Dominance of high‐energy (>150 keV) heavy ion intensities in Earth's middle to outer magnetosphere

Previous observations have driven the prevailing assumption in the field that energetic ions measured by an instrument using a bare solid state detector (SSD) are predominantly protons. However, new near‐equatorial energetic particle observations obtained between 7 and 12 RE during Phase 1 of the Magnetospheric Multiscale mission challenge the validity of this assumption. In particular, measurements by the Energetic Ion Spectrometer (EIS) instruments have revealed that the intensities of heavy ion species (specifically oxygen and helium) dominate those of protons at energies ≳150–220 keV in the middle to outer (>7 RE) magnetosphere. Given that relative composition measurements can drift as sensors degrade in gain, quality cross‐calibration agreement between EIS observations and those from the SSD‐based Fly's Eye Energetic Particle Spectrometer (FEEPS) sensors provides critical support to the veracity of the measurement. Similar observations from the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instruments aboard the Van Allen Probes spacecraft extend the ion composition measurements into the middle magnetosphere and reveal a strongly proton‐dominated environment at L≲6 but decreasing proton intensities at L≳6. It is concluded that the intensity dominance of the heavy ions at higher energies (>150 keV) arises from the existence of significant populations of multiply‐charged heavy ions, presumably of solar wind origin. Plain Language Summary Previous observations have shown that protons tend to be the dominant ion species in the magnetosphere. Because of this, it is generally accepted that “total ion” measurements made by instruments without the capability to determine the ion composition will predominantly measure protons. However, new observations from the Magnetospheric Multiscale (MMS) mission have shown that this is not the case at high energies (>150 keV). The unexpected result was discovered during efforts to cross calibrate the two sensors that comprise the Energetic Particle Detector investigation: the Energetic Ion Spectrometer (EIS) and the Fly's Eye Energetic Particle Spectrometer (FEEPS). Comparison of the FEEPS total ion measurements to the EIS proton‐only measurements showed large disagreement at higher energies. It was concluded that this was due to the prevalence of heavier ions (primarily helium and oxygen) at higher energies. Statistically strong agreement between FEEPS total ion and calculated EIS total ion measurement