Alan H. Nye

The nature of running penumbral waves

A model of a sunspot penumbra, including the effects of magnetic field, compressibility, and buoyancy, is studied in order to identify the mode of running penumbral waves. It is found that the penumbral waves may be identified with gravity-modified magneto-acoustic waves of the ‘plus’ type that are vertically trapped at photospheric levels. Although most of the wave energy is contained in the penumbral photosphere and subphotosphere, the maximum vertical velocity occurs in the chromosphere where (i) the waves are evanescent and (ii) the vertical velocity is in fact observed (in Hα).

Convective instability in the presence of a nonuniform horizontal magnetic field

Newcomb’s criterion for convective stability in the presence of a horizontal magnetic field is written in a form which explicitly shows the effect of vertical variations of the magnetic field strength. It is shown that a nonuniform horizontal magnetic field can be destabilizing as well as stabilizing.

Mass and energy flow near sunspots. II: Linear numerical models of moat flow

Sunspots block the flow of energy to the solar surface. The blocked energy heats the volume beneath the spot, producing a pressure excess which drives an outflow of mass. Linear numerical models of the mass and energy flow around spots were constructed to estimate the predictions of this physical picture against the observed properties of sunspot bright rings and moat flows. The width of the bright ring and moat are predicted to be proportional to the depth of the spot penumbra, in conflict with the observed proportionally of the moat width to the spot diameter. Postulating that spot depths are proportional to spot diameters would bury the moat flow too deeply to be observed, because the radial velocity at the surface is found to be inversely proportional to the depth of the spot penumbra. The radial velocity at the surface is of order a few hundred meters per second after 1 day, in agreement with the observed excess of moat velocities over supergranule velocities.