One of the discoveries made by Parker Solar Probe during first encounters
with the Sun is the ubiquitous presence of relatively small-scale structures
standing out as sudden deflections of the magnetic field. They were called
switchbacks as some of them show up the full reversal of the radial component
of the magnetic field and then return to regular conditions. Analyzing the
magnetic field and plasma perturbations associated with switchbacks we identify
three types of structures with slightly different characteristics: 1. Alfvenic
structures, where the variations of the magnetic field components take place
while the magnitude of the field remains constant; 2. Compressional, the field
magnitude varies together with changes of the components; 3. Structures
manifesting full reversal of the magnetic field (extremal class of Alfvenic
structures). Processing of structures boundaries and plasma bulk velocity
perturbations lead to the conclusion that they represent localized magnetic
field tubes with enhanced parallel plasma velocity and ion beta moving together
with respect to surrounding plasma. The magnetic field deflections before and
after the switchbacks reveal the existence of total axial current. The electric
currents are concentrated on the relatively narrow boundary layers on the
surface of the tubes and determine the magnetic field perturbations inside the
tube. These currents are closed on the structure surface, and typically have
comparable azimuthal and the axial components. The surface of the structure may
also accommodate an electromagnetic wave, that assists to particles in carrying
currents. We suggest that the two types of structures we analyzed here may
represent the local manifestations of the tube deformations corresponding to a
saturated stage of the Firehose instability development.
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