Auroras are common spectacles in Earth's Arctic and Antarctic regions, but on Mars they're rare and not nearly as attention grabbing. Visible only in the ultraviolet, they may not be as entertaining as their earthly counterparts, but after a decade of measurements by ESA’s unmanned Mars Express orbiter, the rare Martian auroras are telling scientists a lot about the local vestigial magnetic fields of the Red Planet.

Earth auroras are worth staying up to see in the high latitudes because of the intensity of our planet's terrestrial magnetic field. This field interacts with the solar wind and, as charged atomic particles from the wind are drawn down along the magnetic field, they collide with different molecules and atoms in the atmosphere to create the stunning light displays.

However, Mars lacks a global magnetic field, so the long curtains of light seen on Earth aren't possible. Instead, the Red Planet has small pockets of magnetic fields in the highlands of the southern hemisphere that are vestiges from ancient times when Mars was more geologically active. First recorded by NASA's now inactive Mars Global Surveyor, these magnetic pockets allow small ultraviolet auroras to form on rare occasions.

Between 2004 and 2014, Mars Express has been observing Martian auroras using its Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) and Analyzer of Space Plasma and Energetic Atoms/Electron Spectrometer (ASPERA). The first Mars orbiter to observe ultraviolet light during the Martian night, a decade worth of observations by Mars Express was necessary to not only log the appearances of auroras, but to gain a better understanding of their properties.

“The ultraviolet auroras turn out to be very rare and transient: they last only a few seconds. Even though Mars Express has passed over each location many times, detections at a given location do not seem to repeat at later times,” says Lauriane Soret of the University of Liège, Belgium.

In all, Mars Express recorded 19 auroras. Sixteen of these were observed while the orbiter was looking straight down, making the auroras more visible, and three were seen at an angle, which allowed scientists to estimate their height at about 137 km (85 mi). Using its ultraviolet sensor, the probe was also able to measure the energy of the electrons hitting the Martian atmosphere and, by matching this against the visual observations, scientists were able to deduce that the auroras formed near the boundary between open and closed magnetic field lines, and that when these lines were oriented to the vertical, the result were bursts of electrons and ultraviolet light.

"It seems that the emissions are controlled by a special shape of the local magnetic field: as it starts to become open, it makes an umbrella shape, allowing access to the energised electrons," says Jean-Claude Gérard of the University of Liège.

Mars Express was launched by the ESA in 2003 and went into orbit around Mars later that year. Its purpose was to study the surface and atmosphere of Mars and to deliver Britain’s ill-fated Beagle 2 lander, which crashed on its landing attempt. During NASA’s Curiosity lander’s descent, Mars Express helped to monitor the maneuver and later made direct radio contact with the rover.

The Mars Express results were published in the Journal of Geophysical Research: Space Physics and Icarus.

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