Aspera-3:Analyser of space plasmas and energetic neutral atoms

The Analyser of Space Plasma and Energetic Neutral Atoms (ASPERA-3) on Mars Express (MEX) has now completed one martian year in orbit. The experiment has performed very well and has yielded many interesting findings about the interaction of the solar wind with the planet Mars. ASPERA-3 has revealed that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the MEX pericentre (250-300 km) above the dayside planetary surface. This is quite deep in the ionosphere and atmosphere, implying strong solar wind plasma forcing. The low-altitude ion energisation and outflow near Mars is surprisingly similar to that over the strongly magnetised planet Earth -from narrow 'mono-energetic' ion beams to beams with a broad energy distribution. The distribution of the accelerated plasma suggests both a direct solar wind energy and momentum exchange, via for instance, waves produced by the shocked solar wind, and an indirect solar wind interaction inducing electric currents and fields in a magnetised plasma environment. In the latter case we also envisage parallel (to the magnetic field) electric currents and electric fields projected onto regions of crustal magnetisation in the nightside ionosphere of Mars. Using a novel measurement technique for energetic neutral atoms (ENAs), ASPERA-3 is exploring a new dimension in the solar wind interaction with unmagnetised planets. Mars is 'shining' in energetic neutral atoms. Part of this is caused by reflections of inflowing hydrogen ENAs from the solar wind, but a large fraction is emitted via charge exchange when hot plasma, of solar wind and planetary wind origin, interacts with the upper atmosphere. In this chapter we briefly review three ENA phenomena related to Mars - ENA albedo, ENA jets and ENA occultation. We also discuss another, yet to be explained, source of ENAs in interplanetary space. This source is most likely related to the heliospheric boundaries, but other explanations should also be considered.