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Why is the corona hundreds of times hotter than the photosphere? Since it is physically impossible to transfer thermal energy from the cooler surface to the much hotter corona, the mechanism for such intense heating has intrigued physicists for many years. However, there is now a general consensus that the Sun's localized, highly variable, and intense magnetic fields are the cause.

Figure 2.20 A false color image of the corona, made from three exposures taken by SOHO. Blue shows plasma at a temperature of 1–1.5 million degrees Celsius, green 1.5–2 million degrees, and red hotter than 2.5 million degrees. A dark coronal hole is visible at upper right.

(ESA/NASA)

Observations show that coronal heating is more intense in regions with stronger magnetic fields. It is also generally accepted that the origin of coronal heating and activity probably lies in the photosphere and the underlying convective zone. Since convection causes rapid, turbulent motion of the gases in the photosphere, the magnetic field lines emerging from the surface are bent or mixed. This may result in the creation of waves along the magnetic field lines, or the formation of magnetic discontinuities and reconnections in the corona.

Several physical processes have been proposed to explain coronal heating. One is “wave‐heating,” involving waves in the gas that propagate along the magnetic field lines and then dissipate in the corona.⁸

Another is “microflare heating,” when a great number of extremely small‐scale flares dissipate magnetic energy in the corona through a process called magnetic reconnection. This occurs when magnetic field lines behave rather like rubber bands, snapping and then reconnecting with lines of opposite polarity.

A third candidate is spicules – the fountain‐like jets of plasma that emanate from the chromosphere (see above). Some fast‐moving spicules reach temperatures of more than 1 million degrees and cause a brightening of the corona. If even some of that super‐hot plasma stays aloft it would make a contribution to coronal heating. Spicules also carry electrical currents and generate magnetic waves.

High resolution X‐ray imaging has revealed the presence of numerous, relatively small explosions, called microflares or nanoflares. These sudden bursts of energy occur within coronal loops – thin magnetic tubes filled with very hot plasma that arch high above the surface. Although they are so small that they cannot be studied individually, so many erupt at the same time that their combined effect is quite dramatic.

Nanoflares are associated with the build‐up of considerable magnetic stress due to random shaking of the magnetic field lines that are rooted in the solar interior. In this scenario, energy is released through the reconnection of neighboring magnetic field lines with locally opposite polarity. This violent release of energy creates two jets of material at the reconnection site which are accelerated and repelled by the reconnected and highly curved magnetic field lines.

The Hinode spacecraft has measured plasma in active regions with temperatures so high that they can only be produced by impulsive energy bursts associated with storms of nanoflares.

Models based on the nanoflare theory suggest that the plasma strands within coronal loops are confined by magnetic field lines. When a nanoflare occurs, a low‐temperature, low‐density strand is rapidly heated to around 10 million degrees Celsius. Heat flows from the hot, upper part of the strand toward the base of the coronal loop, where conditions are cooler. Since the plasma at the base is denser, the heat input is only sufficient to raise its temperature to about 1 million degrees. This dense plasma then expands upward along the strand. Each coronal loop is, therefore, a collection of faint, 5–10 million degree strands, and bright, 1 million degree strands.

Although it seems that these processes play an important, and perhaps dominant, role in coronal heating, there is still some doubt over whether the amount of energy released by the nanoflares is sufficient to heat the corona.

Data from Hinode and NASA's Solar Dynamics Observatory indicate that a special kind of magnetic waves, known as Alfvén waves, may account for much of the coronal heating.⁹ These waves travel at very high speeds along the magnetic field lines that extend from the photosphere and into the corona. However, observations have shown that short‐lived spicules which shoot upward from the chromosphere wiggle sideways up to 1,000 km while they form. Advanced computer simulations suggest that these movements are caused by lateral motion of the magnetic field in the Alfvén waves.

Hinode images have also shown oscillations within solar prominences – large structures of relatively cool plasma that rise through the corona. These oscillations are widely thought to be caused by Alfvén waves propagating along the threads at speeds of about 20 km/s. Like waves crashing on a beach, they dump their energy in the corona, heating the plasma to millions of degrees.