SB PROP @ ARL $ARLP018
ARLP018 The ARRL Solar Report

ZCZC AP18
QST de W1AW
Propagation Forecast Bulletin 18 ARLP018

From ARRL Headquarters

Newington CT May 16, 2025
To all radio amateurs

SB PROP ARL ARLP018
ARLP018 The ARRL Solar Report

After weeks of calm, solar activity is suddenly high again, with two strong solar flares erupting from opposite sides of the sun. A Coronal Mass Ejection (CME) was associated with a strong solar flare on May 12, but modeling shows the ejection passing behind Earth. The geomagnetic field has been at
unsettled to active conditions.


A CME associated with a filament eruption in the northern hemisphere is expected to pass above Earth on May 17, glancing influences will likely
enhance the solar wind field during this time. The co-rotating interacting region (CIR) associated with the large, positive polarity Coronal Hole (CH)
in the southern hemisphere is expected to become geoeffective on May 18 which will further enhance the field.


There remains a 65% chance that M-Class (R1-R2, Minor-Moderate) level flare activity will occur through May 18 with a 30% chance for X-Class (R3-Strong) levels during the same time due to the complex magnetic field within Region AR4087.


The solar wind reflected influences from the Coronal Hole High-Speed Stream
(CH HSS) originating from the negative polarity coronal hole in the southwest part of the sun.


The forecast of Solar and Geomagnetic Activity for May 17 to June 2, 2025:

Solar activity is expected to be predominantly low through the outlook
period, with varying chances for M-class flare activity.


No proton events are expected at geosynchronous orbit.

The greater than 2 MeV electron flux at] geosynchronous orbit is expected to
be at moderate levels until May 28, and again on June 6 and 7. High levels
are expected from May 29 to June 5 as CH HSS influences increase during this time.


Geomagnetic field activity is anticipated to reach minor storm levels on May
28 to June 1 under negative polarity CH HSS influences.


Active levels are likely June 2 and 6. Mostly unsettled levels are likely on May 18 to 21, as well as June 2 and 5. Quiet levels are expected on May 22 to 26.


Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere - May 15, 2025, from F. K. Janda, OK1HH:


"The level of solar activity in the first half of May mostly did not resemble the current peak of the 25th 11-year cycle. The total number of sunspot
groups across the disk ranged from two (May 2) to six (May 11), with no more than one larger group - first the relatively quiet AR4079 and then the eruptively very active AR4086 (whose growth could already be tracked prior to disk rise on the http://jsoc.stanford.edu [ http://jsoc.stanford.edu/ ] - 'Helioseismic Far-Side Imaging'website).


"Ionospheric shortwave propagation conditions were mostly poor to below average. The main culprit was not the slightly lower level of solar
radiation, but was mainly the solar wind. The Earth's ionosphere was under
the influence of elevated free electron concentrations on most days, while
was later bombarded by protons following the increase in flare activity in AR4086 with subsequent CMEs.


"The consequence was not so much lower MUF values as increased attenuation
and scattering in the ionosphere. This was all the more advantageous for stations with higher powers and, in particular, with antenna systems with low radiation angles.


"Further increases in solar activity in the northern half of the solar disk
are still expected in the coming months but cannot be predicted more accurately. In modern times, however, we can monitor it closely - and prepare for it all the better and in good time."


On May 16, Spaceweather.com [ http://Spaceweather.com ] reports that an
Aurora warning is issued for the planet Mars!


The latest solar report from Dr. Tamitha Skov, WX6SWW, can be found on
YouTube at: https://youtu.be/5hRRuaP-bT0?si=bQr5hSz0vc8WPepn [ https://youtu.be/5hRRuaP-bT0?si=bQr5hSz0vc8WPepn ] .


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, visit 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 article "Understanding Solar Indices" from September 2002
QST.

https://bit.ly/3Rc8Njt [ https://bit.ly/3Rc8Njt ]

The Predicted Planetary A Index for May 17 to 23 is 5, 8, 10, 8, 8, 6, and 6, with a mean of 7.3. The Predicted Planetary K Index is 2, 3, 4, 3, 3, 2, and
2, with a mean of 2.7. 10.7 centimeter flux is 115, 115, 120, 125, 125, 125, and 130, with a mean of 122.1.

NNNN
/EX

ARRL The National Association for Amateur Radio(R)
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