| SB PROP @ ARL $ARLP021
ARLP021 The ARRL Solar Report
ZCZC AP21
QST de W1AW
Propagation Forecast Bulletin 21 ARLP021
From ARRL Headquarters
Newington CT June 6, 2025
To all radio amateurs
SB PROP ARL ARLP021
ARLP021 The ARRL Solar Report
Solar activity remained at moderate levels earlier this week. There was an approximately 20-degree filament eruption on June 4 and a possibly related Coronal Mass Ejection (CME). The CME is unlikely to have an Earth-directed component, but analysis is in progress.
The geomagnetic field is expected to be quiet to unsettled on June 7 as High Speed Stream (HSS) activity continues. G1 (Minor) storm levels are likely with the anticipated onset of a CME – that left the Sun on June 3 – around mid-to-late on June 7. On June 8, unsettled to active levels are expected.
Unsettled to active conditions are likely June 10 to 12 due to recurrent negative polarity Coronal Hole High Speed Stream (CH HSS) influences. Unsettled to G1 (Minor) conditions are likely on June 13 to 22 due to positive polarity CH HSS influences. Unsettled to G1 (Minor) conditions are likely on June 23 to 28 due to negative polarity CH HSS influences.
Solar activity is expected to be low with a chance for M-class flares (R1-R2/Minor-Moderate) and a slight chance for an X-class flare (R3-Strong) on June 7.
Solar wind speed parameters increased from approximately 550 km/s to nearly 810 km/s before decreasing to around 760 km/s. This could either be transient influence or a transition back into the Coronal Hole High Speed Streams.
Quiet to active levels are expected to prevail on June 7.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth’s Ionosphere – June 5, 2025, from F. K. Janda, OK1HH:
“When assessing solar activity based on the sunspot, or more modernly based on the intensity of solar radio noise, we can tentatively conclude that the maximum of the 25th cycle occurred last spring and summer, or early autumn (the highest smoothed sunspot number was in October 2024: R12 = 160.8, and then declined). Even so, it was much higher than most astronomers had predicted.
“But that’s not the end of the story. In May of this year in particular, there was a surprising increase in the number and energy of particles in the solar wind, especially during larger solar flares. Particle ionization also affects the Earth’s ionosphere, although not as nicely as we would like given the state of the Earth’s ionosphere. In short, shortwave propagation conditions were rarely good during May and especially early June 2025. They were mostly unstable, disrupted, with irregular daily cycles and frequent occurrences of increased attenuation.
“An exceptional phenomenon is the so-called Forbush effect, also known as the ‘Forbush decrease’ in the intensity of galactic cosmic rays after the arrival of a CME in the vicinity of Earth. The largest decrease in cosmic ray intensity in more than 20 years, by as much as 25%, was recorded on June 1, 2025 (the last time this happened was on October 30, 2003). Particles ejected by the Sun will remain in our vicinity and reduce the intensity of cosmic rays of galactic origin for another week or two.
“A decline is generally expected in the further development of solar activity. Only optimists admit that there will be one more increase this year, probably in the northern half of the solar disk.”
For more information concerning shortwave radio propagation, see www.arrl.org/propagation and the ARRL Technical Information Service web page at www.arrl.org/propagation-of-rf-signals. For an explanation of numbers used in this bulletin, see www.arrl.org/the-sun-the-earth-the-ionosphere. Information and tutorials on propagation can be found at http://k9la.us.
Also, check this: “Understanding Solar Indices” from September 2002 QST.
https://bit.ly/3Rc8Njt
The Predicted Planetary A Index for June 7 to 13 is 10, 8, 5, 15, 12, 10, and 35, with a mean of 13.6. Predicted Planetary K Index is 3, 3, 2, 4, 4, 3, and 6, with a mean of 3.6. Predicted 10.7 centimeter flux is 155, 155, 155, 155, 150, 150, and 155, with a mean of 153.6.
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