Exploration of Pressure Evolution Terms in Anisotropization

This work evaluates pressure evolution inspired by [1], which specializes in the role of velocity shear in generating anisotropy (with certain assumptions). Using the same equation in the study, we investigate how the magnetic field and isotropic compression terms, along with the velocity rate of strain tensors, influence the pressure tensor. We use results from PIC simulations and MMS Observations. Previous studies (e.g. [1]-[4]) have examined heating and other distortions of the pressure tensor near current sheets, vortex sheets and regions of high rate of strain. It has been noted that highly elongated vortex sheets in 2D will coincide with high rates of strain. Furthermore each of these occur in close proximity to current sheets when reconnection is active [5]. It is noteworthy that the model equations employed to study the deformation of the pressure tensors frequently follow the development of [4] which, first, ignores the divergence of heat flux, and second, quickly specializes to gyrotropic pressure tensors via the CGL closure. In the present examination of the pressure anisotropization in collisionless plasma, we re-examine the gyrotropic effects that have been previously studied, but also evaluate the quantitative influence of agyrotropy on pressure. This expands on the work of [6] which found that in some cases (in MMS data), the agyrotropy may have potentially important effects on dissipation. We also examine the relative importance of heat flux in the standard turbulence simulations we employ.

[1]Del Sarto, Daniele, and Francesco Pegoraro., 2018, MNRAS, 475

[2]Franci L., Hellinger P., Matteini L., Verdini A., Landi S., 2016, in Wang L., Bruno R., Mobius E., Vourlidas A., Zank G., eds, Solar Wind 14. ¨ AIP Publishing LCC, p. 04003

[3]Parashar T. N., Matthaeus W. H., 2016, ApJ, 526, 697

[4]Del Sarto D., Pegoraro F., Califano F., 2016a, Phys. Rev. E, 93, 053203

[5]Matthaeus W. H., 1982, Geophys. Res. Lett., 9, 660

[6]Yang, Yan, et al., 2023, ApJ, 944, 148