Two publications in the Journal of Geophysical Research

The Aurora of Mars reveals its secrets



Energetic electrons precipitate into the Martian atmosphere along open magnetic field lines (red spirals). These electrons interact with molecules in the Martian atmosphere, producing a highly localized auroral emission, shown here as a reddish glow. This auroral emission is detected by the IUVS instrument onboard the NASA MAVEN satellite, whose field of view is shown in thin lines. (Credit: Anil Rao, LASP, University of Colorado).

Two new studies conducted by a team of researchers from the Laboratory of Atmospheric and Planetary Physics (LPAP) of the University of Liège in collaboration with the University of Colorado Boulder (USA), reveal important new information on the formation mechanism of the aurorae that occur on Mars. These so-called "discrete" aurorae are very different from those that occur on Earth. These researches have been published in the Journal of Geophysical Research.

A

s we know, Mars does not have a magnetic field similar to the one we have on Earth. Today, the magnetic field on Mars is only a remnant of an active magnetic field from several billion years ago. On Earth, it is this active magnetic field that guides particles to the northern and southern polar regions, creating what are known as the northern and southern lights, depending near which pole they occur. Despite this lack of an active magnetic field, the Red Planet also has aurorae. But the Martian aurorae are not quite the same as those observed on Earth. “Magnetized volcanic rocks under the surface of Mars are at the origin of its residual magnetic field and we were able to observe that the aurorae are concentrated near these zones," explains Lauriane Soret, researcher at the Laboratory of Atmospheric and Planetary Physics (LPAP/STAR Institute) of the University of Liège and author of one of the publications in the Journal of Geophysical Research. As a result, since their discovery, Martian aurorae have always been considered rare and unpredictable.  These so-called "discrete" aurorae, because they are scattered and sporadic, were observed by NASA's MAVEN satellite orbiting the Red Planet, and brought to light in 2005 by the European Space Agency's (ESA) Mars Express satellite.

However, a new study (1) has just demonstrated that the occurrence of these aurorae is not as discrete as all that. According to data transmitted by the Imaging UltraViolet Spectrograph (IUVS), onboard MAVEN, several hundred discrete aurorae have been detected over the last six years, demonstrating that this type of aurora is actually quite common and predictable near areas of intense magnetic field. It is the solar wind (a stream of weakly magnetized particles from the Sun) that strongly influences these auroral events. When solar wind conditions are favorable, it appears that the discrete aurorae occur in the evening for several hours. “A temporal correlation between the detection of an aurora and the presence of an intense precipitation of electrons measured on board MAVEN could be observed, says Jean-Claude Gérard, researcher at LPAP. However, it is not always easy to associate an electron flux with a specific auroral detection since the two measurements are not carried out at exactly the same place.”

The color of Mars aurora

The researchers then set about studying the ultraviolet light emitted by the Martian aurora. Lauriane Soret, Jean-Claude Gérard and Benoît Hubert from the LPAP of ULiège had previously analyzed auroral detections using ESA's Mars Express satellite. Their work had shown how the structure of the planet's residual magnetic field controls the location and intensity of the discrete aurora. This time, thanks to the increased sensitivity of the IUVS spectral imager onboard NASA's MAVEN spacecraft, they were able to describe in detail the ultraviolet emissions, which are invisible to the naked eye. In particular, they were able to demonstrate that they are located in the planet’s upper atmosphere, around 130 km. A detailed study (2) of the spectral composition of the aurora in the ultraviolet shows that they are made up of several emissions, such as carbon monoxide (CO), carbon dioxide (CO 2), nitrogen (N 2) and atomic oxygen. “The sensitivity of the IUVS instrument onboard the MAVEN orbiter has allowed us to go from about 20 detections in ten years to hundreds today, explains Benoît Hubert, a FNRS Research associate at LPAP. It is now possible to draw detailed maps of their location." These analyses indicate that the brightness of the emitted light increases with the intensity of the planet's fossil magnetic field thanks to the funneling effect of the magnetic field lines that converge towards the surface. "This is an important step forward in understanding the mechanisms that control the formation of discrete aurorae. Until now, the relationship with the magnetic field strength was not established. Moreover, the detections were only localized in regions where the magnetic field is intense," emphasizes Lauriane Soret.

The team also observed that the spectrum of the auroral light is variable and that electrons from the solar wind are the trigger for these strange glows. According to their estimates, the green light accompanying these aurorae could be visible to future astronauts on the surface or in orbit around Mars. The nighttime auroral display on Mars could be similar to what we see on Earth. The Mars aurora would be rather greenish but, unlike the terrestrial glow, it is not concentrated at high latitudes. Their brightness would strongly depend on the location on the planet and the solar activity.

Champ magnetique aurore mars 

This planisphere of Mars shows the location of discrete aurorae (white circles) detected by the IUVS spectral imager onboard the NASA's MAVEN spacecraft. The background colors (blue to red) indicate the probability that the planet's fossil magnetic field lines are 'closed', i.e. not connected to interplanetary space. We note the alignment of the auroral detections along the 'magnetic valleys' (clear areas between the red zones), but also their sporadic presence away from them. (Soret et al., 2021).

Scientific references

  1. Schneider, N. M., Milby, Z., Jain, S. K., Gérard, J.-C., Soret, L., Brain, D. A., Weber, T., Girazian, Z., McFadden, J., Deighan, J., Jakosky, B.M. (2021). Discrete Aurora on Mars: Insights into their distribution and activity from MAVEN/IUVS observations, Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2021JA029428
  2. Soret, L., Gérard, J.-C., Schneider, N., Jain, S., Milby, Z., Ritter, B., et al. (2021). Discrete aurora on Mars: Spectral properties, vertical profiles, and electron energies, Journal of Geophysical Research: Space Physics, 126, e2021JA029495. https://doi.org/10.1029/2021JA029495

Contacts

Lauriane SORET

Jean-Claude GÉRARD

Benoît HUBERT

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