Energetic neutral atoms at Mars 1. Imaging of solar wind protons

Abstract

[1] Energetic neutral atom (ENA) images are computed from an empirical solar wind proton model and recent models of the Martian exosphere. The proton model describes the proton bulk velocity in the interaction region and the neutral model the profiles of H, H 2 and O (hot) and O (thermal). We assume a Maxwellian proton velocity distribution. The ENA production model is analytical; thus an ENA image can be constructed for any vantage point by line of sight integration, and we present examples of such images. In this work we study the influence of different parameters in the input models on the generated images and examine the relative importance of the different parameters on the global ENA production rate as well as on image morphology for particular vantage points. All together our parameter set includes 14 variables for solar wind conditions, flow geometry, and the exosphere model. Changing the parameters also makes it possible to mimic various features of the solar wind-Mars interaction, such as some of the effects caused by the recently observed magnetic anomalies. It is found that the exobase temperature of atomic hydrogen is the parameter that affects the ENA production and images most. We also investigate the backscattering of ENAs that precipitate on the Martian atmosphere. It is found to be an important effect for views of Mars in the antisunward direction. The outflux of ENAs from the near Mars region is studied, and two maxima are found. One at an angle of approximately 115°to the Mars-Sun line corresponding to ENAs produced downstream of the bow shock and one at approximately 150°corresponding to ENAs produced upstream of the bow shock in the solar wind.

First ENA observations at Mars: ENA emissions from the martian upper atmosphere

“…Recent observations revealed that the solar wind protons can penetrate up to 270 km ], and this means that this proton-ENA albedo mechanism can be more important than expected. This mechanism is mentioned by Holmström et al (2002). They used a proton precipitation flux of 10 4 -10 5 cm −2 s −1 [Brecht (1997)] and concluded that the proton-ENA albedo mechanism can be negligible.…”

“…We call this mechanism 'proton-ENA albedo process' hereafter. The ENA flux generated by this proton-ENA albedo process was mentioned by Holmström et al (2002). They used the proton precipitation flux calculated by Brecht (1997), and concluded that the flux of ENAs associatively produced is small enough to be neglected.…”

“…Therefore ENA sources around Mars are expected to be quite different from those around the Earth and references therein). Barabash et al (2004) enumerated possible ENA sources: the supersonic solar wind ], the shocked solar wind ], accelerated planetary ions Lichtenegger et al (2002)], solar wind protons interacting with the tenuous Phobos atmosphere [Mura et al (2002)], atmospheric atoms sputtered by picked-up O + ions [Luhmann and Bauer (1992)], and solar wind ENAs backscattered from the Martian exosphere (an ENA albedo process) [Kallio and Barabash (2001);Holmström et al (2002)]. …”

Energetic neutral atom imaging of comets

“…For a more detailed description of the generation of ENA images [see, e.g., Holmström et al, 2002]. To exclude other ENAs than of solar wind origin in our simulations we use a cometopause as an inner boundary-separating the solar wind dominated outer region and the inner region dominated by cometary ions-at 1.8 Á 10 8 m. This value was chosen as the distance where 10% of the solar wind density would be depleted if using a constant solar wind density and velocity in equation (2).…”

ENA detection in the dayside of Mars: ASPERA-3 NPD statistical study

“…ENA albedo is produced when charge-exchange ENAs are generated above the Induced Magnetosphere Boundary (IMB; Lundin et al, 2004), cross the IMB and are back-scattered against the exobase (Kallio and Barabash, 2001). A fraction of SW protons cross the IMB due to finite Larmor radius effect and experience a back-scattering process (proton-ENA albedo, Holmström et al, 2002), thereby potentially contributing to the overall signal. In addition, spatially constrained and highly directional ENA fluxes (ENA jets) up to 7×10 5 cm -2 sr -1 s -1 , in the 0.3-3 keV energy range, emanating from a compact region of the sub-solar exosphere, have been detected by the NPD (Futaana et al, 2006b).…”