:Product: Forecast Discussion
:Issued: 2026 Mar 18 0030 UTC
# Prepared by the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center
#
Solar Activity

.24 hr Summary...
Solar activity reached moderate levels due to a solo M-class flare. Four
sunspot groups were present on the visible solar disk, with NOAA/SWPC
Region 4392 (S16E01, Csi/beta-gamma) the source of an M1.3/Sn (R1-Minor)
solar flare at 17/0904 UTC. The region had been variable with periods of
growth and decay the past two days; however, today it has primarily
undergone decay while it retained a mildly mixed magnetic configuration.
Region 4391 (N07W47, Axx/alpha) reemerged, but stalled in any further
development. The remaining regions were mainly stable and changed
little.

An approximately 10 degree filament eruption from just SW of center disk
was observed around 17/0300 UTC in GONG H-alpha and GOES SUVI imagery.
An associated narrow CME became visible in STEREO COR2 at 17/0838 UTC
and is faintly visible in LASCO C2 at 17/1101 UTC CT. Analysis shows
glancing influences on 21 Mar but passage proximity and influences from
a coincident coronal hole high speed stream (CH HSS) could strengthen
any geomagnetic response.

.Forecast...
Solar activity is expected to be low, with a chance for M-class (R1-R2;
Minor-Moderate) level events 18-20 Mar due primarily to the minor
instability and variability of Region 4392.

Energetic Particle

.24 hr Summary...
The greater than 2 MeV electron flux reached high levels again, with a
peak of 2,230 pfu at 17/1715 UTC. The greater than 10 MeV proton flux
continued at background levels.

.Forecast...
The greater than 2 MeV electron flux is likely to reach high levels yet
again on 18 Mar. A brief return to normal-moderate levels on 19-20 Mar
is expected due to a likely coronal mass ejection (CME) arrival. The
greater than 10 MeV proton flux is expected to continue at background
values, with a slight chance for an S1 (Minor) level event 19-20 Mar due
to possible CME shock enhancement influences.

Solar Wind

.24 hr Summary...
Solar wind parameters were indicative of a gradual return to a slow
regime. Total IMF strength was at normal levels of mainly 4-5 nT and the
Bz component was mainly northward. Solar wind speed decreased from peaks
at 510 km/s to around 425-450 km/s. The phi angle was predominantly
positive.

.Forecast...
The solar wind environment is anticipated to transition to a near normal
regime through 18 Mar. An anticipated CME arrival near midday on 19 Mar
is expected to enhance solar wind speed and total IMF strength. CME
passage is likely to continue into 20 Mar with the additional factors of
two other nearby CMEs passing by close to Earth, as well as a possible
solar sector boundary crossing (SSBC). All this is likely to lead to a
complicated and amplified solar wind regime 19-20 Mar and beyond.

Geospace

.24 hr Summary...
The geomagnetic field was quiet.

.Forecast...
Quiet to unsettled conditions are expected on 18 Mar as positive
polarity CH HSS influences end. The geomagnetic field is likely to
become disturbed, with G2 (Moderate) storm levels likely upon and after
the anticipated CME arrival on 19 Mar. Active to G1 (Minor) storm
conditions are likely on 20 Mar due to continuing, but weakening CME
passage influences.