Van Allen Probes EMFISIS Waves
NYE Whistlers 2015-12-31

We could "hear" these New Year's Eve fireworks high above the planet! These signals were recorded by the Van Allen Probes A spacecraft EMFISIS Waves instrument on New Year's Eve, December 31, 2015, at about 12:04 UTC. The measurements of three orthogonal magnetic antennas Bu, Bv, and Bw were combined to make the 12 second stereo audio recording.

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. As a bonus, there are a few distinct proton whistlers which are the low frequency rising tones just following the more common falling tone. These proton whistlers rise in frequency to just below the proton cyclotron frequency, a characteristic frequency of the local ionized gas.

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.