Evidence for Nonadiabatic Oxygen Energization in the Near‐Earth Magnetotail From MMS

We present Magnetospheric Multiscale (MMS) mission observations of substorm‐related ion injections at magnetotail positions between ∼7.5 RE and 8.5 RE. Energization of supra‐thermal ions (50 keV–1,000 keV) is strongly species dependent, with oxygen reaching significantly higher peak energies than protons. Using a previously established correlation technique, we confirm that the highest energy (>400 keV) oxygen ions are multiply charged of solar wind origin, enabling them to reach high energies in rough proportion to their charge states. Significantly, we conclude that oxygen ions between 130 keV and 330 keV are singly charged, and that they sometimes achieve higher energizations relative to protons, that is, higher than expected based on their charge states. We conclude that nonadiabatic processes can boost the energies of the oxygen ions. The technique does not depend on “before injection” and “after injection” spectral comparisons, and therefore likely represents the most definitive test yet of nonadiabaticity. Plain Language Summary Earth's space environment, its magnetosphere, forms an invisible comet‐like shape, with a “magnetotail” extending away from the Sun. Satellite observations there often show “injections”; sudden enhancements of ions from 10 to 100s of kilo‐electron volts. The ions of hydrogen (H), helium (He), and oxygen (O) come from both the solar wind and from the Earth's ionosphere. Oxygen from the solar wind is more strongly ionized (charge state often +6) than that from the ionosphere (charge state +1). In the magnetosphere, ions gyrate around in spirals in Earth's magnetic field. Because singly charged oxygen (O+) gyrates more slowly and with larger gyrating orbits than other ions, we expect that such ions can be more strongly accelerated compared to lighter ions during injections. But, that expectation is difficult to prove from a single spacecraft. Here, we present a method that more definitively demonstrates that O+ is sometimes preferentially energized compared to H+ during injections. Key Points We revisit correlations of 50 keV –1,000 keV ion dynamics to infer charge‐ and mass‐dependent acceleration during magnetotail injections Singly charged oxygen ions (≤300 keV) are sometimes preferentially energized compared to protons, likely by nonadiabatic processes The technique does not depend on time‐separated spectral comparisons and so is likely the most definitive test yet of nonadiabaticity