The electric field of a New Mexico dust devil

Abstract

On August 21, 1962, a continuously recording inverted field mill, installed on an open semidesert area near the NMIMT campus, recorded the passage of a fairly large dust devil at a time when I was present to make visual observations. Its closest approach to the field mill was about 450 m, established by reference to power line poles and roads. The bulk of the visible dust cloud was contained roughly in a vertical cylinder estimated to have a diameter of 20 m and a height of 120 m. Experience acquired in observation of many other dust devils suggests that the vortex must have extended far above the height of 120 m; in this case the part above this height was not visible against the mountain background and the dust density in it was probably relatively low. The bulk of the dust seen was probably outside the vortex proper.The transcribed potential gradient record for this event is shown as the solid curve in Figure 1, where it is seen that the gradient was depressed for 10 rain; the lowest value was about 111 v m -z below the level of 49 v m -• prevailing before and after the event. The average general wind speed was about 4 m sec -•, measured by an anemometer near the field mill. The dotted curve of Figure I is a theoretical one representing the effect of a straight line passage, at 450 m, of a simple vertical dipole with a ground speed of 4.0 m sec -z. The electric moment of this hypothetical dipole is 1.13 coul m. (No implication regarding the actual structure of the dust devil is intended at this point;with a structure having a height of the order of 100 m, at distances of 450 m and greater, it is only the nei electric moment of the structure and its image that is of concern.) The agreement between the two curves preceding the minimum is excellent; the moderate divergence beginning about 2 rain after the minimum suggests a diminishing intensity of the dust devil, or an increasing ground speed, though the maxi-mum amount of the divergence is within the range of the 'noise' present.The simplest model having the apparent electric moment would be a uniform distribution of negative space charge density in the approximately cylindrical dust cloud (taken together with its image, of course). If the cloud dimensions are those estimated, this model is required to contain a space charge density of 1.6 X 106 el cm -3 (el --elementary charge).If it is assumed that the diameter was underestimated and that it actually was as large as 30 m, the corresponding space charge density is about 0.7 X 106 el cm -3. If the lower part of the visible dust column was occupied by positive space charge, a possibility discussed below, the negative charge density in the remaining part of the column would have to have a value even larger. Space charge densities of the order of 106 el cm -3 may seem at first glance to be improbably high, in view of the densities usually encountered in the atmosphere. It is quite possible, however, that the dust particle density in o -20

Quasi‐electrostatic field analysis and simulation of Martian and terrestrial dust devils

“…The time evolution of space charge density is very similar to the initial accumulation of the thunderstorm space charge in the QE heating model before a conventional breakdown is reached [Pasko et al, 1997]. This maximum electric field for a typical terrestrial dust devil is difficult to compare with experimental measurements, all of which have strongly saturated at a few kV/m [Freier, 1960;Crozier, 1964Crozier, , 1970Farrell et al, 2004], but the predicted value of 150 kV/m is not inconsistent with these measurements. Very recently, Jackson and Farrell (submitted man- Melnik and Parrot [1998], the dissipation of dust grain charge due to atmospheric conductivity is fully considered in our FE simulation.…”

“…The height and diameter of this generator region can be specified arbitrarily and we assume that the generator region fills the entire dust devil. Although more complicated charge structures can be included in this model, the dipole model of a dust devil is thought to be reasonable and has been used to estimate space charge densities [Crozier, 1964;Farrell et al, 2004]. The dipole charge model has a uniform distribution of negative space charge density at the dust devil top and a uniform distribution of positive space charge density at the dust devil bottom in a modeled cylindrical dust cloud.…”

“…[Crozier, 1964[Crozier, , 1970Farrell et al, 2004]. To study the evolution of field and charge in a dust devil and how the tribocharging process is saturated by atmospheric conductivity that controls the exponentially growth of the maximum in-devil electric field, we perform following simulations of quasi-static electrification with our finite element model for Martian and terrestrial dust devils of different size, conductivity, and time rate of charging.…”

“…Most recently, Jackson and Farrell (submitted manuscript, 2005) measured near 120 kV/m QE fields of dust devils with a carefully calibrated electrometer. A selfregulating mechanism as suggested by Crozier [1964], or a saturation mechanism such as the instantaneous charge dissipation and electric field relaxation due to atmosphere electric conductivity should be introduced properly into the analytical or numerical models [Desch and Cuzzi, 2000].…”

Electric and magnetic signatures of dust devils from the 2000–2001 MATADOR desert tests

“…Terrestrial dust devils provide some of the strongest indirect evidence for Martian dust devil electrification. Freier [1960] and Crozier [1964] found that modest terrestrial dust devils of $10-m diameter and $100-m height possess a substantial DC electric field on the order of 0.6 kV/m. These fields were consistent with the cloud possessing a macroscopic electric dipole moment, M, on the order of $1 C-m oriented toward the nadir.…”