Episode 10 · VLF and Solar Flares · Series 01: The Listening Station

How does a very low frequency signal show a solar flare?

A frame from the draft of episode 10, VLF and Solar Flares

Coming soon

Release: Dec 15, 2026
Length: 3:12
On the dial: near 25 kHz
Signal: Navy VLF transmitters

What it covers

A solar flare can show up on a plain radio. Navies talk to submarines with very low frequency radio. That signal travels between the ground and the bottom of a layer in the sky, and a flare changes that layer within minutes. I explain how that works, how a small program of mine measures six navy transmitters once a second, and what one real day of my log shows next to the X-rays that a NOAA satellite measured. I will be honest: the flare of that day left no clear mark, and I explain why.

The written guide: set this up yourself

Chapters

0:00Intro
0:21Very low
0:49A ceiling in the sky
1:16A flare
1:40My logger
2:09One real day
2:30The flare test

Frames from the draft

VLF and Solar Flares: Very low — 02 Very low

VLF and Solar Flares: A ceiling in the sky — 03 A ceiling in the sky

VLF and Solar Flares: A flare — 04 A flare

VLF and Solar Flares: My logger — 05 My logger

VLF and Solar Flares: One real day — 06 One real day

VLF and Solar Flares: The flare test — 07 The flare test

Gear in this episode

RX-888 Mk II	the receiver that hears all of shortwave at once (mine is from OpenSourceSDRLab)
MLA-30+ loop antenna	the small active loop antenna for shortwave
HP EliteDesk small form factor PC	the shortwave computer I call Raphael (mine was rescued from e-waste)
CyberPower CP1500PFCLCD UPS (1500 VA, sine wave)	the battery backup that keeps almost all of my station running through a power cut

Software and references

NOAA Space Weather Prediction Center: GOES X-ray fluxswpc.noaa.gov	the sun's X-rays, measured by satellite, live and free. My monitor reads this data
Stanford SOLAR Center: the SID space weather monitor programsolar-center.stanford.edu	how to detect solar flares with a very low frequency receiver, written for schools
NOAA: solar flares and radio blackoutsswpc.noaa.gov	what a flare does to the layer in the sky, and the flare classes
ka9q-radiogithub.com	the program that cuts the channel out of the receiver's stream (episode 5)

What is simplified, and what is invented

The logger is a program I wrote. It is not public, so there is no link.
The curves in scenes 5 and 6 are one real day from my log, as 5-minute and 1-minute means, next to public NOAA data for the same day. The video shows the hour of the day and no date.
On that day my line showed no clear mark of the flare. The small drop near the flare began before the X-rays rose, and drops of that size happen with no flare.
That the drops at sunrise and sunset come from the layer forming and fading is the usual explanation. One day does not prove it.
The X-ray curve is clipped at the bottom of its panel where the satellite data drops out for a few minutes.
Sunrise and sunset are computed for Seattle. The map marks my region only, at Seattle, and the dashed lines start there.
The side views of the earth and the layer are simplified and not to scale. The day and night heights of the layer are a sketch.
The signal trace in scene 3 and the signal bar in scene 2 are illustrations, not measurements.
In scene 4 the frequencies and the widths of the measuring windows are to scale. The noise and the shape of the hump are an illustration.
Five of the six transmitters were not heard on that day. My antenna is a small loop that is still indoors.
World coastline: Natural Earth (public domain).