Terrestrial Energetic Neutral Atom Emissions and the Ground‐Based Geomagnetic Indices: Implications From IBEX Observations. Issue 11 (16th November 2019)

• Record Type: Journal Article
• Authors: Ogasawara, K.; Dayeh, M. A.; Fuselier, S. A.; Goldstein, J.; McComas, D. J.; Valek, P.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 11(2019)
• Page Start: 8761
• Page End: 8777
• Publication Date: 2019-11-16
• Subjects: energetic neutral atom -- interstellar boundary explorer -- field‐aligned current; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026976
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We 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. Plain Language Summary: Energetic neutral atom (ENA) observations are useful for future space weather predictions. ENAs are produced when energetic ions interact with the cold exosphere of the Earth, and their emission profile suggests the energetic ion distributions in geospace. In this study, 1‐ to 10‐keV ENAs that originated from the Earth are observed from a new vantage point, enabling observations over an unprecedented length of time. The results showed that the ENA time profile correlated with Earth's auroral activities. Furthermore, the results suggested faster loss processes in this energy range than previously known. Key Points: IBEX's unique vantage point provides the first daylong terrestrial ENA observations that can be correlated with geomagnetic indices The correlations imply that the process of the strongest terrestrial ENA emissions is highly related to field‐aligned current formation The 2‐ to 4‐hr ENA decay time compared to AE suggests that the Coulomb collision loss can occur over charge exchange at <10 keV