Simulating photospheric Doppler velocity fields

A method is described for constructing artificial data that realistically simulate photospheric velocity fields. The velocity fields include rotation, differential rotation, meridional circulation, giant cell convection, supergranulation, convective limb shift, p-mode oscillations, and observer motion. Data constructed by this method can be used for testing algorithms designed to extract and analyze these velocity fields in real Doppler velocity data.     

The photospheric velocity field of Procyon

BUSS observations of the profiles of two well observed spectral lines in the ultraviolet spectrum of CMi (Procyon; F5 IV–V) are analysed with a Fourier transform method in order to determine values of various parameters of the velocity field of the upper photosphere. We find a microturbulent line-of-sight velocity componentL = 0.9 ± 0.4 km s^–1, a macroturbulent velocity componentL _M = 5.3 ± 0.2 km s^–1, and a rotational velocity componentv _R sini=10.0±1.2 km s^–1. In these calculations a single-moded sinusoidal isotropic macroturbulent velocity function was assumed. The result appears to be sensitive to the assumed shape of the macroturbulence function: for an assumed Gaussian shape the observations can be described withv _R sini=4 km s^–1 andL _M = 11.6 ± 2.7 km s^–1. A comparison is made with other results and theoretical predictions.     

On the determination of the photospheric velocity distribution from profiles of weak Fraunhofer lines

The steady-state vertical-velocity response of an isothermal atmosphere to pressure fluctuations of arbitrary period and horizontal wavelength at its base is derived in the approximation of dissipationless polytropic motion in the atmosphere. It is pointed out that, since only upward modes can be excited in an isothermal atmosphere perturbed from below, the infinite response found by Worrall (1972) at the critical frequency _g does not occur. The correct behavior of the response is presented in some detail.Comparison of the response of the model, for the case of isothermal osculations, with observed features of the photospheric oscillations indicates that, in addition to the evanescent photospheric oscillations which occur at the compression-wave propagation cut-off frequencies and which have horizontal wavelengths 3000 km, in the lower photosphere there are also smaller-scale evanescent oscillations which have horizontal wavelengths 1000 km, periods ranging from 200 to 400 s, amplitudes comparable to that of the larger-scale oscillations, and in which the phase of the vertical velocity oscillation leads the phase of the pressure oscillation.     

Temperature effects on measurements of photospheric magnetic fields

The effects of line profile variations, due to the temperature enhancements connected with photospheric magnetographic measurements are evaluated in a quantitative way: taking into account these results, the differences found by many authors between the values of the field obtained with different Fraunhofer lines can be reduced within 10–30%.     

Two-dimensional spatial power spectra of photospheric velocity fluctuations

Two-dimensional spatial autocorrelation functions and power spectral density distributions were obtained from high-resolution velocity spectroheliograms. Although the autocorrelation functions indicate the existence of velocity cells of size roughly 2500 to 3500 km, the power spectra fail to reveal them because the cells are not strictly spatially periodic.     

Las campanas observatory seeing measurements

Instrumentation built to record seeing data automatically via image motion measurements of bright stars in small telescopes is described. The centroid of the star image is found 256 times s^-1 in one dimension and is analyzed on-line. The device works over a range of FWHM values as would be seen through a large telescope between <0.1 and 3.0 arcsec. The first results for two identical instruments set up at two locations near the duPont Telescope at Las Campanas Observatory are reported. For a total of 61 nights of data (450 h at each site), the median seeing is 0.6 arcsec, with quartiles at 0.4 and 0.8 arcsec. These are FWHM values referred to 5000 Å at the zenith. So far, the two sites are indistinguishable on average.     

A sodium experiment for photospheric velocity field observations

A method for studying small scale photospheric velocity fields with a balloon experiment is proposed. A sodium resonance cell is used with a diffraction limited telescope. Good pointing accuracy is not necessary. Preliminary results of ground-based observations are given.     

Some results of the photospheric large-scale velocity research from Belgrade observations

On the basis of the possibility of sight-line velocity observations by a special equatorial solar spectrograph, a research programme for detection of photospheric large-scale velocities has been initiated. The first series of observations in the FeI 6302 Å absorption line has been limited to the central meridian.The combined limb effect assumed to incorporate an unresolved stationary photospheric motion, has been evaluated. The observed asymmetry of the obtained curve is mainly explained by dB ₀/dt.The remaining sight-line velocities along the central meridian, taken as random, gave an r.m.s. value of 32 m s^–1. In a few cases a certain kinematic situation in some areas along the central meridian lasted for 2 to 4 consecutive days. It is assumed that such velocity features belong to the kinematic picture of a large-scale photospheric motion which, as a whole, has not yet been clearly seen.     

The determination of the velocity amplitude of the total photospheric motion field

By the analysis of the profiles of 20 weak lines observed at five centre-to-limb positions on the solar disk, radial and tangential components of the velocity amplitude of the photospheric motion field are derived in the range of optical depth - 3.0 ? lgτ5 ? +0.5.

     

Relationships between photospheric plasma angular velocity and solar activity

An analysis of relationships between latitudinal fine structures of the photospheric plasma differential rotation and solar activity shows that sunspot activity seems to be lower (as measured by the number and extension of sunspot groups) at latitudes where minima of angular velocity appear.     

On depth-dependence of photospheric oscillations

We report on results from photographic observations of photospheric oscillations as a function of depth. Using rms-values and power-spectra from shifts of entire line-profiles, we find qualitatively an increase of the velocity-amplitude with increasing height. We get more quantitative informations by comparing measured asymmetries of line-profiles with calculated ones derived from Voigt-functions containing a depth dependent velocity-field.We find the scale-height H ₀ of photospheric velocity oscillations to be 930±100 km. This result is to be compared with H ₀ = 1100±200 km obtained by Canfield (1976), who used velocity weighting functions of the line centres.Further, we show that a general observed line asymmetry of medium strong lines (c-shape) does not depend on the phase of oscillations.Mitt. aus dem Kiepenheuer-Institut Nr. 178.     

On some properties of the photospheric structure

New observations made on board the stratospheric solar observatory (SSO) in 1973 confirm the conclusion of the previous paper (Krat, 1972) showing a more rough structure of the chromosphere in comparison with the photosphere.     

On the use of mosaic direct-vision prism for radial velocity measurements

Mosaic direct-vision objective prisms satisfy the three conditions necessary for a whole-sky survey of spectroscopic binaries and short-period variables. We point out that the use of rectangular component prisms instead of square ones may cut by half the light loss caused by frame obstruction. The portion of the aperture not covered by the prism can be used to produce a direct image which can serve as a reference point in the spectral line measurement, thus dispensing with any rotating mechanism.     

On possible correlations in the photospheric magnetic field

We have searched for correlations between photospheric magnetic field changes in the north and south hemispheres of the Sun. Both active region logs and analysis of Mount Wilson magnetograms were employed. No correlations were found, and we infer that local convective turbulence is more important than dynamo processes with regard to the appearance of individual active regions.     

On photospheric and chromospheric penumbral waves

The full magnetoatmospheric wave equation is numerically solved for the particular penumbral model of Nye and Thomas (1974). For chromospheric running penumbral waves it is found that the maximum vertical velocity occurs at the base of the Hα region (z ≈ 300 km) much lower than previous WKB estimates have suggested. The maximum ‘vertical’ kinetic energy occurs at z ≈ - 130 km. Very significant horizontal velocities are also found for these waves, and, in the absence of shear flow it appears that previous estimates of photospheric vertical velocities of order 10^?1 km s^?1 could be substantial underestimates. For the photospheric events of Musman et al. (1976), a high vertical velocity maximum is found in the corona, and the modes appear highly dispersive for periods ≥ 220 s. The effects of a sinusoidal shear flow profile on running penumbral waves are examined, and it is found that the eigenvalues (horizontal wavenumber k) are changed little, but the eigenmodes become significantly distorted; the position of the vertical velocity peak rises compared to the zero flow case, and the velocity below that peak drops significantly. This effect may well cancel the increased estimates based on zero flow.