Indian scientists studying data from the country’s first dedicated solar mission, Aditya-L1, have made an important breakthrough in understanding one of astronomy’s most puzzling questions — why the Sun’s outer atmosphere, or corona, is dramatically hotter than its visible surface.
The corona, despite being much farther from the Sun’s extremely hot core, reaches temperatures of around 2 million degrees Celsius and, during particularly intense solar events, can become as hot as 40 million degrees Celsius. This is in stark contrast to the Sun’s photosphere, or visible surface, where temperatures are around 5,500 degrees Celsius. The Sun’s core, meanwhile, has an estimated temperature of nearly 15 million degrees Celsius.
A recent study led by Prof R Ramesh of the Indian Institute of Astrophysics (IIA) and published in the Astrophysical Journal Letters has used observations from Aditya-L1 to examine how the corona is heated and how it manages to retain its enormous energy despite repeatedly releasing vast quantities of energy into space.
The corona is also the birthplace of some of the Sun’s most powerful eruptions, including solar flares and coronal mass ejections (CMEs). During a CME, huge quantities of magnetised plasma and gas are expelled from the Sun. These eruptions can travel towards Earth and trigger geomagnetic storms, potentially disrupting satellites, communication systems and power infrastructure, while also producing spectacular auroras.
The frequency of CMEs varies with the Sun’s activity. During relatively quiet periods, the Sun may produce two or three such eruptions each day. At the peak of its approximately 11-year solar cycle, however, the number can rise to 10 or more CMEs in a day.
The persistence of the corona’s extreme temperature has therefore remained a major scientific puzzle. According to Prof Ramesh, if the enormous energy released during these eruptions were not replenished, the Sun would progressively lose its energy, with catastrophic consequences for the solar system.
Scientists have traditionally considered two major mechanisms that could supply energy to the corona. The first involves the continuous bubbling and boiling-like movements taking place on the Sun’s surface. These motions generate waves that travel outward through the solar atmosphere and transport energy towards the corona.
The second mechanism is linked to the Sun’s highly complex magnetic field. Magnetic field lines in the solar atmosphere constantly twist, stretch and become entangled. When these magnetic structures suddenly break and reconnect, enormous amounts of energy can be released. Such processes are closely associated with powerful solar eruptions and frequently occur around sunspots, where magnetic fields are particularly intense.
The new study has attempted to quantify the contribution made by both mechanisms. Researchers examined observations of a particularly energetic CME that occurred on August 5, 2024. The event was observed using the Visible Emission Line Coronagraph (VELC), one of the key instruments aboard Aditya-L1, which is positioned to continuously observe the Sun without the interruptions caused by Earth’s atmosphere.
The observations showed that within about 10 hours of the CME, the distorted magnetic field structures had returned towards their earlier configuration through reconnection. According to the researchers, this process enabled the energy in the corona to be rapidly reorganised and replenished.
The findings indicate that magnetic reconnection is the dominant contributor to maintaining the corona’s extreme temperatures. The waves generated by the Sun’s surface motions do transport energy outward, but their contribution appears to be comparatively small. The study estimates that these waves account for only about 7 per cent of the energy required to sustain the corona.
Prof Ramesh said the findings do not mean that surface-generated waves are unimportant, but demonstrate that they cannot by themselves explain the extraordinary heating of the solar atmosphere. Instead, the repeated breaking and reconnection of magnetic field lines across the Sun appears to provide the bulk of the required energy.
The researchers believe the observations provide an important reference point for future investigations into how energy is generated, transported and released in the Sun’s atmosphere. The findings could also improve scientific understanding of solar eruptions and the space-weather events they trigger.
With Aditya-L1 continuing its observations of the Sun, scientists expect that the mission will provide further evidence to help resolve fundamental questions surrounding solar physics. The latest results, they say, bring researchers closer to explaining how the Sun maintains an atmosphere that is vastly hotter than its own visible surface despite continuously releasing enormous amounts of energy into space.
