Direct SDO/AIA imaging in 2011 established that ubiquitous low-frequency Alfvénic waves permeate the quiet solar transition region and corona with an energy flux (~100–200 W m⁻²) sufficient, in principle, to heat the quiet corona and accelerate the fast solar wind. What remains unresolved — and what fifteen years of follow-up work has not settled — is the causal chain itself: where in the lower atmosphere and by which physical processes these waves are launched, and how, where, and into which particle species their energy is dissipated, together with quantified dissipation rates, filling factors, the dominant damping physics, and the full (including high-frequency) wave energetics in active regions. The problem is to turn the demonstrated energetic sufficiency of the wave reservoir into an identified, quantitatively verified heating and wind-acceleration mechanism.

Background

The solar corona — the Sun's outer atmosphere, at a temperature of order 10610^6 K — must be continuously heated to balance radiative and conductive losses, and above regions of open magnetic field the plasma must additionally be accelerated to hundreds of km s⁻¹ to form the fast solar wind. Alfvén waves are transverse, incompressible oscillations of plasma and magnetic field that propagate along field lines; because they are hard to damp, they can carry magneto-convective energy from the cool, dense lower atmosphere (photosphere and the partially ionized chromosphere, separated from the corona by the sharp transition region) up into the tenuous corona. Before 2011, coronal detections (e.g. with the ground-based Coronal Multi-channel Polarimeter, CoMP) found Doppler amplitudes of only ~0.5 km s⁻¹, far below the ~100–200 W m⁻² energy flux required to heat the quiet corona and drive the fast wind, leaving wave (AC) heating in doubt relative to reconnection-based (DC) alternatives. McIntosh et al. (2011, Nature 475, 477–480), using high-cadence extreme-ultraviolet (EUV) imaging from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), directly imaged ubiquitous outward-propagating Alfvénic swaying of spicules (thin, jet-like chromospheric–transition-region features) and coronal fine structure, with amplitudes of order 20 km s⁻¹ and periods of 100–500 s. Combining amplitude, measured phase speed (~200–250 km s⁻¹), density, and an assumed volume filling factor f≈1f\approx 1 through EA=fρv2VAE_{\mathrm{A}}=f\rho v^2 V_{\mathrm{A}} (with ρ\rho the plasma density, vv the wave amplitude, and VAV_{\mathrm{A}} the Alfvén speed at which the waves propagate along the field), they estimated a wave energy flux of ~100–200 W m⁻² at ~15 Mm height in coronal-hole and quiet-Sun regions (coronal holes are patches of open magnetic field with a low-density corona; the quiet Sun is the magnetically quiescent remainder of the disk outside them and outside active regions, the strong-field, sunspot-associated patches where the corona is brightest) — energetically sufficient — and proposed a two-stage picture in which lower-atmospheric mass injection (spicules) is followed by wave dissipation that sustains coronal temperatures and accelerates the wind. The authors explicitly identified what this demonstration did not establish: (1) the generation of the waves — where in the lower atmosphere and by which processes (magneto-convective shaking, magnetic reconnection, or mode conversion) they are launched; (2) the dissipation — the mechanism(s) and spatial locations where wave energy is converted to heat or bulk flow, in particular the partition of that energy between ions and electrons; (3) the active-region regime, where bidirectional propagation, smaller spatial scales, and shorter periods may cause the 8-s-cadence AIA measurements to under-report amplitudes and phase speeds, leaving the full frequency spectrum and its energetic contribution unmeasured (their active-region flux estimate of order 100 W m⁻² falls far short of the ~2000 W m⁻² active-region requirement, plausibly an observational artifact); and (4) quantitative dissipation rates, the effective filling factor across heights (assumed, not measured), and identification of the dominant damping physics — turbulent cascade, resonant absorption (damping by energy transfer to fine structure across the field), phase mixing (neighbouring field lines drifting out of phase because the wave speed varies across the field), or nonlinear processes. Later observations with CoMP, with the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard the Japanese Hinode satellite, and since 2022 the 4-m Daniel K. Inouye Solar Telescope (DKIST, whose Cryogenic Near-Infrared Spectropolarimeter, Cryo-NIRSP, measures the coronal Fe XIII 1074 nm line with order-of-magnitude better cadence and resolution) have confirmed the fluctuations and extended the measured spectrum above 10–100 mHz, but, as reviews through 2020–2023 state, the exact processes that transport and dissipate the wave energy are still not understood.

Problem Statement

Establish, at observational/quantitative certainty, the physical chain by which the ubiquitous low-frequency Alfvénic waves detected throughout the quiescent solar atmosphere power the quiet corona and the fast solar wind. Concretely, determine: (i) the generation of these waves — where in the photosphere/chromosphere/transition region they are launched and by which processes (magneto-convective driving, reconnection, mode conversion), including the relative contribution of each channel to the observed coronal wave flux; (ii) the dissipation — the mechanism(s) and spatial locations where the wave energy is converted into heat and bulk flow, and the partition of that energy between ions and electrons; (iii) the wave energy budget in active regions, including the higher-frequency, smaller-scale, bidirectional contributions that the original 8-s-cadence EUV imaging could not resolve, sufficient to decide whether waves can meet the ~2000 W m⁻² active-region requirement or whether other processes dominate there; and (iv) quantitative, height-resolved dissipation rates, the effective magnetic filling factor of the wave-carrying structures, and the identification of the dominant damping physics (turbulent cascade, resonant absorption, phase mixing, or nonlinear processes). Resolving the chain requires connecting the identified generation and dissipation physics to the measured wave fluxes and to the observed coronal energy budget and wind acceleration — i.e., closing the loop between the demonstrated energetic sufficiency of the waves and a verified heating/acceleration mechanism.

Each of (i)–(iv) is an independently refutable sub-question: a determination of any subset of them is a partial result in its own right, while a resolution of the statement as a whole additionally requires the chain closure described above.

References

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