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.