BEXUS30 – ELFI: Measuring Schumann resonances in the atmosphere

The ELFI project was one of nine BEXUS experiments carried in two stratospheric balloons in 2021. The aim of the experiment was to develop a system for the non-stationary measurement of electromagnetic waves in the extremely low frequency range. The Schumann resonances that are part of this range are especially important for meteorological research. For the planned use of the system on a stratospheric balloon, various requirements and aspects regarding the measurement environment had to be considered during the development. The system is based on a magnetic loop antenna connected to a signal processing unit, the Analog Front-End. The antenna has special characteristics to enable the measurement of Schumann resonances. Due to the necessary high sensitivity of the antenna, a deployment mechanism was developed to lower the antenna for the measurement, thus reducing the influence of interference from the electronics or actuators of other experiments on the gondola. After the balloon is launched, the mechanism is extended, and the antenna is lowered below the gondola. The Analog Front-End has several stages that filter, amplify and digitalize the signal measured with the antenna. An on-board computer, built from reliable general-purpose hardware, performs the measurement, organizes and stores the measurement data, and provides communication with the ground station. Hence, monitoring and control of the experiment through the ground station was possible. In addition, an algorithm for automatic gain control was integrated to allow flexible measurement of different amplitudes. In several testing periods the system was validated for functionality and reliability. Through numerous preliminary tests, frequencies from reference sources could be detected, e.g., 50 Hz of the power supply network or 16.67 Hz of the railroad power supply. Underground measurements confirmed that the system is suitable for detecting low frequencies. Furthermore, the system was tested and confirmed to be usable under extreme conditions like low temperatures and low air pressures. The developed deployment mechanism with scissor arms was proved to be robust and flexible. Both hardware and software worked as expected and are reliable and adaptable to different conditions. During final tests in an almost interference-free area our system was able to record optimal signals, in which the Schumann resonances could be detected. Based on these successful results, the system was ready to be deployed on the stratospheric balloon to perform measurements in the atmospher