(C) ARRL Letter
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The ARRL Solar Update [1]
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Date: 2025-09
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 near 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 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
http://www.arrl.org/propagation and the ARRL Technical Information
Service web page at,
http://arrl.org/propagation-of-rf-signals. For
an explanation of numbers used in this bulletin, see
http://arrl.org/the-sun-the-earth-the-ionosphere . Information and
tutorials on propagation can be found at,
http://k9la.us/ .
Also, check this:
https://bit.ly/3Rc8Njt
"Understanding Solar Indices" from September 2002 QST.
[END]
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