On the night of the 10th May 2024, observers of the night sky were treated to an exceptional spectacle: intense aurora borealis illuminated all of France (Figure 1) for the first time in nearly 20 years. This auroral storm, which lasted almost 20 hours, could be admired by those in North America as the night progressed.
More than just a poetic spectacle, the auroras are the visible part of a chain of fascinating physical processes, which can occur throughout the solar system and beyond. Understanding such processes inspires a large community of researchers, but they are also often the object of reductive simplifications and confusion, particularly in the press.
Figure 1 : Images of the aurora observed on the night of 10-11 May 2024 in Touraine (left, credits : N. Biver) or at Mont Ventoux (right: credits : K en B photography).
The lights in the upper atmosphere
The polar aurora, borealis in the North and australis in the South, are luminous emissions which are produced in the high atmosphere, between 80 km and several hundreds of km above sea level. The aurora are found neighbouring the magnetic poles, hence the use of the adjective “polar”. Seen from space, they are concentrated along two magnetically connected high-latitude ovals with an average position of between 60° and 70°. The aurora are produced by the influx of energetic, electricaly charged particles – electrons and ions – into the magnetosphere, the magnetic cavity which surrounds the Earth (a schematic is seen in Figure 2). When these particles reach the atmosphere – the more energy they have, the lower they penetrate – they transfer a part of their kinetic energy to the local atoms and molecules which re-emit it in the form of light. The observed colours in the visible range and their altitude are thus characteristic of the chemical composition of our atmosphere: the green and red emissions are produced by atomic oxygen at both low and high altitudes, the red and blue/purple emissions by neutrall or ionised molecular Nitrogen at lower altitudes (Table 1).
Table 1 : Main lines and bands of visible aurora (taken from Mottez, 2017).
Figure 2 : Artist’s representation of the terrestrial magnetosphere. The blue lines show the magnetic field lines which connect the Northern and Southern magnetic poles.
A proxy of solar-terrestrial interactions
Let’s now look at the origin of the particles which produce the aurora, which has only been understood since the dawn of the space era. It is often said that the aurora are produced directly by solar wind (see below) particles, but this is not exactly true (see this compilation of misconceptions on the aurora by F. Mottez) As discussed above, they come directly from the magnetosphere. This cavity is produced by the interaction between the Earth’s magnetic field and the solar wind, this magnetised stream of charged particles that constantly flows throughout the solar system. As shown in Figure 2, it is compressed on the dayside, where it extends to more than 10 Earth radii, and is elongated on the nightside. Charged particles enter the magnetosphere from two reservoirs; the lesser being the upper, ionised part of the terrestrial atmosphere (the ionosphere), and the larger being the solar wind. Depending on its configuration, the solar wind can provide more or less particles, as we will see below. As these particles circulate around the magnetosphere, they can easily acquire sufficient energy to enter the atmosphere. The aurorae are therefore produced almost permanently, but their low intensity and/or high altitude renders them generally poorly visible to an observer on the ground. Nevertheless, the auroral activity periodically intensifies with bright, intense arcs during events known as “substorms”; the triggering of which depends on a principal ingredient: the orientation of the solar magnetic field as seen from the position of Earth.
Figure 3 : An auroral substorm photographed by the POLAR spacecraft on the 12 March 2014. The aurora, here observed in the UV domain, intensify on the nightside (in the upper-right of each image) of the magnetosphere. Crédits : NASA.
The terrestrial magnetosphere provides a shield that protects us against the solar wind. The solar wind has a magnetic field however, and when it is oriented towards the South the shield of the magnetosphere becomes less effective. Under such circumstances, a magnetic connection is established and permits solar wind particles to enter the magnetosphere. These particles are transported over the poles and accumulate at the equator on the nightside of the magnetosphere where they are accelerated in bursts towards the Earth, producing intense aurorae on the nightside which have a wide extent in latitude. The substorm cycle, described here in only a few lines, is a complex physical phenomenon that has been studied by researchers for more than half a century. The understanding of such events has been the objective of numerous space missions, most recently including the satellite constellations of Themis and MMS. Figure 3 shows an example of aurora during the development of a substorm.
Figure 4 : Animation of the aurora borealis (top) and aurora australis (bottom) observed on the 10th May 2024 by the DMSP spacecraft. The transition between a thin oval around +65° of latitude to a wide, intense oval reaching latitudes < 50° is spectacular and quite rare. Credits : JhuAPL, NOAA. https://ssusi.jhuapl.edu/gallery_AUR
The solar wind can also, but a lot more occasionally, produce particularly intense aurorae when it violently compresses the terrestrial magnetosphere. This is known as a geomagnetic storm, which induces substorms and bright auroral features that reach low latitudes. This is what occurred on the 10th May, when spacecraft measured the auroral morphology seen in figure 4, showing bright aurora reaching latitudes below 50°. Two days earlier, the sun had emitted a series of six coronal mass ejections, bubbles of dense, rapid plasma which coalesced and reached the Earth near midday on 10th May and triggered a major (G5 class) geomagnetic storm, the most intense since 2003.
Happy as Ulysses
These auroral storms are therefore directly linked to solar activity, and the next two years, corresponding to the next peak in solar activity, should bring their share of major solar flares. This should present plenty of opportunities to observe the manifestations of interactions between the magnetic field of our planet and the plasma emitted by our star, including such auroral displays. Amateurs can follow the solar and auroral activity in real time on the dedicated websites such as https://www.spaceweatherlive.com
Auroral emission can also be observed in other wavelength ranges on Earth (from radio to X-rays) and, more generally, on magnetised planets and stars, making it possible to study their magnetospheres. These auroral processes have been analysed in detail on the giant planets using polar probes such as Cassini/Juno or the Hubble space telescope and on distant stars with large ground-based radio telescopes. Their study is one of the LAM’s research priorities.
A risk for industry
A more tangible consequence of the compression of the magnetosphere is the impact that the solar activity can have on us on the ground at Earth. The observation and prediction of solar activity and the consequences at Earth have given birth to the discipline of space weather, defined thus by the European space agency: “Space weather studies the environmental conditions of the thermosphere, ionosphere, and terrestrial magnetosphere caused by the Sun and the solar wind, which can affect the operation and reliability of systems or services on the ground or in space, or endanger human health or property.”. Various French research bodies are involved in these aspects, which are beyond the scope of this article, including the French organisation for space weather applications research.
L. Lamy
Astronome-adjoint, LAM/Aix-Marseille Université et LESIA/Observatoire de Paris.
J. Waters
Chercheur post-doctoral CNES, LAM/Aix-Marseille Université
References : Aurores polaires, la Terre sous le vent du soleil, F. Mottez, Belin, 2017.
