Experimental Characterisation of the Electric and Magnetic Fields Generated by Dust Devils

Lofted particulate in dust devils becomes charged through triboelectrification, that is, the exchange of charge in collision between grains. Electric fields from charged dust were first detected in the mid 17^th century, with quantitative measurements recording in the region of kilovolts per metre. Magnetic field observations of dust devils are much less common, with the only published terrestrial measurement from 2001 in Arizona. The most complete magnetic field dataset associated with dust devils comes from NASA’s InSight mission to Mars, with 1200 sols of near-continuous observation, and over 15000 convective events detected, likely to be dust devils.  

To better understand the expected electric and magnetic fields generated by these aeolian features, a new apparatus was developed, building upon previous experimental work. The Terrestrial Experimental appaRatus for Investigating the Electric and magnetic fields of dust devils (TERIE) consists of a multi-instrumented 1000 mm diameter, 1200 mm tall tank, lined externally with grounded aluminium foil to act as a Faraday cage, and internally with sand to reduce the impact of tribocharging from particle-wall collisions. 

The apparatus records electric field strength at 4 vertical positions, and the (vector) magnetic field at 3 vertical positions. Through photodiodes, the optical thickness of the dust devil column can be evaluated, and offline sampling of the suspended particles can be used to understand the distribution through the profile of the simulated event. By incorporation of different mast positions, the radial profile of the generated field can also be investigated. 

Initial results from the new experimental apparatus show electric fields exceeding 40~kVm^-1 were generated by the rotation of sand, with the distribution of the field broadly matching that expected from simulation. Some low frequency, sub-nanotesla variations in magnetic field were detected in the presence of rotating charged sand, consistent with expectations from models and previous experiments.