NASA Goddard's July 2018 press release NASA’s MAVEN Spacecraft Finds That “Stolen” Electrons Enable Unusual Aurora on Mars says:
But two puzzles make this type of aurora seem impossible at first glance: how did these protons get past the planet’s “bow shock,” a magnetic obstacle which normally diverts the solar wind’s charged particles around the planet? And how could the protons give off light, since atoms need electrons to do so?
“The answer was thievery,” said Justin Deighan, of the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder, lead author of a paper on this research appearing July 23 in Nature Astronomy. “As they approach Mars, the protons coming in with the solar wind transform into neutral atoms by stealing electrons from the outer edge of the huge cloud of hydrogen surrounding the planet. The bow shock can only divert charged particles, so these neutral atoms continue right on through.” When those high-speed incoming atoms hit the atmosphere, some of their energy was emitted as ultraviolet light, which is invisible to the human eye but detectable to instruments like the IUVS on MAVEN. In fact, one incoming atom can collide with molecules in the atmosphere hundreds of times before it slows down, giving off a slew of ultraviolet photons.
It's an interesting story; some of the protons from the sun become neutrals at the bow shock, then "tunnel through" to areas of increased density where they collisionally decelerate producing UV auroral light.
But I'd never heard of this huge cloud of hydrogen, likely produced from sublimation of near-surface ice on Mars. It must have a long residence time since the sublimation rate must be quite low these days.
- Just how huge is this cloud of hydrogen around Mars? Is there something like a scale height, and a total mass or at least a partial pressure at a given altitude?
If it's really the decelerating, recently neutralized protons that emit the UV light as described in the quote, then what role does this huge cloud of Hydrogen play?
oh, the answer to the second part is the charge exchange in the p-H2 collisions.
NASA GIF from the same article:
This animation shows a proton aurora at Mars. First, a solar wind proton approaches Mars at high speed and encounters a cloud of hydrogen surrounding the planet. The proton steals an electron from a Martian hydrogen atom, thereby becoming a neutral atom. The atom passes through the bowshock, a magnetic obstacle surrounding Mars, because neutral particles are not affected by magnetic fields. Finally, the hydrogen atom enters Mars' atmosphere and collides with gas molecules, causing the atom to emit ultraviolet light. Credits: NASA/MAVEN/Goddard Space Flight Center/Dan Gallagher