[1] An apparatus has been fabricated to simulate terrestrial and Martian dust devils. Comparisons of surface pressure profiles through the vortex core generated in the apparatus with both those in natural dust devils on Earth and those inferred for Mars are similar and are consistent with theoretical Rankine vortex models. Experiments to determine particle threshold under Earth ambient atmospheric pressures show that sand (particles > 60 mm in diameter) threshold is analogous to normal boundary-layer shear, in which the rotating winds of the vortex generate surface shear and hence lift. Lowerpressure experiments down to $65 mbar follow this trend for sand-sized particles. However, smaller particles (i.e., dust) and all particles at very low pressures ($10-60 mbar) appear to be subjected to an additional lift function interpreted to result from the strong decrease in atmospheric pressure centered beneath the vortex core. Initial results suggest that the wind speeds required for the entrainment of grains $2 mm in diameter (i.e., Martian dust sizes) are about half those required for entrainment by boundary layer winds on both Earth and Mars.

Abstract. Discovery of dust devil vortices in Mars

In-situ (mobile) sampling of 33 natural dust devil vortices reveals very high total

Terrestrial dust devils are known to be triboelectric generators: electric charge is generated and exchanged via the mixing and colliding of moving grains. Static electric fields from the structures have been previously reported, this in excess of 1/2 kV/m at a couple hundred meters from the devil. In this work, we report on AC magnetic measurements made during the passage of an intense ∼10‐m wide, couple hundred‐meter tall dust devil in the Nevada desert. The devil‐related magnetic activity appears in two forms: Impulsive extremely‐low‐frequency (ELF: 30–300 Hz) static discharges to the instrument as the sensor became immersed directly into the devil's electrified dust and ultra‐low‐frequency (ULF: 3–30 Hz), continuous emissions that were sensed remotely as the storm approached and receded from the mobile station. The latter is a new finding not previously reported. Such measurements not only prove that individual dust grains within the devil are charged, but that the grains are transported in bulk to form an extended, coherent radiation source.

a b s t r a c tDust devils -convective vortices made visible by the dust and debris they entrain -are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites.We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements.Daily (10:00-16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction.The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain.