Mars has auroras without a global magnetic field, and we finally know how

Earth’s auroras are a glorious wonder, but our planet isn’t the only place in the Solar System where these phenomena can be found.

An atmospheric glow, albeit sometimes at invisible wavelengths, has been detected on all the planets except Mercury, and even on some of Jupiter’s moons… and even on a comet. But Mars is where it gets interesting. The Red Planet is famous for its missing global magnetic field, an ingredient that plays a crucial role in the formation of auroras elsewhere.

But that doesn’t mean that Mars is totally free of magnetism. Regions of localized magnetic fields sprout from some regions of the crust, particularly in the southern hemisphere. New analysis has confirmed that these small, local magnetic fields interact with the solar wind in interesting ways to produce the discrete (or structured) ultraviolet auroras on Mars.

“We have the first detailed study looking at how solar wind conditions affect auroras on Mars,” said physicist and astronomer Zachary Girazian of the University of Iowa.

“Our main finding is that within the strong crustal field region, the rate of aurora occurrence depends primarily on the orientation of the solar wind magnetic field, whereas outside the strong crustal field region, the rate of occurrence depends mainly on the dynamic pressure of the solar wind”.

Here on Earth, we have a pretty good handle on how the aurora borealis and australis occur. They appear when solar wind particles collide with Earth’s magnetosphere and are then accelerated along magnetic field lines at high latitudes, where they rain down into the upper atmosphere.

There, they interact with atmospheric particles to produce the bright lights that dance across the sky.

The evidence suggests that the phenomena form in a similar way in other bodies. For example, Jupiter’s powerful and permanent auroras are also facilitated by the massive planet’s complex magnetic field.

But Mars’ global magnetic field decayed fairly early in the planet’s history, leaving only patches of magnetism preserved in magnetized minerals in the crust. Ultraviolet images of Mars at night have revealed that auroras tend to form near these crustal magnetic fields, which makes sense if magnetic field lines are required for particle acceleration.

Girazian and his team’s work also takes into account solar wind conditions. They analyzed data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, which has been collecting ultraviolet images of the Red Planet since 2014. It’s also equipped with an instrument called the Solar Wind Ion Analyzer, which, unsurprisingly, analyzes the solar wind. .

They compared data on the dynamic pressure of the solar wind, as well as the strength and angle of the interplanetary magnetic field, with ultraviolet data on Martian auroras. They found that, outside the crustal magnetic field regions, the dynamic pressure of the solar wind plays an important role in the detection frequency of auroras.

However, the pressure of the solar wind seems to play a minor role in the brightness of these auroras. This suggests that space weather events such as coronal mass ejections, where masses of charged particles are ejected from the Sun and are associated with increased solar wind pressure, can trigger Martian auroras.

Within the magnetic field regions of the crust, the orientation of the magnetic field and the solar wind appear to play an important role in the formation of auroras on Mars. At certain orientations, the solar wind appears to favor magnetic reconnection events, or the acceleration of particles required to produce the ultraviolet glow.

These results, the researchers said, reveal new information about how interactions with the solar wind can generate auroras on a planet stripped of its global magnetic field. This information can be used to help better understand the formation of discrete auroras on very different worlds.

“Now is a very fruitful and exciting time to investigate auroras on Mars,” Girazian said.

“The database of discrete auroral observations we have from MAVEN is the first of its kind, allowing us to understand basic features of auroras for the first time.”

The research has been published in the Journal of Geophysical Research: Space Physics.

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