From Spaceweather.com
March 23, 2023:
Something rare and strange happened last month. On Feb. 23rd, growing sunspot AR3234 produced an M-class solar flare. It was nearly midnight in Florida when the explosion occurred, so you’d expect no one there to notice. On the contrary, in the community of High Springs, FL, amateur radio astronomer Dave Typinski recorded a strong shortwave radio burst.
“You CAN see the sun at midnight in Florida… sometimes,” says Typinski. This is what his instruments recorded while the flare was underway:
A double wave of static washed over Florida, filling the radio spectrum with noise at all frequencies below 25 MHz. “The Sun was 69° below the horizon when this happened,” he marvels.
How is this possible? The entire body of our planet was blocking the event from Typinski’s antenna. It’s called “antipodal focusing.” First postulated by Marconi more than 100 years ago, antipodal focusing is a mode of radio propagation in which a signal starts out on one side of the planet, gets trapped between Earth’s surface and the ionosphere, and travels to the opposite hemisphere. Waves converging at the antipode can create a surprisingly strong signal.
Right: This diagram from a declassified US Gov.report shows the basic geometry of antipodal focusing.
“This is the second or maybe third midnight solar radio burst I’ve seen in ten years, but it’s by far the strongest,” says Typinski. “The previous events happened at the height of Solar Cycle 24. They’re quite rare.”
Pause: Yes, solar flares can produce radio signals. Typinski’s midnight burst was a “Type V,” caused by streams of electrons shooting through the sun’s atmosphere in the aftermath of the flare. Plasma waves rippling away from the streams emited intense bursts of natural radio static. The burst was first observed in broad daylight at the Learmonth Solar Observatory in Australia, then it curved around Earth to reach Typinski.
Above: An example of antipodal focusing of seismic waves caused by the Chicxulub asteroid impact. The geometry is the same as for radio waves. [more].
“This propagation mode was used during the Cold War,” notes Typinski. “The U.S. would park a SIGINT ship in the south Pacific to grab signals from the Eastern Bloc. The Soviets probably did the same thing, parking in the southern Indian ocean.”
Turns out, this method of spying works for radio astronomers, too. Would you like to record an event like this? NASA’s Radio JOVE program makes it easy. Off-the-shelf radio telescope kits allow even novices to monitor radio outbursts from the sun, which are becoming more frequent as Solar Cycle 25 intensifies.
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