Browsing by Author "Gershman, D. J."

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    Electron energy dissipation in a magnetotail reconnection region
    (Physics of Plasmas, 2023-08-08) Burch, J. L.; Genestreti, K. J.; Heuer, S. V.; Chasapis, A.; Torbert, R. B.; Gershman, D. J.; Bandyopadhyay, R.; Pollock, C. J.; Matthaeus, W. H.; Nakamura, T. K. M.; Egedal, J.
    The four Magnetospheric Multiscale spacecraft encountered a reconnection region in the Earth's magnetospheric tail on 11 July 2017. Previous publications have reported characteristics of the electron diffusion region, including its aspect ratio, the reconnection electric field, plasma wave generation from electron beams in its vicinity, and energetic particles in the Earthward exhaust. This paper reports on the investigation of conversion of electromagnetic energy to electron kinetic energy (by J·E) and the ensuing conversion of electron beam energy to electron thermal energy via the pressure–strain interaction. The main result is that omnidirectional, compressive dissipation of electron energy dominates in the positive J·E region, while incompressive parallel dissipation dominates in the inflow region where J·E is small. The existence of parallel electric fields in the inflow region supports previous suggestions that electron trapping by these fields contributes to the parallel dissipation. All of the results are reproduced quantitatively within a factor of two with a 2.5-D particle-in-cell simulation.
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    Scaling of Ion Bulk Heating in Magnetic Reconnection Outflows for the High-Alfvén-speed and Low-β Regime in Earth's Magnetotail
    (The Astrophysical Journal, 2024-08-14) Øieroset, M.; Phan, T. D.; Drake, J. F.; Starkey, M.; Fuselier, S. A.; Cohen, I. J.; Haggerty, C. C.; Shay, M. A.; Oka, M.; Gershman, D. J.
    We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-β and high-Alfvén-speed regime of the Earth's magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfvén speeds (800–4000 km s−1) and inflow ion β (0.002–1) covered by this study is in a plasma regime that could be applicable to the solar corona and flare environments. We find that the observed ion heating increases with increasing inflow (upstream) Alfvén speed, VA, based on the reconnecting magnetic field and the upstream plasma density. However, ion heating does not increase linearly as a function of available magnetic energy per particle, . Instead, the heating increases progressively less as rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfvén-speed and low-β regime scales linearly with , with a scaling factor nearly identical to that found for the low-VA and high-β magnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstream VA, suggesting that the energy partition between ions and electrons in reconnection exhausts could be a function of the available magnetic energy per particle. Finally, we find that the observed difference in ion and electron heating scaling may be consistent with the predicted effects of a trapping potential in the exhaust, which enhances electron heating, but reduces ion heating.
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    Turbulent Energy Transfer and Proton–Electron Heating in Collisionless Plasmas
    (Astrophysical Journal, 2022-12-19) Roy, S.; Bandyopadhyay, R.; Yang, Y.; Parashar, T. N.; Matthaeus, W. H.; Adhikari, S.; Roytershteyn, V.; Chasapis, A.; Li, Hui; Gershman, D. J.; Giles, B. L.; Burch, J. L.
    Despite decades of study of high-temperature weakly collisional plasmas, a complete understanding of how energy is transferred between particles and fields in turbulent plasmas remains elusive. Two major questions in this regard are how fluid-scale energy transfer rates, associated with turbulence, connect with kinetic-scale dissipation, and what controls the fraction of dissipation on different charged species. Although the rate of cascade has long been recognized as a limiting factor in the heating rate at kinetic scales, there has not been direct evidence correlating the heating rate with MHD-scale cascade rates. Using kinetic simulations and in situ spacecraft data, we show that the fluid-scale energy flux indeed accounts for the total energy dissipated at kinetic scales. A phenomenology, based on disruption of proton gyromotion by fluctuating electric fields that are produced in turbulence at proton scales, argues that the proton versus electron heating is controlled by the ratio of the nonlinear timescale to the proton cyclotron time and by the plasma beta. The proposed scalings are supported by the simulations and observations.