[SFDXA] First X-Class Major Solar Flare of Solar Cycle 25 Blacks Out HF on July 3 -ARRL

[Bill]

>
>     First X-Class Major Solar Flare of Solar Cycle 25 Blacks Out HF on
>     July 3
>
> 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.
>
> In this instance, it caused what NOAA’s Space Weather Prediction 
> Center (*SWPC 
> <https://www.swpc.noaa.gov/sites/default/files/images/NOAAscales.pdf>*) 
> calls an R3-level or “strong” radio blackout (on a *scale 
> <https://www.swpc.noaa.gov/sites/default/files/images/NOAAscales.pdf>* 
> 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/
>
http://arrl.org/news/first-x-class-major-solar-flare-of-solar-cycle-25-blacks-out-hf-on-july-3
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