VLF signal strengths are very reactive to the state of the ionosphere. Sudden Ionospheric Disturbances (SIDs) are often caused by the X-Rays associated with solar storms when they impact the day-side of the earth. They can cause the received signal strength of VLF transmissions to either increase of decrease for minutes, hours or even days in the event of a really severe solar storm. Additionally, frequencies in MF and HF would also be affected in varying degrees. Noise from severe solar storms can also affect frequencies up into the GHz with high levels of noise.
Several factors will be at play to cause either an increase or decrease of the signal strength of a particular VLF signal.
These would be:
– Frequency of transmission
– Distance from transmitter to receiver, hence the phase between ground wave and sky wave (if beyond ground wave distance only the sky wave would be present)
– Solar altitude
– Position of the terminator wrt transmitter and receiver
The plot above covers the past 24 hours. A SID (most often cause by a solar flare) would be identifiable as a fast dip (or rise) in signal strength simultaneously of several VLF stations, followed by a slow recovery. This happens in sync with the arrival of the solar X-Rays impacting the ionosphere, which cause an increase in the ionospheric D-layer absoption, which can be enough to stop or at least attenuate the propagation of radio signals. Think of a shark fin (or an inverted shark fin) and you’ll quickly recognise it on the VLF plot. This station has so far detected solar flares as weak as C4 at the beginning of Solar Cycle 25!
Quite often, VLF stations will change transmission power (instant changes in signal level will be seen) or switch off completely for maintenance or repairs. Not all the transmitters listed in the colour key on the image are monitored as many are off air or are no longer receivable. Likewise, this feed may stop and start at any time due to local circumstances. It’s widely thought that SIDs can only be detected during daylight hours, but this station has in the past identified SIDs on the VLF plot occurring several hours after local sunset. Night-time VLF signals tend to be more disturbed. With no solar activity the daytime signals are normally relatively smooth.
My VLF reception system
Location: Plymouth, UK.
10m long wire plugged directly in to the mic input of my PC’s sound card. The sound card is running at 192kbs, which gives 96kHz monitoring bandwidth (0-96 kHz). DL4YHFs Spectrum Lab is programmed to monitor the soundcard input and plots the signal strengths on the plotter.
Learn more about the uses of VLF stations here and here (external links).
VLF Frequency List (external link)
GOES XRay Flux (external link)
LISIRD Penticton Solar Radio Flux Plot 10.7cm Time Series (external link)