Nonlinear Coupling of Kinetic Alfven Waves and Ion
Acoustic Waves in the inner Heliosphere
Abstract
We study the nonlinear coupling of kinetic Alfv\’{e}n
waves with ion acoustic waves applicable to the Earth’s radiation belt
and near-Sun streamer belt solar wind using dynamical equations in the
form of modified Zakharov systems. Numerical simulations show the
formation of magnetic field filamentary structures associated with
density humps and dips which become turbulent at later times,
redistributing the energy to higher wavenumbers. The magnetic power
spectra exhibit an inertial range Kolmogorov-like spectral index value
of \(-5/3\) for
\(k_\perp\rho_i
< 1\), followed by a steeper dissipation range
spectra with indices \(\sim
-3\) for the radiation belt case and
\(\sim-4\) for the near-Sun
streamer belt solar wind case, here
\(k_\perp\) and
\(\rho_i\) represent the
wavevector component perpendicular to the background magnetic field and
the ion gyroradius, respectively. Applying quasilinear theory in terms
of the Fokker-Planck equation in the region of wavenumber turbulent
spectra, we find the particle distribution function flattening in the
superthermal tail population which is the signature of particle
energization and plasma heating.