ISRO Studies the Sun and Discovers It’s Still Full of Surprise.

One of the longest standing questions in solar physics is a seemingly impossible one: why is the Sun’s outer atmosphere, the corona, millions of degrees hotter than its visible surface? Scientists have known for decades that the corona can reach temperatures of around 2 million degrees Celsius, and in some circumstances much higher, even though the Sun’s visible surface is roughly 5,500°C. Now, observations from India’s first dedicated solar mission, Aditya-L1, are providing new clues about how the corona generates and replenishes the enormous amount of energy it loses during powerful solar eruptions. Researchers from India say their latest findings, published in the Astrophysical Journal Letters, help quantify the contribution of different mechanisms responsible for heating the corona. A Solar Mystery That Has Puzzled Scientists The Sun has several distinct layers. At its centre is the core, where temperatures reach around 15 million°C. Moving outward, energy passes through the radiation and convection zones before reaching the photosphere, the visible surface, where the temperature is approximately 5,500°C. Beyond the surface lies the corona, the Sun's extremely hot outer atmosphere. Its temperature is typically around 2 million°C and can rise dramatically during periods of intense solar activity. The temperature difference is one of the major unresolved problems in solar physics. Scientists have proposed several mechanisms to explain how the corona becomes and remains so hot. Solar Eruptions Add to the Puzzle The mystery becomes even more complicated during major solar eruptions. The corona is the region where events such as solar flares and coronal mass ejections (CMEs) originate. During a CME, enormous quantities of magnetised plasma and energy are expelled into space. These eruptions can have consequences far beyond the Sun. When CME interact with Earth's magnetic field, they can produce geomagnetic storms capable of disrupting satellites, communications systems and power infrastructure. They can also contribute to spectacular auroras. During relatively quiet periods, the Sun can produce a few CMEs each day. During the peak of its approximately 11-year solar activity cycle, the number can rise considerably. This raises a fundamental question, if the corona repeatedly loses enormous amounts of energy, what mechanism replenishes that energy and keeps the atmosphere extremely hot? Two Major Energy Sources Scientists have generally considered two important mechanisms. The first involves the Sun turbulent surface. The bubbling and boiling motion of material beneath the photosphere generates waves that travel outward through the solar atmosphere, transporting energy toward the corona. The second involves the Sun magnetic field. The magnetic field lines in the solar atmosphere can become twisted and tangled. When these magnetic structures break and reconnect, they can release substantial amounts of energy into the surrounding plasma. This process is known as magnetic reconnection and is closely associated with many forms of solar activity. Aditya-L1 Provides Important Measurements Researchers led by solar astrophysicist Prof R. Ramesh of the Indian Institute of Astrophysics studied observations of a particularly energetic CME that occurred on August 5, 2024. The event was observed by Aditya-L1 Visible Emission Line Coronagraph (VELC), an instrument designed to study the solar corona and its dynamics. According to the researchers, their analysis allowed them to estimate how much energy was supplied to the corona by the two proposed mechanisms. Their results suggest that the energy associated with waves generated by turbulent motions contributes around 7% of the required energy, while magnetic restructuring and reconnection account for approximately 93%. Magnetic Reconnection Takes Centre Stage The researchers observed that following the CME, the tangled magnetic field structures were able to reconnect and return toward their earlier configuration within roughly 10 hours. According to the study, this process helped replenish energy in the corona. The findings do not mean that surface generated waves are irrelevant. Instead, they suggest that although these waves transport energy outward, they may not provide enough energy on their own to explain the corona's extreme temperatures. The study therefore points toward magnetic field restructuring as the dominant contributor in the particular event examined. Why the Findings Matter Understanding how the corona is heated is not simply a question of solving an astronomical puzzle. The corona is also the source region of space weather, which can influence Earth technological environment. Strong solar eruptions can disturb satellites, navigation systems, radio communications and electrical networks. Better knowledge of how energy builds up and is released in the solar atmosphere could therefore improve scientists understanding of how major solar eruptions develop. It may eventually contribute to better space weather forecasting, although significant research remains necessary before scientists can reliably predict individual eruptions. A Major Role for Aditya-L1 Launched by the Indian Space Research Organisation (ISRO), Aditya-L1 was designed to observe the Sun continuously from a location near the Sun,Earth L1 point. Its instruments allow scientists to examine different aspects of solar activity, including the corona, magnetic environment and solar wind. The mission is giving Indian researchers access to continuous observations that can be used alongside data from other solar observatories around the world. The latest research demonstrates how measurements from the mission can be used not only to observe spectacular solar events but also to investigate fundamental questions about how stars work. More Questions Remain The findings provide an important benchmark, but they do not necessarily solve every aspect of the coronal heating problem. Solar activity is highly complex, and different events can involve different physical processes. Researchers will need to study additional eruptions and compare observations across different conditions to determine how broadly the latest conclusions apply. Future observations from Aditya-L1 and other solar missions could help scientists test whether magnetic reconnection consistently accounts for the majority of energy supplied to the corona. Conclusion The latest Aditya-L1 observations have provided scientists with valuable evidence about how the Sun's corona may maintain its extraordinarily high temperature despite repeatedly losing energy through powerful eruptions. The study estimates that magnetic field restructuring and reconnection supplied about 93% of the energy needed in the event examined, compared with around 7% from waves generated by turbulent surface motion. While the Sun still has many mysteries left to reveal, India's solar mission is giving researchers new data with which to investigate some of the most fundamental question about our nearest star.