The ionospheric D-layer (60 and 90km) is also called the “ignorosphere”, clearly indicating the level of knowledge on this atmospheric layer. Its electron content follows a daily cycle, but also the seasonal cycle of the solar UV flux radiation. In addition to these long-period variations, the D-layer electron density is submitted to much faster forcing, like geomagnetic storms, solar eruptions (hours) or lightning and Transient Luminous Events (seconds or less). The electron content of the D-layer controls the absorption of some radio waves and links the neutral atmosphere to the magnetosphere and radiation belts. The disturbances of the D-layer electron density can lead to HF communications disruption, a threat now identified by the International Civil Aviation Organization. The project focuses on two natural hazards: solar flares because they produce the strongest forcing of the D-layer, and the lightning strokes and Transient Luminous Events because their links to the D-layer are largely unclear. 

The D-layer is too high for balloons and too low for satellites. Therefore, it is inaccessible to continuous in situ measurements. The most widely used technique to study the D-layer consists of measuring the VLF emissions from man-made or natural origin. VLF modes propagate in the waveguide formed by the Earth and the ionosphere, with reduced loss (~2 dB/Mm), thus propagating very long distances. An increase in electron density alters the modes: the amplitude and phase are then affected, which provides an efficient way to detect, in real time, electron density variations. 

During this talk, we will detail the project VLF4IONS, and in particular, the network that is being built worldwide (including Tenerife).