Scaling of Ion Bulk Heating in Magnetic Reconnection Outflows for the High-Alfven-speed and Low-β Regime in Earth's Magnetotail

We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-beta and high-Alfven-speed regime of the Earth's magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfven speeds (800-4000 km s(-1)) and inflow ion beta (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) Alfven speed, V-A, 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, m(i)V(A)(2). Instead, the heating increases progressively less as m(i)V(A)(2) rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfven-speed and low-beta regime scales linearly with m(i)V(A)(2), with a scaling factor nearly identical to that found for the low-V-A and high-beta magnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstream V-A, 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.