Ultra Low Frequency Waves of Earth’s Magnetosphere

The Earth generates a significant magnetic field, the effects of which extend out into space. This field interacts with the solar wind, the stream of charged particles constantly emitted from the Sun that fills the solar system. Earth’s magnetic field has an associated magnetic pressure, which pushes back against the incoming solar wind, forming a cavity around the Earth known as the magnetosphere, which extends sunward in average conditions to approximately 10 Earth radii (∼65,000 km). Inside this cavity, the plasma (ionized gas) dynamics are dominated by the effects of the Earth’s magnetic field. Other magnetized planets (e.g., Mercury, Jupiter, and Saturn) also support magnetospheres in response to the solar wind. The solar wind is not a steady flow; therefore, the delicate balance of pressure that defines Earth’s magnetosphere is constantly being perturbed. In this way, the inhomogeneous solar wind acts as a driver of large-scale perturbations of the magnetosphere. Akin to beating the skin of a drum, this launches waves throughout the magnetosphere known as ultra-low-frequency (ULF) waves. These large length scale, long timescale (periods of 1–1,000 s) waves communicate these changes to the equilibrium as they propagate. They drive electric currents that cause particles to precipitate into the upper atmosphere generating auroral displays. They interact with highly energetic particles trapped in Earth’s magnetosphere in a region known as the radiation belts. These particles can drastically impact orbiting spacecraft, and in this way ULF waves tie in critically to space weather: the real-time plasma conditions in near-Earth space and their effect on humanity. It has also been shown that ULF waves can directly drive electric currents on the ground, which can affect power grids. ULF waves can further be used as a remote-sensing tool, with their frequency measured on the ground permitting the inference of the plasma properties in space. There are also several sources of ULF waves internal to the magnetosphere—for example, those driven by interaction with energetic particles. Through these mechanisms, ULF waves impact almost every area of the magnetosphere and are therefore a critical piece of the overall plasma dynamics.