Van Allen Probes EMFISIS Waves
May the Fourth Be with You! 2017-05-04

They may sound like movie sound effects, but these are natural low-dispersion whistlers recorded by the EMFISIS Waves instruments on NASA's Van Allen Probes B spacecraft. Recorded on May 4th (aka Star Wars day) 2017 at about 22:29 UTC combining the measurements of three orthogonal magnetic search coils with Bu in the right channel, Bv in the left channel, and Bw mixed equally between the two. Close your eyes and imagine yourself on the spinning spacecraft with the signals propagating up from the Earth, or, if you like, imagine sitting in the gun turret of some sci fi spaceship spinning to fire your blasters. (Of course, you can hear the battle in space, thanks to your plasma wave audio system!)

These signals, called whistlers, are created by lightning discharges in Earth's atmosphere which then propagate back and forth along magnetic field lines with lower frequencies being delayed as they travel through the thin charged gas of space. The resulting spread in frequency produces a whistling tone with the dispersion related to the quantity of plasma the signal has traversed. In this case, the spacecraft was fairly close to Earth, so there is very little dispersion. The interesting low-frequency component heard frequently in this recording consists of proton whistlers, caused by protons spiraling along the magnetic field lines.

The video presents a frequency-time spectrogram for the three antennas, with a moving cursor that shows the time position of the audio track. Time advances from left to right along the horizontal axis, frequency ranges from low to high frequencies along the vertical axes, and the amplitude of the signals is color coded with blue indicating weak signals and red indicating strong signals.