Swarm Absolute Scalar Magnetometer burst mode: from instrument validation to routine observation of intense lightning activity

Over the past 10 years, the ESA Swarm mission led to many scientific successes sometimes way beyond its primary science targets. One such example is the unanticipated science allowed by the experimental 250 Hz burst mode magnetic scalar data provided by the Absolute Scalar Magnetometers on board the satellites. This burst mode was originally meant to be run briefly for in-orbit calibration and validation activities during the initial months of the Swarm mission. However, and despite the fact that the 250 Hz sampling rate can only access the ELF part of the electromagnetic spectrum, numerous whistler signals could be detected. After carefully assessing the possibility of routinely detecting and characterising such whistlers, it was thus next decided to conduct campaigns of one-week duration every month on Swarm Alpha and Bravo. These started in 2019 and still continue today, the corresponding Burst mode data now being a routine product of the mission.In this presentation, we will review the main scientific results of these campaigns: geographical and temporal distribution of whistler events as well as the detectability of whistler signals, which we assessed using joint campaigns on the Alpha and Bravo satellites during the recent counter-rotating phase of the Swarm mission. Using ground ionosondes and in-situ electron density measurements we also demonstrated the promising possibility of using whistlers to measure ionosphere plasma density parameters along their path up to the Swarm satellites. Finally, using data from the ground based ELF measurements from WERA, and data from the lightning detection network WWLLN, the originating strikes could also be identified. We found that only the most powerful lightning strikes produce detected whistlers, and that these strikes can sometimes occur several thousands of kilometres away from the satellites.