Auroras on Mars?

Green skies on a red planet?  You bet.

Last December, a NASA spacecraft named "Maven", detected what scientists call "evidence of widespread Aurora's in the Northern Hemisphere". They said the auroras wrapped around the equator like a string of Christmas lights. NASA said if that display happened here, the auroras would have been seen as far South as Florida.  

The following is from a NASA article:

"It really is amazing," says Nick Schneider who leads MAVEN's Imaging Ultraviolet Spectrograph (IUVS) instrument team at the University of Colorado.  "Auroras on Mars appear to be more wide ranging than we ever imagined."

A map of MAVEN's Imaging Ultraviolet Spectrograph (IUVS) auroral detections in December 2014 overlaid on Mars’ surface. The map shows that the aurora was widespread in the northern hemisphere, not tied to any geographic location. The aurora was detected in all observations during a 5-day period. Credits: University of Colorado  

This isn't the first time a spacecraft has detected auroras on Mars.  Ten years ago, the European Space Agency's Mars Express found an ultraviolet glow coming from "magnetic umbrellas" in the southern hemisphere.

Unlike Earth, Mars does not have a global magnetic field that envelops the entire planet.  Instead, Mars has umbrella-shaped magnetic fields that sprout out of the ground like mushrooms, here and there, but mainly in the southern hemisphere.  These umbrellas are remnants of an ancient global field that decayed billions of years ago.   

"The canopies of the patchwork umbrellas are where we expect to find Martian auroras," says Schneider. "But MAVEN is seeing them outside these umbrellas, so this is something new."
Auroras occur, both on Earth and Mars, when energetic particles from space rain down on the upper atmosphere.

On Earth, these particles are guided toward the poles by our planet's global magnetic field.  That's why auroras are seen most often around the Arctic and Antarctic. On Mars, there is no organized planetary magnetic field to guide the particles north and south—so they can go anywhere.
"The particles seem to precipitate into the atmosphere anywhere they want," says Schneider. "Magnetic fields in the solar wind drape across Mars, even into the atmosphere, and the charged particles just follow those field lines down into the atmosphere."

According to the MAVEN data, solar particles that caused the "Christmas lights" penetrated deeply into the Martian atmosphere---sparking auroras less than 100 km from the surface.  That's lower than auroras on Earth, which range from 100 km to 500 km high.