The Sun’s corona is the vast outer atmosphere of our star, a layer of superheated plasma visible only during a total solar eclipse. This low-density plasma extends millions of kilometers into space. Despite its distance from the Sun’s core, the corona maintains an astonishing temperature, reaching between 1 and 3 million Kelvin. This extreme heat presents a fundamental conflict with basic thermal physics, challenging astronomers to understand how energy is generated and transferred within its magnetic environment.
The Coronal Heating Paradox
The most perplexing puzzle is the dramatic reversal of the Sun’s temperature gradient, known as the coronal heating paradox. The visible surface of the Sun, the photosphere, registers approximately 6,000 Kelvin. Just above this surface, the temperature suddenly skyrockets by a factor of hundreds.
This temperature increase defies the expectation that heat energy should dissipate as it moves away from its source. Heat should spontaneously flow from the hotter photosphere to the cooler outer layers, but this is clearly not happening. Instead, an immense, non-thermal energy source must continuously feed the corona to maintain its million-degree state, overriding natural cooling processes.
The energy required must originate from below the surface and be transported upward by a mechanism other than simple heat conduction. This mechanism must bypass cooler lower atmospheric layers, such as the chromosphere, before depositing energy high in the rarefied corona. Two competing theories attempt to explain how the Sun’s powerful magnetic fields are involved in this extraordinary superheating.
Proposed Mechanisms for Energy Transfer
One leading hypothesis for the temperature jump is wave heating, involving the propagation of plasma waves from the Sun’s turbulent interior. Convective motion beneath the photosphere continually generates various waves, including magnetoacoustic and Alfvén waves. These magnetic waves travel along the Sun’s field lines, carrying mechanical energy upward into the atmosphere.
As these waves enter the low-density corona, they are predicted to grow in amplitude and eventually dissipate. The energy released by this process is transferred into the coronal plasma, heating it to extreme temperatures. Observations from recent missions support the existence of these magnetic waves, suggesting they contribute significantly to the total energy budget.
A second major theory centers on magnetic reconnection, a process associated with impulsive, small-scale events called nanoflares. The Sun’s magnetic field lines are constantly tangled and stressed by the motion of surface plasma. When oppositely directed field lines come into close proximity, they spontaneously rearrange, converting stored magnetic energy into intense heat and kinetic energy.
Nanoflares are theorized to be miniature reconnection events, individually too small and brief to be observed directly. However, if billions of these tiny explosions occur every second, their collective energy release could continuously heat the corona. Evidence for these events exists in the form of “nanojets,” which are fast jets of plasma that signal underlying magnetic reconnection.
The Acceleration Mystery of the Solar Wind
Beyond the heating paradox, astronomers seek to understand the powerful engine driving the solar wind, the continuous outflow of charged particles from the corona. This stream of plasma is powerful enough to escape the Sun’s gravity and fill the entire solar system. The mystery lies in how this material is rapidly accelerated to its observed speeds.
The solar wind is categorized into two types: the slow wind (300 to 400 kilometers per second) and the fast wind (up to 800 kilometers per second). The fast solar wind typically originates from coronal holes, regions where magnetic field lines are open and extend directly into interplanetary space.
Simple models of thermal expansion cannot fully account for the high speed of the fast solar wind, requiring an additional energy input. The rapid acceleration occurs surprisingly close to the Sun, within the first few solar radii above the surface. This suggests that a mechanism like the pressure gradient from Alfvén waves, or other non-thermal processes, must actively provide the necessary momentum to overcome gravity and push the plasma outward.
Observational Tools Seeking Answers
Addressing these solar mysteries requires gathering measurements directly within the Sun’s atmosphere, a challenge met by dedicated space missions. The Parker Solar Probe (PSP) is the first spacecraft designed to fly into the low solar corona, making it the closest human-made object to the Sun. Its mission is to take direct, in situ measurements of the plasma, magnetic fields, and energy flow.
PSP is equipped with instruments like the Solar Wind Electrons Alphas and Protons (SWEAP) investigation, which measures solar wind particle properties. The FIELDS instrument suite measures electric and magnetic fields, providing data on wave activity and localized magnetic field reversals known as “switchbacks.” These direct measurements provide the first physical evidence of the processes responsible for heating the corona and accelerating the solar wind.
The European Space Agency’s Solar Orbiter (SolO) works in tandem with PSP to provide a broader context for the Sun’s behavior. SolO captures high-resolution images of the Sun’s surface and provides remote measurements of the corona. Coordinated observations between the two spacecraft allow scientists to trace the evolution of plasma elements from the Sun’s surface to PSP’s up-close readings.