干旱区生态环境知识资源中心

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). Furthermore, 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 center dot 10(13) and 6 center dot 10(13) Omega m and to correlate with changes in electric field. Vertical DC electric fields with amplitudes up to 6kVm(-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 7ms(-1), or around twice the background value. We suggest that low-frequency emissions, below similar to 200Hz, 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 multilayer model that relates the electric field to the charge distribution. For example, a nonlinear variation of the electric field observed by the FM sensors below 50cm is simulated by a near-surface layer of tens of centimeters 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 10(12) and 5 center dot 10(13) electrons m(-3).