Subject: Weekly Highlights of Solar and Geomagnetic Activity, 11 - 17 December 2023

:Product: Weekly Highlights and Forecasts
:Issued: 2023 Dec 18 1406 UTC
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# Weekly Highlights and Forecasts
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Highlights of Solar and Geomagnetic Activity
11 - 17 December 2023

Solar activity ranged from low to high levels. Of the 19 numbered
active regions that were observed on the visible disk this week,
Region 3514 (N05, L=141, class/area=Ekc/470 on 14 Dec) was the
primarily responsible for the most significant activity. The region
produced an X2.8 flare (R3-Strong) at 14/1702 UTC, the strongest
since Sep 2017. Associated with the event were Type II (est. 2118
km/s) and Type IV radio sweeps, a radio burst values with a
Castelli-U profile, a Tenflare, and extremely high radio burst
fluxes reported by the USAF RSTN observatories. Subsequent
coronagraph imagery contained a CME signature with a halo shock and
ejecta that was primarily headed towards the SW. This ejecta was
modeled alongside other CME-producing events, an M5.8 flare (R2 -
Moderate) at 14/0744 UTC, an M2.3 flare (R1 - Minor) at 14/1348 UTC
and double peak M6.9 flare at 15/0734 UTC. The resulting WSA-Enlil
output suggested anticipated CME influence for late on 16 Dec and
through 17 Dec.

Other activity of note included coronal dimming on late on 11 Dec
and 12 Dec from the vicinity of Region 3514. The X-ray response from
these events were below the R1 event threshold but signatures in
coronagraph imagery were identified following the events. Modeling
and analysis of these events suggested CME influence likely around
14-15 Dec.

With high levels observed on 14-15 Dec, and moderate levels observed
on 17 Dec (all from Region 3514), the remainder of the summary
period was at low solar activity levels. Other CME activity observed
in coronagraph imagery was determined to not be along the Sun-Earth
line.

The GOES-16 greater than 10 MeV proton flux briefly exceeded the S1
(Minor) threshold at the very end of the 15 Dec UT day. Flux levels
reached a peak of 13.9pfu shortly after at 16/0015 UTC. The proton
enhancement was thought to be produced by the combined contribution
of the CMEs associated with the X2.8 flare at 14/1702 UTC and the
M6.9 flare at 15/0734 UTC.

The greater than 2 MeV electron flux at geosynchronous orbit was at
high levels on 11-12 Dec. Due to an increase in geomagnetic
activity, electron flux levels remained at normal to moderate levels
from 13-17 Dec.

Geomagnetic field activity ranged from quiet to G1 (Minor) storm
levels. Quiet levels were observed over 11 Dec. An increase to
active levels was observed after a brief period of southward Bz
reached -12 nT. This was likely caused by a filament eruption on the
Sun over 08 Dec. Quiet to unsettled conditions on 13 Dec increased
to active levels again on 14 Dec (southward Bz reached ~-11 nT). On
15 Dec, only quiet to unsettled conditions followed the arrival of a
CME associated with activity on the Sun over 11-12 Dec. An increase
to active conditions was observed on 16 Dec when solar wind speeds
increased to above 500 km/s. Activity would increase to G1 (Minor)
geomagnetic storm levels over 17 Dec as CME influence from activity
on the Sun over 14-15 Dec passed by Earth. Total magnetic field
strength increased to a peak 17 nT and Bz reached as far south -14
nT as the CME passed. Solar wind speeds increased to roughly 550
km/s before decreasing to below 500 km/s by the end of the 17 Dec UT
day.

Forecast of Solar and Geomagnetic Activity
18 December - 13 January 2024

Solar activity is expected to be low levels, with a chance for
moderate activity (R1-R2/Minor-Moderate) throughout the outlook
period.

No proton events are expected at geosynchronous orbit.

The greater than 2 MeV electron flux at geosynchronous orbit is
expected to reach high levels due to multiple, recurrent CH HSSs on
20-22 Dec, 03-05 Jan, and 08-10 Jan. The remainder of the outlook
period is likely to be at normal to moderate levels.

Geomagnetic field activity is expected to reach G1 (Minor) storm on
18 Dec due to waning CME influence. Active levels are likely on 19
Dec as solar wind influence is anticipated to transition to a CH
HSS. Unsettled levels due to recurrent CH HSSs are also likely on
20-21 Dec, 23 Dec, 30 Dec-02 Jan, and 08-10 Jan. The remainder of
the outlook period is expected to be mostly quiet.


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