|A full halo CME arrived at earth|
|Sunspot group 06 (NOAA 0669) is still in the running as it released an M-flare on September 12, together with an earth-directed CME.|
|Nederlandstalige versie / Version française||posted: Sept 14, 2004|
After a series of backsided CMEs last week, sunspot group 12 (NOAA 0672) finally sent out a CME in the direction earth. The plasma cloud left the Sun during a long duration event on September 12 and was first visible in LASCO/C3 at 01:18UT (see the figures above or a movie). We didn't have to wait a long time for the shock front to arrive at earth: less then 43 hours. SOHO/Celias detected on September 13, around 19:30UT a shock in several physical quantities like the solar wind speed, density, thermal speed (speed of random motions of particles in gas cloud) and flow angle. The ACE-satellite measured some disturbances in the L1 point of the interplanetary magnetic field and the north-south component of it. The orientation of this magnetic field is crucial for the onset of a geomagnetic storm. This process is called magnetic reconnection: coupling of the interplanetary and the geomagnetic field. The strength of geomagnetic disturbances is measured in terms of an index K ranging between 0 and 9. The groundbased station Izmiran measured a local K of 5 at the arrival of the shock front, this is a minor storm.