Whistlers in the ELF recorded by Swarm satellites: results from recent regular ASM burst sessions

Every month since August 2019 the ASM instrument onboard one of the Swarm satellites has been operated in burst mode during one week. ASM measures the intensity of the magnetic field at the satellite position, at about 450 km height for Swarm A and 500 km for Swarm B. From the nominal 1 Hz, the sampling frequency has been increased to 250 Hz, to observe Extremely Low Frequency (ELF) signals.The most remarkable phenomena observable in this part of the electromagnetic spectrum are whistlers, generated by the lightning activity occurring in the troposphere.

Due to the local time drift of Swarm high-inclination satellites, this is the first time that a collection of whistlers in the ELF is recorded worldwide at any local time by a single satellite mission.

A large variability in the number of events has been observed, and comparison with the very limited ASM burst dataset available for high solar activity, seems to indicate a strong solar activity dependence on the occurrence of these events.

Whistler dispersion as received at the satellite's position depends on the propagation through the ionospheric plasma, increasing with an increase of electron density.

A selection of remarkable whistlers events have been analysed. We identified the originating lightning strike recorded on the ground by the World ELF Radiolocation Array (WERA) and characterised the wave propagation through the ionosphere. For that purpose, events recorded within 600 km from a ground ionosonde have been analysed, providing electron density profiles needed for ray-tracing calculation of the propagation path up to the satellite location. The in-situ measurements of electron density made by Swarm were used to adjust the topside profile. Comparisons with the IRI model, constrained or not with experimental data indicate that whistler dispersion is sensitive to the F2 peak electron density.