Is the Weibel instability enhanced by the suprathermal populations, or not?

TL;DR

This paper investigates how the shape of anisotropic distribution functions, especially the novel product-bi-Kappa distribution, influences the Weibel instability's growth rate and threshold, revealing increased susceptibility with certain parameters.

Contribution

Introduces the product-bi-Kappa distribution to analyze the Weibel instability, showing it can enhance growth rates and lower thresholds compared to previous models.

Findings

Maximum growth rates are higher with the product-bi-Kappa distribution.

The instability threshold is significantly reduced for small Kappa values.

The shape of the distribution function critically affects the instability's behavior.

Abstract

The kinetic instabilities of the Weibel-type are presently invoked in a large variety of astrophysical scenarios because anisotropic plasma structures are ubiquitous in space. The Weibel instability is driven by a temperature anisotropy which is commonly modeled by a bi-axis distribution function, such as a bi-Maxwellian or a generalized bi-Kappa. Previous studies have been limited to a bi-Kappa distribution and found a suppression of this instability in the presence of suprathermal tails. In the present paper it is shown that the Weibel growth rate is rather more sensitive to the shape of the anisotropic distribution function. In order to illustrate the distinguishing properties of this instability a \emph{product-bi-Kappa distribution} is introduced, with the advantage that this distribution function enables the use of different values of the spectral index in the two directions, $\kappa_{\parallel} \ne \kappa_{\perp}$. The growth rates and the instability threshold are derived and contrasted with those for a simple bi-Kappa and a bi-Maxwellian. Thus, while the maximum growth rates reached at the saturation are found to be higher, the threshold is drastically reduced making the anisotropic product-bi-Kappa (with small kappas) highly susceptible to the Weibel instability. This effect could also rise questions on the temperature or the temperature anisotropy that seems to be not an exclusive source of free energy for this instability, and definition of these notions for such Kappa distributions must probably be reconsidered.