Earth’s aurorae form when charged particles from the magnetosphere strike molecules in the atmosphere, energizing or even ionizing them. As the molecules relax to the ground state, they emit a photon of visible light in a characteristic color. These colliding particles—largely electrons—are accelerated by localized electric fields parallel to the local magnetic field occurring in a region spanning several Earth radii.
Evidence of these electric fields has been provided by sounding rocket and spacecraft missions dating to as far back as the 1960s, yet no definitive formation mechanism has been accepted. To properly discriminate between a number of hypotheses, researchers need a better understanding of the spatial and temporal distribution and evolution of these fields. When the European Space Agency’s (ESA) Cluster mission lowered its perigee in 2008, these observations became possible.
Cluster consists of four identical spacecraft, flying with separations that can vary from tens of kilometers to tens of thousands. Simultaneous observations between the four craft enable space physicists to deduce the 3D structure of the electric field.
Marklund and Lindqvist collect and summarize the contributions of Cluster to our understanding of the auroral acceleration region (AAR), the area of space in which the above-described processes take place.
By collecting a large number of Cluster transits through this region, physicists have deduced that the AAR can generally be found somewhere between 1 and 4.4 Earth radii above the surface, with the bulk of the acceleration taking place in the lower third. Despite this relatively broad “statistical AAR,” the acceleration region at any given moment is usually thin; in one observation, for example, the AAR was confined to an altitude range of 0.4 Earth radius, whereas the actual layer was likely much thinner than that. The observations cannot uniquely determine the thickness of the actual layer, which could be as small as the order of 1 kilometer, the authors say. Such structures are observed to remain stable for minutes at a time.
Cluster measurements also have shed light on the connection between the observed shape of the electron acceleration potential and the underlying plasma environment. So-called S-shaped potentials arise in the presence of sharp plasma density transitions, whereas U-shaped ones are related to more diffuse boundaries. However, the dynamic nature of space plasma means that the morphology of a boundary can shift on timescales of minutes, as exemplified by a case study.
In sum, 2 decades of Cluster observations have significantly improved our understanding of the processes—both local and broad—that result in our planet’s beautiful aurorae. With the missions extended through 2022, we can expect more insight in the coming years. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2021JA029497, 2021)
—Morgan Rehnberg, Science Writer