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SB PROP @ ARL $ARLP034
ARLP034 The ARRL Solar Report

ZCZC AP34
QST de W1AW
Propagation Forecast Bulletin 34 ARLP034

From ARRL Headquarters

Newington CT September 12, 2025
To all radio amateurs

SB PROP ARL ARLP034
ARLP034 The ARRL Solar Report

Solar activity has been at very low to low levels this past week.

Low-level C-class flaring was observed from Regions AR4210 and AR4207. Slight growth was observed in Regions AR4213 and AR4214. The rest of the spotted regions were either stable or in decay. Solar activity is expected to remain
at low levels, with a chance for isolated M-class flares
(R1-R2/minor-moderate) through September 13.


A CME (Coronal Mass Ejection) was observed just beyond the Western limb on September 9. No impacts are expected, and no Earth-directed CMEs were
observed.


Solar wind parameters continued to be enhanced under positive polarity
coronal hole high-speed stream (CH HSS) influences. Solar wind speed
increased from approximately 450 km/s to nearly 550 km/s. Elevated solar wind conditions are expected to continue into September 12. Another enhancement is likely on September 14 with the onset of negative polarity CH HSS influences and the possible glancing effects of the September 11 CME.


Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere, September 11, 2025, by F. K. Janda, OK1HH:


"During a seven-day interval (August 26 to September 1), when solar flux
values were above 200 s.f.u., there was a decline to below 120 s.f.u. during the first ten days of September, which also occurred during the previous
solar rotation, on August 17-19.


"Solar flare activity simultaneously decreased significantly, while all
sunspot groups moved to the western half of the solar disk. Instead, we are
now observing a large coronal hole, located mostly north of the solar
equator, whose boundary has already crossed the central meridian.


"The intensified solar wind, which is definitely blowing from its western
edge, is expected to reach Earth since September 13. It will likely
contribute to increased geomagnetic activity, which, combined with relatively low solar activity, will have negative consequences for the state of the ionosphere. MUF values will decrease, attenuation will increase, and
shortwave propagation conditions will deteriorate.


"Starting in the first half of September, however, solar activity will begin
to increase, although not as significantly as it did in August. In addition, geomagnetic activity appears to be declining at the same time. The
approaching Autumn Equinox will also have a significant impact on the state
of the ionosphere. These are already three reasons why shortwave propagation conditions should gradually improve."


The Predicted Planetary A Index for September 13 to 19 is 5, 12, 20, 15, 8,
8, and 8, with a mean of 10.9. Predicted Planetary K Index is 2, 4, 5, 4, 3,
3, and 3, with a mean of 3.4. 10.7-centimeter flux is 120, 125, 130, 135,
135, 140, and 145, with a mean of 132.9.


For more information concerning shortwave radio propagation, see www.arrl.org/propagation [ http://www.arrl.org/propagation?utm_source=Informz&utm_medium=Email&utm_campaign=ARRL
] and the ARRL Technical Information Service web page at www.arrl.org/propagation-of-rf-signals [ http://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 [ http://arrl.org/the-sun-the-earth-the-ionosphere ] . Information and
tutorials on propagation can be found at http://k9la.us [ http://k9la.us/ ] .


Also, check this: "Understanding Solar Indices" from September 2002 QST. https://bit.ly/3Rc8Njt [ https://bit.ly/3Rc8Njt ]

NNNN
/EX

ARRL The National Association for Amateur Radio(R)
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