2. Sudden Ionospheric Disturbance (SID) Receiver

SIDrX - Solar Flare Receiver (schematic)

My Sudden Ionospheric Disturbance (SID) receiver is a modified Stokes' Gyrator tuned VLF receiver based on the circuit board available from FAR circuits (see Communications Quarterly Spring 1994 pp.24-26). I live in Richardson, TX just north of Dallas. From there I monitor NAA from Cutler, ME at 24kHz. This station goes off the air every Monday (see plot above) but is otherwise always available.

The antenna is 50 turns of solid core phone staking-wire wound on hooks among rafters in the attic aligned appropriately for Maine. The shape is a triangle and has an enclosed area of perhaps 9 square feet. The lead-in is the same wire as a loosely twisted pair. This gave me a terrific hum problem with the monitored signal carrying a large degree of power line from the neighborhood. I (mostly) solved this by balancing the lead-in with a 600 ohm CT transformer, the output being single ended (see schematic). The antenna resonated right at 24kHz without adding capacitance -- saved me from trudging back up into the attic and removing turns! I also added a buffer amp and meter to the original design, also changing the original integrator design slightly.

I have had the receiver up and running since early June 1999.

The output signal level is logged by an Onset Computer Hobo data logger I bought through Radio Shack. I have it set up to take samples every 15 seconds.

I have had problems with the receiver going into oscillation. In some cases, this seemed to be caused by nearby lightning strikes. I have attacked this problem by placing back to back silicon diodes across the input to the receiver and shielding the lead to the tuning pot. This seems to help a great deal.

Below is a three day log for the receiver showing many flares during the daytime hours.

The time scale on these plots is CDT; so, add 5 hours to my plots to compute UTC. The vertical scale is arbitrary (the Hobo logs 0 to +2.5 volts) and was selected to provide decent dynamic range. Here are a few events to look at:

• The morning transition events are clearly visible the first two days: a peak at about 4:20 AM followed by a trough at about 5:40 AM.
• The received signal is sensitive to solar X-ray events during the daylight hours. These are seen above as the rounded peaks with occasional small peaks. Examine the GOES satellite data below and you will see that the receiver is clearly indicating many of the X-ray flux peaks picked up by the satellites. If you check carefully, you will see that the sensitivity falls off as the daytime progresses.
• There is a strong and broad peak between 7 AM and 1PM on August 20. Compare to the GOES plot below. There may also be some delay in response visible as well.

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Other plots

• Thunderstorms move through the area
• Magnetic storm

Also, check out the Moore Observatory at the University of Louisville, KY which makes their SID receiver output and archives available on line at http://moondog.astro.louisville.edu/flares/

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