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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://www.youtube.com/watch?v=a7mWOPWe0NA ] .


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://www.swpc.noaa.gov/products/solar-cycle-progression ]


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 [ 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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