John H. Thomas

Sunspots : theory and observations

Preface. I: Introduction. The Theory of Sunspots J.H. Thomas, N.O. Weiss. II: Setting the Scene. Starspots P.B. Byrne. The Evolution of Sunspots C. Zwaan. III: Overall Structure of Sunspots. Continuum Observations and Empirical Models of the Thermal Structure of Sunspots P. Maltby. Observations of the Mesoscale Magnetic Structure of Sunspots A. Skumanich. Magnetohydrostatic Equilibrium in Sunspot Models K. Jahn. The Fate of the Heat Flux Blocked by Spots H.C. Spruit. IV: Fine Structure of Sunspots. Fine Structure of Umbrae and Penumbrae R. Muller. High Resolution Observations of the Magnetic and Velocity Fields of Simple Sunspots A.M. Title, Z.A. Frank, R.A. Shine T.D. Tarbell, K.P. Topka, G.B. Scharmer, W. Schmidt. Magnetoconvection M.R.E. Proctor. The Cluster Model of Sunspots A.R. Choudhuri. V: Waves and Oscillations in Sunspots. Sunspot Oscillations: Observations and Implications B.W. Lites. Magnetohydrodynamic Waves in Structured Magnetic Fields B. Roberts. Theory of Umbral Oscillations and Penumbral Waves S.M. Chitre. Sunspot Seismology: The Interaction of a Sunspot with Solar p-Modes T.J. Bogdan. VI: The Relation of Sunspots to the Global Solar Magnetic Field. The Formation of Flux Tubes at the Base of the Convection Zone D.W. Hughes. The Motion of Magnetic Flux Tubes in the Convection Zone and the Subsurface Origin of Active Regions F. Moreno-Insertis. VII: Concluding Summary. The Sunspot Phenomenon: A Commentary E.N. Parker. Index.

Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae

Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.

The Vector Magnetic Field, Evershed Flow, and Intensity in a Sunspot

We present results of simultaneous observations of the vector magnetic field, Evershed flow, and intensity pattern in a nearly axisymmetric sunspot, made with the Advanced Stokes Polarimeter at the Vacuum Tower Telescope at NSO (Sacramento Peak). The vector magnetic field is determined from the Stokes profiles of the magnetically sensitive lines Fe I 630.15 and 630.25 nm, and Doppler velocities and intensities are measured in several lines including the weak C I 538.03 nm line, formed in the deepest layers of the atmosphere. The strength of the magnetic field decreases with increasing zenith angle (angle of inclination to the local vertical), and this decrease is nearly linear over most of the range of values in the sunspot. Magnetic field strength and continuum intensity are inversely related in the sunspot in a manner similar to the characteristic nonlinear relationship found by Kopp & Rabin in the infrared line Fe I 1564.9 nm. A different relationship is found between magnetic field strength and core intensity (in Fe I 630.25 nm), however, with the curve doubling back to give two distinct values of field strength at the same core intensity in the penumbra—the higher and lower field strengths corresponding to the inner and outer penumbra, respectively. In the penumbra the magnetic field pattern consists of spokelike extensions of stronger, more vertical magnetic field separated by regions of weaker, nearly horizontal magnetic field, as found by Degenhardt & Wiehr and Lites et al. The penumbral magnetic field extends outward beyond the outer continuum boundary of the sunspot, forming a canopy at the height of formation of Fe I 630.25 nm. Our results for the Evershed flow confirm the discovery by Rimmele that this flow is generally confined to narrow, elevated channels in the penumbra. In the Fe I 630.25 nm line and other strong photospheric lines we see isolated, radially elongated channels of Evershed flow crossing the outer penumbra. These flow channels lie in regions of the penumbra where the magnetic field is very nearly horizontal. In the weak C I 538.03 nm line (formed at a height h = 40 km) the flow pattern shows small, isolated patches of upflow, lying at the inner end of the Fe I flow channels where the magnetic field is more inclined to the horizontal. These patches presumably correspond to the upstream footpoints of the arched magnetic flux tubes carrying the Evershed flow. For some of the flow channels we find isolated patches of strong downflow in the C I line just outside the penumbra that might correspond to the downstream footpoints of these flux tubes. There is a weak association between the Evershed flow channels and the dark filaments seen in continuum intensity in the penumbra, but a much stronger association between the flow and the dark filaments seen in core intensity measured in the same spectral line.

The Evershed effect in sunspots as a siphon flow along a magnetic flux tube

The Evershed effect—a wavelength shift and profile asymmetry in the spectral lines observed from the outer regions of sunspots (the penumbra)—has been interpreted as a radial outflow of gas from the sunspot, but the dynamics of the flow have not been fully understood. Although the Evershed effect seems to stop abruptly at the outer edge of the penumbra, the outflow itself must continue, though tracing its path has proved difficult. Theoretical, and observational studies have suggested that much of the continuing flow may follow magnetic field lines that go below the visible surface of the Sun at or just beyond the edge of the penumbra, and recent observations have now confirmed this picture. Here we show, using theoretical calculations based on a more realistic model, that the flow acts like a siphon which is driven along a magnetic flux tube by the pressure drop between the endpoints of the tube.

THE ORIGIN OF PENUMBRAL STRUCTURE IN SUNSPOTS: DOWNWARD PUMPING OF MAGNETIC FLUX

This paper offers the first coherent picture of the interactions between convection and magnetic fields that lead to the formation of the complicated filamentary structure of a sunspot penumbra. Recent observations have revealed the intricate interlocking-comb structure of the penumbral magnetic field. Some field lines, with associated Evershed outflows, plunge below the solar surface near the edge of the spot. We claim that these field lines are pumped downward by small-scale granular convection outside the sunspot. This mechanism is demonstrated in numerical experiments. Magnetic pumping is a key new ingredient that links several theoretical ideas about penumbral structure and dynamics; it explains not only the abrupt appearance of a penumbra as a pore increases in size but also the behavior of moving magnetic features outside a spot. Subject headings: MHD — Sun: magnetic fields — Sun: photosphere — sunspots On-line material: color figures

Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae

The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Suns surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot.

Solar Interface Dynamo Models with a Realistic Rotation Profile

Extensions of the interface dynamo model of Parker are considered through two-dimensional numerical simulations in spherical geometry. In the interface model, the production of the poloidal and toroidal components of the magnetic field occur in two separate regions coupled by diffusion. A large discontinuous jump in the diffusivity at the interface allows the production of a sufficiently strong toroidal magnetic field in the lower region while avoiding the difficulty of alpha quenching. When the rotation rate is assumed to vary only radially, dynamo waves that closely resemble the analytical solutions in Cartesian geometry found by Parker are found propagating along the interface. However, when a fit to the solar rotation profile—as determined from helioseismology, with both latitudinal and radial dependence—is included, no fully satisfactory solar-like oscillatory solutions are found. For an appropriately large diffusivity contrast, only steady modes are found for negative dynamo number, and only purely decaying solutions are found for positive dynamo number. Here the effect of the latitudinal variation of rotation is to suppress the oscillatory interface modes driven by the radial shear. Oscillatory solutions can be found for a small diffusivity contrast, but these solutions have field strengths that are too low for the solar case. The hybrid mode of Charbonneau & MacGregor found from similar calculations is shown to result from an incorrect boundary condition imposed at the interface and thus is not a valid solution.

Localized sources of propagating acoustic waves in the solar photosphere

A time series of Doppler measurements of the solar photosphere with moderate spatial resolution is described which covers a portion of the solar disk surrounding a small sunspot group. At temporal frequencies above 5.5 mHz, the Doppler field probes the spatial and temporal distribution of regions that emit acoustic energy. In the frequency range between 5.5 and 7.5 mHz, inclusive, a small fraction of the surface area emits a disproportionate amount of acoustic energy. The regions with excess emission are characterized by a patchy structure at spatial scales of a few arcseconds and by association (but not exact co-location) with regions having substantial magnetic field strength. These observations bear on the conjecture that most of the acoustic energy driving solar p-modes is created in localized regions occupying a small fraction of the solar surface area.

The absorption of p-modes by sunspots: Variations with degree and order

A spherical harmonic decomposition of the p-modes into inward and outward propagating waves is employed to investigate the absorption of solar p-modes by an isolated sunspot. The absorption coefficient (averaged over frequency and azimuthal order) is found to increase with increasing horizontal wavenumber k over the range 0-0.8/Mm. For larger horizontal wavenumbers, in the range 0.8-1.5/Mm, the absorption coefficient decreases with increasing k. The absorption along each individual p-mode ridge tends to peak at an intermediate value of the spherical harmonic degree in the range 200-400. The highest absorption is found along the p(1) ridge, and the absorption decreases with increasing radial order.

Siphon flows in isolated magnetic flux tubes

The paper considers steady siphon flows in isolated thin magnetic flux tubes surrounded by field-free gas, with plasma beta greater than or equal to 1, appropriate for conditions in the solar photosphere. The cross-sectional area of the flux tube varies along the tube in response to pressure changes induced by the siphon flow. Consideration is also given to steady isothermal siphon flows in arched magnetic flux tubes in a stratified atmosphere. Applications of the results to intense magnetic flux tubes in the solar photosphere and to the photospheric Evershed flow in a sunspot penumbra are addressed.