An understanding of how turbulent energy is partitioned between ions and
electrons in weakly collisional plasmas is crucial for modelling many
astrophysical systems. Using theory and simulations of a four-dimensional
reduced model of low-beta gyrokinetics (the `Kinetic Reduced Electron Heating
Model’), we investigate the dependence of collisionless heating processes on
plasma beta and imbalance (normalised cross-helicity). These parameters are
important because they control the helicity barrier, the formation of which
divides the parameter space into two distinct regimes with remarkably different
properties. In the first, at lower beta and/or imbalance, the absence of a
helicity barrier allows the cascade of injected power to proceed to small
(perpendicular) scales, but its slow cascade rate makes it susceptible to
significant electron Landau damping, in some cases leading to a marked
steepening of the magnetic spectra on scales above the ion Larmor radius. In
the second, at higher beta and/or imbalance, the helicity barrier halts the
cascade, confining electron Landau damping to scales above the steep
`transition-range’ spectral break, resulting in dominant ion heating. Wir
formulate quantitative models of these processes that compare well to
simulations in each regime, and combine them with results of previous studies
to construct a simple formula for the electron-ion heating ratio as a function
of beta and imbalance. This model predicts a `winner takes all’ picture of
low-beta plasma heating, where a small change in the fluctuations’ properties
at large scales (the imbalance) can cause a sudden switch between electron and
ion heating.

Dieser Artikel untersucht Zeitreisen und deren Auswirkungen.

PDF herunterladen: