halo.press 21,January,1997
Guenter Brueckner
Press release for the NASA press conference on the Solar-Terrestrial event,
06 Jan.1997 - 10 Jan.1997.
A "Halo" coronal mass ejection as the cause
of geomagnetic activity.
Washington,DC---An international group of space scientists has
observed a "Halo" coronal mass ejection at the Sun.This event developed
into a giant magnetic bubble,or magnetic cloud,which travelled to the
Earth,where it caused a large magnetospheric storm.
The group of American,German,French,and British scientists,under the
direction of Dr.Guenter Brueckner of the Naval Research Laboratory (NRL),
says,that these observations are crucial to the understanding of the
magnetic clouds,their origin on the Sun and future predictions of
geomagnetic storms,as early as 4 days before their occurance.
The group used observations from NRL's Large Angle Spectrometric
Coronagraph (LASCO) instrument on borard the ESA-NASA Solar and
Heliospheric Observatory (SOHO) satellite.
LASCO consists of three "coronagraphs",which can image the very dim
corona out to 32 solar radii distance from the Sun.These coronagraphs need
to be space born to avoid the glare of the Earth' atmosphere.Furthermore,
they are specially designed telescopes,which reject the glaring light from
the visible solar disk.
Coronal mass ejections,which are flying toward the earth,can be seen
in a coronagraph as "Halo" or "Ring" events.Because the coronagraphs use a
blocking disk in front of their aperture lens,any event,moving toward the
Earth,cannot be seen directly against the solar disk.But these events
become visible as a faint ring around the occulting disk,after they have
expanded enough to be larger than the apparent diameter of the occulting
disk,which is 2.24 Million kilometer,projected against the solar disk.After
they have expanded larger than this,they appear as a faint ring from behind
the occulting disk.
At this point,the coronal mass ejection has already the typical
features of a magnetic cloud:A strong magnetic field forms a magnetic
bubble.In front of it,the moving bubble forms a shock wave.Internally,the
strong magnetic pressure balances the gas pressure,which results in a lower
density.At the surface of the magnetic cloud,the density increases and the
material is visible as a faint ring against the sky.
First signs of the Halo event were seen on 06 Jan.,97 at 17:30 UT.The
full event shows up in the inner coronagraph at 18:50 UT of the same day.
(Fig.1).It showed up in the outer coronagraph appr. 3.5 hours later.(Upper
left picture of Fig.2)
The Halo could be tracked for another 4.5 hours in
the outer coronagraph.(Fig.2)
It moves slightly in a south- westerly
direction.By the time the magnetic cloud reaches the earth 92 hours (3
days,20 hours) later,the magnetic cloud's diamter had expanded to 26
Million kilometers (18 tinmes the Sun's diameter).This large dimension
ensures,that the cloud engulfes the Earth.
From the expansion speed of the cloud,one can estimate that it left
the solar surface appr. on 06 Jan.at 15:50 UT.The first sign of the cloud
were observed from the Wind spacecraft on 10 Jan.at 04:45 UT,moving with a
speed of 450 km/second.If one assumes that the cloud travelled with
constant speed in a straight line from the Sun to the Earth,one can project
backward its departure from the solar surface on 06 Jan.at 9:00 UT.The
difference of 4:15 hours between these two times is very small when
compared with the total travel time of 92 hours.There were no other events
observed during this period.Consequently,a certain relationship between the
Halo event and the magnetic cloud seen at the Earth exists.
Large coronal mass ejections can be seen more easily,when they are
ejected in the plane of the sky,either above the east or above the west
limb of the Sun.But these events never reach the Earth.Figure 3 shows a
large event being ejected over the west limb.Its magnetic cloud never
reached the earth.It is for this reason,that the Halo events are more
interesting to study their relations with magnetic clouds,observed at the
Earth.
This Halo event is the second one ever observed during the minimum of
solar activity and the first one,which has been seen during solar minimum
which caused a major magnetospheric storm.
The LASCO instrument is far more sensitive than any of its
predecessors.This makes observations of these Halo events possible.
But the appearance of a Halo event can not be used by itself for a
prediction of geomagnetic activity.Only those magnetic clouds react with
the magnetosphere,which have a magnetic field direction anti-parallel to
the magnetic field of the Earth.The coronagraph observations alone do not
show the magnetic cloud's magnetic field direction.