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You wait for an aurora on Mars and MAVEN spots two arriving at the same time

Boffins baffled: There's, like, zero magnetic field and very little atmosphere. What gives?

A coronal mass ejection from our Sun sent a shower of charged particles to Mars, generating two types of ultraviolet auroras astronomers have never seen at the same time before.

It's the first time such a solar wind has been detected by Martian orbiter MAVEN – that's the Mars Atmosphere and Volatile Evolution probe – during its eight years of service, NASA said. On Earth, auroras are caused when highly energetic charged particles spewed from the Sun slam into particles in our atmosphere. Our planet's magnetic field steers the solar wind toward the polar regions to create the familiar Northern and Southern Lights. 

Auroras on Mars are a more mysterious phenomenon. The Red Planet lacks a magnetic field these days and has a very weak, thin atmosphere, yet it still produces dazzling displays of lights streaking across its skies. Scientists are still trying to figure out the exact mechanisms powering Martian auroras. Now, NASA's MAVEN spacecraft has provided more data and detected not one but two types of aurora occurring simultaneously.

On August 27, an active sunspot expelled a series of coronal mass ejections (CME). The event sent a surge of charged particles into space that rained down on Mars, creating a proton aurora and a diffuse aurora at the same time. 

A diffuse aurora is caused by energetic electrons colliding with and exciting particles in Mars' atmosphere, causing them to emit UV light.

A proton aurora appears only when conditions are right after the end of a Martian dust storm. Dust storms heat up the atmosphere, pushing up water vapor into higher altitudes where the molecules are split into their oxygen and hydrogen parts. When a gust of solar wind hits the hydrogen atoms, they interact and release rays of ultraviolet light. 

Sumedha Gupta, a postdoctoral physics researcher at the University of Colorado, Boulder, first noticed both types of aurora during a routine check of MAVEN's data. "I was so surprised to see proton aurora at the same time as diffuse aurora, because it had never happened before," she said in a statement. "They're both increasing with solar activity, so we hope it keeps happening."

The CME in August produced one of the brightest solar energetic events observed by MAVEN yet. Astronomers believe the spacecraft will probably spot even more Martian auroras as the Sun approaches its solar maximum in 2024 to 2025, and solar activity reaches its peak during its 11-year cycle. The Sun will spew more CMEs and shower Mars with more solar energetic events to create green swaying across its skies. 

"It's exciting to still be observing 'firsts' like these simultaneous aurora so many years into the mission," said Shannon Curry, MAVEN's principal investigator at the University of California, Berkeley. "We have so much to learn about the atmosphere and how solar storms affect the Red Planet. Our team cannot wait for the next few years of observing the most extreme conditions during the MAVEN mission's lifetime."

In May, the spacecraft was recovered from safe mode after engineers managed to fix a navigation systems issue plaguing MAVEN for months. MAVEN is expected to continue operating through the next decade. ®

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