Terrestrial Energetic Neutral Atom Emissions and the Ground‐Based Geomagnetic Indices: Implications From IBEX Observations

Abstract

AbstractWe report the first daylong continuous observations of bright terrestrial energetic neutral atom (ENA) emissions by the Interstellar Boundary Explorer (IBEX). The unique vantage point of IBEX, up to ∼48 Earth radii (Re) from the dawn‐dusk side, allowed it to monitor ENAs from 0.3 to 6.0 keV over an unprecedented length of time. We have taken advantage of this capability to discover correlations between auroral electrojet (AE) indices and ENA emissions. Although it was difficult to separate the source from low‐altitude emissions and ring current emissions using IBEX's one‐pixel observations, the characteristics of these correlations and e‐folding times hint at the processes behind the ENA emissions. By introducing exponential e‐folding times ranging from 2 to 4 hr in the AE index temporal profiles, the correlation coefficient was maximized to around 0.75−0.85. This indicates (1) that the building and trapping of parent ions, accompanied by injections, cause AE enhancements and (2) that the shorter e‐folding time (typically ∼7–8 hr for energetic ring current ions, assuming charge exchange reactions) supports a more efficient decay process by Coulomb collisions in this energy range. On the other hand, no correlation was found with Sym‐H, while Asy‐H correlated for one orbit. Long‐term continuous ENA observations can also be applied to future space weather monitoring outside the magnetosphere. Based on our results, we suggest that comparisons among multiple spacecraft observations with high‐angular‐resolution ENA sensors or ENA imagers will be required to determine magnetospheric activity more accurately due to the expected asymmetry of the ENA sources.