One Way to Heat the Corona

The Sun’s corona is incredibly hot, with temperatures of 1-2 million degrees, and yet we don’t really understand how it’s heated. Thermonuclear fusion keeps the center of the Sun comparably hot, but between the Sun’s interior and its outer coronal layers lies the much cooler photosphere and chromosphere, where the temperature drops to a chilly 6000 degrees or so.  This cool layer sandwiched between the Sun’s interior furnace and its hot corona presents a problem for heating the corona. Heat can’t flow in its normal way through this cool zone, and so it’s difficult to understand how the energy can get from the inside to the outside. It’s like trying to cook a steak on the top of an igloo by setting a fire inside.

One possible way to move energy through the cool zone involves the magnetic fields that thread from the photosphere into the corona. The Sun’s turbulent motion wiggles these magnetic fields, producing waves that slide along the field lines, much like waves across a pond. These waves pass through the cooler zone relatively unattenuated only to steepen and “crash” when they reach the corona.

The situation is somewhat like that of an earthquake-produced tsunami. Such a wave can travel thousands of miles across open ocean, and only deposits its destructive energy when it encounters the shores of some far-off land. In the magnetic field case, the rarefied plasma of the corona plays the role of the unfortunate coastline, absorbing the energy of the waves.

One of our eclipse experiments is designed to detect the observable signature of these crashing waves. Theoretical models indicate that the waves likely to provide the most energy to the corona should have a period between about a half second and two or three seconds. We will look for flashes in the brightness of the corona with the same range of periods. These periodic brightenings may be the signs of each successive wave crashing into the corona.

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