Sun-as-a-star Observation of White-Light Flares

TL;DR

This study analyzes long-term solar irradiance data to demonstrate that all solar flares emit white-light continuum radiation, primarily at ~9000K, contributing significantly to their total energy output.

Contribution

It provides evidence that all solar flares produce white-light emission and quantifies its spectral and energetic contribution, using extensive satellite data analysis.

Findings

White-light emission is detectable in moderate flares during the impulsive phase.

White-light continuum accounts for about 70% of the total radiated energy in flares.

The continuum spectrum is consistent with a blackbody at approximately 9000K.

Abstract

Solar flares radiates energy at all wavelengths, but the spectral distribution of this energy is still poorly known. White-light continuum emission is sometimes observed and the flares are then termed "white-light flares" (WLF). In this paper, we investigate if all flares are white-light flares and how is the radiated energy spectrally distributed. We perform a superposed epoch analysis of spectral and total irradiance measurements obtained since 1996 by the SOHO and GOES spacecrafts at various wavelength, from Soft X-ray to the visible domain. The long-term record of solar irradiance and excellent duty cycle of the measurements allow us to detect a signal in visible irradiance even for moderate (C-class) flares, mainly during the impulsive phase. We identify this signal as continuum emission emitted by white-light flares, and find that it is consistent with a blackbody emission at ~9000K. We estimate for several sets of flares the contribution of the WL continuum and find it to be of ~70% of the total radiated energy. We re-analyse the X17 flare that occurred on 28 October 2003 and find similar results. This paper brings evidence that all flares are white-light flares and that the white-light continuum is the main contributor to the total radiated energy; this continuum is consistent with blackbody spectrum at ~9000K. These observational results are important in order to understand the physical mechanisms during flares and open the way to a possible contribution of flares to TSI variations.