[SFDXA] ARLP037 The ARRL Solar Report

[Bill]

ARLP037 The ARRL Solar Report


	

SB PROP @ ARL $ARLP037

ARLP037 The ARRL Solar Report

ZCZC AP37

QST de W1AW

Propagation Forecast Bulletin 37 ARLP037

 From ARRL Headquarters

Newington CT, October 3, 2025

To all radio amateurs

SB PROP ARL ARLP037

ARLP037 The ARRL Solar Report

Solar activity, which has been at low levels, has returned to moderate 
levels. Region AR4232 produced the largest event of the period, an 
impulsive M3.6 flare (R1-minor) on October 1, 2025.

An associated Coronal Mass Ejection (CME) was observed in subsequent 
SOHO LASCO/C2 coronagraph imagery that began on September 29. Analysis 
and modelling of the ejecta is ongoing.

Newly numbered Region AR4237 remained relatively quiet as it developed. 
Region AR4263 also produced an R1 event with an M1.0/Sf flare near the 
end of the reporting period.

It is important to note that, after careful assessment of recent 
magnetograph imagery, a spot previously associated to region AR4239 is 
now considered part of the AR4230, explaining its reported size 
increase. A new region appeared but remained unnumbered until further 
observation.

With multiple regions continuing to exhibit growth, flare activity is 
likely to remain at moderate levels (R1-R2/minor-moderate) with a slight 
chance for an X-class flare (R3-strong) through October 30.

Weak enhancements in the solar wind environment, that began September 
29, are likely to continue as influence from a negative polarity Coronal 
Hole High Speed Stream (CH HSS) persists.

The CH HSS influences on the solar wind conditions near Earth are 
expected to persist through October 5, as the planet continues to be 
embedded at the CH associated co-rotating interactive region (CIR). A 
recurrent negative CH was observed rotating into the East limb of the 
visible solar disk during the period, but no impact is expected from it 
in the upcoming days.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's 
Ionosphere, October 2, 2025, by F. K. Janda, OK1HH:

"During September, a relatively large number of sunspot groups were 
observed on the solar disk, but there were almost no energetically 
significant eruptions. It seemed to be the calm before the storm. The 
question was what would cause the storm and how the disturbance would 
develop. Important factors were: the proximity of the Autumn Equinox and 
the configuration of areas on the solar disk.

"The following phenomena were key to further developments:

- The Russell-McPherron effect, in which the magnetic field lines of the 
Sun and Earth can connect around the equinox.

- The source of the intensified solar wind heading towards Earth.

"The second condition was also met - we observed coronal holes No. 83 
and 84 on the solar disk near active regions No. 4230 and 4238, with the 
space between them being close enough to the central meridian.

"The disturbance began with a so-called positive phase on the afternoon 
of September 29 UTC, while the critical frequencies in the F2 
ionospheric layer initially rose for several hours. In the following 
days, the solar wind speed gradually increased to an impressive 800 
km/s. The intensity of the geomagnetic disturbance reached grade G3 
(three-hour K indices rose to 6 to 7).

"The return of the ionosphere to normal conditions will be slow, as 
solar activity is expected to gradually decline in the first half of 
October. In addition, after a slight lull, another disturbance can be 
expected around October 7, and even after that, the situation will not 
be very calm."

The latest report from Dr. Tamitha Skov, WX6SWW, can be found on YouTube 
at https://www.youtube.com/watch?v=a7mWOPWe0NA 
<https://arrl.informz.net/z/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU2Ng/index.html>.

The NOAA Space Weather Prediction Center has a webpage that discusses 
the current solar cycle progression: 
www.swpc.noaa.gov/products/solar-cycle-progression 
<https://arrl.informz.net/z/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU2Nw/index.html>

The Predicted Planetary A Index for October 4 to 10 is 8, 8, 8, 5, 5, 5, 
and 5, with a mean of 6.3. The Predicted Planetary K Index is 3, 3, 3, 
2, 2, 2, and 2, with a mean of 2.4. 10.7 centimeter flux is 170, 165, 
165, 165, 155, 155, and 155, with a mean of 161.4.

For more information concerning shortwave radio propagation, see 
www.arrl.org/propagation 
<https://arrl.informz.net/z/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU2OA/index.html> 
and the ARRL Technical Information Service web page at 
www.arrl.org/propagation-of-rf-signals 
<https://arrl.informz.net/z/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU2OQ/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/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU3MA/index.html>. 
Information and tutorials on propagation can be found at http://k9la.us 
<https://arrl.informz.net/z/cjUucD9taT0zNDUzMzc2JnA9MSZ1PTUyNTQ3MzY2NCZsaT0zOTk2NDU3Mg/index.html>.

NNNN

/EX

	


	

ARRL The National Association for Amateur Radio®

	

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