Auroras may appear calm and gentle as they shimmer under starry skies—but that placid beauty belies how dynamic they are as a system. At times, they can suddenly intensify, like a growing storm; as seen from space, a soft arc over the polar cap of Earth can turn into a glowing, writhing crown.
Now Tanaka et al. have modeled this process in simulations that can reproduce the precise sequence of events in these global “auroral substorms” as they grow and envelop the planet. These models have enabled the authors to examine the underlying physics of a substorm, including the complex set of currents that flow through Earth’s ionosphere during a storm. The results are already challenging some long-standing conventional wisdom.
That auroral substorms are a global phenomenon is a relatively recent discovery, made in 1964 by Japanese researcher Syun-ichi Akasofu, then a graduate student. At that time, scientists knew that under the right conditions, an aurora could surge in brightness that seems to migrate westward across the globe in a matter of hours. But by studying all-sky images of auroras from different sites, Akasofu realized that auroras weren’t simply a local phenomenon but part of a global system of electrical currents surging from space, under which Earth rotates.
These substorms originate in bursts of energy that come speeding down the tail of Earth’s magnetic field, where the field lines are blown out over the nightside of the planet by the solar wind. In the traditional picture, the magnetic field lines meet again at the tip of the tail, and when they reconnect, they snap together explosively, releasing energy like rubber bands and flicking charged particles down the field lines toward the nightside of the planet and back up again. Schematically, this forms a neat picture of what space physicists call the “current wedge,” traced out by the magnetic field lines.
But the simulations by the authors paint a different view. They are able to reproduce the precise sequence of events: the initial onset of a soft arc that intensifies into a westward surge in a matter of minutes. But auroras and currents don’t always follow the field lines exactly. Instead, they are driven by more complex dynamics in Earth’s magnetic field. As the magnetic field lines wrap themselves around the nightside of Earth, they create a plasma regime that exerts a dynamo effect, generating electromagnetic energy that powers the current—and the aurora’s expansion.
In this picture, the current wedge is a misleading concept: The particles raining down are not the cause of the field-aligned current but the result of the dynamo. This is a more comprehensive and elegant explanation of substorms, the authors argue. At the very least, it represents a challenge to the prevailing wisdom that could lead to a new wave of substorm research. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2017JA024102, 2017)
—Mark Zastrow, Freelance Writer