The Sun is structured into several distinct layers that make up its interior and atmosphere. The visible surface, the photosphere, is the lowest layer of the solar atmosphere and primarily emits the light we see. Above the photosphere lies the chromosphere, a reddish layer, and extending far beyond that is the corona, the Sun’s vast, tenuous outer atmosphere. Scientists face a profound challenge in understanding how energy flows through these layers. While the energy source is the Sun’s core, the outermost atmospheric layers are dramatically hotter than the surface below them. This long-standing puzzle, known as the coronal heating problem, requires understanding mechanisms that transport and release immense energy far from the Sun’s interior.
The Solar Atmosphere’s Temperature Mystery
The natural expectation for any body radiating heat is that temperature should decrease as distance from the heat source increases. This rule holds true for the Sun’s interior and visible surface, but it is violated in the outer atmosphere. The photosphere maintains an average temperature of approximately 5,800 Kelvin (K). Moving just a few hundred kilometers higher, the temperature briefly drops to a minimum before beginning a steep and unexpected climb.
The chromosphere, immediately above the photosphere, experiences a temperature increase to between 8,000 K and 20,000 K. This temperature gradient becomes even more extreme in the corona. The corona routinely reaches temperatures exceeding 1,000,000 K and can climb to several million degrees Kelvin during active periods. This tremendous temperature jump—from thousands of degrees at the surface to millions in the outer atmosphere—is the core of the mystery. This suggests that a process other than simple radiation or thermal conduction must be injecting substantial energy into the outer layers.
Heating Mechanism: Energy Transport by Waves
One major theory proposes that the energy needed to heat the corona is carried upward from the Sun’s interior by various types of waves. The constant turbulent motion of the plasma in the convection zone, the layer beneath the photosphere, acts like a giant acoustic drum, generating a multitude of waves. These waves travel outward along the Sun’s magnetic field lines, carrying mechanical energy into the upper atmosphere.
A particularly important disturbance is the Alfvén wave, a specific magnetic wave that travels through the highly conductive solar plasma. These waves are generated by the churning plasma and are guided by the pervasive magnetic fields that permeate the Sun’s atmosphere. As the waves travel higher into the corona, the sharp drop in plasma density causes the waves to grow in amplitude.
This growth makes the waves unstable, causing them to dissipate their energy through a process resembling turbulence. The mechanical energy carried by the waves is converted into thermal energy, heating the surrounding plasma. This mechanism, often referred to as AC heating, provides a steady, distributed source of energy that could account for the overall background temperature of the corona.
Heating Mechanism: Magnetic Field Reconnection
A second, highly energetic theory proposes magnetic field reconnection, focusing on the dynamic nature of the Sun’s magnetic field. The Sun’s surface is constantly in motion, causing the magnetic field lines extending into the corona to become stressed and tangled, a phenomenon referred to as magnetic braiding. This braiding stores enormous amounts of energy within the magnetic field structure.
When oppositely directed magnetic field lines are pushed together, they rapidly break and snap back into a simpler, lower-energy configuration. This sudden realignment, or magnetic reconnection, converts the stored magnetic energy into heat and kinetic energy released into the surrounding plasma. While this process is responsible for the largest solar flares, scientists theorize it also occurs on a much smaller, more frequent scale.
These miniature events are called nanoflares, a concept first proposed by Eugene Parker. Individually, a nanoflare is a burst of energy too small and brief to be easily observed by current instruments. However, if millions or billions of these tiny magnetic reconnection events occur continuously, their collective energy release could sustain the corona’s high temperatures. This impulsive, localized release of energy is often termed DC heating.
Synthesizing the Solutions
The scientific community agrees that the heating of the solar atmosphere is not due to a single process but a combination of mechanisms. Both the continuous dissipation of waves and the impulsive heating from magnetic reconnection likely contribute to the corona’s energy budget. The central challenge is determining the relative contribution of each mechanism and understanding how they interact across different regions of the Sun.
Modern solar missions are designed to gather the data needed to resolve this complex interplay. The NASA Parker Solar Probe and the ESA/NASA Solar Orbiter are making coordinated observations closer to the Sun than ever before. These spacecraft provide both in-situ measurements of the plasma and remote sensing observations of the large-scale structures.
Recent joint observations suggest that turbulence, a key outcome of wave dissipation, plays a significant role in transferring energy to the coronal plasma. Data is continuously being analyzed to confirm the presence and frequency of nanoflares, which remain difficult to detect directly. Solving this coronal heating problem is important for understanding our own star and comprehending the physics governing the atmospheres of stars throughout the universe.