Superdiffusion of energetic particles at shocks: A Lévy Flight model for acceleration
arXiv:2408.03638 · doi:10.1051/0004-6361/202451765
Abstract
In the Heliosphere, power-law particle distributions are observed e.g. upstream of interplanetary shocks, which can result from superdiffusive transport. This non-Gaussian transport regime may result from intermittent magnetic field structures. Recently, we showed that a Lévy flight model reproduces the observed features at shocks: power-law distributions upstream and enhanced intensities at the shock. We extend the Lévy flight model to study the impact of superdiffusive transport on particle acceleration at shocks. The acceleration time scale and spectral slope are compared to Gaussian diffusion and a Lévy walk model. The fractional transport equation is solved by sampling the number density with the corresponding stochastic differential equation that is driven by an alpha-stable Lévy distribution. For both Gaussian and superdiffusive transport we use a modified version of CRPropa 3.2. We obtain the number density and energy spectra for constant and energy-dependent anomalous diffusion and find, compared to the case of Gaussian diffusion, harder energy spectra at the shock as well as faster acceleration. The spectral slope is even harder than predicted for Lévy walks. Lévy flight models of superdiffusive transport lead to observed features in the Heliosphere. We further show that superdiffusive transport impacts the acceleration process by changing the probability to escape the shock. The flexibility of the Lévy flight model allows for further studies in the future, taking the shock geometry and magnetic field structure into account.
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