© ESA/Yuri Shprits
European Space Agency Cluster II satellites observe equatorial noise waves inside the Earth's magnetosphere.
Since the early 1970s, orbiting satellites have picked up on noise-like plasma waves very close to the Earth's magnetic field equator. This "equatorial noise," as it was then named, seemed to be an unruly mess of electric and magnetic fields oscillating at different frequencies in the form of plasma waves.
Now a team from MIT, the University of California at Los Angeles, the University of Sheffield, and elsewhere has detected a remarkably orderly pattern amid the noise.
In a region of space about 12,000 miles from Earth's surface, two spacecraft separated by a narrow patch of space—about as wide as Rhode Island—identified a region where the tangle of plasma waves gives way to a very regular structure
. The scientists detected the invisible structure using a spectrogram—a visual representation of the spectrum of frequencies in space. Through this lens, they observed a stack of 13 equally spaced, zebra-like stripes
The team also observed something more curious: Each stripe, or plasma wave, appeared to be a multiple, or harmonic, of a proton gyrofrequency—the frequency at which protons gyrate around the Earth's magnetic field line
. The researchers performed some calculations to estimate the growth rate of each plasma wave, and discovered that the very orderly waves likely originated from the gyrating protons.
Yuri Shprits, a visiting associate professor in MIT's Department of Earth, Atmospheric and Planetary Sciences, says the striped structure could indicate a region in space in which new and different interactions may take place
"This structure is pretty close to the Earth, which is important because people want to understand the environment where satellites operate," Shprits says. "Usually plasma undergoes a number of different instabilities, and waves tend to move from one region in space to another, so everything you see is noisy, very short-lived, and on smaller scales. But this structure seems to be very persistent, highly coherent in space, and was remarkably organized and structured, which we didn't know could exist to such high degree."
Shprits and his colleagues, including Benjamin Weiss, a professor of planetary sciences at MIT, have published their results today in the journal Nature Communications