Modeling the whistler-heat flux instability in the solar wind using skew kappa distribution functions

Abstract:

Understanding the electron heat-flux regulation in the solar wind has been an open problem during the last decades. The mechanism that causes this macroscopic parameter to have values below the collisional limit given by the Spitzer-Härm law is still unknown. Considering that the solar wind is a weakly collisional plasma, the most accepted interpretation to explain this phenomenon is a non-collisional regulation of the electron heat-flux by wave-particle interactions. Due to the field-aligned skewness present in solar wind electron velocity distribution functions (eVDFs), one of the main candidates for this non-collisional regulation is the whistler-heat flux instability (WHFI), although, the wave that potentially regulates this parameter is still under debate. Over the years, the solar wind eVDF has been modeled in different ways taking into account these characteristics. All these approaches assume that the electron population is composed of different sub-populations, each described by its own distribution function. The most used distributions correspond to Maxwellian, Bi-Maxwellian or Kappa functions, and their linear combination allows, in particular, to adequately model the skewness seen in the solar wind eVDF. In this research we take a different approach and model the eVDF using a Kappa distribution to which an asymmetry term has been added. This function is able to describe the field aligned skewness by itself, allowing us to model the solar wind electrons as a single population. Taking this into consideration, we solve the Vlasov linear dispersion relation and analyze the effect of these skewed electrons on the excitation of parallel propagating whistler modes in the linear regime. We also obtained the marginal stability thresholds of the WHFI and study the effect that different plasma parameters, specially the heat flux parameter, have on the excitation of this instability.