Coronal mass ejections (CMEs) are giant eruptions of the Sun's atmosphere from its 'surface' which are ejected out into space. They are many times larger than the Earth and typically contain over a billion tonnes of matter. CMEs travel away from the Sun at speeds of up to several million km/h and can impact on comets, asteroids, and planets - including the Earth. Our planet is normally protected from CMEs by the terrestrial magnetic field, but the twisted magnetic fields carried by CMEs can break through this protective shield, causing particles to stream down over the Earth's polar regions and create auroras

The scientists came together to study one event in great detail in an attempt to gain an enhanced understanding of CMEs, to gain an insight into their prediction and more importantly, when and how they may interact with and cause effects on and in the vicinity of the Earth. After a painstaking analysis of the observations and measurements from all the different spacecraft and facilities on the ground, they have assembled an incredibly detailed picture.

They chose an eruption which lifted off from the Sun on the 13th May 2005 and headed in our direction. As it approached our planet, it interacted with the solar wind, the material which is constantly flowing out from the Sun at relatively steady rates. This particular CME deflected some of the solar wind northward as it headed in the direction of Earth and was itself slowed as a result of the solar wind ahead of it.

The mass expelled in the event was not that different from many other solar eruptions but its magnetic field was very intense, and as such, this event caused the largest geomagnetic storm (rapid changes in the shape and strength of the Earth's magnetic field) during the year 2005. At that time solar activity was in decline from the maximum period between the years 2002 and 2004 to the recent minimum between 2008 and 2010.

Data used to conduct this study came from many sources and in many forms. These included images of the Sun and its vicinity from instruments aboard the SOHO spacecraft; radio-burst data from the Wind spacecraft, GOES satellite, and ground-based instrumentation, solar wind measurements from the SOHO, ACE, and Wind spacecraft and measurements of the Earth’s magnetosphere and ionosphere from the Cluster and IMAGE spacecraft and ground-based magnetometers.

At the start of the event the outburst was thought to be a ‘simple CME’, but the unprecedented coverage revealed it to be extremely complex, with many small parts which when looked at individually, make up the bigger picture from its launch through to its arrival at the Earth. The event was caused by multiple flare-type events near the solar surface which released magnetic energy and mass out into the solar wind in the form of the CME.

The material then travelled through interplanetary space out towards the Earth (in this phase it is described as an Interplanetary CME or ICME).  With the magnetic field frozen inside it in the form of a ‘flux rope’, or ‘magnetic cloud’ (MC), when the ICME reached our planet it began to compress the Earth’s magnetic field in to a distance of about 38000 km (in comparison, the field on the Sun-ward side would normally extend to 95000 km). The arrival of the CME also caused some minor effects on satellites and communications as well as wonderful auroral displays.

Dr Bisi sees the new analysis as a key step forward in our understanding of the way solar eruptions develop and affect the Earth.  “We learned an enormous amount from the 2005 event.  Even an apparently simple CME turned out to be incredibly complex.  And the intense reaction of the Earth’s magnetic field to a fast but not particularly powerful event was a surprise.”

Source: Royal Astronomical Society