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Thread: EHD Model of Dust Devils

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    EHD Model of Dust Devils

    Dust devils are typically considered to be simple convective currents, sorta like Rayleigh-Benard cells, except on steroids. Under a cloudless sky, the surface of the Earth gets heated, and a layer of hot air builds up. Then all of a sudden all of that hot air starts rising. In extreme cases, the updraft can exceed 30 m/s, and can hoist many tons of dust over .5 km above the ground. The dust makes a significant contribution to the volume of airborne particulate matter in the atmosphere, which affects weather by providing the nuclei of condensation for rain (i.e., natural cloud seeding).

    Examined more critically, dust devils shouldn't be possible with the given mechanisms. It is true that hot air rises. But that's the problem. Hot air rises. It doesn't get heated, and want to rise, but then realize that if it just waits around for a little while, it can rise all at once in a dust devil. No, it just rises. In a related thread, I presented a model of inferior mirages, which occur in the same conditions, and wherein the same reluctance for hot air to rise is displayed. The Sun photo-ionizes the surface of the Earth, leaving it positively charged. Electrons liberated from the surface are captured by molecules in the air (especially water vapor), establishing a Coulomb force between the air and the ground. This Coulomb force offsets the buoyancy of the air, allowing an unusual amount of thermal potential to develop without producing an updraft. The significance previously considered was that the hotter air, being less dense, is capable of refracting sunlight, resulting in an inferior mirage. In the present thread, another implication will be considered: an all-or-nothing updraft.

    If the surface has been heated to over 40 C, and the air near the surface has been heated to over 30 C, there is the potential for an updraft into the surrounding air, which is typically far cooler (< 20 C). But the electric force is 39 orders of magnitude more powerful than gravity (which is the fundamental force responsible for cool, dense air falling and for hot air, with less density, rising). So how is an updraft possible? The Coulomb force should prevent the hot air from rising, until the Sun goes down and the ionization process stops, at which time the opposite charges will recombine faster than they are getting created.

    Yet there is a limit to how strongly ionized the surface of the Earth can become. Incoming EM waves can liberate weakly-bound electrons from molecules directly exposed to the sunlight, but once a net charge has developed across the surface of the Earth, the electric force will prevent the escape of electrons into the air. This limits the amount of space charge that can develop. Yet the temperature of the air can continue to increase. If the buoyancy of the hot air overpowers its electrostatic attraction to the ground, a small parcel of air will break away, ascending to an altitude appropriate for its low density. As the electric force falls off with the square of the distance, the rising parcel will experience less downward force the higher it goes.

    The vacuum created at the surface by the first rising parcel will be filled by air sliding along the ground, still bound by the electric force, yet responding horizontally to the low pressure. Once the lateral inflow achieves the point at which the first parcel rose, the low pressure aloft, combined with the parcel's buoyancy, overpowers the electric force, allowing the convergent inflow to rise as well. So the first rising parcel triggers a continuous flow of air along the ground and then upward at the point of convergence.

    Since the hot air layer is so shallow (~10 cm), we might think that the updraft will quickly run out of thermal energy in its immediate vicinity. If cool air from above is drawn in, the updraft will fail. But charged air has a lower viscosity, because electrostatic repulsion prevents the particle collisions that instantiate friction.24,25,26 So the ionized air will flow more easily than the cool, neutrally charged air above it. Hence the vacuum near the ground will be filled with more hot air, even if the hot air has to travel a greater distance than cool air from above. This creates the possibility of hot air from a broad area flowing along the ground to get into a single, organized updraft.

    If the updraft is stationary, its intensity and duration are limited by the distance from which hot air can be drawn, with its friction still being lower than the friction of drawing in higher-viscosity air from above. If the terrain is flat and smooth, we can expect the effective inflow to come from further away, as skin friction will result in boundary vortexes that will be relatively small, and turbulence will not reach the full depth of the hot air layer. Hence the laminar flow in the hot air layer will present little friction. Rougher surface conditions will reduce the effective inflow radius of the updraft.

    If the updraft can get more hot air from one direction than from another, it will move in that direction, and instead of the air moving to the updraft, the updraft will move to the air that can rise. This removes the restriction on the duration of the updraft. Regardless, the intensity of the updraft is still limited by the rate at which it can pull air from the hot air layer without pulling in cool air from above.

    If the converging lines of motion are not perfectly radial, a spiraling inflow pattern will emerge, and the updraft will become a vortex. The direction of the rotation is random, and it is common for both cyclonic and anti-cyclonic dust devils to occur in the same area, with the same conditions. It is also possible for the same dust devil to switch directions and continue on. This is true in both the northern and southern hemispheres of the Earth. Hence there is no reason to believe that the rotation is being encouraged by the Coriolis effect, or by Lorentz force acceleration, both of which would prefer one direction over another, and which would be hemisphere-specific.

    If the air moves fast enough, it will start to kick up dust at the surface. The dust has the charge of the Earth. When mixed with the oppositely charged air, the net charge of the hot air becomes zero, completely freeing the air from its electrostatic attraction to the Earth. Hence the dusty air will rise far more vigorously than the clear air that initiated the process. This explains the rapid intensification in the instant that the dust devil becomes visible due to airborne particulate matter.

    The corollary also appears to be true, that while the presence of dust might help free the space charge from its attraction to the ground, the space charge might be responsible for hoisting far more dust into the atmosphere than would be predicted simply on the basis of wind speeds and durations.214,215

    The charge separation mechanism (ionization from sunlight), combined with the consolidated convection, accounts for the huge voltages detected in dust devils. The 10 kV/m potentials that have been measured are typically attributed to triboelectric charging from particle collisions within the vortex, but this doesn't explain why there would be any triboelectric charging at all when particles of similar constitution collide, nor why there would be that many collisions anyway in the laminar inflow to a vortex, nor why other vortexes of similar intensity (such as gustnadoes) do not develop similar potentials.

    It is not likely that the reduced pressure inside the vortex (which will lower the electrical resistance of the air) will result in any significant "fair weather current" in the presence of the fair weather electric field (100 V/m), as some have contended. It is also not likely that at the distances and speeds in question, there is any significant increase in temperature due to skin friction.

    As concerns dust devils on Mars, the question is not so much a matter of how a temperature inversion occurred, with a layer of hot CO2 under cool CO2, and where the total buoyancy, if all consolidated into a vortex, could create a dust devil. Rather, the first and biggest question is how that much work could be performed at all in an atmosphere that is so thin. This can be answered with the same mechanism. A charge separation, instantiating an electric field, could create a space charge in which the atmospheric pressure is far greater than normal. Then, if surface heating increases the buoyancy beyond that which can be contained by the electric force, an updraft occurs. In these conditions, there is no cooler layer above the hot layer, so the intensity of the dust devil is not limited to how fast it can pull in hot CO2 without pulling in cool CO2 from above as well, extinguishing itself in the process. Hence dust devils of great size and speed become possible.

    Positing the existence of a major charge separation that gets neutralized by the mixing of charged CO2 with oppositely charge dust also explains the flashes that have been observed at the base of Martian dust devils. Heat from the discharges might also contribute to the buoyancy of the updraft, though there's no reason to suspect that this is a necessary condition.

    Dust devil on Mars, taken by rover Spirit on sol 486, courtesy NASA. devil).gif


    24. Nishida, K., Kiriyama, K., Kanaya, T., Kaji, K., and Okubo, T., 2004: Theoretical calculation of the reduced viscosity of aqueous suspensions of charged spherical particles. Journal of Polymer Science Part B: Polymer Physics, 42 (6): 1068-1074.

    25. Wood, T. L., Corke, T. C., and Post, M., 2010: Plasma actuators for drag reduction on wings, nacelles and/or fuselage of vertical take-off and landing aircraft. United States Patent Application, 20100224733.

    26. El-Khabiry, S., and Colver, G. M., 2011: Drag reduction by DC corona discharge along an electrically conductive flat plate for small Reynolds number flow. Physics of Fluids, 9(3): 587.

    214. Sanders, R., 2002: Stalking Arizona dust devils helps scientists understand electrical, atmospheric effects of dust storms on Mars.

    215. Renno, N. O., Abreu, V. J., Koch, J., Smith, P. H., Hartogensis, O. K., De Bruin, H. A. R., Burose, D., Delory, G. T., Farrell, W. M., Watts, C. J., Garatuza, J., Parker, M., and Carswell, A., 2004: MATADOR 2002: A pilot field experiment on convective plumes and dust devils. Journal of Geophysical Research, 109, E07001.

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