Solar activity ranged from low to high.  Region 4087 produced the strongest event of the period, an impulsive R3 flare on May 14 at 0825 UTC near the NE limb.  The region also produced R2 flares at 0325 and 1119 UTC.  Finally, three R1 events were produced by the region on May 14 and 15.  Region 4086 also produced an R3 event, with a flare observed on May 13 at 1538 UTC.

No proton events were observed at geosynchronous orbit.  However, a weak enhancement, which peaked below the S1 threshold, was observed on May 13 following the R3 event from Region 4086.

Geomagnetic field activity was at quiet levels on May 12 and 13. Late on May 16, enhancements in solar wind parameters, associated with the onset of a positive polarity CH HSS and possible influence from the southern periphery of the CME that left the Sun on May 12, increased activity to active levels.

Forecast of Solar and Geomagnetic Activity May 19 - June 14 2025:

Solar activity is expected to be at low levels, with a chance for R1-R2 (minor-moderate) events, over the outlook period.

No proton events are expected at geosynchronous orbit.

The remainder of the outlook period is expected to be at normal to moderate levels.

Geomagnetic field activity is likely to be elevated above quiet levels for most of the next 27 days due to multiple, recurrent, coronal hole features.  G2 (moderate) conditions are likely on May 29 and June 13; G1 (minor) conditions likely on May 19, May 28, and June 14; active conditions are likely on May 30 - June 01, and June 10; unsettled levels are likely on May 20-23, May 27, June 02-07, and June 11-12.  Quiet conditions are expected for the few remaining days of the outlook period.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere - May 22, 2025, from F. K. Janda, OK1HH.

Compared to previous months, there was a significant decrease in solar activity in May.  For example, on May 2, there were only two groups of spots on the entire solar disk observed.  After that, spot activity in the northern half of the solar disk increased slightly, but solar flux dropped significantly during the second third of the month.  Additionally, a large coronal hole appeared in the southern hemisphere of the sun.  Although this coronal hole was observed during previous rotations in March and April, it is much larger this time.

Consistent with this observation, after geomagnetic activity quieted down at the end of April, we experienced several days of strong geomagnetic disturbances in the first third of May.  Starting in the second third of May, the solar flux dropped significantly, reaching values not seen since October 2024.

The combination of low solar activity and high geomagnetic activity resulted in a significant deterioration of ionospheric shortwave propagation conditions, manifested by a drop in the critical frequencies of the F2 layer and an increase in attenuation and scattering.  This occurred especially on May 7-12 and May 14-18, and is likely to continue in the coming days.

The latest attempts to forecast further developments suggest that conditions should improve by the end of May.  The summer ionosphere of the northern hemisphere of Earth is characterized by lower maximal frequencies and higher lows, not counting the sporadic layer E surprises.

The Predicted Planetary A Index for May 24 to 30 is 6, 5, 5, 8, 25, 30, and 20, with a mean of 14.1.  The Predicted Planetary K Index is 2, 2, 2, 3, 5, 6, and 4, with a mean of 3.4.  10.7 centimeter flux is 120, 120, 120, 130, 135, 135, and 140, with a mean of 128.5.  

For more information concerning shortwave radio propagation, see http://www.arrl.org/propagation and the ARRL Technical Information Service web page at, http://arrl.org/propagation-of-rf-signals . For an explanation of numbers used in this bulletin, see http://arrl.org/the-sun-the-earth-the-ionosphere  . Information and tutorials on propagation can be found at, http://k9la.us/ .

Also, check:  

"Understanding Solar Indices" from September 2002 QST:

https://bit.ly/3Rc8Njt