Activity-brightness Correlations for the Sun and Sun-like Stars

The Astrophysical Journal Letters (Impact Factor: 5.6). 09/2011; 739(2):L45. DOI: 10.1088/2041-8205/739/2/L45

ABSTRACT We analyze the effect of solar features on the variability of the solar irradiance in three different spectral ranges. Our study is based on two solar-cycles' worth of full-disk photometric images from the San Fernando Observatory, obtained with red, blue, and Ca II K-line filters. For each image we measure the photometric sum, Σ, which is the relative contribution of solar features to the disk-integrated intensity of the image. The photometric sums in the red and blue continuum, Σr and Σb, exhibit similar temporal patterns: they are negatively correlated with solar activity, with strong short-term variability, and weak solar-cycle variability. However, the Ca II K-line photometric sum, ΣK, is positively correlated with solar activity and has strong variations on solar-cycle timescales. We show that we can model the variability of the Sun's bolometric flux as a linear combination of Σr and ΣK. We infer that, over solar-cycle timescales, the variability of the Sun's bolometric irradiance is directly correlated with spectral line variability, but inversely correlated with continuum variability. Our blue and red continuum filters are quite similar to the Strömgren b and y filters used to measure stellar photometric variability. We conclude that active stars whose visible continuum brightness varies inversely with activity, as measured by the Ca HK index, are displaying a pattern that is similar to that of the Sun, i.e., radiative variability in the visible continuum that is spot-dominated.

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    ABSTRACT: [1] The SIM (Spectral Irradiance Monitor) on SORCE (Solar Radiation and Climate Experiment) provides more spectrally complete daily SSI (spectral solar irradiance) measurements than ever before, allowing us to explore chemical and physical processes in the Earth's ocean and atmosphere system. However, the newly observed SSI instigated controversies in the Sun-climate community on whether the SIM-observed trends are true solar variations and on whether climate responses are in phase or out of phase with solar forcing. In this study, we focus on resolving two apparently contradictory results published on possible temperature responses to SIM-derived solar forcing. When applying extreme scenarios of SIM-based spectral solar forcing in a radiative-convective model (RCM), we find that some apparently contradictory results can be explained by the different methods used to apply the SIM SSI data. It is clear that accurate SSI data are essential for accurate climate simulations and that climate modelers need to take care how they apply these data.
    06/2013; 118(12). DOI:10.1002/jgrd.50506
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    ABSTRACT: The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that treats stars as hypothetical Suns with coverage by magnetic features different from that of the Sun. The presented model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and bright faculae. A stellar spectrum is calculated from spectra of the individual components, by weighting them with corresponding disc area coverages. The latter are obtained by extrapolating the solar dependences of spot and facular disc area coverages on chromospheric activity to stars with different levels of mean chromospheric activity. We have found that the contribution by starspots to the variability increases faster with chromospheric activity than the facular contribution. This causes the transition from faculae-dominated variability and direct activity--brightness correlation to spot-dominated variability and inverse activity--brightness correlation with increasing chromospheric activity level. We have shown that the regime of the variability also depends on the angle between the stellar rotation axis and the line-of-sight and on the latitudinal distribution of active regions on the stellar surface. Our model can be used as a tool to extrapolate the observed photometric variability of the Sun to Sun-like stars at different activity levels, which makes possible the direct comparison between solar and stellar irradiance data.
    Astronomy and Astrophysics 06/2014; 569. DOI:10.1051/0004-6361/201323086 · 4.48 Impact Factor
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    ABSTRACT: Measurements from the Solar Irradiance Monitor (SIM) onboard the SORCE mission indicate that solar spectral irradiance at Visible and IR wavelengths varies in counter phase with the solar activity cycle. The sign of these variations is not reproduced by most of the irradiance reconstruction techniques based on variations of surface magnetism employed so far, and it is not clear yet whether SIM calibration procedures need to be improved, or if instead new physical mechanisms must be invoked to explain such variations. We employ three-dimensional magneto hydrodynamic simulations of the solar photosphere to investigate the dependence of solar radiance in SIM Visible and IR spectral ranges on variations of the filling factor of surface magnetic fields. We find that the contribution of magnetic features to solar radiance is strongly dependent on the location on the disk of the features, being negative close to disk center and positive toward the limb. If features are homogeneously distributed over a region around the equator (activity belt) then their contribution to irradiance is positive with respect to the contribution of HD snapshots, but decreases with the increase of their magnetic flux for average magnetic flux larger than 50 G in at least two of the Visible and IR spectral bands monitored by SIM. Under the assumption that the 50 G snapshots are representative of quiet Sun regions we find thus that the Spectral Irradiance can be in counter-phase with the solar magnetic activity cycle.
    The Astrophysical Journal 04/2014; 788(2). DOI:10.1088/0004-637X/788/2/151 · 6.28 Impact Factor


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