08/15/2024

By: Frank Donovan, W3LPL

Editor's note: This article was written earlier in the week. The latest
measurements show even higher numbers. See this week's K7RA Solar Report in The
ARRLÿLetter for latest information.ÿ

Record High Solar Cycle 25 Solar Maximum Sunspot Numbers Have Improved HF
Propagation Since Mid-July and Possibly Bringing Worldwide 6 Meter F2
Propagation This Fall.

According to the NOAA Space Weather Prediction Center (SWPC), Solar Cycle 25
likely reached its highest sunspot number yet of at least 299 on August 8th.

www.swpc.noaa.gov/news/solar-cycle-25-likely-reached-highest-sunspot-number-ove
r-20-years[1]

The World Data Center - Sunspot Index and Long-Term Solar Observations (SILSO)
publishes near-real-time Estimated International Sunspot Number (EISN) reports
based on its global network of reporting stations. Daily EISN reports during
Solar Cycle 25 were consistently well below 200 until suddenly rising to 218 on
July 14th and reaching 289 on July 18th, but then declining to 178 on July
22nd.ÿ

While occasional brief daily EISN increases are not unusual during solar
maximum, after only five days, the daily EISN suddenly rose to 212 on July 27th
reaching a Solar Cycle 25 record high 297 on August 8th and remaining mostly
well above 200 through this writing on August 11th.. Daily EISN reports are
likely to remain well above 200 during most days through late September and
possibly much longer.

Since February 2002, worldwide 6-meter propagation has been mostly limited to
sporadic occurrences of trans-equatorial propagation (TEP) near the equinox
months and occasional sporadic-E propagation reaching many thousands of miles
during June and July. Worldwide 6-meter F2 propagation may again occur --
perhaps very frequently -- starting in late October 2024 if daily EISN reports
consistently remain well above 200. See www.sidc.be/SILSO/eisnplot[2].


[1] https://www.swpc.noaa.gov/news/solar-cycle-25-likely-reached-highest-sunspot-number-over-20-years

[2] http://www.sidc.be/SILSO/eisnplot


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