Do all Flares have White Light Emission?

TL;DR

This study presents high-resolution observations of a solar flare, revealing that white light emission is a small, localized, and common feature in all solar flares, caused by radiative back warming.

Contribution

It provides detailed high-resolution evidence that white light emission occurs in all solar flares and links it to magnetic field changes and radiative back warming.

Findings

White light brightening is localized and below the resolution of most space telescopes.

White light emission occurs only during the impulsive stage of the flare.

White light emission is caused by radiative back warming.

Abstract

High-cadence, multiwavelength optical observations of a solar active region (NOAA 10969), obtained with the Swedish Solar Telescope, are presented. Difference imaging of white light continuum data reveals a white light brightening, 2 min in duration, linked to a co-temporal and co-spatial C2.0 flare event. The flare kernel observed in the white light images has a diameter of 300 km, thus rendering it below the resolution limit of most space-based telescopes. Continuum emission is present only during the impulsive stage of the flare, with the effects of chromospheric emission subsequently delayed by approximately 2 min. The localized flare emission peaks at 300% above the quiescent flux. This large, yet tightly confined, increase in emission is only resolvable due to the high spatial resolution of the Swedish Solar Telescope. An investigation of the line-of-sight magnetic field derived from simultaneous MDI data shows that the continuum brightening is located very close to a magnetic polarity inversion line. Additionally, an H-alpha flare ribbon is directed along a region of rapid magnetic energy change, with the footpoints of the ribbon remaining co-spatial with the observed white light brightening throughout the duration of the flare. The observed flare parameters are compared with current observations and theoretical models for M- and X-class events and we determine the observed white-light emission is caused by radiative back warming. We suggest that the creation of white-light emission is a common feature of all solar flares.