Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...284..593s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 284, no. 2, p. 593-598
Astronomy and Astrophysics
Astrophysics
22
Helical Flow, Magnetohydrodynamics, Solar Atmosphere, Solar Flares, Solar Magnetic Field, Force-Free Magnetic Fields, Plasma Jets, Strouhal Number
Scientific paper
The possibility is discussed that the build-up of dc magnetic energy in the solar atmosphere, which is needed as the energy source for explosive events such as flares and may also contribute to the general atmospheric heating, is due to the alpha effect. In active regions, the plasma-magnetic field system evolves quasistatically through a succession of force-free equilibria. We consider the force-free equilibrium field as the mean field and the deviations from the force-free state as the fluctuations, or turbulence, in the sense of mean-field magnetohydrodynamics. The energy input into the atmosphere occurs in the form fluctuations, namely wavelike disturbances propagating upward from the photosphere, and energy to be accumulated in the mean magnetic field must be picked up from the fluctuations. Since the fluctuations represent a wave turbulence (and not a convective one -- the atmosphere is convectively stable), the Strouhal number is small and so the application of the first order smoothing approximation well justified. For the build-up of mean-field (dc) magnetic energy the existence of both mean-field and turbulent current helicities and their relation is of particular importance. The role played by current helicity allows a comparison of results obtained by using the two-scale approach of mean-field theory with concepts which assume the conservation of magnetic helicity (as the Taylor relaxation concept) or consider inverse cascades of magnetic helicity as important for inverse energy cascades, respectively. Under the assumption that either the magnetic or the velocity fluctuations are statistically stationary and homogeneous, the turbulent part of the magnetic helicity is constant in time and its mean-field part must grow if the mean-field energy is to grow, in accordance with the helicity cascade concept, since magnetic helicity, as energy, is injected through the photosphere on the scale of the fluctuations. Evidence for or against the mechanism proposed may be supplied by further observational determinations of current helicity in the atmosphere.
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