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1. Kinetic entropy-based measures of distribution function non-Maxwellianity: theory and simulations

   https://doi.org/10.1017/S0022377820001270  

   Liang, Haoming ; Barbhuiya, M. Hasan ; Cassak, P. A. ; Pezzi, O. ; Servidio, S. ; Valentini, F. ; Zank, G. P. ( October 2020 , Journal of Plasma Physics)

   null (Ed.)

   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. 

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2. Characterizing velocity–space signatures of electron energization in large-guide-field collisionless magnetic reconnection

   https://doi.org/10.1063/5.0082213  

   McCubbin, Andrew J. ; Howes, Gregory G. ; TenBarge, Jason M. ( May 2022 , Physics of Plasmas)

   Magnetic reconnection plays an important role in the release of magnetic energy and consequent energization of particles in collisionless plasmas. Energy transfer in collisionless magnetic reconnection is inherently a two-step process: reversible, collisionless energization of particles by the electric field, followed by collisional thermalization of that energy, leading to irreversible plasma heating. Gyrokinetic numerical simulations are used to explore the first step of electron energization, and we generate the first examples of field–particle correlation signatures of electron energization in 2D strong-guide-field collisionless magnetic reconnection. We determine these velocity space signatures at the x-point and in the exhaust, the regions of the reconnection geometry in which the electron energization primarily occurs. Modeling of these velocity–space signatures shows that, in the strong-guide-field limit, the energization of electrons occurs through bulk acceleration of the out-of-plane electron flow by the parallel electric field that drives the reconnection, a non-resonant mechanism of energization. We explore the variation of these velocity–space signatures over the plasma beta range 0.01≤βi≤1. Our analysis goes beyond the fluid picture of the plasma dynamics and exploits the kinetic features of electron energization in the exhaust region to propose a single-point diagnostic, which can potentially identify a reconnection exhaust region using spacecraft observations. 

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3. Characterizing Velocity-Space Signatures of Electron Energization in Large-Guide-Field Collisionless Magnetic Reconnection

   McCubbin, Andrew J. ; Howes, Gregory G. ; TenBarge, Jason M. ( December 2021 , ArXivorg)

   Magnetic reconnection plays an important role in the release of magnetic energy and consequent energization of particles in collisionless plasmas. Energy transfer in collisionless magnetic reconnection is inherently a two-step process: reversible, collisionless energization of particles by the electric field, followed by collisional thermalization of that energy, leading to irreversible plasma heating. Gyrokinetic numerical simulations are used to explore the first step of electron energization, and we generate the first examples of field-particle correlation (FPC) signatures of electron energization in 2D strong-guide-field collisionless magnetic reconnection. We determine these velocity space signatures at the x-point and in the exhaust, the regions of the reconnection geometry in which the electron energization primarily occurs. Modeling of these velocity-space signatures shows that, in the strong-guide-field limit, the energization of electrons occurs through bulk acceleration of the out-of-plane electron flow by parallel electric field that drives the reconnection, a non-resonant mechanism of energization. We explore the variation of these velocity-space signatures over the plasma beta range 0.01 < beta_i < 1. Our analysis goes beyond the fluid picture of the plasma dynamics and exploits the kinetic features of electron energization in the exhaust region to propose a single-point diagnostic which can potentially identify a reconnection exhaust region using spacecraft observations. 

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4. Dissipation measures in weakly collisional plasmas

   https://doi.org/10.1093/mnras/stab1516  

   Pezzi, O ; Liang, H ; Juno, J L ; Cassak, P A ; Vásconez, C L ; Sorriso-Valvo, L ; Perrone, D ; Servidio, S ; Roytershteyn, V ; TenBarge, J M ; et al ( June 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The physical foundations of the dissipation of energy and the associated heating in weakly collisional plasmas are poorly understood. Here, we compare and contrast several measures that have been used to characterize energy dissipation and kinetic-scale conversion in plasmas by means of a suite of kinetic numerical simulations describing both magnetic reconnection and decaying plasma turbulence. We adopt three different numerical codes that can also 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, whose definition is related to energy transfer in a fluid description of a plasma, and (ii) four distribution function-based parameters, requiring knowledge of the particle velocity distribution function. There is an overall agreement between the dissipation measures obtained in the PIC and continuum reconnection simulations, with slight differences due to the presence/absence of secondary islands in the two simulations. There are also many qualitative similarities between the signatures in the reconnection simulations and the self-consistent current sheets that form in turbulence, although the latter exhibits significant variations compared to the reconnection results. All the parameters confirm that dissipation occurs close to regions of intense magnetic stresses, thus exhibiting local correlation. The distribution function-based measures show a broader width compared to energy-based proxies, suggesting that energy transfer is co-localized at coherent structures, but can affect the particle distribution function in wider regions. The effect of interparticle collisions on these parameters is finally discussed. 

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5. Particle-in-cell simulations of the magnetorotational instability in stratified shearing boxes

   https://doi.org/10.1093/mnras/stae959  

   Sandoval, Astor ; Riquelme, Mario ; Spitkovsky, Anatoly ; Bacchini, Fabio ( April 2024 , Monthly Notices of the Royal Astronomical Society)

   The magnetorotational instability (MRI) plays a crucial role in regulating the accretion efficiency in astrophysical accretion discs. In low-luminosity discs around black holes, such as Sgr A* and M87, Coulomb collisions are infrequent, making the MRI physics effectively collisionless. The collisionless MRI gives rise to kinetic plasma effects that can potentially affect its dynamic and thermodynamic properties. We present 2D and 3D particle-in-cell (PIC) plasma simulations of the collisionless MRI in stratified discs using shearing boxes with net vertical field. We use pair plasmas, with initial β = 100 and concentrate on subrelativistic plasma temperatures (kBT ≲ mc2). Our 2D and 3D runs show disc expansion, particle and magnetic field outflows, and a dynamo-like process. They also produce magnetic pressure dominated discs with (Maxwell stress dominated) viscosity parameter α ∼ 0.5–1. By the end of the simulations, the dynamo-like magnetic field tends to dominate the magnetic energy and the viscosity in the discs. Our 2D and 3D runs produce fairly similar results, and are also consistent with previous 3D MHD (magnetohydrodynamic) simulations. Our simulations also show non-thermal particle acceleration, approximately characterized by power-law tails with temperature-dependent spectral indices − p. For temperatures $k_\mathrm{ B}T \sim 0.05-0.3\, mc^2$, we find p ≈ 2.2–1.9. The maximum accelerated particle energy depends on the scale separation between MHD and Larmor-scale plasma phenomena in a way consistent with previous PIC results of magnetic reconnection-driven acceleration. Our study constitutes a first step towards modelling from first principles potentially observable stratified MRI effects in low-luminosity accretion discs around black holes.

    

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