We have new insight into a wonderful Martian phenomenon, thanks to a collaboration between two orbiting space probes.
NASA's Mars Atmosphere and Volatile Evolution (MAVEN) and the United Arab Emirates' Hope Probe have joined forces to study the ultraviolet proton auroras that dance and glow high up in Mars' atmosphere.
The new research reveals that these daytime events aren't always diffuse, featureless and evenly distributed, but highly dynamic and variable, containing fine-scale structures.
"EMM's (Emirates Mars Mission) observations suggested that the aurora was so widespread and disorganized that the plasma environment around Mars must have been truly disturbed, to the point that the solar wind was directly impacting the upper atmosphere wherever we observed auroral emission," says planetary scientist Mike Chaffin of the University of Colorado Boulder.
"By combining EMM auroral observations with MAVEN measurements of the auroral plasma environment, we can confirm this hypothesis and determine that what we were seeing was essentially a map of where the solar wind was raining down onto the planet."
Proton aurorae – the most common aurorae on the red planet – were first described in 2018, as seen in MAVEN data. They form fairly similarly to how aurorae on Earth form; however, since Mars is a very different beast, without an internally-driven magnetosphere like Earth's, the end result is unique to Mars.
The closest the red planet does have to a global magnetic field is a flimsy one induced by the buzz of charged particles decelerating as they smack into the atmosphere. As weak as it is, it is usually adequate enough to deflect many of the high speed protons and neutrons raining down from the Sun.
The proton aurorae form when positively charged protons in the solar wind collide with Mars' hydrogen envelope and become ionized, stealing electrons from hydrogen atoms to become neutral.
This charge exchange allows the neutral particles to bypass the magnetic field's bow shock around Mars, raining down in the upper atmosphere and emitting ultraviolet light.
It was thought that this process reliably produced uniform auroral emission over Mars' dayside. The new observations show otherwise.
Rather than the smooth profile expected, the data from the Hope Probe shows that, sometimes, the aurora is patchy, suggesting that there can be unknown processes at play during the formation of these aurorae.
This is where MAVEN enters the picture. NASA's orbiter carries a full suite of plasma instruments, for probing the solar wind, magnetic environment, and thermal ions in the space around Mars.
It simultaneously took measurements while Hope imaged the strange aurorae, and the combined data allowed the scientists to reconstruct the reason behind it.
"By examining multiple Emirates Mars Mission observations of patchy aurora that have different shapes and locations, and combining these images with plasma measurements made by NASA's Mars Atmosphere and Volatile EvolutioN mission, we conclude that a number of processes can produce patchy aurora," the researchers write in their paper.
"This patchy aurora is mostly the result of plasma turbulence, which under some circumstances leads to direct deposition of the solar wind across the entire Martian dayside."
In other words, a rare chaotic interaction between Mars and the solar wind is responsible for the patchy aurora; although it's not entirely clear what the impact is on the Martian surface.
It's possible, however, that there are implications for long-term atmosphere and water loss; without a global magnetic field, Mars is continuing to lose both.
Interestingly, the proton aurorae – both smooth and patchy – can help us understand at least one of these, since the hydrogen involved is partially being created by water in the Martian atmosphere leaking into space.
"Many future data and modeling studies will be needed," the researchers write, "to tease out the full implications of these conditions for Mars atmospheric evolution."
The research has been published in Geophysical Research Letters.