Earth’s auroras are a glorious wonder, but our planet is not the only place in the solar system where these phenomena can be found.
An atmospheric glow, albeit sometimes in invisible wavelengths, has been observed on every planet except Mercury, and even some moons of Jupiter… and even a comet. But Mars is where it gets interesting. The red planet is known for its lost global magnetic field, an ingredient that plays a critical role in the formation of aurora elsewhere.
But that doesn’t mean Mars is completely free of magnetism. Regions with localized magnetic fields sprout from some parts of the crust, particularly in the southern hemisphere. New analysis has confirmed that these small, localized magnetic fields interact in interesting ways with the solar wind to produce Mars’ discrete (or structured) ultraviolet auroras.
“We have the first detailed study looking at how solar wind conditions affect auroras on Mars,” said University of Iowa physicist and astronomer Zachary Girazian.
“Our main finding is that within the region of the strong crustal field, the rate of occurrence of the aurora mainly depends on the orientation of the magnetic field of the solar wind, while outside the region of the strong crustal field, the frequency of occurrence largely depends on the dynamic pressure from the solar wind.”
Here on Earth, we have a pretty good idea of how auroras — borealis and australis — happen. They appear when particles from the solar wind collide with Earth’s magnetosphere and are then accelerated along the lines of the magnetic field to high latitudes, where they rain down into the upper atmosphere.
There they interact with atmospheric particles to produce the glittering lights that dance across the sky.
There is some evidence that the phenomena form in similar ways on other bodies. For example, Jupiter’s powerful, permanent auroras are also made possible by the massive planet’s complex magnetic field.
But Mars’ global magnetic field decayed quite early in the planet’s history, leaving behind only bits of magnetism preserved in magnetized minerals in its crust. Ultraviolet images of Mars at night have revealed that auroras tend to form near these magnetic fields in the Earth’s crust, which makes sense if magnetic field lines are needed for particle acceleration.
Girazian and his team’s work also takes solar wind conditions into account. 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 is 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 Mars’ auroras. They found that, beyond the magnetic field regions of the Earth’s crust, 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 little role in the brightness of said auroras. This suggests that space weather events, such as coronal mass ejections, in which masses of charged particles are ejected from the sun and associated with higher solar wind pressures, could trigger Martian auroras.
Within the magnetic field regions of the Earth’s crust, the orientation of the magnetic field and solar wind appear to play an important role in the formation of auroras on Mars. At certain orientations, the solar wind appears to favor the magnetic reconnection events or particle acceleration needed to produce the ultraviolet glow.
These results, the researchers said, reveal new information about how interactions with the solar wind could generate auroras on a planet stripped of its global magnetic field. This information can be used to better understand the formation of discrete auroras on very different worlds.
“This is a very fruitful and exciting time for exploring aurora on Mars,” Girazian said.
“The database of discrete aurora observations we have from MAVEN is the first of its kind, allowing us to understand the basic features of the aurora for the first time.”
The research is published in the Journal of Geophysical Research: Space Physics†
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