September 27-??, 1997
Solar Observations from SOHO
SOHO LASCO/EIT observed a halo CME starting late in the day on Sept 27th.
The activity in EIT began around 22:00-23:00 UT in the active region about
30 degrees north of central meridian. There was no obvious coronal shock
wave observed with this event, just a series of expanding loops.
The event appeared in the C2 field of view at 01:27 UT on Sep. 28th.
The leading edge first appeared to the east and north and gradually
developed to form a complete circle. We measured the leading edge
to the NE and came up with a velocity of ~240 km/sec in the plane
of the sky.
From G. Brueckner,
(1) EIT:Exploding loop at 0928 0051 in north-east corner of active region,
followed by dimming of the corona above the explosion,but limited to
a small area.
HALO#1:Visible above occulter at 0928 0149,speed:297km/sec.Projected
lift off time from surface:0109.This seems to be within the error
of the exploding loop timing.
HALO#2:Visible above occulter at 0928 0230,same speed,but weaker event.
Other than a brightening loop,no counterpart in the EIT at 0150.
HALOS were weak (not visible in the raw data,but clearly visible
in the running differences).
(3) Flares:B1 at 0928 0047,timing would fit the exploding loop.It is a
longer duration event (several hours) but very weak.
Comments:The absence of a Coronal EIT Wave is suspicious,but we have not
proven,that each CME originates with a coronal shock wave. All the rest
seems to fit together for a weak event at the Earth. However,there was
considerable activity on Sept.27-29 at the back side of the Sun. Several
large CMEs went off in the plane of the sky in both east and west
direction. The brightest CME,which we ever have seen,went off to the east on
Sept.29.For that reason the HALO events could well have originated on the
back side of the Sun.
If something happens at the Earth,it should be not earlier than 1001 1000.
(Courtesy of Alan McAllister, Helio Research/HAO)
On September 27, 1997 Sara Martin of HelioResearch was observing filaments
in the northern hemisphere. These were located between roughly 15-30 N and
10E - 20W, slanting towards the SW (based on eyeball estimates from an BBSO
Ha image at 16:14UT, by AHM). They had formed in a winding filament channel
connecting and threading through and elongated region of older plage. There
were two main filament sections, one to the east was at roughly 30 degrees
from a N/S line and extended for perhaps 150,000 km. Although still a thin
filament, it had developed barbs and could be identified as a dextral
filament, the dominant pattern in the northern hemisphere. The other filament
to the west was smaller and more nearly E/W, being about 60,000 km long. Late
in the day the longer filament erupted, followed by a classic two
ribbon flare. The flare is estimated to have started between 22:00 and 22:45.
The location of the filament indicates that the associated CME will very
likely intersect Earth, sometime on September 30, assuming roughly average
transit time. The season, being just after the equinox, is most favorable
for geomagnetic activity associated with northern hemisphere events.
The global magnetic field rooted near the north polar hole, must be southward.
The extended plage, has a positive leading region and a negative trailing
region (new cycle) with the leader having the same polarity as the pole. The
relative relation of the plages, suggests a left-skewed connection in the
corona, as expected for the northern hemisphere. Yohkoh and EIT images do
show a left-skewed arcade over the filament matching that expected from the
filament chirality based on work by Martin and McAllister. Also in the EIT
304 image from Sept 27, the main shape of the filament is a backwards S as
expected in the north (Rust and Kumar).
The alignment of the plage fields would predict an roughly east-west axis
for the CME (or possible magnetic cloud) with the axial field pointing to
the east. We can predict (see Martin and McAllister, Chapman CME proceedings)
that the CME will have *left-hand helicity* (again being the dominent northern
pattern found by Bothmer, Schwenn, and Rust. The slight NE-SW tilt (if it
is maintained into the interplanetary medium) will lend a slight northern
component, near the core, but the outer layers should have a southward
The following flare was barely above the GOES background at about B0, but
the EIT 195 image from 18:29 UT on September 28, may show its aftermath.
There also appears to be brightening along the edge of the northern coronal
hole. We suggest that both the EIT and Yohkoh teams look for the arcade
signature that should have accompanied the flare. Although this is well on
the disk, it is possible that signatures may be seen by LASCO and HAO Mauna
Addendum: September 28, 1997: Continued observations show an emerging
flux region growing about 15 arcsecs along the channel from the reforming
filament. If this region continues to grow, additional eruptions should
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Last Updated: 09/30/97