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On the Energy Balance of Solar Active Regions
Steinegger, Michael; Vazquez, Manuel; Bonet, Jose A.; Brandt, Peter N.
Astrophysical Journal v.461, p.478 (ApJ Homepage)
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Ground-based observations of the sunspot irradiance deficit Psi and the excess by facular regions in the period 1989 June 3-15 are analyzed using photometric techniques. The results are compared with solar irradiance measurements taken by the Nimbus 7 satellite. The photographic sunspot data were obtained at the Vakuum Newton Teleskop, Observatorio del Teide (Tenerife), and the information on facular emission was derived by means of a "proxy" method from the analysis of full-disk Ca II K spectroheliograms taken at Sacramento Peak Observatory.

The first part of the observing period was characterized by a rather constant activity with sunspot groups of small and medium size, whereas the second part (from June 8 onward) exhibited a drastic increase of activity, dominated by the appearance of the large group NOAA 5528.

By increasing the pixel size of the digitized photographic data, the influence of the variable spatial resolution on the measurements is simulated. Above a certain threshold, the bolometric sunspot contrast falls and the spot area grows; however, the resulting influence on the sunspot deficit Psi and the corresponding umbral parameters (area, contrast, and deficit) is negligible.

Data of areas of sunspot groups (total and umbral) from different observatories are compared with our measurements. Total areas show similar values, but umbral areas from our measurements are clearly larger than those estimated from full-disk images, like those from the Debrecen and San Fernando Observatories.

Analyzing the data of single spots, we confirm earlier findings, namely, that the bolometric contrast is lower than 0.32, the standard value used in the calculation of the photometric sunspot index, and we report that small spots exhibit smaller bolometric contrasts than larger ones. The intensity minimum of the spots is a very appropriate parameter to describe their global thermal properties; it is linearly proportional to the total (alpha) and umbral (alphamu) bolometric contrasts and also correlates with the maximum strength of the magnetic field of the spots and with the spot's size.

The energy balance of individual active regions seems to depend mainly on the area ratio of the plage and the corresponding sunspot group. The different rates at which these areas change produces the temporal evolution of the energy balance.

The daily variations of the global activity indices radio flux at 10.7 cm, Mg II core-to-wing ratio, and the average magnetic field are well correlated with the fluctuations of irradiance and the spot deficit Psi. But on the other hand, the equivalent width of the He I lambda10830 line and the coronal index do not follow this behavior.

The irradiance fluctuations, as measured by Nimbus 7, can be modeled to within 60 parts per million (ppm) using only our ground-based determination of Psi and Phi. By adding the sunspot deficit Psi to the irradiance fluctuations, the result is modeled to within 25 ppm using an empirical law that combines our Phi with a set of global activity indices.

After correcting the irradiance measurements for the facular Phi and spot Psi contributions, a residual remains with an apparent periodicity of 6 days. Different sources to explain this variability are discussed and the importance of an adequate determination of the threshold of Ca II K brightness for the calculation of Phi is explained.

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