Helioseismology in a bottle: an experimental technique

Measurement of the differential rotation of the Sun's interior is one of the great achievements of helioseismology, providing important constraints for stellar physics. The technique relies on observing and analyzing rotationally-induced splittings of p-modes in the star. Here we demonstrate the first use of the technique in a laboratory setting. We apply it in a spherical cavity with a spinning central core (spherical Couette flow) to determine the azimuthal velocity of the air filling the cavity. We excite a number of acoustic resonances (analogous to p-modes in the Sun) using a speaker and record the response with an array of small microphones and/or accelerometers on the outer sphere. Many observed acoustic modes show rotationally-induced splittings which allow us to perform an inversion to determine the air's azimuthal velocity as a function of both radius and latitude. We validate the method by comparing the velocity field obtained through inversion against the velocity profile measured with a calibrated hot film anemometer. The technique has great potential for laboratory setups involving rotating fluids in axisymmetric cavities, and we hope it will be especially useful in liquid metals. Acoustic spectra showing rotationally induced splittings. Top figure is the spectra recorded from a microphone near the equator and lower figure from a microphone at high latitude. Color indicates core's rotation rate in Hz.