A void in the plasma
In the vast, invisible sea of charged particles surrounding Earth, known as plasma, scientists have observed strange, fleeting structures called electron holes. These are small regions, sometimes only tens of meters wide, where electrons are momentarily scooped out, leaving behind a localized pocket of positive electric potential. This positive charge traps some electrons while repelling others, creating a self-sustaining and stable structure that can travel for thousands of kilometers through the magnetosphere.
These are not physical holes but rather vortices in an abstract concept called phase space, which maps the position and velocity of particles. The deficit of electrons in these trapped orbits creates the positive potential that defines the hole. First identified in early computer simulations of plasma, these structures have since been confirmed by laboratory experiments and numerous spacecraft missions. Their scale is minuscule, typically a few "Debye lengths"—a fundamental plasma measurement that can be as small as tens of meters in the magnetosheath. They move at incredible speeds, with observations showing velocities ranging from a few hundred to over 2,000 kilometers per second.
Cosmic particle accelerators
Electron holes are frequently detected by satellites in regions of intense magnetic activity, such as the solar wind, Earth's auroral zones, and areas where magnetic reconnection occurs. The coordinates for this location point near the University of California, Berkeley, which manages the THEMIS mission, an program for studying these phenomena. Launched in 2007, the five THEMIS spacecraft fly in formation to study the triggers for geomagnetic substorms, which power the auroras.
NASA's Magnetospheric Multiscale (MMS) mission, launched in 2015. With its four identical spacecraft flying in a tight pyramid formation, MMS provides unprecedented high-resolution 3D measurements of plasma. This has allowed scientists to fly through electron holes and measure their structure and fields with extreme precision.
The formation of electron holes is linked to instabilities in the plasma, such as when two streams of electrons move at different speeds. The resulting turbulence can cause electrons to become trapped in waves, leading to the formation of these coherent voids. While they are often considered electrostatic, some observations from the THEMIS mission have detected magnetic perturbations associated with them, adding another layer to the puzzle. Understanding these structures is important because they affect the broader dynamics of plasma, including particle transport and heating.