Are solar brightness variations faculae- or spot-dominated? (1602.04447v1)

Abstract: Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability. To understand whether the broader pattern of stellar variations is consistent with the solar paradigm we assess relative contributions of faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness variability as well as its facular and spot contributions depend on the wavelength, timescale of variability, and position of the observer relative to the ecliptic plane. We perform calculations with the SATIRE model, which returns solar brightness with daily cadence from solar disc area coverages of various magnetic features. Moving the observer away from the ecliptic plane increases the amplitude of 11-year variability as it would be seen in Str\"omgren $(b+y)/2$ photometry, but decreases the amplitude of the rotational brightness variations as it would appear in Kepler and CoRoT passbands. The spot and facular contributions to the 11-year solar variability in the Str\"omgren $(b+y)/2$ photometry almost fully compensate each other so that the Sun appears anomalously quiet with respect to its stellar cohort. Such a compensation does not occur on the rotational timescale. The rotational solar brightness variability as it would appear in Kepler and CoRoT passband from the ecliptic plane is spot-dominated but the relative contribution of faculae increases for out-of-ecliptic viewing so that the apparent brightness variations are faculae-dominated for inclinations less than about $i=45^{\circ}$.

Solar Brightness Variations: Faculae vs. Spot Dominance

This paper investigates the relative contributions of faculae and spots to solar brightness variations, with implications for our understanding of solar and stellar photometric variability. Using the SATIRE model, which reconstructs solar brightness based on surface magnetic features, the authors explore how these contributions vary with wavelength, timescale, and observer position relative to the solar ecliptic plane.

Key Findings

1. Timescales of Variability:
   • On the 11-year solar activity cycle, faculae dominate brightness variations in the UV and visible spectra, except beyond 1200 nm in the infrared where spots prevail. The transition between these regimes is influenced by wavelength-dependent factors like Planck function sensitivity and Fraunhofer line effects.
   • On the rotational timescale, solar brightness variability is dominated by spots in the visible and infrared domains. This transition occurs at shorter wavelengths compared to the 11-year cycle due to the spatial distribution of magnetic features, suggesting that brightness variability is governed differently over short and long timescales.
2. Observer Inclination Effects:
   • Moving the observer out of the solar ecliptic plane impacts the amplitude of brightness variations. Long-term variability increases with inclination due to decreased spot contributions, whereas short-term variability decreases, highlighting differing impacts depending on the timescale.
   • For rotational variability, the observer's inclination significantly alters the dominance of facular versus spot signatures, with faculae increasingly dominating at lower inclinations (less than 45°).
3. Stellar Comparisons:
   • The paper compares solar brightness variability in Strömgren filters (often used in observing Sun-like stars) with Total Solar Irradiance (TSI), indicating that solar photometric variations may be underestimated in previous ground-based studies.
   • The authors emphasize caution when extrapolating solar variability patterns to other stars, especially considering factors like stellar metallicity and effective temperature which affect facular contrast and overall variability.

Practical and Theoretical Implications

The results have significant implications for solar and stellar physics, particularly in characterizing the photometric behavior of Sun-like stars and understanding their magnetic activity cycles. This research informs models predicting stellar variability and aids climatological studies by refining estimates of solar-induced climatic effects.

Additionally, the methodology demonstrates the necessity of incorporating observer geometry into models of stellar photometric variability, providing a basis for future studies addressing observational biases in solar and stellar analyses.

Future Directions

This paper outlines potential avenues for future research, including extending models to capture solar variability on sub-daily scales and applying these methods to stars with varying temperatures and metallicities. These developments will enhance understanding of solar variability mechanisms and improve analogs for solar-like stars' behavior.

In conclusion, this paper provides a comprehensive analysis of solar brightness variations, highlighting the roles of faculae and spots. Its insights into the spectrally-dependent dynamics and observer inclination effects pave the way for nuanced interpretations of solar and stellar photometry, crucial for astrophysical and climatological applications.