The Sun had a party - Strong Solar Activity - 2 shocks passed ACE
posted: Sept 26, 2011 - 16:00 LT
Since September 21, the solar activity got a strong boost: the background X-ray radiation was strong (C-level - the answer on the question if there is a chance for C-flares becomes trivial at that moment), a large number of strong light flashes initiated on a particular site on the Sun, a dramatic number of plasma eruptions and a smooth but steady increase of the proton flux measured near the Earth.

Part of the energy stored in a large and dense group of magnetic bundles on the east side of the solar disk was released in the form of a strong light flare on Sept 22. In the same event, a plasma cloud left the Sun. The released energy was also used to accelerate protons gyrating around magnetic field lines that emanate into space. Those field lines give rise to the Parker spiral: the magnetic field lines bend while fanning into space because of the Sun rotation around its own rotation-axis. At that moment of the proton release, the Earth was not yet magnetically connected with that particular site on the Sun: those magnetic field lines did not pass along the Earth. But, the plasma cloud was very broad and large. It was seen as a halo by LASCO onboard SOHO, COR2 onboard STEREO A and COR2 onboard STEREO B. So, the cloud could throw protons towards the magnetic field lines that were connected with the Earth. The bulk of the protons did not reach Earth, only the protons aside of the CME could reach the Earth. That's why the proton flux near Earth rose very gradually. The gradual increase is an argument to conclude that the source region of the protons was situated on the east side of the solar disk.
On Sept 25, the solar wind speed jumped from 300 to 380 km/s. This could fit with a glancing blow of the plasma cloud of Sep 22. The side part of the cloud is in general slower than the front part and the cloud was probably considerably delayed on its trip through space because of the slow surrounding space plasma/solar wind. It's not unreasonable to assume that that part of the cloud had a mean speed of 600 km/s.

This event was followed by a series of similar events. Another (partial) halo was noticed on Sep 24.

More details in the Weekly Bulletin edited by the forecaster who was on duty - ready asap.
While writing, another strong shock is visible in ACE data: stay tuned! We might face a severe geomagnetic party in the Earth's magnetosphere and ionosphere!
See the latest presto-alert on the main page!
See the latest forecast
Plasma Clouds seen by CACTus. Click to enlarge. An overview of the latest CMEs. The proton flux measured by the geostationary satellite GOES. Click ....Go to the latest curve. The in situ solar wind parameters measured by the ACE satellite. Click to ...Go to the latest ACE measurments. The large shock of Sept 26 measured by ACE. ...