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

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, miVA2. Instead, the heating increases progressively less as miVA2 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 miVA2, 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.