Abstract
As the translational energy of particles decreases, so does the wavelength of the waves with which these particles can gyroresonate. Very low energy electrons in the solar wind resonantly interact with waves that contain only a tiny fraction of the turbulent wave energy. Their wavenumbers fall into the "dissipative" range of the solar wind turbulence, above the steepening observed at a fraction of the proton gyrofrequency. The problem is famous for MeV-GeV electrons of small pitch-angle cosine μ and is known as the problem of pitch-angle scattering through μ = 0. It can be solved by calculating the contribution of the nonresonant waves at small μ. At sub-MeV energies, the problem of the pitch-angle scattering of electrons is no longer limited to the small values of μ, but extends to the whole range of pitch angles. For these energies the most frequent assumption thus far has been that the electrons propagate scatter-free. The effect of the nonresonant waves is calculated here at larger μ. It is shown that the particle pitch-angle sines can subdiffuse or supradiffuse in logarithmic space as a function of the elapsed distance along the average magnetic field, and the dependence of this anomalous scattering is related to the spectral shape of the fast-mode wave turbulence. Using typical spectra of compressive turbulence in slow and fast solar wind in the inner heliosphere, as well as in the trailing edge of high-speed streams, it is found that the pitch-angle variation on a fraction of the distance traveled by the electrons is comparable to their initial pitch angle and exceeds, down to at least 0.1 AU, the pitch-angle reduction due to adiabatic focusing in the solar wind magnetic fields. Depending on both the level of nonresonant fast-mode wave turbulence and the injection distribution of the electrons, this can invalidate the scatter-free assumption for the propagation of sub-MeV electrons in the inner heliosphere.
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