Origins and variation of terrestrial energetic neutral atoms outflow

Abstract

[1] Analysis of ENA data from the LENA instrument on the IMAGE spacecraft shows that the terrestrial atmosphere is a copious emitter of energetic neutral atoms (<300 eV) under all conditions. When activity is low, the observed emissions are concentrated close to the Earth and are presumed to be the high-energy tail of the warm oxygen geocorona, with energies <2 eV. When activity increases, the relative abundance of the higher-energy neutrals increases, and the emissions can be seen farther from the Earth. Because of the close correlation between the postperigee ENA flux (fluxes seen 1-2 hours after spacecraft perigee) and Ap and the fact that the postperigee fluxes are seen when no magnetic storm is in progress we conclude that many of the emitted ENA come from the auroral zone and are produced by energized ionospheric ions rather than by precipitating energetic ions. In more spectacular events, such as the Bastille Day storm event (14-16 July 2000), oxygen neutral emissions produced by precipitation of keV ring current oxygen ions can also make an important contribution to the total neutral emission. We conclude that diurnal variation in ENA emissions is a winter hemisphere feature that is absent in the summer hemisphere. As activity increases, the altitude range of the auroral oval ENA emission region increases.

Storm time response of the midlatitude thermosphere: Observations from a network of Fabry‐Perot interferometers

“…Such quiet time nonthermal oxygen atoms are thought to be produced mainly by dissociative recombination of O and NO + [ Hickey et al , ], as opposed to charge exchange with precipitating O + during active times, as discussed in the present paper. Observational evidence of this nonthermal population utilizes various techniques: twilight observations of the 732.0 and 733.0 nm airglow emission from the ground [ Yee et al , ], redline intensity measurements made by the Visible Airglow Experiment on the Atmosphere Explorer‐C satellite [ Schmitt et al , ], red line temperature measurements from the FPI on the Dynamics Explorer satellite [ Hubert et al , ], and in situ measurements made by the low energy neutral atom (LENA) instrument on the IMAGE spacecraft [ Wilson and Moore , ]. This evidence is supported by modeling which suggests that a nonthermal population is present above 200 km, which should increase FPI red line temperature measurements above the ambient temperature [ Shematovich et al , ; Sipler and Biondi , ; Kharchenko et al , ].…”

Energetic oxygen atoms in the polar geocorona

“…Moreover, atmospheric sputtering by the precipitating protons also results in the formation of a flux of escaping energetic oxygen atoms. The upward fluxes of hot oxygen shown in Figures 2c and 3c can be responsible for the energetic neutrals of auroral origin seen by the LENA instrument in both perigee and postperigee images [ Wilson et al , 2003; Wilson and Moore , 2005]. To visualize the polar atmosphere response to the precipitation of magnetospheric protons, we calculated the map of escaping oxygen atoms for the averaged proton parameters on 16 June 2000.…”

“…It is useful to point out that this source also produces the oxygen atoms with energies near 10 eV, which can be seen in low‐latitude regions and far away from the ram direction. Wilson and Moore [2005] suggested, based on the analysis of the low‐latitude perigee LENA data, that the instrument signal consists of low to medium energy (5–30 eV) oxygen atoms produced in and near the cusp region. Results of calculations confirmed this conclusion (see Figure 4).…”

“…In this study we do not consider the neutral polar wind because it mainly results in the formation of atomic hydrogen outflow flux [ Gardner and Schunk , 2004]. Oxygen ion precipitation from the ring current is usually observed for very disturbed conditions such as the extremely strong magnetic storm on Bastille Day of 14 July 2000 [ Wilson and Moore , 2005]. This source of hot oxygen was studied by Bisikalo et al [1995].…”

“…The Low Energy Neutral Atom (LENA) instrument on board the IMAGE spacecraft has provided the first global observations of low‐energy ENA emissions in the terrestrial atmosphere. Analysis of ENA data from LENA shows that the Earth's upper atmosphere is a copious emitter of energetic neutral atoms (<300 eV) under all conditions [ Wilson et al , 2003; Wilson and Moore , 2005]. Two different patterns were found in the ENA images, probably corresponding to different sources of these hot atoms.…”