An Array of Low-Cost, High-Speed, Autonomous Electric Field Mills for Thunderstorm Research

The atmospheric electric field is an important research parameter in understanding storm electrification and energy exchange between lightning and the atmosphere across the globe. The near-surface electric field can range from a few V/m (order of 10-100 V/m), mainly produced by the currents in the global electric circuit and local charge perturbations, to tens of kV/m in the presence of electrified clouds. The electric field mill (EFM), a variable capacitance electrometer, has been the instrument of choice in the atmospheric electricity community studying phenomena associated with the atmospheric electric field. The EFM is particularly useful in following storm movement and evolution, monitoring the fair-weather electric field at distant locations, and measuring the vertical electric field inside clouds with EFM deployments on balloons. In this paper, we describe a new electric field mill ground-based design, which focuses on lowering the manufacturing and operational costs of doing research with an array of EFM instruments while maintaining the scientific capabilities offered by past designs and commercially available devices. The theory of operation, data processing, and calibration of the instrument are also described. Example data from the first generation of these new field mills, deployed in the RELAMPAGO campaign in Argentina, are presented here. The RELAMPAGO deployment and data set illustrate important strengths of this design, for example, cost, autonomy, longevity, and measurement quality.