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SB PROP @ ARL $ARLP036
ARLP036 The ARRL Solar Report

ZCZC AP36
QST de W1AW
Propagation Forecast Bulletin 36 ARLP036

From ARRL Headquarters

Newington CT, September 26, 2025
To all radio amateurs

SB PROP ARL ARLP036
ARLP036 The ARRL Solar Report

Solar activity reached moderate levels this past week. An impulsive M1.0
flare was observed on September 24 from Region AR4217. Slight growth was observed in Region AR4226 with an increase in spots near the leader. New Regions AR4229 and AR4230 were numbered this period. All other regions were either stable or in decay. No Earth-directed CMEs were observed. Solar
activity is expected to return to low activity.


Solar wind parameters were indicative of the arrival of a negative polarity Coronal Hole High Speed Stream (CH HSS). Solar wind speed increased from approximately 310 km/s to near 530 km/s. Total field showed an increase beginning on September 22. The CH HSS influence persisted through September
23 and diminished to background levels on September 25. A trend towards
nominal solar wind conditions is expected through September 26 as Earth moves out of a geoeffective position with the CH HSS.


Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere, September 25, 2025, by F. K. Janda, OK1HH:


"A study published on September 8 in the Astrophysical Journal Letters states that solar activity will increase after approximately sixty years of decline (following the high Solar Cycle No. 19). The next solar cycles should be even higher than the current one and, in particular, higher than the previous one (i.e., than Solar Cycles 24 and 25).


"Short-term developments in the second half of September are in line with forecasts.

Overall, solar activity has increased, although eruptive activity is slightly lower than expected.

In line with the forecast, on September 21, the Earth's magnetic field
entered the common rotation interaction (CIR) region, and the solar wind intensified.

After a two-day increase that began on September 24, geomagnetic field
activity declined again.

Although the forecasts are based primarily on the expected increase in the speed of the solar wind flowing from the edges of coronal holes, which should therefore be less accurate than forecasts based on observed eruptions and
CMEs, they are still relatively reliable.


"Most sunspot groups are now on the solar disk (the visible part of the Sun), but they will gradually begin to set. Solar activity should therefore decline by the end of September, with the decline likely to stop in mid-October. In
the Earth's ionosphere, the decline in maximum usable frequencies (MUF) will begin with a slight delay, similar to the expected rise in the second half of the month."


There is a chance for isolated R1-R2 (Minor-Moderate) radio blackouts until September 28. Unsettled to active levels are likely on September 29 and 30, October 3 to 7, October 11 to 13, and on October 18 due to recurrent CH HSS influences. Quiet to unsettled levels are likely for the remaining days in
this outlook period.


The geomagnetic field is expected to be quiet to unsettled levels on
September 27, and quiet to active levels on September 28.


The latest video report from Dr. Tamitha Skov, WX6SWW, can be found on
YouTube at https://www.youtube.com/watch?v=xzoxXGTDLyM [ https://www.youtube.com/watch?v=xzoxXGTDLyM ] .


The Predicted Planetary A Index for September 27 to October 3 is 5, 5, 15, 8, 5, 5, 5, and 15, with a mean of 8.3. Predicted Planetary K Index is 2, 2, 5,
3, 2, 2, and 5, with a mean of 3. 10.7 centimeter flux is 175, 170, 170, 170, 160, 150, and 140, with a mean of 162.1.


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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