Dissipation measures in weakly-collisional plasmas

The physical foundations of the dissipation of energy and the associatedheating in weakly collisional plasmas are poorly understood. Here, we compareand contrast several measures that have been used to characterize energydissipation and kinetic-scale conversion in plasmas by means of a suite ofkinetic numerical simulations describing both magnetic reconnection anddecaying plasma turbulence. We adopt three different numerical codes that canalso include interparticle collisions: the fully kinetic particle-in-cell VPIC,the fully kinetic continuum Gkeyll, and the Eulerian Hybrid Vlasov-Maxwell(HVM) code. We differentiate between (i) four energy-based parameters, whosedefinition is related to energy transfer in a fluid description of a plasma,and (ii) four distribution function-based parameters, requiring knowledge ofthe particle velocity distribution function. There is an overall agreementbetween the dissipation measures obtained in the PIC and continuum reconnectionsimulations, with slight differences due to the presence/absence of secondaryislands in the two simulations. There are also many qualitative similaritiesbetween the signatures in the reconnection simulations and the self-consistentcurrent sheets that form in turbulence, although the latter exhibitssignificant variations compared to the reconnection results. All the parametersconfirm that dissipation occurs close to regions of intense magnetic stresses,thus exhibiting local correlation. The distribution function-based measuresshow a broader width compared to energy-based proxies, suggesting that energytransfer is co-localized at coherent structures, but can affect the particledistribution function in wider regions. The effect of interparticle collisionson these parameters is finally discussed.