The solar corona, the Sun's tenuous outer atmosphere, is held at temperatures of one to several million kelvin — hundreds of times hotter than the ~5,800 K photospheric surface below it — and must be continuously heated to balance radiative, conductive, and solar-wind losses. Despite a quantified energy budget dating to the 1970s and two mature paradigms (dissipation of magnetohydrodynamic waves and impulsive energy release by magnetic reconnection, including Parker's nanoflares), the field has still not definitively identified which mechanism(s) operate, nor quantified their relative contributions across active regions, the quiet Sun, and coronal holes. Reviews spanning 2015–2025 treat the problem as unresolved; the precise open core is mechanism identification plus quantitative, observationally consistent attribution of the heating across coronal environments.

Background

The Sun's visible surface, the photosphere, sits at about 5,800 K; above it lie the chromosphere (a thin, cooler atmospheric layer), a sharp transition region, and the corona — a tenuous, magnetically structured plasma that spectroscopy (via emission lines of highly ionized metals, identified in the early 1940s) shows to be at one to several million kelvin. Temperature thus rises sharply with height, far from any heat source, which requires a continuous non-radiative energy input: Withbroe & Noyes (1977) quantified the required heating as a non-radiative energy flux that falls steeply from active regions (magnetically strong areas with sunspots and hot loop systems) through the quiet Sun (the magnetically weak regions outside active regions) to coronal holes (open-field regions that are the source of the fast solar wind). De Moortel & Browning (2015) state the problem in its standard form: identification of a heat source that balances losses due to thermal conduction, radiation, and, in open-field regions, the convective (advective) loss to the solar wind.

Because the corona is too tenuous for radiative heating to matter and convection cannot carry heat across the transition region, the energy source must be magnetic. Photospheric convection continuously shuffles and braids the footpoints of coronal magnetic fields, injecting mechanical energy. Two broad paradigms compete. In alternating-current (AC) or wave heating, magnetohydrodynamic (MHD) waves — in particular Alfvén waves, transverse oscillations of magnetic field lines — propagate upward and dissipate through turbulence, resonant absorption, or phase mixing. In direct-current (DC) heating, stressed magnetic fields build up current sheets (thin layers of intense electric current) that release energy impulsively via magnetic reconnection, the reconfiguration of field-line connectivity; Parker (1988) proposed that the X-ray corona is heated by nanoflares, impulsive reconnection events each releasing roughly a billionth of the energy of a major flare, so frequent that their collective effect maintains coronal temperatures.

The central diagnostics are demanding: the corona emits optically thin extreme-ultraviolet and X-ray radiation — radiation that escapes freely, without significant absorption or re-emission — whose intensity measures the emission measure (the line-of-sight integral of electron-density squared) rather than local temperature, so inferring where and how energy is deposited is an ill-posed inverse problem; and the relevant dissipation scales (down to of order 100 km) lie below the resolution of past instruments. Klimchuk (2015) enumerated ten key aspects — including the impulsiveness of all coronal heating, the density of current sheets, the characteristic magnitude and repetition rate of energy release, and the onset conditions — that must be understood before the problem can be considered solved. Cranmer & Winebarger (2019) tabulate scaling relations for the competing wave, turbulence, braiding, and nanoflare theories and note that sub-arcsecond observations give conflicting evidence on braided fields. Viall et al. (2021) emphasize that the chromosphere–corona mass and energy exchange (evaporation and condensation flows along field lines) is an essential, still-poorly-constrained piece of the puzzle. Recent high-resolution observations by Solar Orbiter's Extreme Ultraviolet Imager reveal ubiquitous small transient brightenings ("campfires"), which Chen et al. (2021) reproduce in a three-dimensional radiative-MHD simulation as reconnection-driven events — evidence that impulsive heating operates, but not yet a quantitative attribution of the heating.

Problem Statement

Identify the physical mechanism or combination of mechanisms that heat the solar corona — specifying the energy carrier (waves, stressed magnetic fields, or otherwise), its transport channel, and the dissipation process — and demonstrate quantitatively that the identified mechanism(s) can supply the heating required to balance radiative, conductive, and (in open-field regions) solar-wind losses and to reproduce the observed coronal temperatures and diagnostics. The chromosphere enters as the coupled lower boundary whose mass and energy exchange with the corona must be treated self-consistently (Viall et al. 2021; De Moortel & Browning 2015), not as a separate heating-attribution target. The answer must include the relative contribution of each mechanism across the three main coronal environments (active regions, quiet Sun, coronal holes), shown consistently with accepted observational constraints such as loop scaling laws (empirical relations between the length, maximum temperature, and pressure of coronal loops), emission-measure distributions, spectral-line widths and Doppler signatures, and the statistics and energetics of transient brightenings. This is the standard coronal heating problem as posed in the primary literature (De Moortel & Browning 2015; Klimchuk 2015; Cranmer & Winebarger 2019; Viall et al. 2021): the mechanism identification and its quantitative, environment-resolved attribution are one linked objective. Klimchuk (2015) states the completion criterion for the magnetically closed corona (active regions and quiet Sun): ten key aspects of the heating must be understood before the problem can be considered solved, not merely that a candidate process be shown feasible in one setting; in his framing, heating of the open corona (coronal holes and the solar wind) is "a separate question, possibly with a different answer". Cranmer & Winebarger (2019) pose the open target jointly — "the physical processes that heat the corona and accelerate the solar wind" — and it is this joint framing, together with the environment-resolved energy budget of Withbroe & Noyes (1977), that places all three environments inside the completion boundary here.

References

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