Effects of Tilt and Demodulation on Balloon Born Electric Field Measurements

Balloon-borne electric field measurements are one of the few ways to observe the charge geometry and magnitude inside thunderclouds. These instruments have used the same basic design since the 1970s, which simply measures the charge difference on two metal spheres. To get the direction of the field from this measurement, the spheres are spun about a horizontal axis using a motor, and also rotated about the vertical axis using vanes. This modulates the vertical electric field by the spin rate, and the horizontal electric field by the rotation rate. It is then possible to extract all three Cartesian components of the thunderstorm electric field from this modulated signal. However, while there are multiple demodulation methods, none of them are well documented. In addition, the demodulation methods all assume that the package remains vertical, something which may not happen in a turbulent storm environment. In this presentation, we will review how to demodulate the signals recorded by these instruments, and investigate the effects of tilt and pendulum motion on both the modulated signals and demodulated measurements. Examples will be shown from recent field campaigns, as well as from simulated soundings through simulated storms.