[SFDXA] ARLP040 The ARRL Solar Report

[Bill]

ARLP040 The ARRL Solar Report

	

SB PROP @ ARL $ARLP040

ARLP040 The ARRL Solar Report

ZCZC AP40

QST de W1AW

Propagation Forecast Bulletin 40 ARLP040

 From ARRL Headquarters

Newington CT October 24, 2025

To all radio amateurs

SB PROP ARL ARLP040

ARLP040 The ARRL Solar Report

Solar activity reached moderate levels this past week, but has now 
returned to, and remains at, lower levels. Region 4248 produced an M1.1 
flare on October 20, which was the largest event of the period. Region 
4261 was numbered this period as it rotated into better viewing conditions.

Region 4262 was numbered this period as well, splitting it from Region 
4257. Additionally, new spots were noted near N08W00 and N09E67 but went 
unnumbered due to a lack of flaring and time of emergence. No 
Earth-directed CMEs were observed in available coronagraph imagery. 
Solar activity is expected to remain low, with a chance for isolated 
M-class flares (R1-R2/minor-moderate).

Additional notable activity included a type II radio sweep with an 
estimated speed of 2,474 km/s and a type IV radio sweep that began on 
October 21. Significant field line movement, appearing to originate from 
beyond the northwestern limb, was observed in GOES SUVI imagery 
beginning on October 21. The subsequent coronal mass ejection (CME) was 
then first seen on LASCO C2 coronagraph imagery on October 21. This 
event has been analyzed as a far-sided asymmetric halo.

Solar wind parameters were mildly enhanced this period due to negative 
polarity coronal hole high speed stream (CH HSS) influences. Solar wind 
speeds exhibited a decreasing trend from approximately 600 km/s to under 
500 km/s by the period's end. Phi was predominantly positive while 
undertaking brief excursions into the positive solar sector. Mildly 
enhanced solar wind conditions and waning negative polarity CH HSS 
influences were expected to continue.

Solar activity is expected to be at moderate levels from October 31 to 
November 15 due to the return of Region 4246. Low levels are expected to 
prevail to October 30, and then on November 14 and 15. Geomagnetic field 
activity is expected to reach G1-G2 (Minor-Moderate) storm levels on 
October 28 to 30.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's 
Ionosphere, October 23, 2025, by F. K. Janda, OK1HH:

"Although it may not seem so at first glance, solar activity is still at 
the peak of its 25th cycle. Although the number of sunspot groups fell 
to six in mid-October, a week later it had risen to ten. Few eruptions 
were observed on the solar disk, or the side of the Sun facing Earth, 
including only a few that could be classified as moderately strong based 
on the intensity of X-ray emissions.

"However, something is brewing on the far side of the Sun. This is 
evidenced by the occurrence of three CMEs, the first on October 21 and 
the other two on October 22. The increased concentration of protons with 
an energy of 10 MeV (and a lower concentration of 100 MeV) in the solar 
wind confirms that this is significant activity. To analyze the 
development of the relevant activity, we will have to wait until it 
appears on the eastern limb of the solar disk.

"The geomagnetic field has calmed down in recent days. The next 
disturbance is expected around October 28. At first glance, this is very 
good news for shortwave propagation conditions during the weekend of 
October 25-26, but if geomagnetic activity increases during these days, 
it will come as no surprise.

"In fact, with the right timing of the disturbance, it could be followed 
by an increase in MUF and an overall improvement. Although a significant 
deterioration in propagation conditions is expected with a relatively 
high probability only after the aforementioned disturbance (i.e., around 
October 29), it may occur earlier."

The Predicted Planetary A Index for October 25 to 31 is 10, 5, 5, 25, 
35, 25, and 15, with a mean of 17.1. Predicted Planetary K Index is 3, 
2, 2, 5, 6, 5, and 4, with a mean of 3.9. 10.7-centimeter flux is 150, 
150, 150, 145, 145, 145, and 140, with a mean of 146.4.

For more information concerning shortwave radio propagation, see 
www.arrl.org/propagation 
<https://arrl.informz.net/z/cjUucD9taT0zNDcxNTQwJnA9MSZ1PTUyNTQ3MzY2NCZsaT00MDI1ODUzMw/index.html> 
and the ARRL Technical Information Service web page at 
www.arrl.org/propagation-of-rf-signals 
<https://arrl.informz.net/z/cjUucD9taT0zNDcxNTQwJnA9MSZ1PTUyNTQ3MzY2NCZsaT00MDI1ODUzNA/index.html>. 
For an explanation of numbers used in this bulletin, see 
www.arrl.org/the-sun-the-earth-the-ionosphere 
<https://arrl.informz.net/z/cjUucD9taT0zNDcxNTQwJnA9MSZ1PTUyNTQ3MzY2NCZsaT00MDI1ODUzNQ/index.html>. 
Information and tutorials on propagation can be found at http://k9la.us 
<https://arrl.informz.net/z/cjUucD9taT0zNDcxNTQwJnA9MSZ1PTUyNTQ3MzY2NCZsaT00MDI1ODUzNg/index.html>.

NNNN

/EX

	

	

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