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 [
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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