First meeting of new BAA Very Low Frequency Radio Astronomy Sub-Group Monday 16 March at 20:00 UTC
andrew....@googlemail.com
Dear All,
I would like to invite you to join me at the first meeting of the new BAA VLF group – this is a group dedicated to discussing the detection of sudden ionospheric disturbances (SIDs) using the very low frequency radio range.
The link for the meeting is below – hopefully you can you join us on the day.
Microsoft Teams meeting
Join: https://teams.microsoft.com/meet/357127117415?p=HsB0ITEukhGEDeLGBK
Meeting ID: 357 127 117 415
Passcode: 4mh9we9A
Dial in by phone
+44 20 3321 5208,,453456086# United Kingdom, City of London
Phone conference ID: 453 456 086#
What is a SID? ☀️
A SID stands for Sudden Ionospheric Disturbance. It refers to a rapid change in the ionosphere caused by intense solar radiation, usually from a solar flare. These disturbances are commonly detected by monitoring Very Low Frequency (VLF) radio signals.
What happens during a SID? ☀️
When a solar flare occurs (often on the Sun observed by missions such as GOES weather satellites):
- The flare emits strong X-rays and extreme ultraviolet radiation.
- These reach Earth in about 8 minutes.
- The radiation increases ionization in the D-layer of the ionosphere.
- This suddenly changes how VLF radio waves propagate.
- Receivers on Earth see a sharp spike or dip in signal strength from distant transmitters.
This effect is what radio astronomers and space-weather monitors call a SID.
Frequency range used for SID monitoring 📡
SID monitoring typically uses Very Low Frequency transmitters:
VLF range:
- 3 kHz – 30 kHz
Most amateur SID monitors listen between:
- 15 kHz – 30 kHz
This is because many powerful navigation and time transmitters operate there.
Examples include transmitters such as:
- **NAA VLF Transmitter – 24.0 kHz
- **DHO38 VLF Transmitter – 23.4 kHz
- **GBZ VLF Transmitter – 19.6 kHz
A SID receiver measures signal strength changes from these transmitters rather than decoding their content.
Typical SID equipment
Many amateur setups include:
- A long wire or loop antenna
- A VLF receiver
- A data logger connected to a computer
A well-known example is the Stanford Solar Center SuperSID monitor, used worldwide by schools and amateur observers. Another is the UKRAA's VLF receiver, also widely used by the amateur radio astronomy community.
✅ Key Idea.
A SID is not the signal itself—it’s the ionospheric reaction to a solar flare, detected by monitoring VLF signals (≈3–30 kHz).
Andy
Dr Andrew Thornett, M6THO.
Lichfield Radio Observatory.