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February 22, 2010 | Solar scientists have long debated why the Sun's corona, or atmosphere, is millions of degrees hotter than its surface. Images retrieved by the Hinode satellite, launched in 2006, are shining some light on this paradox.
NCAR's Scott McIntosh, a solar physicist, and colleagues have discovered a new type of solar spicule in the satellite imagery. Solar spicules are supersonic jets of plasma that burst from the Sun at speeds up to 50,000 miles per hour (81,000 kilometers per hour), reaching heights of 3,000 mi (5,000 km). They are difficult to study due to their relatively short lifespans (about five minutes) and small sizes (diameters measuring about 300 miles, or 500 kilometers).
Because the plasma in these spicules does not reach the temperatures found in the solar corona, scientists have not credited them with heating the corona. However, by analyzing data from Hinode, McIntosh was able to trace Type II spicules—very subtle columns of plasma that are hotter, faster, and shorter-lived than the previously known spicules, now called Type I. Evidence indicates that the new spicules, which researchers have dubbed radices, may play an important role in supplying and replenishing the corona's heat, helping explain the temperature difference between the corona and solar surface.
"Our calculations indicate that radices can fill the corona with hot plasma even if only one to five percent of the radices reach coronal temperatures," McIntosh says.