The Sun’s visible surface – the photosphere – is 5500° C. Outside it is a tenuous layer of hot, ionized gas (plasma) – the corona, which is familiar to anyone who has seen a total solar eclipse, since it glimmers white around the hidden Sun. The solar corona get hotter – up to nearly 4 million degrees C, rather than colder – the farther it is from the Sun’s surface. This fact has puzzled solar astronomers for decades. A suborbital rocket mission that launched in July 2012 has provided a partial answer. The High-resolution Coronal Imager (Hi-C) verified one of the mechanisms that pumps energy into the corona: the complex process of magnetic reconnection.
The Sun’s activity, including solar flares and plasma eruptions, is powered by magnetic fields. The Sun’s surface is like a collection of thousand-mile-long magnets scattered around after bubbling up from inside the Sun. Magnetic fields poke out of one spot and loop around to another spot. Plasma flows along those fields, outlining them with glowing threads. The images from Hi-C showed interweaved magnetic fields that were braided just like hair. When those braids relax and straighten, they release energy. Hi-C witnessed one such event during its flight.
It also detected an area where magnetic field lines crossed in an X, then straightened out as the fields reconnected. Minutes later, that spot erupted with a mini solar flare. Hi-C showed that the Sun is dynamic, with magnetic fields constantly warping, twisting, and colliding in bursts of energy. Added together, those energy bursts can boost the temperature of the corona to 4 million degrees C when the Sun is particularly active.
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