A new electric field mill array with each of the mill’s rotor controlled precisely by a GPS module: Equipment and initial results

We have newly designed an electrostatic sensor, called an electric field mill (EFM), to simplify the estimation of the charge position and charge amount transferred by lightning discharges. It is necessary for this remote estimation of the transferred charge to measure electric field changes caused by charge loss at the time of a lightning strike at multiple locations. For multiple-station measurement of electric field changes, not only speed but also phase for exposure and shielding of the sensing plates inside each EFM of the array should be synchronized to maintain the sensitivities of the deployed instruments. Currently, there is no such EFM with specified speed and phase control performance of the rotary part. Thus, we developed a new EFM in which the rotary mechanism was controlled consistently to within 3% error by a GPS module. Five EFMs had been distributed in the Hokuriku area of Japan during the winter season of 2022–2023 for a test observation. Here we describe the design and a simple calibration method for our new EFM array. Data analysis method based on the assumption of a simple monopole charge structure is also summarized. For validation, locations of assumed point charges were compared with three-dimensional lightning mapping data estimated by radio observations in the MF-HF bands. Initial results indicated the validity to estimate transferred charge amounts and positions of winter cloud-to-ground lightning discharges with our new EFM array.