Composition dependence of ion heating in dayside magnetopause reconnection: MMS observations

Introduction Motivated by observed composition dependence in reconnection energy outflow and by theoretical and simulation studies predicting differences between heating of H+ and heavier ions, we investigated the ion composition dependence of heating associated with reconnection in 28 dayside magnetopause crossings in Magnetospheric Multiscale (MMS) data. Methods We applied Least Squares fitting to analyze the relationship of temperature change across the magnetopause exhaust to available magnetic energy. Available magnetic energy per ion-electron pair flowing into the magnetopause from the magnetosheath and magnetosphere ranged between a few tens of eV and ∼1750 eV. Results The individual fits for composition-nonspecific ions and for H+ were significantly lower than the empirical scaling relationship found previously between the temperature change and the inflowing magnetic energy; the fit for He++ was higher, with marginal significance. A composite data product combining H+ and He++ agreed with the empirical scaling relationship to within 95%. Discussion Although comparisons between heating of H+ and He++ are suggestive of enhanced heating of heavy ions, differences could not be identified conclusively due to high scatter and a small number of events with adequate densities of heavy ions.