Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsa12a..03b&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SA12A-03
Physics
5443 Magnetospheres (2756), 6225 Mars, 6297 Instruments And Techniques
Scientific paper
The ASPERA-3 instrument onboard ESA Mars Express mission comprises four instruments, two energetic neutral atom (ENA) sensors and an electron and ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux in the energy range 0.1 - 60 keV with no mass and energy resolution but a comparatively high angular resolution of 4.6° ×11.5° . The Neutral Particle Detector (NPD) provides measurements of the ENA flux in the energy range 0.1 - 10 keV, resolving velocity and mass (H and O) with a coarse angular resolution of 5° ×30° . The ENA detection technique is based on the atom - surface interaction. We review the latest ASPERA-3 results on ENA measurements at Mars focusing on three types of observations (1) hydrogen atoms backscattered from the Martian upper atmosphere, (2) ENA occultation, and (3) possible observations of oxygen ENAs. NPD sensors detected intensive fluxes of hydrogen atoms at an energy of 1 keV from the nadir direction around pericenter (approximately 270 km). The atoms originate from the solar wind protons precipitating on the upper atmosphere and going through a cascade of charge exchange - stripping reactions and finally elastically scattered back. This interpretation is confirmed by local measurements by the ASPERA-3 ion spectrometer of solar wind protons at heights as low as pericenter. ENA images of this component indicate that the solar wind can reach large areas of the upper atmosphere delivering mass, energy, and momentum to atmospheric atoms. ENA resulting from the charge - exchange of the undisturbed solar wind on the exosphere outside the Martian magnetosheath can propagate through the upper atmosphere at the terminator and experience scattering within a large solid angle. The fluxes of these scattered ENAs are observed in the sunward direction deep inside the eclipse resembling occultation geometry. The profile of the signal can be used to define atmospheric parameters at the terminator region. Finally, fluxes of very slow atoms recorded in NPD time-of-flight (TOF) spectra may result from low energy oxygen atoms (down to the instrument lower energy limit for oxygen, approximately 300 eV) emitted from altitudes of few hundred km.
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