Exploring a Terrestrial X‐Ray Source and Auroral Emissions Using NICER

The International Space Station (ISS)‐hosted Neutron Star Interior Composition Explorer (NICER) aims to observe distant astrophysical sources. As its field‐of‐view passes through Earth's magnetosphere, NICER detects local magnetospheric X‐ray emissions. While monitoring a distant astronomical source, NICER detected an unexpected X‐ray signal. This signal contained distinct lines at 0.537 and 0.404 keV, near the K‐alpha lines of oxygen and nitrogen, respectively. These emissions have been proposed to be generated locally either by solar wind charge‐exchange, auroral X‐ray emissions, or photoionization/photoexcitation. To differentiate between these sources, X‐ray spectral characteristics were put into the context of particle precipitation, cusp boundary models, solar wind parameters, and geomagnetic activity. To account for variations in the line‐of‐sight and the magnetospheric conditions, observations from NICER utilized the same astrophysical target on multiple different dates. The findings are most consistent with a combined source composed of auroral emissions and solar wind charge‐exchange (SWCX). The prominent nitrogen and oxygen lines support the auroral component, while the line‐of‐sight's passing through the cusp and the broadness of the oxygen feature suggest a contributing component of SWCX. These emissions are positive detections of local (within Earth's magnetosphere) X‐ray generation mechanisms.