CME Cannibalism
posted: Sept 13, 2011
Happenings on the Sun and in Space on Sep 06, 2011
On Sept 06, NOAA 1283 released an M5.3 flare (peak at 01:50UT) and blew a plasma cloud - CME into space. The active region was at that moment located near the central meridian (a virtual line on the solar disk in front of the Earth), slightly above the solar equator: N14W07. These are the perfect ingredients for an Earthward directed CME recipe. If you watch the plasma cloud in the coronograph COR2 onboard STEREOA, for example, you see that the cloud however is directed well above the ecliptic: no really straight to Earth. But a CME is something big, very big. When the CME is faster compared to the ambient solar wind, a shock front is formed (think of a speed boat). A shock has even a broader spatial extension compared to the CME itself. Because a CME is so huge, there is a big chance that (a part of) the CME indeed passes the Earth even if it is not directed straight away to the Earth. This CME was not fast: we estimated the speed to be between 300 and 400 km/s.

What happened next : on that same day, the same active region, released an X2.1 flare flash peaking at 22:20UT. The Sun rotated such that the action region was located at N14W18: we consider this still near the central meridian. The CME associated with this flare was faster: from coronographic images of STEREO A/COR2, we estimated the speed to be around 800 km/s.

We think that this second CME caught up the first one, bumped on it and slowed down. This is cannibalism in space. After the clash, the two CME continue to speed through space as one big cloud with a larger mass then of its constituents. It's just adding mass.
Consequences for the Earth
The satellite ACE, in front of Earth, passed through this cannibalistic monster on Sept 09 and 10: first the shock and secondly the cloud itself. ACE measures in situ the conditions of the solar wind. It saw first a strong sudden increase of the solar wind speed, the magnetic field strength, the temperature and the density followed by a plasma with magnetic irregularities. On Sept 10, we see a rather smooth rotation of the magnetic field first pointing in one direction and after the rotation, pointing in the opposite direction.

The cannibal clashed on the Earth magnetosphere causing our magnetic shield to wobble. This caused a geomagnetic storm on Sept 09 and 10.

STEREO A/COR2 - the first halo CME. Click to enlarge. STEREO A/COR2 - the second halo CME. Click to ... The in situ measurements of ACE. Click .... The Kp index estimates the disturbance of the magnetic field of the Earth on a planetary scale. ....