The Winter Solstice, the shortest day of the year in the Northern
Hemisphere occurred at 0327 UTC on December 22. It is the start of
summer in the Southern Hemisphere.

Solar activity increased over the last reporting week (December
14-20), with eleven new sunspot groups emerging.

One new sunspot group appeared on December 15, four more on the
following day, another on December 17, three more on December 18,
and two more in December 19-20.

Average daily sunspot number rose from 110.3 to 137.4, solar flux
from 129.8 to 162.7, planetary A index 5.6 to 18.4, and middle
latitude A index from 4.6 to 13.7.

The most active day was Sunday, December 17 when the planetary A
index was 36, and Alaska's college A index was 88.

The cause was what Spaceweather.com reported as the strongest flare
of the current solar cycle, an X2.8 class, and it caused a radio
blackout.

Here is a video of the brief flash:

https://bit.ly/3RP3xCw

Spaceweather.com reported on Wednesday that another flare is coming
from sunspot group AR3529, and here is a movie they posted:

https://bit.ly/3tipAbr

Predicted solar flux is 190, 188 and 186 on December 22-24, then
182, 180, 170 and 165 on December 25-28, 145 on December 29-30, 150
on December 31, then 145, 140 and 138 on January 1-3, 2024, then 136
on January 4-5, then 140, 145 and 148 on January 6-8, 145 on January
9-12, then 150, 147, 145, 140, and 138 on January 13-17, 136 on
January 18-19, then 140, 145 and 148 on January 20-22, then 145 on
January 23-26, then 150, 145, 140 and 138 on January 27-30.

Predicted planetary A index is 5, 12 and 8 on December 22-24, 5 on
December 25-29, 8 on December 30-31, then 10 and 8 on January 1-2,
2024, 5 on January 3-7, 10 on January 8-9, 8 on January 10, 5 on
January 11-13, 15 on January 14, 12 on January 15-16, and 8 on
January 17-19, then 5 on January 20-25, and 8 on January 26-27.

Jon Jones, N0JK wrote, from Kansas:

"Some winter 6 meter Es December 18-19. N7BHC (EL15) and KD5CAF
(EL18) into EM28 for me on FT8 around 0100 UTC December 19. Earlier
stations in Colorado, Kansas, and Missouri had ZL7DX in on 6 meters
at 2200 UTC December 18."

Here is a new, long video from Dr. Tamitha Skov, WX6SWW, from
earlier this week:

https://bit.ly/3GPRYET

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - December 21, 2023 from F.K. Janda, OK1HH:

"Astronomical winter began in the Northern Hemisphere at the moment
of the Winter Solstice: December 22 at 0327 UTC. On this day is the
longest night and, of course, the shortest day. The total effect of
solar X-ray and ultraviolet radiation on the ionosphere of our
hemisphere was thus relatively the smallest of the entire year, and
the effects of changes in the solar wind were all the more
effective. This is also one of the reasons why, despite relatively
high solar activity, the shortwave propagation conditions are worse
than we would like and then we expected.

"Over the next six months, the length of the day will increase until
the Summer Solstice on June 20. Slowly at first, then faster,
fastest around the Spring Equinox on March 20. It is certain that
then the propagation conditions will be significantly better than
now. It is even possible that the maximum of the eleven-year cycle
will occur as early as next year, although it would be better for us
if it did not occur until 2025.

"Although we have not observed any particularly large sunspot groups
in recent weeks, there were always one or two active regions among
them, whose magnetic configuration allowed the development of a
medium-sized eruption, possibly even with a CME - after all we
observed several of these. The exception was the X2.8 class eruption
on December 14 at 1702 UTC, the strongest so far since the beginning
of the 25th solar cycle, or since the major disturbances in
September 2017.

"The eruption originated in AR3514, which was approaching the
western limb of the Sun. Even though it hurled a fast-moving CME
into space, it was relatively unlikely to cause strong geomagnetic
storms here on Earth. Eventually, the CME either missed Earth or hit
so weakly that it was not detected by satellite sensors.

"During the rise of the solar flux from 126 on December 12 to 195 on
December 20, with the corresponding increase in solar X-ray
radiation, shortwave propagation improved only slightly, actually
fluctuating, which was expected.

"The last geomagnetically quiet day was December 13, after which the
Earth's magnetic field was unsettled to active (more precisely:
active around last weekend). However, most days until the end of
this year should be geomagnetically quieter, while the solar flux
will remain elevated. Therefore, we can expect slightly better
propagation."

Here are a number of articles about a Big Flare:

https://bit.ly/3RQG4Rb

https://bit.ly/3RRzBpe

https://bit.ly/48tJtuH

https://bit.ly/3TAeybV

https://bit.ly/48pIpbo

https://bit.ly/488c88X

https://bit.ly/3tymsrK

https://bit.ly/3RwJzLh

https://bit.ly/3RThBuQ

https://bit.ly/3RRzzh0

An article about Radio Blackout:

https://bit.ly/3v5b5Il

Send your tips, reports, observations, questions and comments to
k7ra@arrl.net. When reporting observations, don't forget to tell us
which mode you were operating.

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 .

An archive of past propagation bulletins is at
http://arrl.org/w1aw-bulletins-archive-propagation . More good
information and tutorials on propagation are at http://k9la.us/ .

Also, check this article about understanding solar indices:

https://bit.ly/3Rc8Njt

Instructions for starting or ending email distribution of ARRL
bulletins are at http://arrl.org/bulletins .

Sunspot numbers for December 14 through 20, 2023 were 126, 130, 163,
129, 137, 144, and 133, with a mean of 137.4. 10.7 cm flux was
155.1, 144.3, 149, 154.6, 161.4, 179.3, and 195.3, with a mean of
162.7. Estimated planetary A indices were 16, 12, 14, 36, 28, 12,
and 11, with a mean of 18.4. Middle latitude A index was 13, 8, 10,
32, 16, 10, and 7, with a mean of 13.7.