For decades, astronomers and astrophysicists have grappled with one of the most counterintuitive puzzles in solar science: why does the Sun’s outermost atmospheric layer, the corona, reach temperatures of millions of degrees Celsius—far hotter than the star’s visible surface, the photosphere? This stark temperature gradient across solar layers outright defies established thermodynamic logic, and until now, scientists have never been able to definitively quantify what drives the corona’s extreme heat and ability to replenish energy after massive eruptions.
New groundbreaking research from India, based on data collected by the country’s landmark Aditya-L1 space solar observatory, has brought the global scientific community closer than ever to solving this enduring mystery. In a new paper published in the peer-reviewed *Astrophysical Journal Letters*, a team led by leading solar astrophysicist Professor R Ramesh of the Indian Institute of Astrophysics (IIA) presents the first quantified breakdown of the two leading energy mechanisms thought to sustain the corona’s heat, revealing that magnetic field reconnection is responsible for nearly all of the layer’s energy replenishment.
To put the sun’s unusual temperature structure in context: the Sun generates all of its energy via nuclear fusion in its dense, superheated core, which hits roughly 15 million degrees Celsius. Moving outward from the core to the Sun’s visible photosphere, temperatures drop dramatically to around 5,500 degrees Celsius. Intuitively, temperatures should continue to fall as one moves further from the energy-producing core—but the opposite happens in the corona, where temperatures skyrocket back to between 2 million and 40 million degrees Celsius.
This anomaly becomes even more puzzling when accounting for the extreme energy the corona regularly sheds. The corona is the birthplace of catastrophic solar events including solar flares and coronal mass ejections (CMEs): massive expulsions of magnetized plasma and energy that the Sun launches into interplanetary space. While these ejections create striking auroras near Earth’s polar regions, they also pose serious risks to modern infrastructure, triggering geomagnetic storms that can disrupt satellite communications, interfere with GPS signals, and even bring down entire power grids.
During quiet periods of solar activity, the Sun produces between two and three CMEs per day. At the peak of its 11-year activity cycle, that number jumps to 10 or more CMEs daily. If the corona could not replenish the energy it loses during these frequent eruptions, the Sun would quickly cool, and Earth would be plunged into an irreversible deep freeze—an outcome that has never occurred, proving a powerful energy replenishment mechanism must be at work.
For years, scientists have hypothesized two primary processes that could supply the corona with energy. The first suggests that churning, convection-driven motions on the Sun’s surface generate energy-carrying waves that travel outward to the corona, much like ocean waves carry energy toward a shoreline. The second explanation centers on the Sun’s tangled atmospheric magnetic field lines: these lines constantly snap apart and reconnect, a process that releases massive amounts of heat energy.
Professor Ramesh’s team was the first to quantify the exact contribution of each mechanism, using high-resolution observations of an extremely energetic CME that occurred on August 5, 2024, captured by Aditya-L1’s Visible Emission Line Coronagraph (Velc) instrument. The team tracked the corona’s recovery in the 10 hours following the eruption, observing how tangled magnetic field lines snapped during the CME, then reconnected and reverted to their original configuration to replenish the corona’s lost energy.
Their calculations revealed a stark divide: just 7% of the corona’s total energy requirement comes from surface-generated waves, while a full 93% is supplied by the repeated snapping and reconnection of solar magnetic field lines.
While surface motion waves do contribute some energy, the study confirms their input is far too small to sustain the corona’s extreme temperatures on their own. Professor Ramesh emphasized that magnetic reconnection occurring across the entire solar atmosphere is the undisputed primary driver of the corona’s anomalous heat.
These new quantified findings provide a critical benchmark for future solar research, bringing scientists closer to answering a fundamental physics question that has defied explanation for generations. As India’s first dedicated solar space mission, Aditya-L1 continues to deliver groundbreaking data that advances global understanding of our host star, with implications for space weather forecasting and fundamental astrophysics research.