Why Sun’s outer atmosphere is millions of degrees hotter than its surface

Scientists have found the answer to one of the most long-standing mysteries of physics: Why the Sun’s outer atmosphere or the corona, is millions of degrees hotter than its surface.

They have discovered a major source of hot gas that replenishes the corona: narrow jets of plasma, known as spicules, shooting up from just above the Sun’s surface.

“By identifying that these jets insert heated plasma into the Sun’s outer atmosphere, we gain a greater knowledge of the corona and possibly improve our understanding of the Sun’s subtle influence on Earth’s upper atmosphere,” said Scott McIntosh, a scientist at the National Center for Atmospheric Research (NCAR).

Research in the 1980s found that spicule plasma did not reach coronal temperatures but in 2007, Bart De Pontieu, the lead author and a solar physicist at LMSAL, and McIntosh identified a new class of spicules that shoot upward at high speeds, often in excess of 60 miles per second (100km per second), before disappearing.

“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,” said McIntosh.

De Pontieu believes that understanding the interface region between the Sun’s visible surface, or photosphere, and its corona is a key step.

Another NASA mission, the Interface Region Imaging Spectrograph (IRIS) in 2012 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.

The study is published in Science.