ILGEW project intends to improve the scientific returns of the ESA Earth Explorer Swarm Mission that measures the ever changing Earth magnetic field using a constellation of Low Earth Orbiter (LEO) satellites.
We analyse events recorded during specific measurement campaigns of one of Swarm instruments, the Absolute Scalar Magnetometer (ASM). These are whistlers, electromagnetic signals resembling whistles, that are received by the satellites. Whistlers are excited by the lightning activity on the Earth surface and they can propagate deeply into the Earth ionosphere, interacting strongly with the ionised medium. We focus on the Extremely Low Frequency band accessible by Swarm instruments, mainly between 20 and 125 Hz. It is a part of the electromagnetic spectrum that has not yet been systematically studied from space and it still present many challenges for a complete understanding of the ground-ionosphere interactions.
After a lightning strike, the whole electromagnetic spectrum is excited by a transient short signal. When this signal crosses the boundary between the neutral atmosphere and the ionosphere, at about 90 km height, this frequency content is spread in time by its interaction with the electrons and ions present in the ionosphere. These charged particles are bound to the Earth’s magnetic field lines, thus they produce various changes of the signal.
Whistlers received by Swarm satellites can be characterised by their dispersion, D, which is a measure of the relation between the signal frequency and its propagation time inside the ionosphere. The state of the ionosphere at the time of this event is therefore a key parameter to determine the whistler’s dispersion: day/night conditions, low-high solar activity, all contribute to the variability of the dispersion. As a general indication, the more charged particles are present along the propagation path, the higher dispersion will be produced.
Example of whistler recorded by Swarm satellite B on 19/01/2014 around 11:30:57 UT. Left panel: high-pass filtered time series on 3 s window. Right panel: spectrogram of the time series, revealing the lowering frequency, typical of whistlers.
ILGEW project has three main objectives:
To address its scientific objectives, ILGEW uses both space-based and ground-based instruments:
In the development of the project we are building a dataset of whistlers and related parameters. This dataset will be publicly accessible through the project website. If interested in these data, contact us.
We are developing an ELF ray tracing algorithm to reproduce the path followed by each frequency of the lightning signal to reach Swarm satellites. This algorithm uses empirical models to generate the background environment for electromagnetic wave propagation.
An adaptation of IRI model to experimental measured conditions is realised using the Swarm EFI in-situ measurements. During this project we will also use ionosonde data, whenever available near Swarm orbit, in order to obtain the most realistic representation of the ionosphere.
The comparison between experimental and synthetic whistler’s dispersions would eventually allow us to validate the ionospheric background model and provide new information for model improvements.
Coïsson, P., V. Truhlik, V., J. Mlynarczyk, G. Hulot, R. Madelon, O. Bonnot, P. Vigneron, L. Brocco, D. Burešová, J. Chum, P. Rzonca and A. Kulak: Reconstructing the propagation of Whistlers observed in ELF during ASM burst sessions from the lightning strikes to their detection and validation of IRI model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10018, https://doi.org/10.5194/egusphere-egu2020-10018, 2020
Coïsson,P., G. Hulot, R. Madelon, P. Vigneron, O. Bonnot, V. Truhlik, D. Buresova, J. Chum, J. Mlynarczyk, A. Kulak and P. Rzonca Assessing 0+ whistlers in ELF domain to constrain the ionosphere along the propagation path: initial results, IRI workshop 2019, Nicosia, Cyprus, 9-13 Septembers 2019.
Coïsson,P., G. Hulot, R. Madelon, P. Vigneron, O. Bonnot, V. Truhlik, D. Buresova, J. Chum, J. Mlynarczyk, A. Kulak and P. Rzonca ELF whistlers analysis for ionospheric modelling: Initial results of the ILGEW project Swartm 9th data quality workshop, Prague, Czech Republic, 16-26 September 2019.
This project is developed under a programme of, and funded by, the European Space Agency in the framework of Earth Observation (EO) Science for Society (Contract No. 4000126708/19/NL/IA)
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