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SB PROP @ ARL $ARLP035
ARLP035 The ARRL Solar Report

ZCZC AP35
QST de W1AW
Propagation Forecast Bulletin 35 ARLP035

From ARRL Headquarters

Newington, CT, September 19, 2025
To all radio amateurs

SB PROP ARL ARLP035
ARLP035 The ARRL Solar Report

Solar activity was at low levels this past week. Low-level C-class flaring
was observed from Regions AR4216 and AR4217, as well as from Region AR4213
just beyond the Southwest limb. Slight growth was observed in all spotted regions. No Earth-directed CMEs were observed. Solar activity is expected to
be low, with a chance for an isolated M-flare (R1-R2/minor-moderate), through September 20.


Solar wind parameters were indicative of coronal hole high-speed stream (CH HSS) influences. Solar wind speed increased to near 760 km/s early on
September 15 before subsiding to between 600-700 km/s. Solar wind parameters are expected to continue under the influence of a negative polarity Coronal Hole High Speed Stream. Solar wind speed is likely to gradually diminish on September 20. The geomagnetic field is expected to be at quiet levels on September 20 and quiet to active on September 21.


Spaceweather.com [ http://Spaceweather.com ] for September 19 reports that a co-rotating interaction region, or CIR, could hit Earth on September 22.


Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere, September 18, 2025, by F. K. Janda, OK1HH:


"The development of phenomena on the Sun and subsequently in the Earth's magnetosphere and ionosphere during the second decade of October was interesting and exciting.

First, solar activity passed through a 27-day minimum on October 10-13, and
the subsequent increase is likely to continue for most of the month.

Above all, however, we observed a large butterfly-shaped coronal hole on the Sun.

Predictions of when the intensified solar wind from its edges would reach
Earth rarely coincided on Sunday, September 14.


"And so, it happened after 1600 UTC. The CME of September 11 (1030 UTC) contributed to the intensification of the solar wind, while the 'Russell-McPherron effect,' which connects the Sun's magnetic field with the Earth's magnetic field during the weeks around the equinox, contributed to
the impact. During the evening, the positive phase of the disturbance
continued with an increase in the MUF of the F2 ionospheric layer.


"The geomagnetic disturbance intensified to G3 on September 15, while on September 15-16, we experienced its negative phase with a significant
decrease in MUF in the middle and higher latitudes of the globe.

And, of course, the overall deterioration in shortwave propagation conditions was unmistakable.

The improvement began slowly on September 17 and will continue, as we expect further growth in solar activity for most of September.

As usual, the fact that we are in the period around the Autumnal Equinox will also contribute to the improvement.


"We could also tell that Summer in the ionosphere was over by the significant decrease in the activity of the sporadic-E layer, which was still quite frequent in mid-latitudes in the first half of September."


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 ] .


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 ]


The Predicted Planetary A Index for September 20 to 26 is 5, 5, 5, 5, 5, 5,
and 5, with a mean of 5. Predicted Planetary K Index is 2, 2, 2, 2, 2, 2, and 2, with a mean of 2. 10.7 centimeter flux 135, 135, 135, 130, 125, 130, and 125, with a mean of 130.7.

NNNN
/EX

ARRL The National Association for Amateur Radio(R)
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