Why Is The Sun’s Corona So Hot? Aditya-L1 May Have The Answer

One of the biggest puzzles in solar physics is why the Sun's outer atmosphere, or corona, is millions of degrees hotter than its visible surface. New observations from India's Aditya-L1 mission may have brought scientists closer to answering that question, with researchers finding that the Sun's constantly changing magnetic fields could provide most of the energy needed to keep the corona so hot.

The findings, led by Prof R Ramesh of the Indian Institute of Astrophysics (IIA), have been published in the Astrophysical Journal Letters. The study also attempts to quantify the contribution of two competing mechanisms that scientists believe heat the corona.

Why Is The Sun's Corona So Hot?

The temperature inside the Sun follows an apparently logical pattern — until scientists reach its outer atmosphere.

The Sun's core reaches around 15 million°C, while its visible surface, or photosphere, is about 5,500°C. Yet the corona, which lies far above the surface, can reach around 2 million°C and, during particularly active periods, temperatures can rise much higher.

This phenomenon is known as the coronal heating problem and has remained one of the major unanswered questions in solar physics.

The mystery becomes even more complicated because the corona is constantly losing enormous amounts of energy through violent eruptions, including coronal mass ejections (CMEs) and solar flares.

CMEs can release huge clouds of magnetised plasma into space. When directed towards Earth, they can trigger geomagnetic storms capable of disrupting satellites, communications and power infrastructure.

During relatively quiet periods, the Sun produces around two to three CMEs a day. That number can rise to 10 or more during the peak of its roughly 11-year solar activity cycle.

So what keeps replenishing the corona's energy?

Two Mechanisms Could Be Heating The Corona

Scientists have traditionally considered two major mechanisms.

The first involves the Sun's constantly moving surface. The bubbling and boiling motions of plasma generate waves that travel upwards through the solar atmosphere, carrying energy towards the corona.

The second involves the Sun's magnetic field lines. These lines can become twisted and tangled before suddenly snapping and reconnecting. The process releases enormous amounts of energy and is closely associated with solar eruptions.

But after an eruption, the magnetic field lines can reconnect and reconfigure themselves, potentially replenishing the energy lost from the corona.

What Did Aditya-L1 Find?

The researchers used observations from Aditya-L1's Visible Emission Line Coronagraph (VELC) to study a particularly energetic CME that occurred on August 5, 2024.

The observations showed that within about 10 hours of the CME, the tangled magnetic field lines had returned towards their previous configuration through reconnection.

The researchers then calculated how much energy was supplied to the corona by the two mechanisms.

Their finding was striking: waves generated by the Sun's surface contributed only about 7% of the energy requirement, while magnetic-field reconfiguration accounted for roughly 93%.

In other words, while waves generated by the Sun's bubbling surface do transport energy into the corona, the study suggests that magnetic reconnection is the dominant mechanism replenishing the energy lost during eruptions.

"Though the waves generated as a result of the bubbling, boiling motions on the Sun's surface generate and transport energy, their contribution is very little - they supply only 7% of the energy requirement," Ramesh said.

"The remaining 93% comes because the Sun reconfigures itself and replenishes the lost energy."

Why Does This Matter?

The findings do not necessarily solve the coronal heating problem on their own, but they provide a quantitative benchmark for future research into how energy is generated and transported through the Sun's atmosphere.

The observations are particularly significant because Aditya-L1 allows Indian scientists to continuously study the Sun from space, providing data that can be used to examine phenomena that are difficult to observe from Earth.

The study suggests that the Sun's magnetic field is not merely responsible for producing spectacular eruptions. Its constant rearrangement may also be central to keeping the corona at its extraordinarily high temperature.

Understanding that process could also improve scientists' ability to predict space weather — solar eruptions that can affect satellites, communication systems, navigation and power networks on Earth.