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SOLAR SYSTEM

Why is the Sun’s atmosphere hotter than its surface?

The Sun’s atmosphere is hotter than its surface because of energy transported and deposited by the Sun’s magnetic field. The magnetic field is created in the solar interior and carried outwards to fill the atmosphere by the motion of hot bubbles of plasma in the Sun. In the atmosphere, the field lines are constantly moved by the motions of the magnetic footpoints at the Sun’s surface. These motions cause waves and reconfiguration of the atmospheric field. These in turn act to heat the atmospheric plasma.

Since the atmosphere is a plasma – a gas of electrons and protons not combined into atoms – the magnetic field and the gas are interconnected; if you move the field, you move the gas, and vice versa. There are many competing ideas as to how changes in the magnetic field lead to a heating of the plasma, and this is the focus of intense current research. New observations from both space missions and ground-based telescopes are now enabling scientists to address coronal heating, and a more clear picture should arise in the coming decade.

Dr Huw Morgan, director of research and head of Solar System physics, Aberystwyth University

ASTROPHYSICS