:Product: Weekly Highlights and Forecasts
:Issued: 2001 Apr 24 2112 UT
Highlights of Solar and Geomagnetic Activity
16 - 22 April 2001

Solar activity was at moderate levels during most of the period.
Region 9415 (S22, L = 001, class/area Eko/880 on 07 April) had a
deceptively quiet west limb passage on 16 April. However, it
produced a flare from beyond the west limb (roughly W115) on 18
April associated with a Type II radio sweep, a C2 X-ray flare (peak
at 18/0214 UTC), a solar proton event, and an impressive coronal
mass ejection (CME), most of which was directed anti-Earthward.
April 18 also saw the return of old Region 9393, which was
renumbered as Region 9433 (N17, L = 155, class/area Fki/790 on 21
April). This region was the likely source for isolated, optically
uncorrelated low-level M-class flares on 16 - 17 April (please refer
to the Energetic Events listing for flare specifics). Region 9433
continued to produce isolated low-level M-class flares during the
rest of the period, the largest of which was an M3/1n at 22/2044 UTC
with an associated 270 SFU Tenflare and a Type II radio sweep. Most
of the flares in Region 9433 have occurred within the trailer
portion of the group, where new magnetic flux emerged during the
last few days of the period resulting in the formation  of a
magnetic delta configuration on 22 April. Region 9433 continued
(ominously) to increase in size and magnetic complexity as the
period ended. The remaining regions were small- to moderate-sized
and simply structured.

Data were available from the Advanced Composition Explorer (ACE)
spacecraft for most of the period. Two CMEs passed the spacecraft
during the period. The first shock, associated with the X14/2b flare
of 15 April, reached ACE at 18/0005 UTC associated with abrupt
increases in solar wind velocity, temperature, and total IMF field
intensity. IMF Bz was variable following the shock with brief,
intermittent periods of southward IMF Bz with maximum southerly
deflections to minus 21 nT observed at 18/0320 UTC and 18/0430 UTC.
The second weaker shock, likely associated with the 18 April C2
flare from beyond the west limb, passed ACE at 21/1500 UTC and was
associated with relatively modest increases in velocity, density,
and total IMF intensity. IMF Bz turned mostly northward following
the shock, then took a southward turn at approximately 22/0330 UTC.
IMF Bz remained southward for the rest of the day with maximum
deflections to minus 11 nT.

Proton events at greater than 100 MeV and greater than 10 MeV were in
progress as the period began in the wake of an X14/2b flare that
occurred on 15 April. The greater than 100 MeV event began at
15/1405 UTC, reached a peak of 146 PFU at 15/1525 UTC, and ended at
approximately 17/0510 UTC. The greater than 10 MeV event began at
15/1410 UTC, reached a peak of 951 PFU at 15/1920 UTC, and ended at
17/1700 UTC. Proton events at greater than 100 MeV and greater than
10 MeV also followed the C2 event from beyond the west limb on 18
April. The greater than 100 MeV event began at 18/0255 UTC, reached
a maximum flux of 12 PFU at 18/0600 UTC, and ended at approximately
19/0325 UTC. The greater than 10 MeV event began at 18/0315 UTC,
reached a peak of 321 PFU at 18/1045 UTC, and ended at 20/0820 UTC.
Ground-level events and polar-cap absorption events were associated
with these proton events.

The greater than 2 MeV electron flux at geosynchronous orbit was at
moderate to high levels during 16 - 17 April. Electron fluxes
decreased to mostly normal levels for the remainder of the period.

The geomagnetic field was disturbed on 18 and 21 - 22 April due to
CME passages at Earth. The 18 April disturbance began with a sudden
storm commencement (SSC) at 18/0048 UTC (50 nT, Boulder USGS
magnetometer) followed by active to major storm levels with severe
storm levels at high latitudes. The storm subsided to mostly quiet
to unsettled levels after 18/1200 UTC. The 21 - 22 April disturbance
began at around 21/1530 UTC, following a relatively weak
interplanetary shock passage. Field activity increased to active
levels at around 21/2100 UTC. A subsequent increase to active to
minor storm levels occurred during 22/0400 - 1800 UTC with periods
of major to severe storming detected at a few high latitude
monitoring stations. This disturbance continued as the period ended.

Forecast of Solar and Geomagnetic Activity
25 April - 21 May 2001

Solar activity is expected to range from moderate to high levels
during the period. Region 9433 is expected to produce M-class flares
during the period. This region could also produce isolated major
flare activity before departing the visible disk on 02 May. Old
Region 9415 is due back on 30 April and may provide an increased
chance for isolated major flare activity. The same is true for (Old)
Region 9433, which will return to the visible disk on 14 May.

Solar proton events will be possible during the period.

The greater than 2 MeV electron flux at geosynchronous orbit is
expected to be at normal to moderate levels during most of the
period.

Geomagnetic field activity is expected to be at mostly unsettled
levels during the period, barring an Earth-directed CME.


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