An analysis of the electromagnetic "hiss" that surrounds Earth reveals it's not just static; there's a signal hidden within, which may help scientists uncover its source.

In space, no one can hear you scream—but if you have the right radio equipment, you can “hear” the electromagnetic waves undulating through the void. Now, scientists have found previously unheard signals in this static that might help them uncover the source of a particular kind of hiss.

All of the waves examined in this study were generated by the particles of plasma trapped in Earth’s magnetic field. They spiral, gyrate, resonate, and stock up energy and release it—which creates tiny ripples in the electric and magnetic field surrounding Earth. When these waves are converted to sound—like a radio playing FM broadcasts—the space around Earth sounds like a jungle filled with different species of particles and electromagnetic behaviors, all emitting distinctive calls. For example, lightning can trigger waves called whistlers, which, as the name suggests, sound like whistling falling tones. Spectacular auroral displays amplify the so-called dawn chorus—chirpy waves that sound similar to birds in the morning (listen to a sample here).

One of the most mysterious of these noises is plasmaspheric hiss—an ever-present sibilance in the inner regions of Earth’s magnetic field. It sounds like pure static spanning 100 Hz to several kilohertz, a frequency range roughly equivalent to that produced by the middle third of a piano. Scientists know that plasmaspheric hiss plays a crucial role in shaping the structure of Earth’s radiation belts, disrupting them by knocking their energetic particles out into the atmosphere.

However, the source of the hiss is unknown. One theory says that it is the direct result of spiraling electrons high over Earth’s equator. Others propose that it consists of the remnants of distant whistlers or chorus waves that devolve into incoherence, like the expressionless chop far out at sea.

Previously, scientists assumed that this hiss was random white noise with no coherent features. However, when Summers et al. analyzed NASA satellite measurements of the hiss from 2013, they found something quite different. After breaking down the noise into its spectrum of frequencies, they discovered barely detectable rising and falling tones similar to the whistlers, at frequencies rising to roughly middle C and falling for about two octaves. The authors say that this detection was made possible by the high resolution of the instruments on the satellites, NASA’s Van Allen Probes, and their particularly useful orbit, which keeps them mostly within Earth’s radiation belts.

Although the waves within plasmaspheric hiss resemble whistler tones and may share similarities in mathematical wave theory, the physical mechanism that generates the hiss is still wide open for debate. The authors expect that this fine structure will renew interest in the subject and may contain the clues to pin down its source. (Journal of Geophysical Research: Space Physics, doi:10.1002/2014JA020437, 2014)

—Mark Zastrow, Freelance Writer

Citation: Zastrow, M. (2015), New clues to mysterious hiss in Earth’s plasmasphere, Eos, 96, doi:10.1029/2015EO037985. Published on 26 October 2015.