|Earth AKR in Stereo|
Cluster is a set of four Earth-orbiting spacecraft that fly in a formation that allows us to detect the differences between time and space features of the signals we measure. In this instance the wideband plasma wave instruments, WBD, on Cluster 3 (Samba) and Cluster 4 (Tango) simultaneously measured a radio signal known as "Auroral Kilometric Radiation" or AKR, allowing for a spatially separated stereo audio recording. AKR radio emissions are associated with the energetic electrons that collide with molecules high in the atmosphere above Earth's polar regions and produce the aurora. These signals occur at frequencies far above the range of human hearing, but this mode of the WBD instrument samples the frequency range from 251 kHz to 261 kHz at a rate that can be played back as sound in the audible range from 1 to 11 kHz. Additionally, this recording is slowed down by a factor of two so that the 18-second sample takes 36 seconds to play and the top frequencies are shifted down to 5.5 kHz. Complex structure can be heard in AKR samples, and combining the measurements from all four spacecraft yields new insights into how these structures are distributed in time and space.
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The Radio and Plasma Wave Group, Department of Physics & Astronomy, College of Liberal Arts & Sciences.