Weekly Highlights of Solar and Geomagnetic Activity, 20 - 26 September 2021

[SWPC.We...@noaa.gov]

:Product: Weekly Highlights and Forecasts
:Issued: 2021 Sep 27 0235 UTC
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# Weekly Highlights and Forecasts
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Highlights of Solar and Geomagnetic Activity
20 - 26 September 2021

Solar activity was at very low to moderate levels. Region 2871 (S27,
L=357, class/area=Dao/310 on 21 Sep) produced the largest flare of
the period, an M2/1n flare at 23/0444 UTC. Associated with the flare
was a Type II radio sweep (966 km/s) followed by a Type IV radio
sweep. A subsequent CME signature was observed in SOHO/LASCO C2
coronagraph imagery. Analysis and modeling of the event suggested
most of the ejecta was away from the Sun-Earth line but a glancing
blow from the periphery of the CME may be possible on 27 Sep. An
additional M1/1n flare was produced by the same region at 23/1528
UTC. A subsequent CME signature associated with the event was also
analyzed with a similar potential for the periphery of the ejecta to
influence Earth around 27 Sep.

Other activity included a Type II radio sweep (739 km/s) associated
with an eruption around Region 2875 (S32, L=053, class/area Bxo/010
on 24 Sep). The associated CME signature was narrow and directed
away from the Sun-Earth line. Most numbered active regions were in
decay and they transited through the W hemisphere with most decaying
to plage by the end of the period. The remaining numbered regions in
the E hemisphere were relatively simple and quiet.

No proton events were observed at geosynchronous orbit.

The greater than 2 MeV electron flux at geosynchronous orbit was at
normal to moderate levels throughout the reporting period.

Geomagnetic field activity ranged from quiet to active levels.
Active levels were observed on 22 Sep in response to a SSBC followed
by influence from positive polarity CH HSS. Unsettled conditions
were observed on 21 Sep, and 23-25 Sep due to coronal hole activity.
The remainder of the reporting period was quiet.

Forecast of Solar and Geomagnetic Activity
27 September - 23 October 2021

Solar activity is expected to range from very low to low levels. Low
levels are likely on 27-29 Sep and very low levels are likely for
the remainder of 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 moderate to high levels. High levels are likely on
29 Sep - 02 Oct, 05-08 Oct and 20-21 Oct due primarily to influence
from multiple, recurrent CH HSSs. Normal to moderate levels are
expected for the remainder of the outlook period.

Geomagnetic field activity is expected to range from quiet to G2
(Moderate) geomagnetic storm conditions. G1-G2 (Minor-Moderate)
conditions are likely on 27 Sep in response to combined effects of
an anticipated positive polarity CH HSS and possible glancing blow
from a CME that left the Sun on 23 Sep. Active conditions associated
with coronal hole activity are likely on 28-29 Sep, 05 Oct, 10 Oct
and 19 Oct. Unsettled conditions are likely on 04 Oct, 18 Oct and
20-21 Oct. The remainder of the outlook period is expected to be at
quiet levels.