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In near future a large spaceship is built. It is nuclear powered, has a crew of two dozen, and is propelled by an advanced ion drive like VASIMR.

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electromagnetic thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat a propellant. Then a magnetic field accelerates the resulting plasma to generate thrust (plasma propulsion engine). It is one of several types of spacecraft electric propulsion systems.

The ship departs to Mars from Low Earth orbit.

Will the engine exhaust cause an aurora?

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  • $\begingroup$ I wouldn't normally do this, but I've upgraded the tag from reality-check to science-based as I think this question warrants the higher standard. $\endgroup$ – Separatrix Oct 24 '18 at 9:11
  • $\begingroup$ @Separatrix I consider it a mistake. Reality check explicitly tells "no may be an acceptable answer". On the other hand, science based has no such implication and standard "try to avoid frame challenges and saying no" applies. $\endgroup$ – Mołot Oct 24 '18 at 12:00
  • $\begingroup$ @Mołot, the wording of the question specifically wants a "yes" or "no" as the fundamental answer, so "no" is still valid. $\endgroup$ – Separatrix Oct 24 '18 at 12:37
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Technically yes, in practice no.

Most auroras occur between 90 and 130 km above ground level, though some occur hundreds of kilometers high. Source

NASA considers Low Earth Orbit to be between 160 km and 1000 km above ground, so you have sufficient overlap to be safe in that respect.

An aurora is the emission of photons by gas particles in the atmosphere. In order to be considered an aurora there is the obvious requirement that it be visible from the ground. This translates to the requirement that many gas particles emit photons at the same time over a large area of the sky.

The exact mechanisms that make auroras possible are still under investigation. A general idea is that the sun is constantly emitting charged particles (mostly electrons) in all directions, and many of these get trapped above the Earth's atmosphere in what's called the magnetosphere.

enter image description here

Image taken from NASA's tumblr page on magnetospheres

The size of the magnetosphere depends on the relative strengths of the solar wind and the Earth's magnetic fields. When the solar winds rapidly increase in strength it contracts and you can observe the consequence of an onslaught of charged particles being flung into the Earth's atmosphere.

This is typically observable at the poles where the magnetic fields are the strongest and attract the charged particles most strongly.

Because of this, auroras are massive.

Aurora Australis image from NASA

See a beautiful animated version of this image here

As you can now imagine, the sheer scale of the event is absolutely massive and caused by forces that no spaceship will even need.

So, yes, an ion drive which emits plasma will cause the atmospheric particles surrounding the thruster to gain energy and release it in the form of photons and cause a light display much in the same way that the particles trapped in the magnetosphere do.

However there would not be any sustained excitation of the atmosphere, and so instead of generating an aurora that covers the sky, the space-ship would leave a quickly fading, much smaller trail of lights reminiscent of the lights of an aurora.

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  • $\begingroup$ An ion engine fires an electron beam into its exhaust plasma jet, neutralising it (and keeping the space craft from getting charged). I assume the plasma jet is dense enough for the electrons and cations to be able to recombine, so what eventually hits the atmosphere are neutral particles. $\endgroup$ – Karl Oct 24 '18 at 22:31
  • $\begingroup$ @Karl I believe that what hits the atmosphere is still charged plasma based on this diagram by the Ad Astra rocket company $\endgroup$ – Thymine Oct 25 '18 at 5:50
  • $\begingroup$ This diagram is very simplified. If you fire a positively charged plasma out of a rocket drive, you definitively have to get rid of the residual negative charges, also know as electrons. Otherwise you quickly have a highly charged rocket, and the plasma doesn't get very far. $\endgroup$ – Karl Oct 25 '18 at 16:56

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