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Schlagwörter:
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Zusammenfassung:
Whistlers are generated by the electromagnetic signal from lightning discharges leaking into the ionosphere. They propagate upward through the ionosphere where they are detected by the Absolute Scalar Magnetometer (ASM) of the satellites of the Swarm mission at orbital altitudes (Alpha at ~475 km, Bravo at ~510 km). Spectral analysis allows us to determine the propagation group delay of whistlers as function of their frequency.
We investigate a theoretical development of the refractive index under the cold plasma hypothesis within the Extremely Low Frequency (ELF) band. It suggests a proportionality relation between the group delay of the whistler and the integral of the square root of the charged particles densities along the propagation path. We name this value the Total square-Root Electron Content (TREC).
Ray tracing through an ionosphere based on the International Reference Ionosphere 2016 (IRI), in a dipolar magnetic field derived from the IGRF-13, provide us with an estimate of the propagation path. In conjunction with the whistler analysis from Swarm, we are able to calculate the TREC for a given whistler.
This method is tested on 397 whistlers that occurred between April 2020 and December 2022, within 500 km of one of three active ionosondes: Ramey, Boa-Vista and Santa-Maria. TREC of Swarm are compared against TREC computed by integration of the ionospheric profiles from IRI and from the ionosondes. Improved ionospheric profiles with topside correction using in-situ measurements of the plasma density by the Swarm Langmuir probes are also tested. Encouraging results are obtained, which will be discussed.
