Sun-as-a-star Spectral Irradiance Observations of Transiting Active Regions

TL;DR

This study analyzes spectral irradiance from different solar active regions to understand their magnetic and thermal properties, revealing how various wavelengths correlate with magnetic flux and how they can diagnose stellar flare precursors.

Contribution

It provides a comprehensive multi-wavelength analysis of transiting solar active regions, highlighting new diagnostic potential for stellar magnetic fields and plasma conditions.

Findings

Visible and total irradiance darken with sunspot centrality

UV bands correlate with magnetic flux variations

EUV and X-ray amplitudes increase with temperature

Abstract

Major solar flares are prone to occur in active region atmospheres associated with large, complex, dynamically-evolving sunspots. This points to the importance of monitoring the evolution of starspots, not only in visible but also in ultra violet (UV) and X-rays, in understanding the origin and occurrence of stellar flares. To this end, we perform spectral irradiance analysis on different types of transiting solar active regions by using a variety of full-disk synoptic observations. The target events are an isolated sunspot, spotless plage, and emerging flux in prolonged quiet-Sun conditions selected from the past decade. We find that the visible continuum and total solar irradiance become darkened when the spot is at the central meridian, whereas it is bright near the solar limb; UV bands sensitive to the chromosphere correlate well with the variation of total unsigned magnetic flux in the photosphere; amplitudes of EUV and soft X-ray increase with the characteristic temperature, whose light curves are flat-topped due to their sensitivity to the optically thin corona; the transiting spotless plage does not show the darkening in the visible irradiance, while the emerging flux produces an asymmetry in all light curves about the central meridian. The multi-wavelength sun-as-a-star study described here indicates that such time lags between the coronal and photospheric light curves have the potential to probe the extent of coronal magnetic fields above the starspots. In addition, EUV wavelengths that are sensitive to the transition-region temperature sometimes show anti-phased variations, which may be used for diagnosing plasmas around starspots.