SB PROP @ ARL $ARLP039
ARLP039 Propagation Forecast
ZCZC AP39
QST de W1AW
Propagation Forecast Bulletin 39 ARLP039
From ARRL Headquarters
Newington, CT September 25, 2026
To all radio amateurs
SB PROP ARL ARLP039
ARLP039 The ARRL Solar Report
Solar activity is likely to continue at low levels with isolated to occasional C-class flare activity, along with a slight chance for R1-R2 (Minor-Moderate) M-class flare events, through September 25, according to NOAAís Space Weather Prediction Center. The SWPC reports that nine sunspot regions are on the visible disk, including a new Region 4541. There were three solar flares observed in the past week, none of which was directed toward Earth.
A disturbed solar wind environment was expected to arrive late yesterday, followed by positive polarity coronal hole high-speed stream (+CH HSS) influences continuing through September 25.
Unsettled to active geomagnetic field conditions levels are expected as of today under early +CH HSS effects, including isolated periods of G1 (Minor) geomagnetic storming today and tomorrow.
The Australian Space Weather Forecasting Centreís global HF propagation forecast for middle latitudes is for mild degradation today due to forecast geomagnetic activity, followed by normal propagation in the days that follow. We are currently in a period of equinoctial propagation, during which conditions are relatively uniform around the world.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere, September 24, 2026, by F. K. Janda, OK1HH:
After a prolonged period of calm activity on the Sun as well as in Earthís magnetosphereóspanning the entire second third of Septemberósolar activity began to rise first, followed by an increase in geomagnetic activity starting on September 24. On September 18 only, there were no sunspots on the Sun. The rise in solar activity on the 24th was accompanied by an intensification of the solar wind,
while the geomagnetic disturbance began its so-called positive phase (the critical frequencies f0F2 increased significantly between 9:00 and 13:00 UT). Improved shortwave propagation conditions were observed on September 23ñ24.
Solar activity will remain at a higher level for several more days, while the geomagnetic disturbance will continue only briefly in the so-called negative phase of development. A calm period is expected to follow during the last few days of the month. Recovery from the disturbance will likely be rapid, while a return to improved shortwave propagation conditions can be expected for most of the remaining days of the month.
The Predicted Planetary A Index for September 25 to October 1 is 18, 12, 8, 5, 5, 5, and 5 with a mean of 8.3. The Predicted Planetary K Index is 5, 3, 3, 2, 2, 2, and 2 with a mean of 2.7. 10.7 centimeter flux is 105, 105, 100, 105, 110, 115, and 115 with a mean of 107.9.
For more information concerning shortwave radio propagation, see www.arrl.org/propagation and the ARRL Technical Information Service web page at www.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. Information and tutorials on propagation can be found at http://k9la.us.
Also, check this: "Understanding Solar Indices" from September 2002 QST. https://bit.ly/3Rc8Njt
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