Gary A. Chapman

Solar Variability and the Relation of Facular to Sunspot Areas during Solar Cycle 22

The total irradiance of the Sun has been found to vary mostly because of changes in the areas of dark sunspots and bright faculae. Improved observations, such as those discussed in this paper, are needed to understand better the interplay between these two competing features. In this paper, faculae are determined by observations using a filter centered at the Ca II K line (393.4 nm) with a bandpass of 0.9 nm. This filter allows the detection of faculae across the entire solar disk rather than just at the limb, as is the case for white-light faculae. Sunspots are detected with a filter at 672.3 nm with a bandpass of 9.7 nm. The mean ratio of facular to sunspot area was found to be 16.7 ± 0.51 for a 7½ year period during solar cycle 22 but showed a significant increase as the solar cycle progressed. This ratio suggests that the irradiance excess associated with faculae outweighs the irradiance deficit associated with sunspots by about 50%. The facular area also exhibited a quadratic dependence on sunspot area, as suggested by Foukal, but there is no clear evidence of a turnover in facular area at large sunspot areas. Lagged cross-correlations between facular and sunspot areas showed a clear rotational modulation extending to lags of five to six rotations when spots led faculae. Lags in the opposite direction, however, showed the rotational modulation falling abruptly after about two rotations.

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.

Solar irradiance variations from photometry of active regions

The Extreme Limb Photometer (ELP) has been used to measure the irradiance fluctuation of the Sun due to selected active regions. Forty-five active regions that were completely scanned at various disk positions are included in the analysis. The contribution of these active regions to a global solar irradiance fluctuation has been correlated with photometric sunspot and facular indices (PSI and PFI) using published values of sunspot and calcium plage areas. The measured ELP fluctuations are converted to a global brightness fluctuation, ΔB/B. The sunspot component of ΔB/B correlates with PSI with r = 0.95. The facular component of ΔB/B correlates with PFI with r - 0.72. The expression for PFI is important to the question of energy balance between sunspots and faculae and the results presented here are not incompatible with energy balance between the two phenomena; that is the energy deficit of sunspots may be balanced by the energy excess of faculae.

Precise ground-based solar photometry and variations of total irradiance

Variations in the total solar irradiance measured by the active cavity radiometer irradiance monitor (ACRIM) on SMM have been correlated with measures of magnetic activity on the solar disk. Quantitative indices of magnetic activity were derived from ground-based, full-disk, photometric images of the Sun at red (6723 A) and violet (3934-A K line) wavelengths. The red images have been obtained on a daily basis at the San Fernando Observatory since 1985, and the K line images since 1988. Sunspot irradiance deficits are calculated directly from the red images while proxy measures of facular irradiance excesses are derived from the K line images. The images analyzed here were made during 21 days between June 20 and July 14, 1988, a period centered on the disk passage of a large sunspot group. The best two-parameter multiple correlation coefficient between the ACRIM data and the photometric data is R² = 0.97 (21 data points, 18 degrees of freedom). The zero point S0 = 1367.27 W m−2 agrees well with the solar irradiance measured by ACRIM/SMM during the 1986 activity minimum; the residual standard deviation was 0.13 W m−2 (about 100 ppm). The multiple correlations were extended to include measures of the irradiance contribution of “network” magnetic fields, unassociated with active regions. NOAA 9 spacecraft observations of UV Mg II lines at 2800 A gave R² = 0.99 (17 degrees of freedom) with S0 = 1366.68 + 0.08 W m−2. The index of 10.7-cm microwave flux gave R² = 0.98, with S0 = 1366.43 + 0.11 W m−2. We can thus model short-term irradiance changes to within 100 ppm relative precision from ground-based data.

Solar luminosity fluctuations during the disk transit of an active region

On presente des observations photometriques monochromatiques des fluctuations du rayonnement solaire causees par une region active durant tout son transit sur le disque entier, en aout 1982

A study of the contrast of sunspots from photometric images

The thermal contrast α, and the umbra-penumbraAu/Ap, were calculated for 63 sunspots of various sizes and morphologies. Contrary to the assumptions of the PSI model, α andAu/Ap were found to be quite variable. The values of α ranged from 0.1807 to 0.4266;Au/Ap ranged from 0.0089 to 0.4899. The values of α andAu/Ap correlated well (r = 0.6018;p<0.005) and the regression for α andAu/Ap was obtained: α = (0.220 ± 0.016) + (0.340 ± O.06)Au/Ap. The values of α andAu/Ap were then compared with complexity ratings, magnetic field strength, time, and μ. The quantities α andAu/Ap were found to be independent of the complexity, magnetic field strength, and time factors. The correlation between α andAu/Ap lead to the proposed division of α into an umbral thermal contrast αu, and a penumbral thermal contrast αp. These values were calculated from the photometric data: αu = 0.57 ± 0.01 and αp = 0.26 ± 0.006.

Photospheric Faculae and the Solar Oblateness

Photospheric faculae near the equatorial solar limb may provide the excess brightness which Ingersoll and Spiegel showed would explain Dicke and Goldenbergs oblateness measurement. Three lines of evidence support this statement: (1) the excess emission of faculae may arise in optically thin regions, as required by the Ingersoll-Spiegel hypothesis; (2) faculae are sufficiently widespread on the solar surface to account quantitatively for the observed signal; and (3) temporal fluctuations in the expected signal due to faculae in 1966 are correlated with fluctuations in the observed signal at the 1 percent level. (The probability of the correlation coefficient for uncorrelated data exceeding the observed value is less than 1 percent.) Although this evidence clearly demonstrates that faculae make a sizable contribution to the observed oblateness signal, it does not preclude an equally sizable contribution due to true gravitational oblateness. Evidence that faculae may not be the only source of oblateness signal comes from the apparent fact that the ratio of fluctuation amplitude to mean signal amplitude is greater for the facular signal than for the observed oblateness signal. However, this difference may be due to errors in reading the photographs from which the facular signal was derived, or to differences in processing the two sets of data. A better test of our hypothesis cannot be made until the daily oblateness signals and their standard deviations are available. In any case, it appears that further data analysis will be necessary before a reliable value of the solar oblateness can be inferred.

Small-scale and Global Dynamos and the Area and Flux Distributions of Active Regions, Sunspot Groups, and Sunspots: A Multi-database Study

In this work we take advantage of eleven different sunspot group, sunspot, and active region databases to characterize the area and flux distributions of photospheric magnetic structures. We find that, when taken separately, different databases are better fitted by different distributions (as has been reported previously in the literature). However, we find that all our databases can be reconciled by the simple application of a proportionality constant, and that, in reality, different databases are sampling different parts of a composite distribution. This composite distribution is made up by linear combination of Weibull and log-normal distributions -- where a pure Weibull (log-normal) characterizes the distribution of structures with fluxes below (above)