Sunspot photometry and the total solar irradiance deficit measured in 1980 by ACRIM

Until now a simple Photometric Sunspot Index (PSI) model was used (e.g. Willsonet al., 1981) to describe the contribution of sunspots to the solar irradiance deficit measurement by ACRIM. In this work we replace this model by a photometry of sunspot pictures for the period of 19 August to 4 September, 1980 taking into account the individual features, like lightbridges or umbral dots, of each spot. The main results of this preliminary analysis are: (1) theA _u/A _p ratios and alsos the values vary in a wide range and are by no means constant as in the PSI model; (2) the general trend of the irradiance deficit from our analysis agrees well with the ACRIM measurements; (3) on some days there are differences of more than 50% between the deficits derived from our measurements and from the PSI model.Paper presented at the 11th Eurpean Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

On the Intensity Thresholds of the Network and Plage Regions

A period of minimum solar activity in April and May 1996 was used to analyze full-disk CaK spectroheliograms with the aim of deriving the intensity thresholds of the quiet network and the plages by applying the so-called inflexion point method. The average network intensity threshold is found to be influenced mainly by the seeing, whereas the average plage threshold shows an increasing trend from disk center towards the solar limb. Both parameters are compared with the results of other authors.     

On the Determination of the Quiet-Sun Center-to-Limb Variation in Ca II K Spectroheliograms

We present a method for the determination of the center-to-limb variation (CLV) for Caii K spectroheliograms. It is based on the calculation of intensity histograms for rings of constant area. Using a simple cutoff criterion in the cumulative histograms, we are able to derive CLVs of the quiet Sun regardless of the solar activity level. Using an example from the literature, we show in which way different methods of normalization of the quiet-Sun CLV may affect the results.     

On the umbra-penumbra area ratio of sunspots

The area ratio of umbra to penumbra (A_u/A_p) was investigated for a series of 126 sunspots observed around the maximum of solar activity in 1980. Plotting log (A_u) as function of log (A_p) a linear relation is obtained with a slope of 1.10, yielding an average ratio A_u/A_p of 0.24 for small spots and of 0.32 for large spots. This deviates noticeably from the commonly adopted constant value of A_u/A_p = 0.21 (Allen, 1973) used e.g. for modelling the sunspot irradiance deficit (cf. Willson et al., 1981) from the Solar Geophysical Data.Mitteilungen aus dem Kiepenheuer-Institut Nr. 328     

SIMULATION OF SEEING INFLUENCES ON THE PHOTOMETRIC DETERMINATION OF SUNSPOT AREAS

Various methods for measuring areas of sunspots and umbrae are compared to test their robustness against changing seeing conditions. The influence of the seeing is simulated by artificially degrading high-resolution sunspot images. A new procedure – the so-called inflexion point method – for area determinations is described. It is based on the calculation of the inflexion point contours of the two-dimensional sunspot intensity distribution. The areas derived with this method are much more stable against seeing influences than those obtained by the other procedures tested. The effect of the seeing on the measurements of the bolometric sunspot contrast and irradiance deficit produced by a sunspot is also studied.     

Facular excess radiation and the energy balance of solar active regions

Plage areas and intensities derived from CaII K spectroheliograms are used as a proxy for the facular irradiance excess of solar active regions for the period 19 August to 4 September 1980. Using a calibration method proposed by Vrnak et al. (1991), the photospheric facular index (PFI) with constant facular contrastC _p = 0.018 is replaced by a variableC _p , depending on the plage brightness. A sgnificant increase ofC _p from 0.015 to 0.025 is found for plage areas varying from a few to approx. 6 · 10³ millionths hemispheres.Combining the facular irradiance excess with sunspot deficits (as determined for the same period by Steinegger et al. 1990) yields good aggrement with the irradiance variations measured by ACRIM I, using a center-to-limb variation ofC _p according to Chapman and Meyer (1986). The ratio of facular excess to sunspot deficit (integrated over solid angle 2) decreases from values of 1.5 to 2 for regions with sunspot areas below 100 millionths hemispheres to 0.2 for sunspots of areas > 1000 millionths hemispheres,