Does overall thermal equilibrium exist between ions and electrons in a weakly collisional , magnetised , turbulent plasma—and , if not , how is thermal energy partitioned between ions and electrons ?
This is a fundamental question in plasma physics , the answer to which is also crucial for predicting the properties of far-distant astronomical objects such as accretion disks around black holes .
In the context of disks , this question was posed nearly two decades ago and has since generated a sizeable literature .
Here we provide the answer for the case in which energy is injected into the plasma via Alfvénic turbulence : collisionless turbulent heating typically acts to disequilibrate the ion and electron temperatures .
Numerical simulations using a hybrid fluid-gyrokinetic model indicate that the ion-electron heating-rate ratio is an increasing function of the thermal-to-magnetic energy ratio , \beta _ { \mathrm { i } } : it ranges from \sim 0.05 at \beta _ { \mathrm { i } } = 0.1 to at least 30 for \beta _ { \mathrm { i } } \gtrsim 10 .
This energy partition is approximately insensitive to the ion-to-electron temperature ratio T _ { \mathrm { i } } / T _ { \mathrm { e } } .
Thus , in the absence of other equilibrating mechanisms , a collisionless plasma system heated via Alfvénic turbulence will tend towards a nonequilibrium state in which one of the species is significantly hotter than the other , viz. , hotter ions at high \beta _ { \mathrm { i } } , hotter electrons at low \beta _ { \mathrm { i } } .
Spectra of electromagnetic fields and the ion distribution function in 5D phase space exhibit an interesting new magnetically dominated regime at high \beta _ { i } and a tendency for the ion heating to be mediated by nonlinear phase mixing ( “ entropy cascade ” ) when \beta _ { \mathrm { i } } \lesssim 1 and by linear phase mixing ( Landau damping ) when \beta _ { \mathrm { i } } \gg 1 .