The mystery of the Sun’s corona may finally be solved. For years researchers have known – and wondered why – the Sun’s outer atmosphere, or corona, is considerably hotter than its surface. But now, using the combined visual powers of NASA’s Solar Dynamics Observatory and Japan’s Hinode satellite, scientists have made direct observations of jets of plasma shooting off the Sun’s surface, heating the corona to millions of degrees. The existence of these small, narrow jets of plasma, called spicules has long been known, but they had never been directly studied before and were thought to be too cool to have any appreciable heating effect. But a good look with new “eyes” reveals a new kind of spicule that moves energy from the Sun’s interior to create its hot outer atmosphere.
“Heating of spicules to millions of degrees has never been directly observed, so their role in coronal heating had been dismissed as unlikely,” says Bart De Pontieu, the lead author and a solar physicist at LMSAL.
Solar physicst and former Universe Today writer Ian O’Neill (and current Discovery Space producer, and of Astroengine fame) compared the anomaly of the Sun’s atmosphere being hotter than the surface to if the air surrounding a light bulb was a couple of magnitudes hotter than the bulb’s surface. And, he said, you’d want to know why it appears the solar atmosphere is breaking all kinds of thermodynamic laws.
Over the years, experts have proposed a variety of theories, and as De Pontieu said, the spicule theory had been dismissed when it was found spicule plasma did not reach coronal temperatures.
But In 2007, De Pontieu and a group of researchers identified a new class of spicules that moved much faster and were shorter lived than the traditional spicules. These “Type II” spicules shoot upward at high speeds, often in excess of 60 miles per second (100 kilometers per second), before disappearing. The rapid disappearance of these jets suggested that the plasma they carried might get very hot, but direct observational evidence of this process was missing.
Enter SDO and its Atmospheric Imaging Assembly instrument which launched in February 2010, along with NASA’s Focal Plane Package for the Solar Optical Telescope (SOT) on the Japanese Hinode satellite.
“The high spatial and temporal resolution of the newer instruments was crucial in revealing this previously hidden coronal mass supply,” said Scott McIntosh, a solar physicist at NCAR’s High Altitude Observatory. “Our observations reveal, for the first time, the one-to-one connection between plasma that is heated to millions of degrees kelvin and the spicules that insert this plasma into the corona.”
The spicules are accelerated upward into the solar corona in fountain-like jets at speeds of approximately 31 to 62 miles per second (50 to 100 kilometers per second). The research team says that the majority of the plasma is heated to temperatures between 0.02 and 0.1 million Kelvin, while a small fraction is heated to temperatures above one million Kelvin.
A key step in learning more about the Sun, according to De Pontieu, will be to better understand the interface region between the Sun’s visible surface, or photosphere, and its corona. Another NASA mission, the Interface Region Imaging Spectrograph (IRIS), is scheduled for launch in 2012. IRIS will provide high-fidelity data on the complex processes and enormous contrasts of density, temperature, and magnetic field between the photosphere and corona. Researchers hope this will reveal more about the spicule heating and launch mechanisms.
This research appears in the 07 January issue of Science.