Energetic neutral atoms at Mars -: 4.: Imaging of planetary oxygen -: art. no. 1280

[1] Photoionization of the Martian oxygen exosphere/ corona results in the production of planetary oxygen ions. The newborn ions start moving on cycloid trajectories determined by the electric and magnetic fields of the solar wind. The oxygen ions can then charge exchange with the neutral gases (H, H-2, and O) of the Martian exosphere and be converted to energetic neutral atoms (ENAs). Using the empirical model of the solar wind flow near Mars developed by Kallio [1996], we calculate the electric and magnetic fields in the interaction region and obtain the global distribution of the oxygen ions by solving the kinetic equation with source and sink terms. The distribution turns out to be highly asymmetrical with a factor of 1000 excess in the ion column density in the hemisphere to which the interplanetary electric field points. The majority of the oxygen ions within 2R(m) planetocentric distances (R-m is the Martian radius) have energies below 6 keV because the typical size of the system is too small for acceleration to higher energies. About 50% of the oxygen ions experience the change- exchange process that produces ENAs. Making line of sight integration of the ion global distribution, oxygen ENA images for vantage points in the noon- midnight meridian plane are obtained. The morphology of the images clearly reflects the structure of the parent ion distribution. The majority of the oxygen ENAs have energy below 600 eV. For these energies the integral flux is up to 10(4) cm(-2) s(-1) eV(-1). The oxygen ENA differential fluxes are high and reach 10(5) cm(-2) s(-1) sr(-1) eV(-1) in the energy range 0.1- 1.7 keV, which can easily be detected by modern ENA instrumentation. Planetary oxygen ENA imaging at Mars is therefore feasible. The total ENA flux observed at a single vantage point depends on the total amount of oxygen ions in the system and may be used to obtain the instantaneous total ion flux escaping the planet.