Coronal Mass Ejections from Multiple Viewpoints

After many years of observing, measuring and interpreting Coronal Mass Ejections (CMEs) from a single near-Earth viewpoint, we have entered an era in which they are being observed from multiple viewpoints. In October, 1996, NASA launched the STEREO mission, which consisted of two nearly-identical spacecraft in orbit about the Sun – one drifting in front of Earth and the other behind. The drift rate of each spacecraft from Earth is about 22.5 degrees per year, so that in Jan, 2010 they will be about 65 and 70 degrees from Earth or about 135 degrees from each other. Since November, 2007, they have been separated by more than 45 degrees, which easily enables mathematical stereo reconstruction of the CME structure. Although the solar cycle has been extraordinarily quiet, a significant number of CMEs have been observed with the SECCHI instrument, mostly slow and not massive. But they have been observed in isolation, without the confusion of several events occurring at around the same time and merging together. This has enabled the determination of the general CME structure. Virtually all of the CME events are consistent with an idealized “flux rope” - a cylindrical shell in which the dense material encases a low-density magnetic region. These CME events occurred all around the Sun, but some could be tracked toward Earth. During this transit, their shape evolved as it “plowed” into the slower solar wind ahead of it or got pushed from behind by a high speed stream. This tracking of CMEs was enabled by very sensitive telescopes on STEREO/SECCHI imaging the inner heliosphere, the region of space between the Sun and Earth. The CMEs as well as the compression region ahead of the high speed streams, the co-rotating interaction regions (CIRs) can be seen from their formation and tracked all the way to the orbit of Earth. This enables far better predictions of the arrival time of a particular event at Earth – from an imprecise warning of an impact in a few days to an increasingly accurate determination of the impact time. Thus, the STEREO mission, with its unique orbit, is a path-finding mission to demonstrate that it is possible to increase the reliability of such warnings from a mission that was located at the L4 or L5 Lagrangian point, much like the missions in orbit about L1.