Sunspot Magnetic Fields Research Articles

The Relationships of Sunspot Magnetic Field Strength with Sunspot Area, Umbral Area and Penumbra-Umbra Radius Ratio

Stokes profile inversion is very important to get the information on the vector magnetic field. Because the magnetic fields cannot be directly observed, adopting Stokes spectrum analysis to obtain vector magnetic field has become the major technique recently. Therefore, by Stokes profile inversion, we obtained vector magnetic fields of two layers based on the numerical solution (DELO solution, ReEs et al., 1989) to the polarized radiative transfer equation. We analyze the relationships of sunspot magnetic field strength with sunspot area, umbral area and penumbra-umbra radius ratio. By statistical research, it is found that the field strengths of the upper layer and the lower one decrease with the increasing penumbra-umbra radius ratio, and that the logarithmic expression is able to fit well the relationship between the maximum field strength of the upper layer and the sunspot area. Furthermore, we verify the result obtained by Ringnes and Jensen (1961) about the relationship between the maximum magnetic field strength and the umbral area, and the result obtained by Antalova (1991) of the relationship between the field strength and the penumbra-umbra radius ratio.

First Polarimetric Measurements and Modeling of the Paschen-Back Effect in CaH Transitions

We report the first spectropolarimetric observations and modeling of CaH transitions in sunspots. We have detected strong polarization signals in many CaH lines from the A-X system, and we provide the first successful fit to the observed Stokes profiles using the previously developed theory of the Paschen-Back effect in arbitrary electronic states of diatomic molecules and polarized radiative transfer in molecular lines in stellar atmospheres. We analyze the CaH Stokes profiles together with quasi-simultaneous observations in TiO bands and conclude that CaH provides a valuable diagnostic of magnetic fields in sunspots, starspots, cool stars, and brown dwarfs.

Temporal Changes in Sunspot Umbral Magnetic Fields and Temperatures

We have observed high-resolution intensity spectra near the Fe i 1564.8 nm line at a single umbral point corresponding to the darkest position in over 900 sunspots from 1998 through 2005. From these data we determine that the maximum sunspot magnetic fields have been decreasing at about 52 G yr 1 . The same data set shows a concurrent increase in the normalized umbral intensity from 0.60 to 0.75 (corresponding to a blackbody temperature rise from 5137 to 5719 K) and a decrease of more than 50% in the molecular OH line strength. The magnetic field and intensity changes observed over time in the sunspot umbrae from different spots behave in the same way as the magnetic field and intensity changes observed spatially across single sunspots.

Vertical current densities and magnetic gradients in sunspots

Aims. The vertical component of electric current densities and the height dependence of the vertical component of the magnetic field in sunspots were determined. Methods. Full-Stokes magnetograms of eight sunspots obtained from infrared spectro-polarimetric measurements were investigated. The magnetic field strength and the magnetic inclination and azimuth were obtained from an inversion code. Vertical current densities and the vertical derivative of the vertical component of the magnetic field strength resulted from Maxwell equations. Results. It is found that electric current densities and magnetic gradients depend on the finestructure of the sunspots. Typical values for current densities vary in the range ±40 mA m -2 , but their errors are of the same order. Disturbances in the radial structure of the penumbra are related to enhanced current densities up to 149 mA m -2 , and a maximum value of 166 mA m -2 was found in a light bridge. There are indications that the radial structure of the penumbra is related to current densities, but no correlation to the local intensity fluctuations was found in the outer penumbra. The vertical component of the magnetic field decreases by 0.5-1.5 G km -1 in the umbra. Mean values in the inner penumbra are somewhat smaller than in the umbra, and locally dark structures exhibit a faster decrease with height than bright ones. In the outer penumbra the vertical magnetic component increases, independent of the local intensity distribution. Conclusions. Electric current densities could be a diagnostic tool for understanding the penumbral finestructure, although the presently available spatial resolution is probably not good enough to avoid artifacts. Magnetic extrapolations might depend crucially on this problem.

Statistical analysis of the sunspot area and magnetic flux variations in 1996–2005 extracted from the Solar Feature Catalogue

This research presents some statistical properties of sunspots and their magnetic fields extracted in from May 1996 to May 2005 from the SOHO/MDI full disk whitelight images and magnetograms for the searchable Solar Feature Catalogue (SFC) using the automated pattern recognition techniques. A comparison of the total sunspot areas on a given day from the SFC with the daily sunspot areas available from US AF SOON data for 2000–2004 reveal a very good correlation of the datasets with the correlation coefficient of ∼93%. The total sunspot areas in the Northern and Southern hemispheres measured from a single solar image and their cumulative areas during the whole cycle are shown to have a strong North-South asymmetry with the Northern hemisphere prevailing around and after the maximum while the Southern one Schatten taking over towards the coming solar minimum. The similar N-S asymmetry is observed in a total and resulting, or excess, magnetic fluxes. The former is found to follow closely the N-S asymmetry in the sunspot areas while the latter shows a very significant flux separation in the opposite hemispheres. The excess flux is negative in the Southern hemisphere and positive in the Northern one during a long period from 1997 until 2004. During the solar minimum in 1996, the signs of total excess fluxes in the hemispheres are changed to the opposite and a similar change appears in 2003–2004 towards the approaching solar minimum. Since the magnetic field in sunspots is those of the leading polarity, so the excess magnetic flux evolution is believed to show a change of the magnetic field leading polarity during the solar cycle minima, while the asymmetry of the total magnetic flux, possibly, reflects the asymmetry of a poloidal magnetic field, as it is predicted by the oscillatory dynamo models.

The Three-dimensional Structure of a Sunspot Magnetic Field

We report on observations of the three-dimensional structure of a sunspot magnetic field from the photosphere to the chromosphere, obtained with the new visible/infrared spectropolarimeter SPINOR. The observations, interpreted with a non-LTE modeling technique, reveal a surprisingly complex topology with areas of opposite-sign torsion, suggesting that flux ropes of opposite helicity may coexist together in the same spot.

Spatial pattern of photospheric granules in a sunspot neighborhood

Based on high-resolution (∼0.3 arcsec) observations, we studied the behavior of solar granulation in the neighborhood of a sunspot. The bright granules’ spatial distribution and the granules’ surface density as a function of distance from the center of the sunspot umbra were determined. Bright granules distribute delimiting cells of dimensions in the mesogranular scale. The mean diameter of these cells does not show significant variation with the variation of the magnetic field of the sunspot. The granules’ surface density does not show significant variation with distance to the sunspot umbra. Both results point to a very weak, if any, influence of the sunspot magnetic field at distances greater than 20 arcsec.

Technical note: Joint database of sunspot magnetic fields

[1] Preliminary results of the creation of the Pulkovo electron database of sunspot magnetic fields are presented. The database includes observations at seven observatories in the scope of the National Program of Solar Service for 40 years. In different periods of time, Crimea Astrophysical Observatory, Main Astronomical Observatory, Siberian Institute of the Terrestrial Magnetism and Radio wave Propagation (Irkutsk), Institute of Terrestrial Magnetism and Radio wave Propagation (Troitsk, Moscow Region), Shamakha Astrophysical Observatory, Observatory of the Ural State University, and Ussuriisk Astronomical Station participated in these observations; however, only two first observatories from this list conducted continuous observations in 1957–1997. A series of a new activity index, annual mean values of the magnitudes of sunspot magnetic fields in the Pulkovo system, is presented.

The Pulkovo CCD Spectroheliograph - Magnetograph

The CCD spectroheliograph-magnetograph is a focal plane ancillary instrument for Pulkovo horizontal solar telescope ACU-5. The instrument is placed at an exit port of an isothermal high-resolution diffraction-grating spectrograph. The modified Leighton optical scheme for registration of sunspot magnetic fields is used. The instrument provides obtaining FITS digital video cards of radial velocities, magnetic fields and spectroheliogram in any line of spectral region 3900A - 11000A. The time of obtaining of one video card by the size 91″ 154″ is equal 10.24sec. The angular resolution of the instrument is 0″.8; spectral resolution is 0.01-0.03A. There is remote access to a solar telescope in real time on the basis of Internet - process engineerings.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The Influence of Magnetic Index Abnormal Change in Solar Magnetic Field on Climate at Middle Latitude of North Hemisphere

AbstractIn this paper, based on the characteristics of the sunspot number, the time sequence of the magnetic index of the sunspot magnetic field is established. The analysis of temperature field spectrum shows that in the air temperature field of the troposphere and stratosphere at the middle latitude of Northern and Southern hemispheres there generally exist cycles with a 22‐year period. The analysis also shows that the cycle of the air temperature field with a 22‐year period is generated by the solar activity with a 22‐year period.

The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields

Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes V profiles may require very high S/ N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots.

Nonlinear distribution of the sunspot magnetic field during the solar maximum

Using the standard solar w-dynamo equations we formulate appropriate initial and boundary conditions at the moment when the toroidal component is maximum, so that we obtain the toroidal magnetic field component as a function of the solar latitude. We found a new non-homogeneous distribution for the toroidal component along the latitude (〈 ΔB θ / Δθ〉) given by 7G/ degree, 24G/ degree and 34G/ degree for the respectiives latitudes ranges of 10°-15°, 16°-18° and 19°-25°) and this nonhomogenous decay rate is statistically confirmed through a chi-square analysis of the Mt. Wilson Sunspot data for two solar cycles. Based on this result one can expect the most energetic magnetic eruptive phenomena to be, in average, nonlinearly concentrated at latitudes ranging from 10 to 25 degrees in both hemispheres.

SUNSPOT MODELING AND SCALING LAWS

In an early paper Skumanich suggested the existence of a scaling law relating the mean sunspot magnetic field with the square-root of the photospheric pressure. This was derived from an analysis of a variety of theoretical spot models including those by Yun (1968). These were based on the Schliiter-Temesvary (S- T) similarity assumption. To answer criticisms that such modeling may have unphysical (non-axial maxima) solutions, the S-T model was revisited, Moon et al. (1998), with an improved vector potential function. We consider here the consequences of this work for the scaling relation. We show that by dimensionalizing the lateral force balance equation for the S- T model one finds that a single parameter enters as a characteristic value of the solution. This parameter yields Skumanich`s scaling directly. Using an observed universal flux-radius relation for dark solar magnetic features (spots and pores) for comparison, we find good to fair agreement with Yun`s characteristic value, however the Moon et al. values deviate significantly.

Solar activity II: Sunspots and pores

AbstractSunspots and pores appear as a consequence of interactions between strong magnetic fields and moving plasma. A wide variety of small‐scale features, presumably of convective origin, are observed in photospheric layers of sunspots and pores: Umbral dots, light bridges, penumbral filaments, and penumbral grains. Each type of features has specific morphological, photometric, spectral, and kinematic characteristics. Spots and pores modify velocity fields in adjacent photosphere and sub‐photospheric layers. Recent high‐resolution spectral, broad‐band, and helioseismic observations of the structure, dynamics, and magnetic fields of sunspots and pores, together with theoretical interpretations, are discussed in this review.

Structure of a simple sunspot from the inversion of IR spectral data

AbstractAnalysis of spectral data of two neighboring infrared lines, Fe I 15648.5 Å (g = 3) and FeI 15652.9 Å (geff = 1.53) are carried out for a simple sunspot when it was near the solar disk center (μ = 0.92), to understand the basic structure of sunspot magnetic field. Inversions of Stokes profiles are carried out to derive different atmospheric parameters both as a function of location within the sunspot and height in the atmosphere. As a result of the inversion we have obtained maps of magnetic field strength, temperature, line‐of‐sight velocity, field inclination and azimuth for different optical depth layers between log(τ5) = 0 and log(τ5) = –2.0. In this paper we present few results from our inversion for a layer averaged between log(τ5) from 0.0 to –0.5.

Research on sunspot Oscillations – I. The Pulkovo CCD Spectroheliograph-Magnetograph

The CCD spectroheliograph-magnetograph is a focal-plane ancillary instrument for the Pulkovo horizontal solar telescope ACU-5. The instrument is placed at an exit port of an isothermal high-resolution diffraction-grating spectrograph. A modified Leighton optical scheme for registration of sunspot magnetic fields is used. The instrument provides FITS-format digital video maps of radial velocities, magnetic fields, and spectroheliograms in any line of the spectral region 3900–11000 A. The time to obtain one video map of size 91′′×154′′ is equal to 10.24 s. The angular resolution of the instrument is 0.8′′; spectral resolution is 0.01–0.03 A. Since 1996 the Pulkovo CCD spectroheliograph-magnetograph has been used to obtain high spatial and temporal resolution observations of oscillations of radial velocities and magnetic fields in the photospheric layers of sunspots.

Magnetic flux separation in photospheric convection

Three-dimensional non-linear magnetoconvection in a strongly stratified compressible layer exhibits different patterns as the strength of the imposed magnetic field is reduced. There is a transition from a magnetically dominated regime, with small-scale convection in slender hexagonal cells, to a convectively dominated regime, with clusters of broad rising plumes that confine the magnetic flux to narrow lanes where fields are locally intense. Both patterns can coexist for intermediate field strengths, giving rise to flux separation: clumps of vigorously convecting plumes, from which magnetic flux has been excluded, are segregated from regions with strong fields and small-scale convection. A systematic numerical investigation of these different states shows that flux separation can occur over a significant parameter range and that there is also hysteresis. The results are related to the fine structure of magnetic fields in sunspots and in the quiet Sun.

The Sources of Magnetic Field Twist in Solar Active Regions

Observations have revealed that a hemispheric preference of magnetic chirality (handedness) exists throughout the solar atmosphere. For example, the current helicity of active regions is predominantly negative (left-handed twist) in the northern hemisphere and positive (right-handed twist) in the southern. The explanation of this hemispheric tendency is still open to question. In this paper we first review several possible mechanisms and clarify some misunderstandings. In our views, in the photosphere, the differential rotation acting on already emerged sunspot magnetic fields will lead to negative current helicity in the northern hemisphere and positive in the southern, but the same effect caused by the Coriolis force is opposite in sign. In the turbulent convection zone, the Coriolis force acting on the rising magnetic flux tubes will result in negative/positive helicity in the northern/southern hemisphere, but the corresponding action by the differential rotation will give rise to a reversed result. Moreover, in this region the α-effect will produce the wrong sign to explain the observed sense of magnetic twist. It should be noteworthy that the two current helicities generated by the α-effect, that in the mean field and that in the fluctuations, have opposite signs, and the former is positive/negative in the northern/southern hemisphere while the latter is negative/positive in the northern/southern hemisphere. In the overshoot region at the base of the convection zone, the current helicity created by the α-effect has the sign needed. Finally, we suggest that some surface flows (e.g., converging flows that can lead to cancellation of opposite-polarity flux in the photosphere) and magnetic reconnection are also important to the redistribution (or regeneration) of magnetic twist in active regions.

High resolution spectro-polarimetry with a large balloon-borne solar telescope

The balloon mission “Sunrise” consists of a 1m lightweight solar telescope equipped with a powerful spectro-graph-polarimeter and a multi-channel filtergraph for the visible and the UV. The science goals of Sunrise are focussed on, but not limited to, the investigation of the solar surface magnetism. The key questions include the generation and maintenance of the variable magnetic field, the structure and dynamics of the chromosphere, and the interaction of material flow and magnetic field in sunspots.

Stokes polarimetry at the Kodaikanal tower tunnel telescope

A Stokes Polarimeter has been developed using a masked CCD arrangement for the measurement of the vector magnetic field of sunspots. Charge shifting within the CCD is used to record near simultaneous orthogonal polarisation. The testing of the Stokes Polarimeter and the behavior of the integrated system combined with the Kodaikanal tower tunnel telescope will be discussed.