Auroral Storms Across the Solar System

Lighting up a Planetary Sky With a Solar Storm

The MAVEN spacecraft orbiting Mars, as imagined by an artist, as it discovers and measures aurorae on the Red Planet. University of Colorado

Every so often the Sun kicks a bunch of plasma out in the form of a coronal mass ejection, sometimes at the same time as a solar flare. These outbursts are part of what makes living with a star like the Sun so exciting. If that material just fell back into the Sun, we'd have some great views of arching filaments draining their material to the solar surface. But, they don't always stick around. The material rides out from the Sun on the solar wind (a stream of charged particles that moves a few hundred kilometers a second (and sometimes faster)). Eventually it arrives at Earth and the other planets, and when it does, it interacts with the magnetic fields of the planets (and moons, such as Io, Europa, and Ganymede). 

When the solar wind slams into a world with a magnetic field, powerful electrical currents are set up, which can have interesting effects, particularly on Earth. Charged particles sizzle in the upper atmosphere (called the ionosphere), and the result is a phenomenon called space weather. The effects of space weather can be as lovely as a display of northern and southern lights and (at Earth) as deadly as a power outage, communications failures, and threats to humans working in space. Interestingly, Venus experiences auroral storms, even though the planet doesn't have its own magnetic field. In this case, particles from the solar wind slam into the planet's upper atmosphere and the energy-driven interactions make the gases glow. 

These storms have also been seen on Jupiter and Saturn (particularly when northern and southern lights emit strong ultraviolet radiation from those planets' polar regions). And, they've been known to occur on Mars. In fact, the MAVEN mission at Mars measured a very deep-reaching auroral storm on the Red Planet, which the spacecraft began detecting around Christmastime of 2014. The glow was not in visible light, such as we'd see here on Earth, but in the ultraviolet. It was seen in the Martian northern hemisphere and it seemed to extend deep into the atmosphere. O

On Earth, auroral disturbances occur typically around 60 to 90 kilometers up. The Martian aurorae were caused by charged particles form the Sun striking the upper atmosphere and energizing atoms of gas there. That wasn't the first time aurorae had been seen at Mars. In August 2004, the Mars Express orbiter detected an auroral storm in progress over a region on Mars called Terra Cimmeria. Mars Global Surveyor found evidence of a magnetic anomaly in the crust of the planet at the same region. The aurora was likely caused as charged particles moving along magnetic field lines in the area, which in turn caused atmospheric gases to be energized. 

Saturn has been known to sport auroras, as has the planet Jupiter. Both planets have very strong magnetic fields, and so their existence is no surprise. Saturn's are bright in the ultraviolet, visible, and near-infrared spectrum of light and astronomers usually see them as bright circles of light over the poles. Like Saturn's aurorae, Jupiter's auroral storms are visible around the poles and are very frequent. They are quite complex, and sport little bright spots that correspond to interactions with the moons Iio, Ganymede, and Europa. 

Aurorae aren't limited to the largest gas giants. It turns out that Uranus and Neptune also have these same storms caused by interactions with the solar wind. They are detectable with instruments on board Hubble Space Telescope. 

The existence of aurorae on other worlds gives planetary scientists a chance to study magnetic fields on those worlds (if they exist), and to trace the interaction between the solar wind and those fields and atmospheres. As a result of this work, they're getting a much better understanding of the interiors of those worlds, the complexities of their atmospheres, and their magnetospheres.