How Fast? (January 20, 2005)
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On Jan. 15, 2005 SOHO observed a substantial coronal mass ejection (CME) blast out
from the
Sun, heading very much in Earth's direction. But its speed is what captured the
attention
of scientists. When they measured it, they were surprised to find a record
expansion rate
of 2890 kilometers per second in the plane of the sky. LASCO C3 has such a large
field of
view that it gives us time to measure at least two positions for such fast events,
so we
can measure a speed (distance divided by time), while older space-based and
ground-based
coronagraphs were/are limited by much smaller fields of view. (The record speed
was
measured in the plane of the sky, which is not the same as its speed in our
direction, but
the rate at which we see the CME expanding horizontally in our images.) In addition, it is a long way from the Sun to Earth and space is not totally empty. CMEs usually get slowed down by the slower solar wind. The particle cloud from the Jan. 15th CME arrived at Earth about 40 hours later when sensitive instruments detected a strong spike and powerful aurora were observed in many areas around the world. This CME was followed by an even larger one on Jan. 17th (see still) spawned by the same active region. Both can be seen in the video and both CMEs were followed by protracted protons storms. These appear as snowstorms of tiny white specks on SOHO imagers, far more obvious after the Jan. 17th storm. The related story is the speed with which the sunspot group (AR 10720) that generated this storm grew as we watched it. On Jan. 11, 2005, it began to form as a smallish dot. By Jan. 15 it had swollen to about the size of Jupiter (something like 10 Earths across), a most impressive sunspot. Its rate of development was one of the most dramatic that we have witnessed with SOHO. All told it seems that on two different but related fronts speed was the theme of the week. SOHO began its Weekly Pick some time after sending a weekly image or video clip to the American Museum of Natural History (Rose Center) in New York City. There, the SOHO Weekly Pick is displayed with some annotations on a large plasma display. If your institution would also like to receive the same Weekly Pick from us for display (usually in Photoshop or QuickTime format), please send your inquiry to steele.hill@gsfc.nasa.gov. |
