SB PROP @ ARL $ARLP039
ARLP039 The ARRL Solar Report
ZCZC AP39
QST de W1AW
Propagation Forecast Bulletin 39 ARLP039
From ARRL Headquarters
Newington CT, October 17, 2025
To all radio amateurs
SB PROP ARL ARLP039
ARLP039 The ARRL Solar Report
Region AR4246 underwent significant evolution, growing in overall size while gaining multiple new spots. Subsequently, AR4246 was the main provider of activity, which included an M1.2 flare on October 13. Several coronal mass ejections (CMEs) with potential Earth-directed components are in the mix at this time.
Additional modeling efforts are underway to perhaps confirm those suspicions. The largest flare was a long-duration M4.8/2n on October 15. Modeling efforts showed the ejecta to be on a northward trajectory and not on an Earthward course.
Other notable activity included a prominence eruption off the ENE limb-first visible in LASCO C2 imagery on October 14. Given the location of the event, initial analysis suggests this ejecta to be well into foul-ball territory and not Earth-directed. Additional modeling efforts are underway to perhaps
confirm this suspicion.
Slight decay was observed in the intermediate portion of Region AR4248 as it grew in length. Region AR4247 was in decay as well. The remaining regions
were unremarkable in comparison. No new regions were assigned numbers this period. Minor to moderate (R1-R2) radio blackouts are likely for the next 3 days, with a slight chance for an isolated R3 (strong) event, due to the current and potential flare activity of Regions AR4246 and AR4248.
Solar wind parameters continued to reflect the influence of a negative
polarity coronal hole high-speed stream (CH HSS). Solar wind speeds finally dipped below 600 km/s, and phi was predominantly in the negative solar
sector.
Solar wind parameters are expected to continue at enhanced levels, although gradually waning, and continuing through October 17, due to CME arrivals from the October 11 to 13 timeframe originating from AR4246. Solar activity
reached moderate levels due to M-class flare activity.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere, October 16, 2025, by F. K. Janda, OK1HH:
"In line with forecasts, solar activity continued to gradually increase, including several moderately strong flares. These were mostly observed in the active region NOAA 4246, located in the northwest of the solar disk, while approaching its limb.
"The second of the two larger active regions is NOAA 4248, also located in
the northwest, but closer to the central meridian and the solar equator,
while relatively close to the extensive coronal hole No. 87. In the coming days, AR NOAA 4248 will move into the active longitudes. Therefore, its eruptive activity will increase. Between it and the aforementioned coronal hole, a source of intensified solar wind will form, which will hit Earth
during the coming week.
"Even earlier, on October 16, when this text is being written, we expect a G2 geomagnetic disturbance, which will first cause a brief improvement and then
a deterioration in ionospheric propagation of short waves, which have been rather average so far. The exception was a shorter increase in MUF on October 15 between 1000 and 1200 UTC, probably caused by an intensification of the solar wind.
"Solar activity should remain at current levels for the rest of the month. Fluctuations in propagation conditions, mostly around average with occasional deterioration, will depend on the irregular occurrence of geomagnetic disturbances."
The latest video report from Dr. Tamitha Skov, WX6SWW, can be found
on YouTube at:
https://www.youtube.com/watch?v=pAZtua4DZUs [
https://www.youtube.com/watch?v=pAZtua4DZUs ] .
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 October 18 to 24 is 5, 5, 15, 10, 8, 5,
and 5, with a mean of 7.6. Predicted Planetary K Index is 2, 2, 5, 3, 3, 2,
and 2, with a mean of 2.7. 10.7-centimeter flux is 150, 150, 150, 150, 145, 140, and 145, with a mean of 147.1.
NNNN
/EX
ARRL The National Association for Amateur Radio(R)
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