Scattering and energization by large-amplitude whistler-mode waves in
the evolution of solar wind electron distributions and Hamiltonian
analysis of resonant interactions
Abstract
Whistler-mode waves have often been proposed as a plausible mechanism
for pitch angle scattering and energization of electron populations in
the solar wind. Theoretical work suggested that whistler waves with wave
vectors parallel to the interplanetary magnetic field must
counter-propagate (sunward) to the electrons for resonant interactions
to occur. However, recent studies reveal the existence of obliquely
propagating, high amplitude, and coherent waves consistent with the
whistler-mode. Initial results from a particle tracing simulation
demonstrated that these waves were able to scatter and energize
electrons. That simulation was limited and did not examine a broad range
of electron distributions. We have adapted the original particle tracing
code for the solar wind with wave parameters observed by the STEREO
satellites and to model core, halo and strahl electrons. Simulations are
run to record the response of a wide initial phase space volume with
uniform waves and wave packets. Using a Hamiltonian analysis, resonant
responses at different harmonics of the cyclotron frequency are included
in the simulation. A numerical integration scheme that combines the
Hamiltonian analysis and the relativistic 3d particle tracing deployed
on a high performance cluster enables accurate mapping and large-scale
statistical studies of phase space responses. Observations of electron
distributions from WIND at 1 AU are used for normalization. This enables
extrapolations for core, halo, and strahl electrons evolution with the
numerical Green’s function method. Results provide evidence for pitch
angle broadening of the strahl and energization of core and halo
electrons. This model can also provide results that are applicable to a
number of different wave-particle interactions in the heliosphere for
comparison to in-situ measurements.