The heat paradox
The Sun presents a baffling thermal paradox. Its visible surface, the photosphere, has a temperature of about 5,500 degrees Celsius (around 10,000 degrees Fahrenheit). Logic suggests that moving away from this surface should lead to cooler temperatures. Instead, the temperature drops slightly in the next atmospheric layer, the chromosphere, before skyrocketing in the outermost layer, the corona. This tenuous halo of plasma is at temperatures between 1 and 3 million Kelvin, sometimes flaring up to 20 million Kelvin in active regions. This means the corona is hundreds of times hotter than the solar surface just a few thousand kilometers below.
This phenomenon defies the normal behavior of heat transfer. The mystery of how the corona gets so incredibly hot has been a central question in astrophysics since the 1940s. Early clues emerged during a total solar eclipse in 1869, when astronomers detected a mysterious green spectral line in the corona. For decades, it was attributed to a hypothetical new element, "coronium." In the 1940s, Swedish physicist Bengt Edlén identified the source not as a new element, but as highly ionized iron—specifically, iron atoms stripped of 13 electrons (Fe-XIV). For iron to reach such an extreme state of ionization, the surrounding plasma must be at a temperature of millions of degrees, confirming the corona's perplexing heat.
Unraveling the mechanism
Scientists are investigating two primary mechanisms to explain how immense energy is pumped into the corona. The first involves wave heating. The Sun's roiling convective zone generates various types of magnetic waves that travel upwards. One candidate is the Alfvén wave, a type of magnetic vibration that can move through plasma. These waves, first theorized by Hannes Alfvén in the 1940s, are thought to carry energy from the Sun's surface and deposit it in the corona, superheating the sparse plasma. Observations from the Daniel K. Inouye Solar Telescope have provided direct evidence of these small-scale twisting waves.
The second major theory is based on magnetic reconnection. The Sun's surface has a web of magnetic field lines generated by the star's internal dynamo. These field lines twist and stretch, often crossing and abruptly snapping into new configurations. This process, like a stretched rubber band breaking, releases massive bursts of energy. One hypothesis suggests that millions of these small-scale events, often called "nanoflares," are constantly occurring across the Sun. While each individual event is small, their combined effect could be powerful enough to maintain the corona's extreme temperature.
Missions like NASA's Parker Solar Probe are flying directly through the corona to take unprecedented measurements. The probe has detected phenomena like magnetic "switchbacks" and confirmed the existence of a "helicity barrier," which affects how plasma is heated. By sampling the plasma directly, these missions are providing data to test and refine the theories of coronal heating.