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 and solar eruptions (hours) or lightning and Transient Luminous Events (seconds or less). The project VNET4IONS aims to study the D-layer electron density disturbances during eruptive solar events and electric events over stormy regions. It has a societal objective: providing real-time alerts on communication degradations for civil aviation during solar flares. It is thus part of research on the Sun-Earth relationship and space weather (natural hazards).

The D-layer is too high for balloons and too low for satellites. Therefore, it is inaccessible to continuous in situ measurements (few measurements were carried out by sounding rockets during rapid crossings of this layer). 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. The project aims to build a worldwide VLF network to complete the French assets for Space Weather.

The VLF network is based on the AWESOME instrument, developed by the Georgia Institute of Technology. It provides broadband (1 – 450kHz) waveforms @1MHz and 100kHz with an absolute timing accuracy of 20 ns and narrowband time profiles (amplitude and phase) around several VLF-transmitter's frequencies. Measurements are performed along (geomagnetic) North-South and East-West directions simultaneously.

We will present the project in details and some examples of data analysis and modeling performed from the instrument located in Nançay.