Image credit: Hinode/XRT, courtesy NASA Goddard.
A longstanding problem with our understanding of the Sun was finding a reason that would explain why the corona was hotter than the surface. Logically it would seem that the farther one gets from the heat-producing energy of the Sun, the cooler the temperatures would be, but instead the corona -- one of the outermost layers of the atmosphere -- can be as hot as one to two million Kelvin. The solar surface stays at a balmy 6000 K.
However, scientists seem to have finally found the explanation. It appears that a high energy type of spicule -- a jet of plasma that reaches from the surface all the way into the outer atmosphere -- is responsible for heating up the corona. As the NSF notes, spicules have been known about for decades, but the ones scientists observed weren't hot enough to explain coronal temperatures.
The solution was discovered by scientists from Lockheed Martin's Solar and Astrophysics Laboratory (LMSAL), NCAR, and the University of Oslo, supported by NASA and the NSF, NCAR's sponsor. They theorized a new kind of spicule, which they call Type II, that is made of super-hot plasma and shoots out at speeds of over 100 km per second -- also causing them to disappear so rapidly that they were nearly impossible to observe.
Using NASA's Solar Dynamics Observatory and the Japanese Hinode satellite (source of the image above), they were able to observe the Type II spicule. Though there are still questions to be answered -- what causes these super hot spicules, for one -- this is still an exciting development during a time of increasingly productive solar study.