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I just saw the NASA video Mars Proton Aurora below and I'm confused, (which by itself is unremarkable).
Without a planetary magnetic field, what force produces a bow shock in the solar wind at Mars, thereby producing a "proton aurora"? Is it just fluid dynamics and proton-proton collisions? Is large (planetary) scale Coulomb repulsion important as well? Does the dynamics also produce a weak secondary magnetic field that participates?
And what (the heck) is a "hydrogen corona"?
Without a planetary magnetic field, what force produces a bow shock in the solar wind at Mars, thereby producing a "proton aurora"?
As I mention in my answer at https://physics.stackexchange.com/a/335325/59023, Mars does have remnant magnetic fields. Even if Mars lost all of its magnetic fields, it could still generate a standing bow shock due to ionization of the upper atmopshere, which would generate an ionopause like that on Venus (which has no intrinsic magnetic field at all).
As an aside, aurora are just emissions of light generated by the excitation of atoms or molecules from precipitating particles. Thus, a bow shock is not necessary to generate aurora, only an atmosphere.
Is it just fluid dynamics and proton-proton collisions?
It's usually excitation of something like oxygen or nitrogen on Earth. I don't recall specifically which molecule gets excited on Mars but $CO_{2}$ is as likely a candidate as any I can think of off the top of my head.
Is large (planetary) scale Coulomb repulsion important as well?
I am not sure I understand this question. Are you asking if the planetary atmosphere acts like a ball of charge? Remember, the atmosphere exists within a plasma regardless of the existence of a bow shock. Thermal electrons are very fast (e.g., thermal speeds of ~1500-2500 km/s are common in the solar wind) so net, static charges do not live long in space.
Does the dynamics also produce a weak secondary magnetic field that participates?
Yes, the upper atmosphere ionizes like on Venus and that results in an ionosphere, which carries currents, which result in magnetic fields. So if we took away the remnant magnetic fields, you could still get an enhanced magnetic field surrounding the planet due to the ionization and compression of the upper atmosphere. Note that this induced magnetosphere would not be very strong. Venus has a huge, thick atmosphere so lots of stuff to ionize. Even so, the induced magnetosphere only generates magnetic fields in the several 10s of nanotesla (nT). The solar wind near Venus has typical field strengths of ~10-15 nT, for comparison.
The solar wind magnetic field near Mars is much lower at ~1 nT or less. The remnant fields on Mars can be rather large, in some cases exceeding 100 nT [e.g., see doi:10.1134/S0010952517040025] at spacecraft altitudes and upwards of 5000 nT at the surface [e.g., see doi:10.1029/2003JE002048]. The induced magnetic field would be much much lower, like ~4-10 nT.
A stellar corona is the outermost layer of a star's atmosphere.
A geocorona is,
the outermost part of the earth's atmosphere consisting primarily of hydrogen.
Hydrogen corona's have been found on Mars, Ganymede, Europa, Uranus, Callisto.
Hydrogen, being the lightest gas, escapes the surface of a planet easily and can temporarily accumulate in the outermost layer of an atmosphere.
A hydrogen corona is the region of accumulated hydrogen in the outermost layer of an atmosphere.
