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1 HEPPI - LATMOS LATMOS - Laboratoire Atmosphères, Milieux, Observations Spatiales 2 SPRL - Space Physics Research Laboratory Ann Arbor

Abstract : We report results of a field campaign conducted in the Nevada desert with a suite of electric field instruments consisting of a field mill FM and a short dipole antenna SDA. Further, we show that a combination of the measurements of these two instruments allows the estimation of the electric resistivity of air, an important quantity that is extremely difficult to measure near the Earth-s surface. The electric resistivity of air is found to vary between 1.5 · 1013 and 6⋅1013 Ω m and to correlate with changes in electric field. Vertical DC electric fields with amplitudes up to 6 kV m-1 were observed to correspond to clouds of dust blowing through the measurement site. Enhanced DC and AC electric fields are measured during periods when horizontal wind speed exceeds 7 m s-1, or around twice the background value. We suggest that low frequency emissions, below ~200 Hz, are generated by the motion of electrically charged particles in the vicinity of the SDA electrode and propose a simple model to reproduce the observed spectra. According to this model, the spectral response is controlled by three parameters, i the speed of the charged particles, ii the charge concentration and iii the minimum distance between the particle and the electrode. In order to explain the electric fields measured with the FM sensors at different heights, we developed a multi-layer model that relates the electric field to the charge distribution. For example, a non-linear variation of the electric field observed by the FM sensors below 50 cm is simulated by a near surface layer of tens of cm that is filled with electrically charged particles that carry a predominantly negative charge in the vicinity of the soil. The charge concentration inside this layer is estimated to vary between 1012 and 5 · 1013 electrons m-3.

Keywords : Air electric resistivity Dust motion Dust electric charging Atmospheric electricity Electric field

Autor: Elena Seran - Michel Godefroy - Nilton Renno - Harvey Elliott -



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