The optical depth of white-light flare continuum

TL;DR

This paper investigates the optical depth of white-light flare continuum emission, using a new method applied to a specific solar flare, revealing that the emission layer has a small optical depth, consistent with observed spectral edges.

Contribution

It introduces a method to determine the optical depth of white-light flare emission and applies it to a well-studied solar flare, providing new insights into its formation height.

Findings

Optical depth of the flare's visible continuum is small.

The emission layer's properties are consistent with observed spectral edges.

The method can be applied to other flare observations.

Abstract

The white-light continuum emission of a solar flare remains a puzzle as regards its height of formation and its emission mechanism(s). This continuum, and its extension into the near UV, contain the bulk of the energy radiated by a flare, and so its explanation is a high priority. We describe a method to determine the optical depth of the emitting layer and apply it to the well-studied flare of 2002 July~15, making use of MDI pseudo-continuum intensity images. We find the optical depth of the visible continuum in all flare images, including an impulsive ribbon structure to be small, consistent with the observation of Balmer and Paschen edges in other events.