Transition Region Emission from Solar Flares during the Impulsive Phase

TL;DR

This study analyzes UV emission during the impulsive phase of solar flares, deriving luminosities and flow speeds from off-limb measurements, revealing insights into the origins and timing of transition region emissions relative to X-ray data.

Contribution

It provides the first systematic derivation of O VI and Lyα luminosities during solar flare impulsive phases and compares them with X-ray emissions, offering new insights into emission origins.

Findings

O VI and Lyα luminosities are comparable to early X-ray luminosities.

Flow speeds of O VI emitting gas range from 40 to 80 km/s.

Multiple sources contribute to UV emission during flares.

Abstract

There are relatively few observations of UV emission during the impulsive phases of solar flares, so the nature of that emission is poorly known. Photons produced by solar flares can resonantly scatter off atoms and ions in the corona. Based on off-limb measurements by SOHO/UVCS, we derive the O VI $\lambda$1032 luminosities for 29 flares during the impulsive phase and the Ly$\alpha$ luminosities of 5 flares, and we compare them with X-ray luminosities from GOES measurements. The upper transition region and lower transition region luminosities of the events observed are comparable. They are also comparable to the luminosity of the X-ray emitting gas at the beginning of the flare, but after 10-15 minutes the X-ray luminosity usually dominates. In some cases we can use Doppler dimming to estimate flow speeds of the O VI emitting gas, and 5 events show speeds in the 40 to 80 $\rm km s^{-1}$ range. The O VI emission could originate in gas evaporating to fill the X-ray flare loops, in heated chromospheric gas at the footpoints, or in heated prominence material in the coronal mass ejection. All three sources may contribute in different events or even in a single event, and the relative timing of UV and X-ray brightness peaks, the flow speeds, and the total O VI luminosity favor each source in one or more events.