INST PHYSIQUE GLOBE (FR)
Some of the strongest lightning discharges occurring at the Earth surface generate short electromagnetic signals that can propagate at very large distances both at the surface of the planet and in space. During a lightning discharge, the whole electromagnetic spectrum is excited, from radio frequencies to the visible and beyond. The conditions of the atmosphere and the ionosphere crossed by this signal affect its propagation differently in the different electromagnetic frequency bands. More specifically, when the lightning signal crosses the boundary between the neutral atmosphere and the ionosphere, at about 90 km height, the lower frequencies are spread in time due to the interactions with the electrons and ions present in the ionosphere. This phenomenon is called dispersion. It occurs because the charged particles of the ionosphere are bound to the Earth’s magnetic field lines and this induces changes in the direction and speed of signal propagation. When the lightning signal reaches a satellite at low orbital altitude, it is recorded as a whistler, a gliding signal that can be translated into a sound resembling to a whistle.
ILGEW project (May 2019 – September 2020) intends to study these whistlers and improve the scientific returns of the ESA Earth Explorer Swarm mission that measures the Earth magnetic field using a constellation of three satellites.
Whistler events can be recorded by the Swarm’s Absolute Scalar Magnetometer (ASM) only when the acquisition rate of the measurements of the magnetic field intensity is raised from the nominal 1 Hz to a burst-mode at 250 Hz. This allows the investigation of part of the Extremely Low Frequency (ELF) band where whistlers can be received and studied between 20 and 125 Hz. This part of the electromagnetic spectrum has not yet been systematically studied from space and it still presents many challenges for a complete understanding of the ground-ionosphere interactions.
Specific burst-mode measurement campaigns are conducted during the development of ILGEW project, taking advantage of the specific orbital characteristics of Swarm satellites drifting slowly in local time, to explore whistler characteristics under different geophysical conditions.
Whistlers received by Swarm satellites can be characterised by their dispersion, 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 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 will be the whistler’s dispersion.Scientific objectives
ILGEW project has three main scientific objectives:
1. Characterize the whistler dispersion measured from Earth low orbital altitudes, in order to analyse the ionosphere below Swarm satellites, along the propagation path of the whistlers.
2. Constrain the lightning activity that lead to favourable propagation conditions for the generation of detectable ELF whistlers at Swarm altitudes.
3. Establish the benefits that can be obtained for ionospheric models such as the International Reference Ionosphere (IRI) by using the information obtained from whistler’s characteristics.