An international study led by the University of Colorado Boulder (USA) and involving scientists from the Laboratoire de Physique Atmosphérique et Planétaire (LPAP) at the University of Liège has shown that magnetic reconnection - a process by which the topology of a planet's magnetic field is rearranged and some of the energy converted - causes the Martian aurora. This study has been published in Geophysical Research Letters.
T
he aurora borealis is one of the most spectacular natural phenomena. The presence of similar phenomena in the night sky surof Mars is surprising because this planet is smaller than the Earth and has no global magnetic field. Only a region in the southern hemisphere has a magnetic field, a remnant of the past trapped in the planet's crust. A study led by a team from the University of Colorado Boulder - with the participation of researchers from the Université de Liège - has just identified the physical processes behind this type of aurora, which occurs mainly in the southern hemisphere of Mars. The study describes observations based on spacecraft that reveal interactions between local and interplanetary magnetic fields that activate the 'southern lights' on the Red Planet. It shows that the orientation of the solar wind's magnetic field relative to Mars' field lines changes as the planet rotates, creating conditions conducive to magnetic reconnection.
In 2005, the first light emissions from the Martian aurora were detected above magnetised regions. Since then, a number of hypotheses have been put forward as to what causes these aurorae. Members of the Laboratoire de Physique Atmosphérique et Planétaire (LPAP / STAR research unit) at the University of Liège are actively involved in studying auroral phenomena in our solar system, in particular on Earth, Jupiter and its moons and Saturn. They have also studied the aurora on Mars with the Mars Express and Mars Atmosphere and Volatile Evolution (MAVEN) missions. Using spectroscopic instruments on ESA's Mars Express probe, ULiège researchers mapped sporadic UV emissions on the night side of Mars in the planet's southern hemisphere. LPAP is currently participating in NASA's MAVEN and the Arab Emirates' EMM missions, which are acquiring magnificent images of aurorae in the far ultraviolet.
Aurorae, discrete?
In contrast to their name, so-called discrete aurorae are in fact spectacular and can extend for kilometres. The processes responsible for an aurora depend on planetary characteristics, in particular the geometry of the magnetic field and the atmospheric density and composition of the planet. Aurorae on Earth, Jupiter and Saturn are linked to global dipolar magnetic fields generated by convection movements within each planet. EWe think that Mars, in its primitive state, had a similar planetary magnetic field," explains Jean-Claude Gérard, a researcher at LPAP, "but evidence suggests that this field died out several billion years ago.
At the time, it was hypothesised that the discrete aurorae resulted from the interaction between the magnetic field carried by the solar wind and the remanent magnetic fieldsa of Mars. However, the specific physical processes responsible for discrete aurorae remained to be understood. Recently, the University of Colorado's IUVS spectral imager aboard NASA's Mars Atmosphere and Volatile Evolution Orbiter (MAVEN) provided a series of ultraviolet spectral images of localised discrete aurorae illuminating at different locations and times of the night. We observed that the discrete aurorae occur more distinctly in two adjacent regions of the surface. These regions have opposite polarity: the magnetic field lines of one point north, while those of the other point south'' enthuses Lauriane Soret, a researcher at the LPAP and co-author of the article. The interplanetary magnetic field lines can point in different orientations, but the team showed that auroral activity mainly occurs when the field lines are antiparallel to the crustal lines of one of the regions. This alignment occurs at different times of night, just after dusk and just before dawn, for the two magnetic regions.
Magnetic reconnection
This process is known in plasma physics as magnetic reconnection. In simpler terms, it is the tendency of antiparallel magnetic field lines to rearrange themselves in a different order, releasing energy in the process and "opening" field lines that are normally closed and form a kind of magnetic barrier. This opening allows electrons from the solar wind to enter the atmosphere. These electrons collide with the molecules in the Martian atmosphere and excite them, producing aurorae. "Aurorae are ultimately a very widespread phenomenon in the universe. They result from a series of complex physical processes from the Sun to the atmosphere. As on Earth, they appear variable and elusive. This work provides the clearest evidence to date that magnetic reconnection causes the Martian aurora," adds Lauriane Soret. In addition, the glow observed in the ultraviolet must be accompanied by emissions in the visible range. A mission project, called M-MATISSE, has been submitted to the European Space Agency. It is to carry a camera supplied by a Japanese team that will be able to observe these aurorae visible on the night side. Lauriane Soret will be co-leading the scientific team.
Many aspects of the behaviour and origin of the aurora on Mars remain to be elucidated. In particular, we still do not know by what mechanism the auroral particles are precipitated into the Martian atmosphere. Comparison with the Earth and other celestial bodies where aurorae are observed teaches us just how varied the processes involved are", emphasises Jean-Claude Gérard, also a co-author of this study. Studying the interactions of the solar wind with the magnetosphere and atmosphere of Mars will help us to understand how Mars lost most of its atmosphere as a result of solar events, and how it evolved from a once wet planet to the desert we see today," concludes Lauriane.
J. Johnston, N. M. Schneider, S. K. Jain, Z. Milby, J. Deighan, C. F. Bowers, G. A. DiBraccio, J.-C. Gérard, L. Soret, Z. Girazian, D. A. Brain, S. Ruhunusiri, S. Curry, Discrete aurora at Mars: Insights into the role of magnetic reconnection, Geophys. Res.Lett. 50 (2023).
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