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.