Plasma turbulence generated during particle acceleration in reconnection
current sheets with magnetic islands
Abstract
We investigate types of turbulence generated during particle
acceleration in 3D Harris-type reconnecting current sheets (RCSs) with
magnetic islands, using the particle-in-cell approach. When a guiding
magnetic field is present in the RCS, protons and electrons become
separated at ejection into the opposite semi-planes, or footpoints of
reconnecting magnetic loops, due to the opposite gyration. The particles
of the same charge (ions or electrons) ejected from the RCS from the
opposite side where they enter called ‘transit’ particles. They are
strongly energized and form unidirectional beams in the pitch-angle
distribution. While the particles that move back to the same side where
they enter the RCS are called ‘bounced’ particles. They gain less energy
and form more diffusive pitch-angle distributions. In the RCS with
magnetic islands, these two groups of particles are ejected from the
X-nullpoint at the end of the islands forming the similar asymmetric
distributions in the opposite separatrices. The energy difference
between ‘transit’ and ‘bounced’ particles forms ‘bump-on-tail’ velocity
distributions that naturally generate plasma turbulence. Lower-hybrid
waves are generated into the magnetic islands, owing to the two-stream
instabilities. The presence of the anisotropic temperature inside the
RCS can introduce whistler waves. High-frequency fluctuations, upper
hybrid waves or electron Bernstein waves, pile up near X-nullpoints,
which are consistent with MMS observations. We present the wavelet
analysis and energy spectra of the turbulent electric and magnetic field
fluctuations for different frequencies. The results can be beneficial
for understanding in-situ observations of energetic particles in the
heliosphere with modern space missions.