Kinetic entropy-based measures of distribution function non-Maxwellianity: theory and simulations. (October 2020)

• Record Type: Journal Article
• Authors: Liang, Haoming; Barbhuiya, M. Hasan; Cassak, P. A.; Pezzi, O.; Servidio, S.; Valentini, F.; Zank, G. P.
• Is Part Of: Journal of plasma physics. Volume 86:Number 5(2020)
• Publication Date: 2020-10
• Subjects: space plasma physics, -- plasma heating, -- plasma nonlinear phenomena; Plasma (Ionized gases) -- Periodicals; 530.4405
• DOI: 10.1017/S0022377820001270
• Languages: English
• ISSNs: 0022-3778

Abstract

Abstract : We investigate kinetic entropy-based measures of the non-Maxwellianity of distribution functions in plasmas, i.e. entropy-based measures of the departure of a local distribution function from an associated Maxwellian distribution function with the same density, bulk flow and temperature as the local distribution. First, we consider a form previously employed by Kaufmann & Paterson ( J. Geophys. Res., vol. 114, 2009, A00D04), assessing its properties and deriving equivalent forms. To provide a quantitative understanding of it, we derive analytical expressions for three common non-Maxwellian plasma distribution functions. We show that there are undesirable features of this non-Maxwellianity measure including that it can diverge in various physical limits and elucidate the reason for the divergence. We then introduce a new kinetic entropy-based non-Maxwellianity measure based on the velocity-space kinetic entropy density, which has a meaningful physical interpretation and does not diverge. We use collisionless particle-in-cell simulations of two-dimensional anti-parallel magnetic reconnection to assess the kinetic entropy-based non-Maxwellianity measures. We show that regions of non-zero non-Maxwellianity are linked to kinetic processes occurring during magnetic reconnection. We also show the simulated non-Maxwellianity agrees reasonably well with predictions for distributions resembling those calculated analytically. These results can be important for applications, as non-Maxwellianity can be used to identify regions of kinetic-scale physics or increased dissipation in plasmas.