Auroral structures may be caused by reflected electrons

The aurora borealis is a natural phenomenon known for its colorful and fleeting lights, formed when charged particles from the Sun are directed by Earth's magnetic field into the upper atmosphere. These particles collide with atoms and molecules, generating distinct luminous patterns. Although the mechanism that generates the aurora is well understood, some small-scale characteristics still challenge scientists.

To investigate these auroral structures, researchers N. Brindley and D. Whiter used high-resolution optical data and a simple mathematical model. They discovered that certain fine auroral structures may result from electrons that reflect off low-frequency plasma waves, known as Alfvén waves, above the ionosphere ^1,2,3.

The study of auroras offers both challenges and opportunities to understand plasmas and electromagnetic fields. The movement of auroras allows for the estimation of electric fields. Earth-orbiting satellites have already mapped U-shaped electric potentials above auroral arcs, but these observations are limited, as satellites quickly pass over the regions of interest. To understand small-scale fluctuations, researchers used oxygen and nitrogen emissions in the optical and near-infrared spectrum, measured by the Auroral Structure and Kinetics (ASK) instrument in Svalbard, Norway. The instrument provided spatial and temporal data on the evolution of electric fields over a 25-second period, sufficient time to capture behaviors typically missed in satellite-based studies ^2,3.

The researchers used two approaches to map the electric fields, one assuming the presence of a U-shaped potential. The results were largely consistent, and the discrepancies between the two approaches can be explained by electrons accelerated upon reflecting off Alfvén waves above the ionosphere ^1,2,3.

The method of high spatial and temporal resolution optical observation offers a new perspective to understand the aurora at small scales, as well as other space weather phenomena ³.