Mass and energy flow near sunspots

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.     

Mass transport above sunspots

Mass transport in the transition zone above a sunspot has been studied in theCiv line at 1548 Å, formed at 100 000 K. Four spectral rasters have been used, each covering 60×50 are sec. These have been observed with the High Resolution Telescope and Spectrograph during the Spacelab-2 mission in 1985.Flow velocities are derived from multiple Gaussian components fitted to the observed line intensity profiles. Density sensitive lines were used to find a relation between theCiv line intensity and the particle density. This relation is used in the mass flux calculations. The investigation indicates a clear net downward mass flux above sunspots.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Material flow in sunspots

Observations nowadays reveal more and more details about the small‐scale structure of the penumbra and umbra. Recent measurements of the motions in the penumbra indicate that (i) the Evershed flow is confined to a thin photospheric layer and (ii) the material rises from and sinks to subphotospheric layers within the penumbra. This flow pattern solves the question of the mass budget of the Evershed flow in a natural way. The nature of umbral dots and their role for the spot's energy budget remains unclear, since they are obviously unresolved even in the best existing measurements.     

Observations of moving magnetic features near sunspots

The properties of small (< 2″) moving magnetic features near certain sunspots are studied with several time series of longitudinal magnetograms and Hα filtergrams. We find that the moving magnetic features:

1. Are associated only with decaying sunspots surrounded entirely or in part by a zone without a permanent vertical magnetic field.
2. Appear first at or slightly beyond the outer edge of the parent sunspot regardless of the presence or absence of a penumbra.
3. Move approximately radially outward from sunspots at about 1 km s^?1 until they vanish or reach the network.
4. Appear with both magnetic polarities from sunspots of single polarities but appear with a net flux of the same sign as the parent sunspot.
5. Transport net flux away from the parent sunspots at the same rates as the flux decay of the sunspots.
6. Tend to appear in opposite polarity pairs.
7. Appear to carry a total flux away from sunspots several times larger than the total flux of the sunspots.
8. Produce only a very faint emmission in the core of Hα.

A model to help understand the observations is proposed.     

On the topology of filaments and chromospheric fibrils near sunspots

The similarity between the spiral topology of chromospheric fibrils and filaments observed in H near sunspots and the configuration of an axisymmetric force-free magnetic field is examined. It is suggested that some of the observed features could be interpreted in terms of the configuration of lines of force of an axisymmetric force-free chromospheric magnetic field. Implications of the results of analysis to the possible interpretations of other observed topological features near a sunspot are discussed.Visiting scientist at the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Sponsored by the Office of Naval Research, Contract No. N 00014-67-C-0290.     

The topology of force-free magnetic field near bipolar sunspots

The topological character of a new type of solution representing a force-free magnetic field near bipolar sunspots is examined. It is shown that some of the observed topological features of chromospheric fibrils and filaments in H can be interpreted in terms of the configuration of the magnetic lines of force of the present solution. In particular, by the examples considered the observed twisted S-shape topology of lower lying fibrils and the orientation of prominences (higher filaments) associated with sunspots are successfully reproduced.Visiting scientist at the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Naked sunspots

Naked sunspots are spots seen in Hα to be devoid of associated plage. In magnetograms and K-line little if any opposite polarity field is found, and in soft X-ray images a blank appears in the region of the spot. In almost all cases studied in which naked spots resulted the spot groups had emerged in unipolar regions of the same polarity as the naked spot. At least half of the naked spots are associated with coronal holes. The naked spots are long-lived and show rotation rates close to the Newton-Nunn curve. Most of the naked spots had bright rims in Hα, and the one spot observed to disappear left no trace in the background magnetic field. These spots may be a means by which separation of p from f magnetic polarity occurs.     

Structure of sunspots

The sunspot models published so far do not reproduce the observed run of the umbral continuum intensities over the entire spectral range 0.5 < < 4 m. Moreover, in several previous models is the temperature gradient smaller than both the adiabatic and the radiative equilibrium gradient.Agreement between intensities computed from acceptable models and measured intensities can be obtained by introducing an additional opacity for 0.8 m, which is probably caused by the crowding of atomic and molecular lines. We present a new umbral model atmosphere with a wavelength dependent opacity enhancement factor which explains the continuum intensities and also reproduces plausible center-to-limb variations and line profiles. This model is in radiative equilibrium down to _0.5 = 1.5, with an effective temperature of 4000 ± 100K. For the deeper superadiabatic layers a small but probably significant departure from radiative equilibrium is indicated by the intensities in the range 1.5 < < 2.4 m.The uncertainties in the present model and the effect of the additional opacity on line profiles are briefly discussed.     

Flows around sunspots and pores

We report on three sequences of high-resolution white-light and magnetogram observations obtained in the summer of 1989. The duration of sub-arcsecond seeing was three to four hours on each day. Study of the white-light and magnetogram data yields the following results:

1. For all but one of the sunspots we have observed, both dark fibrils and bright grains in the inner part of the penumbra of sunspots move toward the umbra with a speed of about 0.5 km s^-1. In the outer part of the penumbra, movement is away from the umbra. The one exception is a newly formed spot, which has inflow only in its penumbra.
2. Granular flows converge toward almost every pore, even before its formation. Pores are observed to form by the concentration of magnetic flux already existing in the photosphere. The pores (or small sunspots), in turn, then move and concentrate to form bigger sunspot.
3. We followed an emerging flux region (EFR) from 29 to 31 July, 1989 that was composed of a large number of bipoles with magnetic polarities mixed over a large area in the first day of its birth. As time went on, polarities sorted out: the leading polarity elements moved in one direction; the following, the opposite. During the process a large number of cancellations occurred, with some sub-flares and surges observed simultaneously. After about 24 hours, the positive and negative fluxes were essentially separated.
4. We find two kinds of photospheric dark alignments in the region of new flux emergence: (a) alignments connecting two poles of opposite magnetic polarity form the tops of rising flux tubes; (b) alignments corresponding to the magnetic flux of one polarity, which we call elongated pores.

     

Equilibrium and decay of sunspots

We suggest a quantitative sunspot model developed in terms of mean-field magnetohydrodynamics (MHD). The model consistently describes the distributions of magnetic field, fluid velocity, and thermodynamic parameters in a sunspot and the surrounding matter. Two versions of the model allow the MHD equilibrium in sunspots and their slow decay to be analyzed. The baroclinic flow converging to the sunspot plays an important role in the equilibrium. Several calculated characteristics—almost uniform distributions of brightness and magnetic field inside sunspots, their abrupt changes at the boundary, and nearly linear decreases in the area and magnetic flux of decaying sunspots with time—qualitatively agree with the observations.     

Geomagnetic disturbances and complex sunspots

Statistical result shows that a large geomagnetic disturbance is most likely to occur at the time when a complex sunspot is in 6.7° E-19.9° E from the central meridian of the solar disk during the period 1968–1972.     

The structure of sunspots

As a first step in constructing three-dimensional decaying sunspot models we select the relevant observational data. From these we conclude:

1. sunspots, except the smallest, obey a radial and evolutionary similarity;
2. sunspots may be considered as isolated, fairly well defined flux tubes, wrapped in thin current sheets;
3. a substantial number among stable regular spots show a phase of slowest decay whose rate is independent of the spot's area.

Arguments are given that the slowest rate of decay is ultimately determined by Ohmic dissipation in the inner part of the current sheet. Preliminary asymptotic models for the deep layers (deeper than 2000 km below the photosphere) are given which satisfy the above three constraints. To meet the observed rate of slowest decay the current sheet has to be very thin, about 10^?5 to 10^?4 times the umbral radius. Radial large-scale fluid motions are required in the current sheet to maintain the similarity of the structure. The radial motions are linked with the vertical motions which may be connected with the Evershed flow. Finally we discuss details which are less relevant in the large-scale structure of stable sunspots, such as fine structures, twists, the break-down of the similarity and the relation between sunspots and smaller magnetic structures, and the intrinsic scatter in some observed quantities.     

On molecules in sunspots

Spectra of umbrae of several sunspots in the wavelength region 4000–8000 Å scanned photoelectrically at the Göttingen Locarno Observatory were used in order to search for absorption lines of molecules. Several thousands of lines of known molecules were reidentifled (see Table II and Table III). Newly identified molecules are CoH, NiH and H₂O (see Table IV and Wöhl (1969a)). The Zeemann effect on molecular lines (of MgH and CaH; see Table V and Wöhl (1969b)) in spectra of umbrae was detected.     

Lithium abundance in sunspots

The observations of lithium were carried out with the TST-2 telescope at CrAO on August 15–20, 2006. A sunspot model was calculated for the dates of observations. The lithium abundance in a sunspot and in the undisturbed photosphere was determined. It is log(N _Li) = 1.35 for the sunspot and log (N _Li) = 1.05 for the undisturbed photosphere.     

Hydromagnetic flow near an oscillating wall

The flow of a viscous incompressible and electrically conducting fluid produced by harmonically oscillating wall of infinite extent in presence of a transverse magnetic field is considered. Exact solutions for velocity, induced magnetic field, electrical current density and skin-friction are obtained when the magnetic Prandtl number is unity. It is shown that the velocity has a phase lag with respect to the oscillations of the wall. This phase lag is found to be significantly affected by the applied magnetic field.On study-leave from Defence Science Laboratory, Delhi, India.     

Non-radial flow near the cometary surface

It is shown that, as a consequence of the non-uniform temperature distribution of the cometary nucleus, large lateral pressure gradients are set up, which in turn drive strong lateral flows. However, at small heliocentric distances the onset of turbulence within a thin boundary layer destroys these steady lateal flows and the eventual outflow of gas from within the outer boundary of this layer is expected to be more or less radial. On the other hand, at large heliocentric distances, turbulence is unlikely to set in, and the lateral flows that are set up, may persist. Consequently, it is expected that the gas flow out of the cometary nucleus at these large distances to be highly non-radial.     

Vortical distribution of sunspots

A remarkable vortical distribution of sunspots, photographed at National Solar Observatory, Tucson, may be an important indication that a vortex flow is essential in the formation of sunspots. It is argued that the concept of vortex flow around a spot is not a heresy, since a generation mechanism of non-trivial force-free fields requires the presence of time varying vorticity in the photosphere.     

The structure of sunspots

Magnetic field strengths in small umbrae and pores are measured using the line Ti i 6064.6 Å, which is formed purely in umbrae. We find field strengths between 1900 and 2600 G in the darkest parts of small umbrae and of well established pores; the spread is partly intrinsic. The field strength in diffuse transient protopores amounts to 1500 ± 250 G.We demonstrate that usage ofthe well-known magnetic line 6173.3 Å and other Fe i lines yield systematically smaller magnetic field strengths than Ti i 6064.6 Å. This is due to blending ofthe components with the central component due to photospheric stray light and the component. Routine measurements are therefore unreliable for small spots 251-01Based on observations at Sacramento Peak Observatory, Sunspot, New Mexico, U.S.A.     

The depth of sunspots

Good quality photographs of many regular sunspots were obtained in three spectral regions, with the use of narrow band filters. Isodensity contours were used for measurements of the umbra and penumbra size along different axes of sunspots. A parameter based on the morphology of the whole sunspot was defined for a better study of the Wilson effect. Some interesting results are the following: (a) an east-west asymmetry of the Wilson effect was clearly observed, (b) differences in the phenomenon in the three spectral regions were barely significant compared with observational errors and (c) measurements indicate that larger sunspots have greater depth.