Deriving effective sunspot temperatures from SOHO/VIRGO irradiance measurements

Abstract

Abstract.A method is developed to obtain mean effective sunspot temperatures from satellite irradiance deficits. Using a formalism derived from a star spot modelling approach, this method only requires accurate positions and areas of sunspots -but not intensities. The method is applied to SOHO/VIRGO spectral solar irradiance (SSI) at 402, 500, and 862 nm as well as total solar irradiance (TSI) measured during the period 16 July to 17 August 1996, the disk passage of active region NOAA 7981. MDI full disk white-light images are used to extract sunspot positions and areas. First, using the spot pixels, which are identified on MDI images as I s < 0.92, we find the mean effective temperature of NOAA 7981 to be 300 K less than the local photospheric temperature. Because the area is dominated by penumbral pixels, the temperature is close to a value representing the penumbra. Second, separating umbral and penumbral pixels, we obtain a temperature difference of 850 K for the umbra and of 200 K for the penumbra. Finally, if a 5% binning of the spot intensities is applied, a temperature deficit of 1000 K is found for the darkest regions of the spot.

Multiband modelling of the Sun as a variable star from VIRGO/SoHO data

“…The value S L /S min for a confidence level of 0.01 is 1.075, indicating that the confidence intervals for the sunspot temperature deficit and the facular temperature excess are −300 ± 30 K and 40 ± 10 K, respectively, in agreement with the sunspot temperature deficit estimated by Eker et al (2003). Note, however, the extended trade-off between the values of ∆T s and ∆T f for isocontours encompassing larger values of S /S min , which indicates that the solution may become non-unique for larger values of the measurement errors.…”

“…The time is indicated in days from 1st January 1996 on the lower scale and in years on the upper scale. The data subsets range from 13 July 1996 to 11 September 1996 on the left panels, including the intervals previously analysed by Eker et al (2003) and Fligge et al (2000, Fig. 6), and from 5 November 1996 to 4 January 1997 on the right panels, previously analysed by Fligge et al (2000, Fig.…”

“…Therefore, we considered the TSI and SSI time series ranging from 16 July 1996 to 17 August 1996, encompassing the transit of the single active region NOAA 7981 when the irradiance changes were dominated by the modulation of its visibility. The same data subset was selected by Eker et al (2003) to derive the sunspot effective temperature in the framework of a stellar-like approach.…”

“…6 and 7 we display the best fits to some selected data subsets, obtained with ∆T s = −380 K and ∆T f = 60 K. They encompass intervals close to the minimum and the maximum of solar cycle 23, respectively, and include some of the data subsets previously fitted by Fligge et al (2000), Eker et al (2003) and Krivova et al (2003) with different approaches. As in Paper I, the value of the fit for a given measurement is computed with the parameters determined by the best fit of the 14-d data subset whose midpoint is the closest preceding the measurement.…”

A Decade of Hα Transits for HD 189733 b: Stellar Activity versus Absorption in the Extended Atmosphere

“…For the star spot spectra we use PHOENIX model spectra with the same logg and [Fe/H] values but with varying temperatures in step sizes of 100 K. Spots are cooler than the surrounding photosphere with differences in temperature that depend on spectral type, although this dependence may break down at T ef f > T (Eker et al 2003;Berdyugina 2005). We note that we do not distinguish between umbral and penumbral temperatures; they are averaged into a single spot temperature.…”