A modified Kacanov iteration scheme with application to quasilinear diffusion models

The classical Kacanov scheme for the solution of nonlinear variational problems can be interpreted as a fixed point iteration method that updates a given approximation by solving a linear problem in each step. Based on this observation, we introduce a modified Kacanov method, which allows for (adaptive) damping, and, thereby, to derive a new convergence analysis under more general assumptions and for a wider range of applications. For instance, in the specific context of quasilinear diffusion models, our new approach does no longer require a standard monotonicity condition on the nonlinear diffusion coefficient to hold. Moreover, we propose two different adaptive strategies for the practical selection of the damping parameters involved.

Weak Turbulence and Quasilinear Diffusion for Relativistic Wave-Particle Interactions Via a Markov Approach

We derive weak turbulence and quasilinear models for relativistic charged particle dynamics in pitch-angle and energy space, due to interactions with electromagnetic waves propagating (anti-)parallel to a uniform background magnetic field. We use a Markovian approach that starts from the consideration of single particle motion in a prescribed electromagnetic field. This Markovian approach has a number of benefits, including: 1) the evident self-consistent relationship between a more general weak turbulence theory and the standard resonant diffusion quasilinear theory (as is commonly used in e.g. radiation belt and solar wind modeling); 2) the general nature of the Fokker-Planck equation that can be derived without any prior assumptions regarding its form; 3) the clear dependence of the form of the Fokker-Planck equation and the transport coefficients on given specific timescales. The quasilinear diffusion coefficients that we derive are not new in and of themselves, but this concise derivation and discussion of the weak turbulence and quasilinear theories using the Markovian framework is physically very instructive. The results presented herein form fundamental groundwork for future studies that consider phenomena for which some of the assumptions made in this manuscript may be relaxed.

An Equation of the Quasilinear Theory with Wide Resonance Region

An equation of the quasilinear theory is derived. It is based on the same assumptions as the well-known equation in [1]. However, it has another form of the quasilinear operator, which does not contain the longitudinal wavenumber. Due to this, characteristics of the derived equation determine the routes of a quasilinear evolution of the particle distribution function, even when the resonance region determined by the spectrum of longitudinal wavenumbers is wide. It is demonstrated that during the ion acceleration by the ion cyclotron resonant heating, (i) the change of the longitudinal ion energy can be considerable and (ii) the increase of the particle energy may well exceed the increase described by characteristics of the Kennel–Engelmann equation (which are shown, in particular, in [10]), because these characteristics represent the ways of the quasilinear diffusion only when the resonance region is narrow.