Continuum Enhancements in the Ultraviolet, the Visible and the Infrared during the X1 flare on 2014 March 29

TL;DR

This study investigates the sources and energy contributions of continuum emission during an X1 solar flare, combining multi-wavelength observations and modeling to identify chromospheric and photospheric processes involved.

Contribution

It provides a detailed analysis of the emission sources and quantifies the energy of nonthermal electrons needed to produce observed continuum enhancements during the flare.

Findings

Continuum emission originates from both photospheric and chromospheric regions.

Infrared and visible spectra fit blackbody temperatures of 6000-6300 K.

Nonthermal electrons with energies >40 keV can account for the continuum energy.

Abstract

Enhanced continuum brightness is observed in many flares (''white light flares''), yet it is still unclear which processes contribute to the emission. To understand the transport of energy needed to account for this emission, we must first identify both the emission processes and the emission source regions. Possibilities include heating in the chromosphere causing optically thin or thick emission from free-bound transitions of Hydrogen, and heating of the photosphere causing enhanced H$^-$ continuum brightness. To investigate these possibilities, we combine observations from IRIS, SDO/HMI, and the ground-based FIRS instrument, covering wavelengths in the far-UV, near-UV, visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures $\sim$6000-6300 K. The NUV emission, formed in the upper photosphere under undisturbed conditions, exceeds these simple fits during the flare, requiring extra emission from the Balmer continuum in the chromosphere. Thus, the continuum originates from enhanced radiation from photosphere (visible-IR) and chromosphere (NUV). From the standard thick-target flare model, we calculate the energy of the nonthermal electrons observed by RHESSI and compare it to the energy radiated by the continuum emission. We find that the energy contained in most electrons $>$40 keV, or alternatively, of $\sim$10-20% of electrons $>$20 keV is sufficient to explain the extra continuum emission of $\sim4-8 \times 10^{10}$ erg s$^{-1}$ cm$^{-2}$. Also, from the timing of the RHESSI HXR and the IRIS observations, we conclude that the NUV continuum is emitted nearly instantaneously when HXR emission is observed with a time difference of no more than 15 s.