Aditya-L1 mission reveals why Sun's corona is hotter than surface

The Sun defies a fundamental rule of nature: temperatures should decrease as one moves away from a heat source. However, the Sun’s outer atmosphere, known as the corona, is hundreds of times hotter than its visible surface, the photosphere. While the photosphere’s temperature is about 5,500°C, the corona soars to over 1,000,000°C. This baffling phenomenon has puzzled solar physicists for decades and remains one of the most significant unanswered questions in astrophysics.

India’s Aditya-L1 mission, launched by the Indian Space Research Organisation (ISRO), is now offering groundbreaking insights into this mystery. Positioned at the Sun-Earth L1 point, approximately 1.5 million kilometers from Earth, the spacecraft provides an uninterrupted view of the Sun. Its primary goal is to study the Sun’s atmosphere, particularly the layers between the photosphere and corona, to understand how energy moves through these regions.

The corona’s extreme heat is thought to be linked to the Sun’s magnetic fields. As a massive ball of electrically charged plasma, the Sun is threaded with powerful magnetic structures. These magnetic fields twist, reconnect, and release vast amounts of energy. Scientists believe that part of this energy is transferred to the corona, causing its temperature to rise dramatically instead of decreasing with distance from the solar surface.

Two main theories dominate the ongoing research into this phenomenon. The first is magnetic reconnection, where tangled magnetic field lines snap and reconnect explosively, releasing energy. The second involves Alfvén waves, which are magnetic vibrations capable of carrying energy upward through the solar atmosphere. Determining which of these processes plays a more significant role has been a long-standing challenge for researchers.

Aditya-L1 is equipped with advanced instruments, including the Solar Ultraviolet Imaging Telescope (SUIT), which observes the Sun’s photosphere and chromosphere at multiple wavelengths simultaneously. This unique capability allows scientists to track how energy and matter move between different layers of the solar atmosphere. Recent observations from SUIT have captured solar flares, ultraviolet plasma eruptions, and dynamic atmospheric processes with unprecedented detail. These measurements are providing crucial evidence about how magnetic energy moves from the Sun’s lower atmosphere to the corona.

Understanding the mechanisms behind coronal heating has implications beyond academic interest. The same magnetic processes that heat the corona also drive solar flares, coronal mass ejections, and solar storms. These phenomena can disrupt satellites, navigation systems, communication networks, and power grids on Earth. Aditya-L1’s observations aim to shed light on these processes, including chromospheric and coronal heating and the transfer of heat within the Sun’s atmosphere.

By simultaneously studying the photosphere, chromosphere, and corona, Aditya-L1 is helping scientists build a more complete picture of how energy flows through the Sun. More than a year into its operations, the mission is already providing data that were previously inaccessible, bringing researchers closer to solving a mystery that has puzzled astronomers for generations: why the Sun’s atmosphere is hotter than its surface. These findings could also improve predictions of space weather events, safeguarding modern technology from solar disruptions.