Coronal heating problem

The coronal heating problem is the unresolved question of how the Sun’s tenuous outer atmosphere reaches millions of kelvins while its visible surface is about 5,800 kelvins.

Magnetic reconnection: A rearrangement of magnetic field lines that releases stored magnetic energy and can convert it into heat and particle motion. Small reconnection events may dissipate magnetic energy throughout the corona.
Solar corona: The Sun’s tenuous outer atmosphere, extending millions of kilometers into interplanetary space. This is the region whose unexpectedly high temperature defines the problem.
Solar active region: A region of the Sun with concentrated magnetic fields, sunspots, and enhanced activity. Active regions contain hot loops where magnetic heating signatures can be studied.
Parker's nanoflare hypothesis: Eugene Parker’s proposal that numerous tiny magnetic reconnection events heat the solar corona. It predicts impulsive heating too small and frequent to resolve individually in many observations.
Alfvén wave: A transverse magnetohydrodynamic wave that travels along magnetic field lines through a conducting plasma. Alfvén waves can carry energy from the lower atmosphere into the corona.
Solar photosphere: The visible layer of the Sun from which most sunlight escapes. Its much lower temperature makes the corona’s heat contrast striking.
Coronal loop: An arch-shaped structure of hot plasma confined by magnetic field lines in the solar corona. Loop temperatures and densities constrain how energy is deposited along magnetic fields.
Coronal seismology: The use of observed waves and oscillations in coronal structures to infer plasma properties. Wave measurements can estimate energy transport, but the amount ultimately dissipated remains uncertain.
Nanoflare: A hypothesized small, impulsive release of magnetic energy in the solar atmosphere. A multitude of unresolved events could supply sustained coronal heating.
Solar magnetic field: The magnetic field generated by the Sun’s conducting, rotating plasma. Magnetic energy is a leading candidate source for heating the corona.

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