Why is the Sun's outer atmosphere much hotter than its surface?

Space · Published 13 October 2026 Swipe more like this →

In short: The Sun's surface sits at a fiery 5,500°C, yet its outer atmosphere, the corona, reaches millions of degrees. This massive temperature spike defies intuition. NASA's IRIS mission investigates this mystery, studying the interface region where solar material gathers energy and moves through the Sun's lower atmosphere.

Solar physics faces a primary mystery regarding coronal heating: the reason the Sun's million-degree corona exceeds the temperature of the underlying surface by several hundred times. Scientists have long debated whether this heating occurs uniformly or through localized, explosive events. Recent evidence suggests the latter, driven by magnetic reconnection, where magnetic fields in the corona clash and realign explosively.

To untangle this, NASA's Interface Region Imaging Spectrograph (IRIS) focuses on the 'interface region'—the transition between the photosphere and the corona. This area includes the chromosphere and transition region, where temperatures climb rapidly from thousands of degrees to millions. By analyzing ultraviolet light spectra and images, IRIS tracks how solar material moves and accumulates energy as it traverses these layers.

Launched in 2013 as a Small Explorer Mission, the IRIS spacecraft is relatively compact, measuring about 2.1 meters in length with solar panels 3.7 meters across. Despite its size, it carries a 20-centimeter ultraviolet telescope capable of distinguishing solar features as small as 150 miles across. While the mission was originally slated for two years, extensions have allowed for continued study of phenomena like solar flares and nanojets.

Beyond heating, the mission examines how the interface region feeds the solar wind—a constant stream of charged particles—and how magnetic flux emergence influences solar storms. By combining high-resolution observations with 3D supercomputer models, researchers aim to understand the complex magnetic forces that drive the Sun's dynamic, ever-changing atmosphere.

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