A decade of measurements, which turned Swarm into a lightning hunter

The ESA Swarm mission has been designed to provide high-precision registrations of the Earth's magnetic field. Since its launch, the constellation, consisting of three LEO satellites, measures the magnetic signals coming from Earth's core, mantle, crust, oceans, ionosphere, and magnetosphere. Nearly a decade of Swarm observations, allowed us to expand main goals of the mission and improve our capabilities of detecting magnetic field fluctuations triggered by powerful thunderstorms.

Lightning can generate ultra low frequency fluctuations that leak into the upper ionosphere. This means that some lightning bolts are so powerful that they trigger disturbances in Earth's magnetic field and propagate hundreds of kilometers upwards from the thunderstorm, reaching the altitude of Swarm’s orbit. Thanks to two magnetometers,  the Absolute Scalar Magnetometer ASM and the Vector Fluxgate Magnetometer VFM, Swarm turned into a robust lightning hunter. 

Gathered analysis, utilizing mainly the VFM registrations, show that averaged amplitude of lightning generated fluctuations reaches magnitude of 0.5-1.3 nT (peak-to-peak for the scalar field), while typical time delay between the lightning occurrence and the satellite detection is 0.2-0.5 s and this indicates that Swarm detects rather a direct propagation of lightning-generated disturbance than effects of excitation of the ionospheric Alfvén resonance, which would require a longer time scale.

Since Swarm provides full representations of the magnetic field components, we transform data to the so-called MV frame to extract the maximum amplitude of a given fluctuation regardless of the lightning-satellite mutual orientation and wave polarization. In such a way, Swarm data help to comprehensively characterize  wave properties of detected fluctuations, and this is one of fundamental tasks, especially in the large variety of magnetic field disturbances caused by various types of natural hazard phenomena.