The MSFC vector magnetograph

The NASA/Marshall Space Flight Center's solar vector magnetograph system is described; this sytem allows measurements of all components of the Sun's photospheric magnetic field over a 5 × 5 or 2.0 × 2.0 arc min square field-of-view with an optimum time resolution of 100 s and an optimum signal-to-noise of 1600. The basic system components are described, including the optics, detector, digital system and associated electronics. Automatic sequencing and control functions are outlined as well as manual selections of system parameters which afford unique system flexibility. Results of system calibration and performance are presented, including linearity, dynamic range, uniformity, spatial and spectral resolutions, signal-to-noise, electro-optical retardation and polarization calibration. Scientific investigations which utilize the unique characteristics of the instrument are described and typical results are presented.     

Nonlinear calibration of vector magnetograms by the video vector magnetograph at Huairou

We compare the results of two calibration methods for deriving a photospheric vector magnetogram, as applied to the Fei 5324.19 Å line. The first method ignores the dependence of its calibration coefficients on the inclination angle. The second method is a multi-iteration, nonlinear calibration technique developed by [M.J. Hagyard, J.I. Kineke, Solar Phys. 158 (1995) 11], which allows the polarization signals to depend on both field strength and inclination angle. We compare the relationship between the derived solar magnetic field and the Stokes parameters under both methods. We find that the circular polarization signal of the Fei 5324.19 Å line is linearly proportional to the longitudinal strength, B_L, when the field strength ranges from 0 to 1000 Gauss. For B_L > 1000 G and inclination angles ranging from 30° to 90°, deviation from linearity is significant. For the transverse field, B_T, the assumption of linearity only holds for 0 < B_T < 300 G. In contrast to the former method of calibration, the improved calibration method accounts for the nonlinear relationship between polarization signals and the magnetic field strength. Using [A. Skumanich, in: J.H. Thomas and N.O. Weiss (Eds.), Sunspots: Theory and Observations. Kluwer, Dordrecht, 1992, p. 121] dipole field model, we show that the Fei 5324.19 Å line has more linear property than the Fei 6302.5 Å line.     

Joint vector magnetograph observations at BBSO,Huairou Station and Mees Solar Observatory

Joint vector magnetograph observations were carried out at Big Bear Solar Observatory (BBSO), Huairou Solar Observing Station (Huairou), and Mees Solar Observatory (MSO) in late September 1989. Comparisons of vector magnetograms obtained at the three stations show a high degree of consistency in the morphology of both longitudinal and transverse fields. Quantitative comparisons show the presence of noise, cross-talk between longitudinal field and transverse field, Faraday rotation and signal saturation effects in the magnetograms. We have tried to establish how the scatter in measurements from different instruments is apportioned between these sources of error.     

Interpretation of vector magnetograph data including magneto-optic effects

In this paper, the presence of Faraday rotation in measurements of the orientation of a sunspot's transverse magnetic field is investigated. Using observations obtained with the Marshall Space Flight Center's (MSFC) vector magnetograph, the derived vector magnetic field of a simple, symmetric sunspot is used to calculate the degree of Faraday rotation in the azimuth of the transverse field as a function of wavelength from analytical expressions for the Stokes parameters. These results are then compared with the observed rotation of the field's azimuth which is derived from observations at different wavelengths within the Fei 5250 Å spectral line. From these comparisons, we find: the observed rotation of the azimuth is simulated to a reasonable degree by the theoretical formulations if the line-formation parameter η _o is varied over the sunspot; these variations in η _o are substantiated by the line-intensity data; for the MSFC system, Faraday rotation can be neglected for field strengths less than 1800 G and field inclinations greater than 45°; to minimize the effects of Faraday rotation in sunspot umbrae, MSFC magnetograph measurements must be made in the far wings of the Zeeman-sensitive spectral line.     

The chromospheric magnetograph

By comparison of photoelectric magnetograms with high resolution Hα pictures it is possible to formulate a set of rules by which the magnetic field may be derived directly from the filtergrams. This is possible because of the regularities of magnetic field configurations on the sun and because chromospheric morphology is determined by the magnetic field. Off-band pictures (preferably 0.5 Å red) show a well-defined enhanced chromospheric network, the boundaries of which coincide with the 5 G contour of longitudinal field on the Mt. Wilson magnetograms. The actual fields are presumably more concentrated along the dark structure of the network. Higher fields are marked by filled-in cells. Regions of predominantly transverse fields may be inferred from the absence of normal network structure and the presence of chromospheric fibrils. The quiet chromosphere is recognized by the presence of oscillatory motion and the absence of fibrils or strong network structure. Thus, the chromosphere may be divided into three types of regions: enhanced network, horizontal field, and quiet network. The polarity of the magnetic field may be recognized by plage-antiplage asymmetry; that is, the fact that only following magnetic fields show bright plage in the center of Hα.     

A search for sunspot canopies using a vector magnetograph

Using a magnetograph, we examine four sunspots for evidence of a magnetic canopy at the penumbra/photosphere boundary. The penumbral edge is determined from the photometric intensity and is defined to correspond to the value of the average intensity minus twice the standard deviation from the average. From a comparison of the location of this boundary with the location of contours of the vertical and horizontal components of the magnetic field, we conclude that the data are best represented by canopy-type fields close to all four sunspots. There is some evidence that the magnetic inclination in the canopies is 5°–15° with respect to the horizontal and that the canopy base height lies in the middle/upper photosphere. The observations further suggest that the magnetic canopy of a sunspot begins at its outer penumbral boundary.     

The Haleakala Stokes polarimeter

A versatile Stokes polarimeter for solar observations has been developed at the University of Hawaii. Recent improvements to the instrument include a high-resolution echelle spectrometer coupled to the telescope by optical fibers, and 128-element diode array detectors. The on-axis design of the telescope and polarimeter limit instrumental polarization to 10^–4, and the spectrometer detector combination provides spectral resolving power of 160000 for any wavelength between 4000 and 11000 Å. This paper describes the Haleakala polarimeter and in particular the spectrometer with its fiber-optic coupling. Examples of Stokes line profiles observed in a sunspot are presented, together with derived vector magnetic field maps.     

The Mount Wilson magnetograph

Alterations to the Mount Wilson Observatory solar magnetograph were made during 1981. The present state of the instrument, including the spectrograph, is described. The magnetic and Doppler velocity signals and the setup procedure for the magnetogram observation are discussed. The advantages of the new system are described.     

A detection of the Faraday rotation by the solar vector magnetograph

The Faraday rotation in the sunspot atmosphere is statistically detected by examining directions of the linear polarization obtained with the vector magnetograph of the Okayama Astrophysical Observatory. It is very effective near the spectral line center and the azimuth of the linear polarization deviates greatly from the magnetic field azimuth. In the case of the iron line, 5250 Å, the magnetic field azimuth will be obtained with an accuracy better than 15°, if observed in the line wing from 27 to 80 mÅ relative to the line center.     

The lockheed diode array magnetograph

A new magnetograph using a solid state monolithic linear silicon diode array has been constructed at Lockheed Solar Observatory. This magnetograph uses a digital image processor, and makes data available both in digital and analog form. The diode array detector is capable of a signal-to-noise ratio of 2000:1 or better when cooled to a temperature of -40 deg centigrade. Thus, intensity differences of the order of one part in a thousand may easily be detected without signal averaging. This instrument may be considered a prototype for an instrument using a two-dimensional array. The magnetograph is now fully operational, and is being used to produce data for statistical studies of solar magnetic field diffusion.     

Multi-channel magnetograph observations

A new technique for displaying magnetograph observations is presented and applied to the 12-channel magnetograph at Kitt Peak National Observatory. Using the data from a raster scan, a digital spectroheliogram is constructed on the face of a cathode ray tube and photographed. This enables one to recognize patterns in magnetograph data as easily as with conventional photographs. Comparisons with simultaneous spectroheliograms show no qualitative differences and indicate that the magnetograph is quite capable of studying morphology of individual solar features.Kitt Peak National Observatory Contribution No. 499.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Multi-channel magnetograph observations

The Kitt Peak multi-channel magnetograph was used to make raster scans of the super-granulation. The region scanned was carefully selected to be quiet and was located at the center of the disk. Each point in the raster was observed twice (with a time interval of 150 sec) and averaged, thereby cancelling out effects of the 5 min oscillations. Subsequent raster scans were made over a period of 4 h and averaged, further reducing short-lived, nonperiodic modes and enhancing the long-lived super-granulation.The results clearly show vertical supergranular motions. These motions consist of relatively isolated patches of downward flowing material, or downdrafts, with a speed of approximately 0.1 km/sec. These downdrafts are highly correlated with patches of magnetic field, being not only cospatial with them but also approximately linearly proportional to the field strength. The magnetic field strength of these downdrafts is in the range from 50 to 100 G. These downdrafts are also regions of increased temperature (i.e., brighter than average) at all levels observed, from the Caii K line down to the level of the continuum.The observations show no contradiction with the hypothesis that the supergranulation is a convective phenomenon, but they do indicate that the convection would be far more complicated than previously supposed. In particular, the surprisingly large field strengths and the presence of a bright network at all levels suggest that the magnetic field plays a far more active role in the supergranulation.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.     

Multi-channel magnetograph observations

Simultaneous observations of photospheric magnetic fields, Caii K emission, the photospheric network and continuum faculae show that these four quantities are correlated in a complicated manner. The photospheric and calcium networks show increasing contrast with increasing magnetic field strength up to field strengths of about 500 G. Higher values of the magnetic field are found only in pores and sunspots. Continuum faculae also show increasing contrast with increasing magnetic field strength (even at the disk center), but this contrast reaches a maximum at field strengths of about 200 G. At higher field strengths, continuum faculae become monotonically darker until pore or spunspot conditions are reached.Measurements of the photospheric network and the continuum faculae over a wide range of result in families of limb contrast curves. These curves indicate that the dependence on H is as important as the dependence on . They also indicate that the magnetic field has a preferred inclination of about 50°. The facular contrast shows little dependence on resolution. This is interpreted in terms of a geometric model in which faculae are clumps of many individual flux tubes. These tubes are closely packed and unresolvable in the photosphere, but are more widely spaced, and therefore resolvable, in the low chromosphere.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.     

Sky Brightness Measurements at Haleakala, 1955–2002

Measurements of the brightness of the clear daytime sky at Haleakala, Maui are presented for the interval 1955 through 2002. The observations are made near the direction of the Sun, where forward scattering off aerosols dominates the sky brightness. The Haleakala summit at 3054 m is normally above the inversion layer. The Haleakala sky is dark; the observed brightness per airmass has a median of 10 millionths of the solar disk and a mode of 5 millionths, with Rayleigh scattering contributing 1 millionth. There is no demonstrable long-term trend in the data.     

Magneto-optical effects and the interpretation of linearly polarized intensity distributions observed with a vector magnetograph

Linearly polarized intensity distributions observed in sunspots with the Marshall Space Flight Center's (MSFC) vector magnetograph are interpreted taking into account magneto-optical effects. It is shown that these effects can be responsible for the observed spiral configuration in the pattern of linear polarization, even if a purely radial, conventional sunspot model is used.On leave from: Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Fourier parameters and moments of polarization profiles of magnetically active lines. Fourier vector magnetograph

A new method is proposed to determine all components of the solar magnetic fields using the cumulants of the profile of a magnetic sensitive line. The method is based on polarization measurements in a number of points of the line profile and subsequent calculation of the amplitudes and phases of its two first Fourier-harmonics.     

A new type of magnetograph

A solar magnetograph without electronics or any moving parts has been designed. The principle is the use of a subtracting double-dispersion optical system. The exposure time is of the order of 1 sec.Preliminary results obtained at the Swedish Astrophysical Observatory at Anacapri indicate that this method could be convenient to use in space vehicles when aiming at high spatial resolution of solar magnetic fields.     

The representation of magnetic field lines from magnetograph data

Several methods currently used to extrapolate the structure of the solar magnetic field from surface measurements are examined and compared. In particular, the differences between the methods of Schmidt for potential fields and of Nakagawa and Raadu for force-free fields are explained. Suggestions are made regarding the use of these theoretical procedures in making physical conclusions.     

The Imaging Vector Magnetograph at Haleakala –: III. Effects of Instrumental Scattered Light on Stokes Spectra

The scattering of light over the field of view of a solar spectropolarimeter affects all Stokes parameters. The magnetic field vector inferred from the Stokes spectra then has systematic error. The reason is that scattering affects polarized radiation as well as unpolarized. Accurate correction of the Stokes spectra from the Imaging Vector Magnetograph (IVM) of the Mees Solar Observatory illustrates the problem and the solutions.