Abstract
We examine the propagation of Alfvén waves in the solar atmosphere. The principal theoretical virtues of this work are: (i) The full wave equation is solved without recourse to the small-wavelength eikonal approximation (ii) The background solar atmosphere is realistic, consisting of an HSRA/VAL representation of the photosphere and chromosphere, a 200 km thick transition region, a model for the upper transition region below a coronal hole (provided by R. Munro), and the Munro-Jackson model of a polar coronal hole. The principal results are:
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(i)
If the wave source is taken to be near the top of the convection zone, where n H = 5.2 × 1016 cm−3, and if B ⊙ = 10.5 G, then the wave Poynting flux exhibits a series of strong resonant peaks at periods downwards from 1.6 hr. The resonant frequencies are in the ratios of the zeroes of J 0, but depend on B ⊙, and on the density and scale height at the wave source. The longest period peaks may be the most important, because they are nearest to the supergranular periods and to the observed periods near 1 AU, and because they are the broadest in frequency.
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(ii)
The Poynting flux in the resonant peaks can be large enough, i.e. P ⊙ ≈ 104–105 erg cm−2s−1, to strongly affect the solar wind.
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(iii)
¦δv¦ and ¦δB¦ also display resonant peaks.
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(iv)
In the chromosphere and low corona, ¦δv ≈ 7–25 kms−1 and ¦δB¦ ≈0.3–1.0 G if P ⊙≈104-105 erg cm−2s−1.
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(v)
The dependences of ¦δv¦ and ¦δB¦ on height are reduced by finite wavelength effects, except near the wave source where they are enhanced.
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(vi)
Near the base, ¦δB¦ ≈ 350–1200 G if P ⊙ ∼- 104–105. This means that nonlinear effects may be important, and that some density and vertical velocity fluctuations may be associated with the Alfvén waves.
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(vii)
Below the low corona most wave energy is kinetic, except near the base where it becomes mostly magnetic at the resonances.
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(viii)
ϱ0 < δv 2 > v A or < δB 2 > v A/4π are not good estimators of the energy flux.
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(ix)
The Alfvén wave pressure tensor will be important in the transition region only if the magnetic field diverges rapidly. But the Alfvén wave pressure can be important in the coronal hole.
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The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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Hollweg, J.V. Alfvén waves in the solar atmosphere. Sol Phys 56, 305–333 (1978). https://doi.org/10.1007/BF00152474
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DOI: https://doi.org/10.1007/BF00152474