Ultraviolet and Extreme-Ultraviolet Emissions at the Flare Footpoints Observed by Atmosphere Imaging Assembly

Abstract

A solar flare is composed of impulsive energy release events by magnetic reconnection, which forms and heats flare loops. Recent studies have revealed a two-phase evolution pattern of UV 1600 Å emission at the feet of these loops: a rapid pulse lasting for a few seconds to a few minutes, followed by a gradual decay on timescales of a few tens of minutes. Multiple band EUV observations by the Atmosphere Imaging Assembly further reveal very similar signatures. These two phases represent different but related signatures of an impulsive energy release in the corona. The rapid pulse is an immediate response of the lower atmosphere to an intense thermal conduction flux resulting from the sudden heating of the corona to high temperatures (we rule out energetic particles due to a lack of significant hard X-ray emission). The gradual phase is associated with the cooling of hot plasma that has been evaporated into the corona. The observed footpoint emission is again powered by thermal conduction (and enthalpy), but now during a period when approximate steady-state conditions are established in the loop. UV and EUV light curves of individual pixels may therefore be separated into contributions from two distinct physical mechanisms to shed light on the nature of energy transport in a flare. We demonstrate this technique using coordinated, spatially resolved observations of UV and EUV emissions from the footpoints of a C3.2 thermal flare.

Long Duration Flare Emission: Impulsive Heating or Gradual Heating?

“…Instead of computing this loss term using an equilibrium solution (Cargill et al 2012a), we scale this term as being proportional to the mean coronal pressure P á ñ by a scaling constant η. Such proportionality is observed in the decay phase of solar and stellar flares as well as predicted in coronal heating models (Hawley & Fisher 1992;Qiu et al 2013, and references therein). We set η by matching low-temperature emission in AIA 171 channel between model and observation.…”

Comparison of 30 THz impulsive burst time development to microwaves, Hα, EUV, and GOES soft X-rays

“…The presence of nonthermal electrons at the onset of 30 THz emission is suggested by the simultaneous onset of HXRs (inferred from the SXR derivative via the Neupert effect), but the lack of microwave emission until later suggests that there were no electrons in the corona with energies much above a few tens of keV. While the 30 THz emission near the peak is consistent with the standard picture of sudden plasma heating in response to high-energy nonthermal electron bombardment on the dense lower chromosphere regions (as for white light and 1700 Å emission, e.g., Najita & Orrall 1970;Machado et al 1989;Xu et al 2006;Wang et al 2007;Martínez Oliveros et al 2012;Qiu et al 2013), the early rise of 30 THz and 1700 Å emission before the microwaves does not seem consistent with this mechanism because electrons with energies of just a few tens of keV cannot penetrate to the depth required.…”

The Spectral Content of SDO/AIA 1600 and 1700 Å Filters from Flare and Plage Observations

“…Therefore, given the temperature range for the formation of the relevant lines, the AIA UV flare excess emission should reflect the response of the chromosphere to the heating and/or energy deposition. This naturally explains the co-temporal and co-spatial hard Xray and UV emission during flares that has been Qiu et al (2013) assumed that the AIA 1600Å images are dominated by the C IV 1550Å doublet plus continuum contribution, and that the AIA 1700Å images are dominated by continuum emission, ignoring any contribution from other lines. Our results indicate that those assumptions are not entirely valid.…”

“…Due to a lack of knowledge of the spectral content of the AIA UV images, detailed qualitative analyses of flaring plasma using AIA UV images have been rare. A few exceptions are the investigation of plasma cooling (Qiu et al 2010, Cheng et al 2012, Qiu et al 2013) and estimates of the radiative output from the flaring chromosphere (Milligan et al 2014). In contrast, the spectral content in the AIA extreme ultraviolet (EUV) images has been extensively explored.…”