Stephen R. Walton

The Contribution of Faculae and Network to Long-Term Changes in the Total Solar Irradiance

A new database of individual solar features has been compiled from the full-disk photometric Ca II K images taken at the San Fernando Observatory (SFO) during solar cycle 22. The distribution of facular region sizes differs at different phases of the solar cycle; the area coverage of large active regions is reduced by a factor of about 20 at solar minimum compared to solar maximum, while the smaller regions cover about half as much area at minimum as at maximum. The irradiance contribution of large features is about 10 times greater at maximum than at minimum, while that of small features is about twice as large. We have used this data set to model the fraction of variation in the total solar irradiance S that is due to solar features of various sizes. The data show that large-scale bright solar features, i.e., faculae, dominate the ~0.1% change in S between solar maximum and solar minimum. Using a variety of data sets, we conclude that large active regions produce about 80% of the total change.

Processing Photometric Full-Disk Solar Images

Daily, photometric, full-disk digital solar images have been taken at the San Fernando Observatory (SFO) at two resolutions and in several wavelengths for more than eleven years. We describe the standard data processing techniques used for these images, including: calibration, limb fitting, geometric correction, and production of a solar contrast map by limb-darkening removal. The resulting contrast maps have a photometric accuracy which is often a few tenths of a percent. We show that the geometric accuracy of our images, as measured by the reproducibility of disk and sunspot areas, is very high as well. The techniques described in this paper should be applicable to any instrument producing full-disk photometric images.

Differences in the Sun's radiative output in cycles 22 and 23

Analysis of the current solar cycle 23 shows a greater increase in total solar irradiance (TSI) for the early phase of this cycle than expected from measurements of the total magnetic flux and traditional solar activity indices, which indicate that cycle 23 is weaker than cycle 22. In contrast, space observations of TSI from the Solar and Heliospheric Observatory/VIRGO and the Upper Atmospheric Research Satellite/ACRIMII show an increase in TSI of about 0.8-1.0 W m-2 from solar minimum in 1996 to the end of 1999. This is comparable to the TSI increase measured by Nimbus 7/ERB from 1986 to 1989 during the previous cycle. Thus, solar radiative output near the maximum of the 11 yr cycle has been relatively constant despite a factor of 2 smaller amplitude increase for cycle 23 in sunspot and facular areas determined from ground-based observations. As a result, empirical models of TSI based on sunspot deficit and facular/network excess in cycle 22 underestimate the TSI measurements in 1999. This suggests either a problem in the observations or a change in the sources of radiative variability on the Sun.

Flux tube models of solar plages

The spectroscopic properties of model solar plages composed of narrow magnetic flux tubes embedded in undisturbed quiet sun are analyzed. The final models are magnetostatic flux tubes for which the level tau5000 = 1 level is 200 km below the same level in the quiet sun; these models are hotter than the quiet sun at all levels and expand by roughly a factor of three between this level and the quiet sun temperature minimum. These models are tested against a wide range of observational constraints. The assumed flux tube diameter is found to be an important parameter for predictions of the center-to-limb variation of the magnetically insensitive lines and of the visible continuum. A diameter of 300 km at tau5000 = 1 is necessary to give qualitative agreement with observations. A model plage consisting of the derived flux tube model embedded in a quiet sun model derived from the Ca II K line wings reproduces this line in plages at the observed value of mu; one of the models also predicts values of disk center plage contrast at 30, 50, 100, and 200 micron wavelength in good agreement with the observations of Jefferies et al. (1982).

ON THE VARIABILITY OF THE APPARENT SOLAR RADIUS

Full-disk photometric solar images at a wavelength of 672.3 nm have been obtained daily since 1986 using the CFDT1 (Cartesian Full Disk Telescope No. 1). An analysis of these images from 1986 through the end of 2004 December has shown a peak-to-peak variation in the geocentric north-south solar radius of 0.136 ± 0.01, approximately in phase with the solar cycle. The multiple correlation coefficient squared is R2 = 0.0404 (R = 0.2). While this correlation coefficient is small, due to the large number of data points (N = 4042), the level of significance is less than 0.02. The radius had a maximum value near the times of maximum activity for solar cycles 22 and 23.

Weak magnetic fields and solar irradiance variations

NOAA active region 5643 was observed from August 17 to 21, 1989. Sets of video spectra-spectroheliograms including the Fe I line at 6302.5 A were made at least daily with the San Fernando Observatory 28 cm vacuum telescope and vacuum spectroheliograph. These give simultaneous, co-registered digital images representing monochromatic continuum intensity, line core intensity and line-of-sight magnetic field. Three different criteria are used to define the pixels representing the quiet sun and the facular portions of the images. These criteria are the magnetic field strength, the line core intensity, and the distribution of continuum intensities. Each of these definition schemes is used to estimate the irradiance change due to facular emission. The magnetic field and the continuum intensity distribution definitions give estimates which agree closely. The line core intensity definition leads to larger estimates of the facular irradiance contribution. Some model-dependent investigations of the contrasts and sizes of individual facular elements also are presented.

Spatiotemporal Correlations and Turbulent Photospheric Flows from SOHO/MDI Velocity Data

Time series of high-resolution and full-disk velocity images obtained with the Michelson Doppler Imager (MDI) instrument on board SOHO have been used to calculate the spacetime spectrum of photospheric velocity flow. The effects of different methods for filtering acoustic oscillations have been carefully studied. It is found that the spectra show contributions both from organized structures that have their origin in the convection zone and from the turbulent flow. By considering time series of different duration and cadence in solar regions with different line-of-sight projections, it is possible to distinguish the contributions of the spectra from the two different kinds of flows. The spectra associated with the turbulent velocity fields obey power laws characterized by two scaling parameters whose values can be used to describe the type of diffusion. The first parameter is the spectral exponent of the spatial correlation function and the second is a scaling parameter of the time correlation function. Inclusion of the time parameter is an essential difference between the present work and other solar studies. Within the confidence limits of the data, the values of the two parameters indicate that the turbulent part of the flow in the scale range 16-120 Mm produces superdiffusive transport.

Weak Infrared Molecular Lines Reveal Rapid Outflow in Cool Magnetic Sunspot Penumbral Fibrils

New imaging spectropolarimetric observations of the Evershed flow in sunspot penumbrae using weak infrared molecular absorption lines are presented. A plane-polar coordinate system in the sunspot frame is defined, allowing averaging of many raw spectra. Molecular lines show Doppler shifts implying typical horizontal outflow speeds of 6 and up to 9 km s(-1). The Ti I polarimetric spectra show the same rapid outflow and suggest an average penumbral magnetic field strength of 1400 G. While these observations show Doppler shifts of the entire line profile, the velocities are in better agreement with previous measurements from spectral line asymmetries.

The San Fernando Observatory Video Spectra-Spectroheliograph

We describe recent work in the development of the San Fernando Observatory (SFO) Video Spectra-Spectroheliograph (VSSHG), a spectrum-based instrument for the measurement of the solar Stokes profiles. Its most important features are: simultaneous measurement of Stokes I plus one of Stokes Q, U, or V; spatial sampling of 0.5 arc sec; spectral sampling of 8.8 mÅ; and time sampling of one minute (for one pair of Stokes profile) to three minutes (for all four profiles). Routine data processing is carried out using a moments technique; tests of this technique show it to be reasonably accurate. Sample data are shown and briefly discussed: a longitudinal magnetogram and Dopplergram of NOAA 5573 observed on 17 August, 1989, and a vector magnetic field map and Dopplergram of NOAA 6659 observed on 10 June, 1991.