Earth’s auroras are a glorious wonder, but our planet is not the only place in the solar system where these phenomena can be observed.
Atmospheric glow, although sometimes in invisible wavelengths, has been spotted on every planet except Mercury, and even some moons of Jupiter…and even a comet. But it’s on Mars that it gets interesting. The Red Planet is famous for its lost global magnetic field, an ingredient that plays a crucial role in aurora formation elsewhere.
But that doesn’t mean Mars is completely free of magnetism. Regions of localized magnetic fields spring from certain regions of the crust, particularly in the southern hemisphere. A new analysis has confirmed that these small local magnetic fields interact with the solar wind in interesting ways to produce the discrete (or patterned) ultraviolet auroras of Mars.
“We have the first detailed study of how solar wind conditions affect auroras on Mars,” said physicist and astronomer Zachary Girazian of the University of Iowa.
“Our main finding is that inside the strong crustal field region, the rate of aurora occurrence mainly depends on the orientation of the solar wind magnetic field, while outside the crustal field region strong, the occurrence rate mainly depends on the dynamic pressure of the solar wind.”
Here on Earth, we have a pretty good idea of how the aurora borealis and australis occur. They appear when particles from the solar wind collide with the Earth’s magnetosphere, then are accelerated along magnetic field lines to high latitudes, where they rain down into the upper atmosphere.
There, they interact with atmospheric particles to produce the twinkling lights that dance across the sky.
Evidence suggests that the phenomena form similarly on other bodies. For example, Jupiter’s strong and permanent auroras are also facilitated by the huge planet’s complex magnetic field.
But Mars’ global magnetic field disintegrated quite early in the planet’s history, leaving behind only patches of magnetism preserved in magnetized minerals in the crust. Ultraviolet images of Mars at night revealed that auroras tend to form near these crustal magnetic fields, which makes sense if magnetic field lines are needed for particle acceleration.
The work of Girazian and his team 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 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 the Martian auroras. They found that outside the regions of the crustal magnetic field, the dynamic pressure of the solar wind plays an important role in the frequency of aurora detection.
However, solar wind pressure appears to play little role in the brightness of said 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 higher solar wind pressure, can trigger Martian auroras.
Within the regions of the crustal magnetic field, 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 the magnetic reconnection events or particle acceleration needed to produce the ultraviolet glow.
These findings, the researchers say, reveal new insights into 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 for aurora research 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 characteristics of auroras for the first time.”
The research was published in the Journal of Geophysical Research: Space Physics.