An atmospheric glow, albeit sometimes at invisible wavelengths, has been detected on every planet except Mercury and even some moons of Jupiter and even a comet. But it’s Mars where it gets interesting.
Earth’s auroras give us a spectacular visual impression, but our planet is not the only place in the Solar System where these phenomena can be found. The red planet is famous for its missing global magnetic field, a component that plays a crucial role in aurora formation elsewhere.
But that doesn’t mean Mars is completely devoid of magnetism. Localized regions of magnetic fields sprout from some parts of the Earth’s crust, especially in the southern hemisphere. New analysis has confirmed that these small, localized magnetic fields interact with the solar wind in interesting ways 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 physicist and astronomer Zachary Girazian of the University of Iowa.
“Our key finding is that inside the strong crustal field region, the rate of aurora formation depends mostly on the direction of the solar wind magnetic field, while outside the strong crustal field region, the rate of formation depends mostly on the solar wind dynamic pressure.”
Here on Earth, we have a pretty good understanding of how auroras – borealis and australis – happen. They are created when particles from the solar wind collide with the Earth’s magnetosphere and are then 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 occur in similar ways on other bodies. For example, Jupiter’s powerful, persistent auroras are formed due to the massive planet’s complex magnetic field.
But Mars’ global magnetic field broke down quite early in the planet’s history, leaving behind only fragments 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.
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 analyzes the solar wind.
They compared data on the dynamic pressure of the solar wind and the strength and angle of the interplanetary magnetic field with ultraviolet data from Martian auroras. They found that outside the crustal magnetic field regions, the dynamic pressure of the solar wind plays a significant role in the detection frequency of auroras.
However, the pressure of the solar wind seems to play little 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 associated with higher solar wind pressure, could trigger Martian auroras.
Inside crustal magnetic field regions, the direction of the magnetic field and the solar wind appear to play an important role in the formation of auroras on Mars. In certain directions, the solar wind appears favorable for magnetic reconnection events or particle acceleration, which is necessary to produce ultraviolet radiation.
These results reveal new insights into how interactions with the solar wind can produce auroras on a planet stripped of its global magnetic field, the researchers said. This knowledge can be used to help better understand the formation of discrete auroras on very different worlds.