07/07/2021

A lot of radio amateurs may have been wondering, "Where did the bands go?" as
the first X-class solar flare in 4 years blacked out HF propagation for a time
on July 3.

"Many American radio amateurs reported sudden HF propagation blackouts on
Saturday morning, July 3, when solar active region 12838 produced an X1.5 major
solar flare that reached maximum intensity at 1429 UTC, the first X-class solar
flare of Solar Cycle 25 and the first since 2017," Frank Donovan, W3LPL, said.
"HF propagation blackouts are caused when x-ray and extreme ultraviolet
radiation from X-class solar flares strongly ionizes the absorbing D-region in
the Earth's sun-facing dense lower ionosphere," he explained. Such a radio
blackout occurs when a pulse of x-rays ionize the top layer of the atmosphere,
the exosphere.

In this instance, it caused what NOAA's Space Weather Prediction Center
(SWPC[1]) calls an R3-level or "strong" radio blackout (on a scale[2] of R1 -
R5). An R3 incident can cause a "wide-area blackout of HF radio communication
[and] loss of radio contact for about an hour on sunlit side of Earth.
Low-frequency navigation signals degraded for about an hour."

Donovan said that X-class major solar flares are necessary consequences of
steadily increasing Solar Cycle 25 activity. "95% of all X-class solar flares
occur when the solar flux index is 90 or greater. The remaining 5% can occur
any time during the solar cycle," he points out. "X1-class major solar flares
typically degrade HF propagation for only an hour or two at mid and high
latitudes, only on Earth's sunlit side."

X-class major flares are measured on an open-ended scale. The strongest one
ever recorded was an X28 flare in 2003, hundreds of times more powerful than
the July 3 X1.5 solar flare. X10-class and stronger solar flares typically have
effects that last for most of a day and affect the entire sunlit side of the
Earth. Fortunately, X10-class solar flares occur only about once every 20 years
or more.

"Much more severe and long-lasting HF propagation degradations are often caused
by the coronal mass ejections (CMEs) often associated with - but not caused by
- major solar flares," Donovan explained. "HF propagation degradation caused by
CMEs typically begins about 2 days after the effects of the associated solar
flare, the duration of the delay depending on interactions between the CME and
the solar wind."

The CME associated with the July 3 X1.5 solar flare is likely to have little to
no effect on HF propagation going forward, because the active region was very
close to the western edge of the visible solar disk when the CME erupted.
Region 12838 rotated off the visible disk on Sunday, July 4.

Solar flares have no significant effect on VHF ionospheric propagation but can
degrade satellite communications passing through the ionosphere. More frequent,
less powerful M-class medium solar flares produce short-duration degradation at
high latitudes. Very frequent, much weaker A-, B-, and C-class solar flares do
not degrade HF propagation. - Thanks to Frank Donovan, W3LPL 

[1] https://www.swpc.noaa.gov/sites/default/files/images/NOAAscales.pdf
[2] https://www.swpc.noaa.gov/sites/default/files/images/NOAAscales.pdf

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