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The hydrogen exosphere constitutes the uppermost atmospheric layer of the Earth, and its shape may reflect the last stage of the atmospheric escape process. The distribution of hydrogen in the outer exosphere remains unobserved because outer geocoronal emissions are difficult to observe from within the exosphere. In this study, we used the Lyman Alpha Imaging Camera (LAICA) onboard the Proximate Object Close Flyby with Optical Navigation (PROCYON) spacecraft, located outside the exosphere, to obtain the first image of the entire geocorona that extends to more than 38 Earth radii. The observed emission intensity distribution can be reproduced using our analytical model that has three parameters: exobase temperature, exobase density, and solar radiation pressure, which implies that hot hydrogen production in the magnetized plasmasphere is not the dominant process shaping the outer hydrogen exosphere. However, the role of the magnetic effect in determining the total escape flux cannot be ruled out

[1] The dominant neutral constituent in Earth's upper exosphere, atomic hydrogen (H), resonantly scatters solar Lyman-alpha (121.567 nm) radiation, observed as the geocorona. We report here observations of an exospheric response to geomagnetic storms obtained using measurements of the geocorona by Lyman-alpha detectors on the Two Wide-angle Imaging Neutral-atom Spectrometers mission. We introduce a new parameter, NH, the number of H atoms in the spherical shell from a geocentric distance of 3 to 8 Earth radii, to quantitatively characterize in a simplified way global exospheric conditions. Five geomagnetic storms observed during three months in the second half of 2011 are accompanied by abrupt temporary increases, spikes, of NH from 6% to 17%, lasting not longer than a day. These increases seem to show some correlation with the minimum Dst index reached during the peak of each storm.

Energetic neutral atoms are created when energetic magnetospheric ions undergo charge exchange with cold neutral atoms in the Earth's tenuous extended atmosphere (the geocorona). Since they are unaffected by the Earth's magnetic field, these energetic neutrals travel away in straight line trajectories from the points of charge exchange. The remote detection of these particles provides a powerful means through which the global distribution and properties of the geocorona and ring current can be inferred. Due to its 2 × 9 RE polar orbit, the Polar spacecraft provides an excellent platform from which to observe ENAs because it spends much of its time in the polar caps which are usually free from the contaminating energetic charged particles that make observations of ENAs more difficult. In this brief report, we present the first ENA imaging results from Polar. Storm-time ENA images are presented for a northern polar cap apogee pass on August 29, 1996 and for a southern polar cap perigee pass on October 23, 1996. As well, we show with a third event (July 31, 1996) that ENA emissions can also be detected in association with individual substorms.

Energetic neutral atoms (ENAs) were detected by the Energetic Particles and Ion Composition (EPIC) instrument on the Geotail spacecraft during a magnetic storm on October 29–30, 1994. The energetic particles could be identified as neutrals because the direction of the particle flux steadily tracked the direction of the Earth and was uncorrelated with the changing orientation of the ambient magnetic field as the spacecraft viewed the dayside magnetosphere for ∼16 hours. These observations provide the first composition measurements of ENAs, yielding the storm-time evolution of ENA fluxes of hydrogen, helium, and oxygen separately as well as their energy spectra. ENA fluxes and the rate of recovery of Dst are both roughly steady, consistent with charge exchange being a significant energy loss process for the storm-time ring current. For total energies above 200 keV, the intensity of ENA oxygen is the highest, followed by hydrogen, and then by helium. These observations enable us to deduce the line-of-sight (LOS) integrals of the products nHjion and nHfion, where nH=hydrogen geocorona density, jion=differential ion flux, and fion=phase space density (PSD), for ion species H+, He+ and O+ at energies between ∼60 and ∼600 keV. A ∼50–60 keV Maxwellian fits the O+ LOS PSD fairly well but fits the He+ and the H+ only poorly.

Exothermic reactions involving metastable neutrals and ions were recently proposed as sources of hot oxygen atoms in addition to the classical O2+ and NO+ dissociative recombination. The Boltzmann equations for thermal and nonthermal populations of O atoms are solved with a Monte Carlo stochastic simulation method. It is shown that the calculated energy distribution functions of O atoms are significantly in nonequilibrium in the transition region between the thermosphere and the exosphere. It is found that the inclusion of additional sources leads to stronger disturbances of the energy distribution function and, as a consequence, increases the nonthermal fraction of hot O atoms. The variation of the vertical distribution of hot O between solar maximum and minimum conditions is also evaluated and shows good agreement with the available experimental evidence.

This paper investigates new sources of thermospheric non thermal (hot) oxygen due to exothermic reactions involving numerous minor (ion and neutral) and metastable species. Numerical calculations are performed for low altitude, daytime, winter conditions, with moderately high solar activity and low magnetic activity. Under these conditions we find that the quenching of metastable species are a significant source of hot oxygen, with kinetic energy production rates a factor of ten higher than those due to previously considered O2(+) and NO(+) dissociative recombination reactions. Some of the most significant new sources of hot oxygen are reactions involving quenching of O(+)((sup 2)D), O((sup 1)D), N((sup 2)D), O(+)((sup 2)P) and vibrationally excited N2 by atomic oxygen.

[1] The high-energy proton-hydrogen (H+/H) beam associated with proton auroral precipitation transfers momentum in elastic and inelastic collisions with ambient thermal atomic oxygen in the high latitude thermosphere. This process provides a localized novel source of hot oxygen atoms in addition to exothermic photochemistry, charge exchange and momentum transfer from O+ ion precipitation and charge exchange with accelerated ionospheric O+ ions. We suggest that this source contributes to the population of the hot oxygen geocorona and to the flux of escaping oxygen atoms. For an incident proton energy flux of 1 mW m−2 and a mean energy Emean ≈ 5 keV, we calculate a density of hot oxygen atoms with energy above 1 eV of 2.0 × 103 cm−3 and a mean kinetic energy of about 3.5 eV at 700 km. The total upward flux of hot oxygen atoms with energies higher 1 eV is estimated as 3.5 × 108 cm−2 s−1.

[1] X-rays are generated throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and geocoronal neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X-ray photons. A model has been created to simulate this X-ray radiation. Published terrestrial exospheric hydrogen distributions and solar wind speed, density and temperature distributions were used in this model. Simulated images were created as seen from an observation point outside the geocorona. The locations of the bow shock and magnetopause are evident in these images. Perhaps this X-ray emission can be used to remotely sense the solar wind flow around the magnetosphere. Since similar X-rays are produced in the heliosphere, the challenge will be, however, to eliminate this background emission.

[1] During geomagnetic storms a well defined belt of trapped protons and ENAs (energetic neutral atoms) is observed around geomagnetic equator at low L-values. Their source is RC (ring current) protons existing at larger L-values. Through charge exchange with the geocorona RC protons become ENAs and if subjected to a new charge exchange become trapped protons. From low latitude particle observations at four different local times we follow; the RC injection region, the drift of RC-particles through the evening/afternoon into the morning sector, the RC-asymmetry and convection loss to the dayside during the storm initial and main phase, and its development into a symmetric RC in the recovery phase of the storm. INDEX TERMS: 2778 Magnetospheric Physics: Ring current; 2720 Magnetospheric Physics: Energetic particles, trapped; 2730 Magnetospheric Physics:Magnetosphere— inner; 2788 Magnetospheric Physics: Storms and substorms. Citation: Soraas, F., K. Oksavik, K. Aarsnes, D. S. Evans, and M. S. Greer, Storm time equatorial belt – an ‘‘image’’ of RC behavior, Geophys. Res. Lett., 30(2), 1052, doi:10.1029/ 2002GL015636, 2003.

Instrumental improvements responsible for a factor of 25 increase in the sensitivity of the Fabry-Perot interferometer enable us to observe for the first time the short wavelength depletion of the Balmer α spectral profile due to hydrogen escape. These results are shown to be consistent with the implications of OGO-5 observations by Bertaux.