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Journalism of Courage
A recent study has uncovered new insights into a long-standing mystery: why the Sun’s outer atmosphere, or corona, is so much hotter than its surface. While the Sun’s surface temperature, the photosphere, hovers around 10,000°F (5,500°C), the corona reaches a scorching 2 million°F (1.1 million°C).
Led by solar physicist Richard Morton from Northumbria University, the research team used data from the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii—the world’s largest ground-based solar telescope—to investigate “magnetic waves” in the Sun’s atmosphere that could explain this heating phenomenon.
For decades, scientists have struggled to understand how energy generated at the Sun’s surface is transferred to the corona and the solar wind, which travels at speeds over 1 million mph (1.6 million km/h).
The hypothesis is that magnetic waves—specifically Alfven waves—play a key role. Alfven waves are low-frequency, transverse electromagnetic waves that propagate along the Sun’s magnetic field lines. However, these waves had never been directly detected in the corona until this groundbreaking study.
Previous instruments lacked the sensitivity to observe these subtle motions, forcing many models to rely on assumptions about the properties of Alfven waves. The new findings are a significant breakthrough, confirming their existence and potentially revolutionising our understanding of solar dynamics.
DKIST’s 4-meter mirror provides exceptional resolution for solar observations, offering clearer data than previous solar telescopes. The team used the Cryogenic Near Infrared Spectropolarimeter (Cryo-NIRSP) to study coronal Alfven waves. This instrument can visualize movement in the solar corona and measure changes in solar plasma through the Doppler shift effect, which captures the frequency changes as the observer moves relative to the source.
The study uncovered distinct red and blue Doppler shifts, which signaled the presence of Alfven waves. These waves appeared as twisting patterns in the corona’s magnetic field, suggesting their widespread presence throughout the Sun’s atmosphere. Morton emphasized that these waves likely carry significant energy, an important finding in the ongoing debate over how the Sun’s corona gets so hot.
Previous spacecraft data had pointed to magnetic reconnection—where intertwined magnetic fields release energy—as a key mechanism behind coronal heating. But DKIST’s results complicate this picture, showing that both Alfven waves and magnetic reconnection frequently occur together in the Sun’s atmosphere.
The study suggests that Alfven waves may account for at least half of the energy needed to heat the corona, though accurately quantifying their energy remains challenging. The interaction between magnetic reconnection and Alfven wave activity is crucial, not only for understanding solar heating but also for predicting the Sun’s radiative output, which influences the behaviour of other stars.
This research has broader implications for understanding planetary system evolution over long periods and improving short-term solar wind forecasts. Future studies are expected to shed more light on Alfven waves’ characteristics, refining current models and predictions.
