The VLF network GIFDS for a ground-based monitoring of solar disturbances

Abstract

It has been observed that solar flares reflect in the amplitude and phase of radio measurements. As these bursts of X-ray radiation may be harmful, the German Aerospace Center (DLR) operates the Global Ionospheric Flare Detection System (GIFDS) to continuously monitor from the ground the current state of the ionosphere's D region, which is the linking layer affected from the outside in the relevant solar spectral range and in turn taking effect on steadily available VLF signals. A major issue in flare recognition based on the radio signal changes is the non-constant progression of the quiet level over the day. That's why a flare may yield different pictures depending on the time it hits the ionosphere. Taking care of the diurnal variation is crucial for setting up a system capable of raising alerts. In addition, investigating seasonal changes is important also for understanding Earth-originating anomalies like the "October effect", a sharp decrease in signal strength during fall in mid-latitude regions (subject to the project "Analysis of the MEsosphere and Lower Ionosphere fall Effect", AMELIE). Proper flare detection and size estimation rely on identifying the normal amplitude level. As the measurements could suffer also from small-scale oscillations, the instruments were improved and harmonized to suppress noise, resulting in the compact GIFDS receiver.