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.     

Sunspot geometry and pressure balance

Measurements on magnetic canopies extending from sunspots show that, at the outer penumbral edge, heights of the bases are independent of sunspot diameter and average 180 km. This places a lower limit on the outer penumbral base; with an assumed thickness of 250 km, the top is 430 km above z = 0 ( _c = 1) in the photosphere.Chistyakov's (1962) observations require the penumbral surface to be convex in radial section. The Wilson depression, able thus to be found only from limb-side penumbras, is 1360 km from his selected measurements. Averaged over all regular sunspots without special selection, this drops to 1040 km. Thus ^* = 1 in umbras lies around z = -610 km.Magnetic field-strength measurements relate probably to ^* 0.02, some 160 km higher, where z -450 km. The magnetic pressure of the typical 3250 G sunspot field would support the external-axial gas-pressure difference at z = -330 km, the difference of 120 km lying well within the uncertainties. Tension forces, commonly invoked to achieve pressure balance, do not exceed the uncertainties of measurement.Beyond the sunspot, the base of the sunspot field rises only slowly over at least 16 000 km horizontally, whereas Beckers (1963) found the inclination of H superpenumbral fibrils to be some 13°. These results are nicely compatible since the field angle is typically of this magnitude at the minimum heights where H fibrils will be observed, say 1400 km.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Fine structure of solar magnetic fields

The deduction of magnetic fields from chromospheric structure is extended to active regions and transverse fields. Fields independently predicted by these rules from a high resolution H filtergram are compared with a high resolution magnetogram. The H method has the advantage over conventional magnetograms that it shows transverse fields and relates the fields to the real Sun. It has the disadvantage that higher spatial resolution is required and that it is difficult and time consuming in very complicated regions.The response of the chromosphere to magnetic fields is most consistent. Vertical field is invariably marked by bright plage, with brightness roughly proportional to the field strength (except for sunspots). All dark fibrils mark transverse fields and are parallel to field lines. All polarity changes are marked by dark fibrils, which may be transverse fibrils perpendicular to the field boundary, or filaments (prominences) which connect more distant points, and in which the field lines run nearly parallel to the boundary. The asymmetry between preceding and following polarity found by Veeder and Zirin (1970) does not exist; it was due to the low resolution of the Mount Wilson magnetograms.The complexity of active region field structure depends on the history of the region; all flux erupts in simple bipolar form, and lines of force remain connected to sibling spots until reconnection takes place. Thus the complex structure only occurs after eruption of several dipoles which reconnect. The phenomenon of inverted polarity turns out to be due to the emergence of satellite bipolar fields, where the p spot merges with the rest of the p field and the f spot appears as an included f field. Flares usually occur when the field lines from f spot reconnect from its sibling to the main spot.     

Civ plasma flow near active region filaments

Selected Civ V ₀ lines (lines separating adjacent regions of strong blue shift and strong red shift) located near the solar limb and oriented parallel to the radius vector from disk center are shown to be closely aligned with H dark filaments in active regions. The filaments, in turn, are known to lie in the vicinity of magnetic neutral lines. The radial orientation of the V ₀ lines minimizes uncertainties in image registration and their location near the limb ensures that the observed fluid motion has major components paralleling V ₀. It follows that the filaments are located at sites of velocity shear, and, by inference, of magnetic shear. For a case in which a given V ₀ line is observed near both east and west limbs, the gradient of Doppler velocity across the V ₀ line reverses sign from one limb to the other as is expected for horizontal steady flow. Thus, the velocity vectors remain fixed with respect to the filament.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Large-scale motions in the sun

Large-scale solar motions comprise differential rotation (with latitudinal, and perhaps radial gradients), axially symmetric meridional motions, and possible asymmetric motions (giant convective cells or Rossby-type waves or both). These motions must be basic in any satisfactory theory of the changing pattern of solar magnetic fields and of the 22-yr cycle. In the present paper available data are discussed and, as far as possible, evaluated and explained.Rotational measurements are based on the changing positions of discrete features such as sunspots, on Doppler shifts, on geophysical changes and on statistical evaluation of the motions of diffuse objects. The first mentioned, comprising faculae, sunspots, K-corona (to latitudes 45°) and filaments, show agreement better than 0.7 %. A new formula for surface rotation _s , based on faculae and sunspot data, is _s = 14.52 – 2.48 sin² b – 2.51 sin⁶ b deg day^–1, where b is latitude, and validity may extend to about 70°. Errors in Doppler shift measurements and statistical treatments are discussed. There is evidence of a much slower coronal rate at high latitudes, and of a slower sub-surface rate at lower latitudes.Ordered meridional motions have been revealed by statistical investigations of the positions of spot groups, of spots and of filaments. All these results seem explicable in terms of an oscillating hydro-magnetic circulation in each hemisphere. These have both 11-yr and 22-yr components, and these periods are provided by a general dipole field of about one gauss, together with a pair of toroidal fields centred at latitudes ±16° and of average strength of order 10 G.Evidence of large-scale (perhaps 3 × 10⁵ km), irregular surface motions is provided by the distribution of surface magnetic flux, the motions of sunspots, and Doppler-shift observations; it is supported by Ward's theory of the equatorial acceleration. The possibility is suggested that these asymmetric motions also drive the oscillatory meridional motions.     

On the small-scale structure of solar magnetic fields

The small-scale structure of solar magnetic fields has been studied using simultaneous recordings in the spectral lines Fe i 5250 Å and Fe i 5233 Å, obtained with the Kitt Peak multi-channel magnetograph. We find that more than 90% of the magnetic flux in active regions (excluding the sunspots), observed with a 2.4 by 2.4 aperture, is channelled through narrow filaments. This percentage is even higher in quiet areas. The field lines in a magnetic filament diverge rapidly with height, and part of the flux returns back to the neighbouring photosphere. Therefore the strong fields within a magnetic filament are surrounded by weak fields of the order of a few gauss of the opposite polarity. The field-strength distribution within a filament, including the surrounding opposite-polarity fields, seems to be almost the same for all filaments within a given active or quiet region.The analysis of a scan made during an imp. 2 flare showed that observations during and after the flare would give a fictitious decrease of the magnetic energy in the region by a factor of 2–3 due to line-profile changes during the flare.Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On two different types of sunsports-‘vortices’ and ‘sources’-within A U(1)-symmetric Yang-Mills-Higgs system

An intimate link between the structure of the sunspot's magnetic field contiguration and the behaviour of the Higgs field phase at large distances from spot's center is discussed within a U(1)-symmetric Yang-Mills-Higgs gauge theory. It is shown, in particular, that the total magnetic flux of standard sunspots can acquirediscrete values only. A conclusion is also arrived at that, in addition to standard sunspots (magneticvortices), a new sort of sunspots (magneticsources (sinks)), whose total magnetic flux iszero, could principally be observed on the Sun; the two sorts of spots are shown to stand in adual orcomplementary relation to each other. In this connection, finally, a question about the possible role of the divergence of electromagnetic potential — the field violating a gauge invariance of the theory — in physics of two-dimensional solar phenomena is briefly touched.     

Fine structures of chromospheric magnetic field and material flow in a solar active region

In this paper, we analyze the relations between photospheric vector magnetic fields, chromospheric longitudinal magnetic fields and velocity fields in a solar active region. Agreements between the photospheric and chromospheric magnetograms can be found in large-scale structures or in the stronger magnetic structures, but differences also can be found in the fine structures or in other places, which reflect the variation of the magnetic force lines from the photosphere to the chromosphere. The chromospheric superpenumbral magnetic field, measured by the Hline, presents a spoke-like structure. It consists of thick magnetic fibrils which are different from photospheric penumbral magnetic fibrils. The outer superpenumbral magnetic field is almost horizontal. The direction of the chromospheric magnetic fibrils is generally parallel to the transverse components of the photospheric vector magnetic fields. The chromospheric material flow is coupled with the magnetic field structure. The structures of the H chromospheric magnetic fibrils in the network are similar to H dark fibrils, and the feet of the magnetic fibrils are located at the photospheric magnetic elements.     

A relation between magnetic field strength and temperature in sunspots

We present Stokes I Zeeman splitting measurements of sunspots using the highly sensitive (g = 3) Fe i line at = 1.5649 m. The splittings are compared with simultaneous intensity measurements in the adjacent continuum. The relation between magnetic field strength and temperature has a characteristic, nonlinear shape in all the spots studied. In the umbra, there is an approximately linear relation between B ² and T _b, consistent with magnetohydrostatic equilibrium in a nearly vertical field. A distinct flattening of the B ² vs T _brelationship in the inner penumbra may be due to changes in the lateral pressure balance as the magnetic field becomes more horizontal; spatially unresolved intensity inhomogeneities may also influence the observed relation.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

The diatomic molecules BH,BN, and BO in sunspots and the solar abundance of boron

Absorption band spectra of BH and BO have been searched for and not found in spectra of sunspots.Electronic oscillator strengths are available only for the A ¹ -X ^{1 +} system of the BH molecule. The absence of the (0,0) band of BH at 4332 Å reflects a solar abundance of boron logA _B<2.5.The band spectra of BN are several orders of magnitude weaker in sunspots than those of BH and BO.Kitt Peak National Observatory Contribution No. 488.Visiting Astronomer, Solar Division, Kitt Peak National Observatory - Operated by The Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On the position of sunspots in the core of Hα relative to the continuum

The relative position of sunspots as observed in the core of H and in the continuum has been studied in 316 spectra of 84 different sunspots. We find that chromospheric features surrounding sunspots may produce apparent shifts of the spots in the core and in the wing of H. In addition a shift directed towards the limb is found. This shift is found to be a height effect. The difference in height between the levels of the H core and the continuum varies from 2300 km to 1000 km for different sunspots.     

Line formation in a magnetic field and the interpretation of magnetographic measurements

The strong temperature dependence of the line Fei 5250.2 has been studied by calculating line contours and magnetographic calibration curves for different spot models and the BCA. Line contours calculated for arbitrary depth dependence of the magnetic field vector show depolarization effects within the Zeeman components for transversal fields with variable direction and changes of the observed plane of polarization if anomalous dispersion is taken into account.The observed anomalous splitting of the -component may be interpreted best by suggesting discrete inhomogeneities of the magnetic field within sunspots.     

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.

     

Nature of the fine structure of the middle chromosphere

Fine dark H filaments fibrils form at the limb, apparently in most of the middle chromosphere corresponding to an altitude between 1500–2000 km and 4000 km. The space in between filaments is corona and the transition layer. The cool gas in fibrils is protected by the magnetic field against the conductive flux out of the hot corona. Therefore the fibrils stretch up to 4000 km where their temperature is about 18 000 K and the density about 5 × 10⁹ cm^–3. The gas in the fibrils is ionized by electronic collisions and by the external ultraviolet radiation. The second level of the hydrogen atoms in fibrils is populated by recombinations, electronic collisions and by Ly- quanta. The calculated optical thickness of the fibrils in H is about 1, it explains the absorption features on the spectroheliograms. The gas pressure in fibrils is lower than the coronal pressure, and the pressure equilibrium is achieved by a magnetic field of about 1.5–2 G. In the active regions the photospheric fields are stronger, therefore the fibrils in active regions are wider and show more contrast. The emission of the fibrils at the limb is explained by the scattering of the solar radiation. The temperature in arches reaching as high as 5000–6000 km, is stabilized near the top by the HeII emission. Thus the middle chromosphere is essentially a collection of magnetic arches.     

Distribution of sunspots according to their magnetic fluxes

The occurrence frequency distribution of sunspots in different magnetic flux values has been examined. The number of sunspots decreases as ^-1.9 for sunspots with magnetic flux greater than 3 × 10²¹ Maxwell, where is the said flux of a sunspot.     

Effects of magnetic shear,spot, and plage rotation on prominence evolution

We have studied the evolution of two dark H filaments as prominences during their disk passage from 12 to 19 February, 1992 and 6 to 17 March, 1992, using Kodaikanal Observatory H and Caii K spectroheliograms. Both the filaments were well outside the spot regions. However, they were connected to sunspots by small threads. Outside the spot regions, the filaments were also anchored between opposite polarity plage regions. Both the filaments were almost straight in the beginning. However, they acquired a curved shape (inverted U-shape) as the spot and plages underwent rotation. It is shown that rotation of the plage and spot plays an important role in the evolution of prominences, one serving as the anchor and the other imparting necessary shear. Once the shear reaches a critical value it starts unwinding the filaments, resulting in the fine structure of the two prominences studied.     

Penumbral filaments: A dissipation form of the magnetic field

In this paper we suggest that penumbral filaments are a phenomenon of magnetohydrodynamic instability, developed in a stable and uniform magnetic field of sunspots during a dissipation process. We have solved local magnetohydrodynamic disturbance equations and have obtained the necessary condition for filament instability mode, that the ratio of filament length to width must be larger than the ratio of Alfvén speed to sound speed. We have also obtained correlations between two fluctuations from their phase difference. Although there are two correlations between the fluctuation of temperature (or filament intensity) and (1) the fluctuation of magnetic field, and (2) the fluctuation of the flow during the phases of developing and dissipating of the filament, we cannot distinguish whether the correlation is associated with the light filament or dark filament and we cannot decide whether the phase difference is 0° or 180° from tg() = 0. However, we can make a judgment: if the correlation is associated with a light filament during its development phase, it will be associated with a dark filament during its dissipation phase, andvice versa. In addition, there are no correlations between the fluctuations mentioned above for a stable filament, because the phase difference of the filament is changing with time.The phase differences of filaments are related to the existence of a gravitational field.     

Magnetohydrostatic Model of a Bald-Patch Flare

On 18 May, 1994, a subflare was observed in AR 7722 in X-rays by Yohkoh/SXT and in H at National Astronomical Observatory of Japan. The associated brightenings are due to small-scale magnetic energy release, triggered by parasitic fluxes emerging and moving at the edge of leading sunspots. Using the magnetohydrostatic equations derived by Low (1992), we model the magnetic field configuration by extrapolation of the Kitt Peak photospheric field, taking into account the effects of pressure and gravity. H flare kernels are shown to be located at computed separatrices associated with field lines which are tangent to the photosphere, namely 'bald patches (BPs). This is evidence that BPs can be involved in flares, and that current sheets can be dissipated in low levels of the solar atmosphere. The presence of dense plasma which is supported against gravity in the magnetic dips above BPs is correlated to dark elongated features observed in H. Mass flows in these flat fibrils are discussed in the context of energy release in the BP separatrices. The effect of the plasma on the computed magnetic configuration is shown to be of secondary importance with respect to the topology of the field.     

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.     

Helical magnetic fields in filaments

For both even and odd-numbered solar cycles, right-hand heliform filaments predominate at middle and high latitudes in the northern hemisphere while left-handed ones predominate in the south. This recent discovery has prompted a re-examination of past measurements of magnetic fields in prominences. This re-examination indicates that Rust (1967), in his interpretation of solar cycle 20 measurements in terms of the Kippenhahn-Schlüter model, and Leroy, Bommier, and Sahal-Bréchot (1984), in their interpretation of solar cycle 21 measurements in terms of the Kuperus-Raadu model were both misled by the global pattern of helicity. While the original magnetic field measurements are consistent with the new results about heliform magnetic fields in filaments, neither of the well-known classes of two-dimensional models can produce both the proper axial field direction and the observed pattern of helicity. A global, subsurface velocity pattern that would twist the fields before emergence as filaments seems to be required. In this paper a twisted-flux-rope model consistent with the new understanding of filament fields is presented. The model is based on a constant- solution of the magnetostatic equations, where electric current densityj(r) = B(r). The model filament has dimensions in general agreement with observations. It is shown to be stable if the length is less than 140 000 km to 1,400 000 km, depending on the value of. The model also provides a new explanation of eruptive prominences and for the origin of the entrained material.