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Surprisingly long-lived coronal hole
STANFORD -- Stanford researchers have verified the existence of a hole in the sun's corona that persisted for several years. Since the 1960s coronal holes have been identified as the source of high-velocity solar wind streams. But scientists generally had considered them to be a transient feature with a lifetime of months, rather than years.
When the spacecraft Mariner 2, which confirmed that high-speed solar winds originate from coronal holes, monitored solar wind patterns between 1962 to 1974, the longest-lived jet that it found lasted for 14 months.
In light of the Mariner 2 observations, "this long-lasting coronal structure was somewhat surprising," said Taeil Bai, senior research scientist at Stanford's Hansen Experimental Physics Laboratory, who presented the paper.
The first evidence of the long-lived coronal hole came from the Ulysses spacecraft in the early 1990s. As it traveled over the sun's south pole, its instruments observed a 26-day pattern in the flux of energetic particles passing the spacecraft that persisted for two years, and a corresponding variation in solar wind speed that lasted for a year.
The Stanford researchers, using X-ray observations from the Japaese satellite Yohkoh, verified that the source of this pattern was a polar coronal hole that protruded into the
"Such protrusions from the stable coronal hole at the sun's poles have been observed to come and go, but for one to remain for several years is remarkable," Bai said.
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