Extreme ultraviolet late-phase flares as observed by EVE and AIA on board the Solar Dynamics Observatory

TL;DR

This study provides the most extensive statistical census to date of EUV late-phase (ELP) flares observed with SDO/EVE and spatial context from SDO/AIA over 2010–2014. It defines a robust, quantitative set of ELP criteria and identifies 179 ELP events among 1803 flares (≥ C3.0), revealing that ~67% are confined and ~33% eruptive, with a strong positive relation between late-phase delay and duration likely driven by cooling in longer coronal loops. The late-phase strength is highly variable, with a majority (~71.5%) of ELPs having a late-phase peak exceeding the main peak, and the ELP occurrence shows a cycle-dependent decrease toward solar maximum. ELP activity is more common in flares from relatively simple active-region magnetic configurations and exhibits notable links to flare-ribbon morphology and AR productivity, providing a framework to interpret the physical mechanisms behind the EUV late phase and its space-weather relevance.

Abstract

Context. Extreme ultraviolet (EUV) late-phase (ELP) flares exhibit a second peak in warm coronal emissions minutes to hours after the main peak of the flare. This phase is all but negligible, and it is still poorly understood what role it plays across the solar cycle and what governs it. Aims. We present a statistical analysis of ELP flares over four years between May 2010 and May 2014 based on properties such as eruptivity, magnetic configuration, and late-phase duration, delay, and strength in order to understand what influences the likelihood of this class of flares and their behavior on a general scale. Methods. We primarily made use of data from the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE), as well as complementary spatial information provided by the Atmospheric Imaging Assembly (AIA), to assess relationships between the various parameters and to see if ELP flares differ from the general flare population. We quantified the criteria for ELP flare definition and determined the characteristics of the flares. Results. Our analysis shows that about 10\% of all flares with a GOES class $\geq$C3.0 experience an EUV late phase (179 out of 1803). This percentage decreases from solar minimum to solar maximum. C-class flares are considerably less likely to be identified as ELP flares than their higher-energy counterparts, which is in line with previous investigations. The majority of this type of flare are confined (67%), more so than in the general flare population ($\geq$C5.0). There appears to be a (linear) relationship between the late-phase delay and its duration. The ratio of the emission peak of the late and main flare phase lies between 0.3 and 5.9, and exceeds 1 in 71.5% of cases, which is considerably higher than previously reported.

Figures (16)

• Figure 1: Unsmoothed (black) and smoothed (green, yellow, and red) light curves of the EVE emission lines from Table \ref{['tab:lines']} for the X1.8 flare on October 23, 2012. The light curves are arranged according to the corresponding line formation temperature (given above each panel), decreasing from top to bottom. The Fe XX light curves were not smoothed.
• Figure 2: Visual representation of the ELP criteria applied in this study. Depicted are the PFS irradiances of the EVE Fe XX 13.29 nm (blue) and Fe XVI 33.54 nm (green) emission lines for the X1.8 flare on October 23, 2012. The horizontal dotted line represents the pre-flare level. The shaded gray areas correspond to the region in the Fe XVI light curve that satisfies criterion 1 (darker shade of gray) and criterion 2 (lighter and darker shade of gray together) given in Sect. \ref{['subsec:elp_crit']}. The horizontal dashed green line marks the limit corresponding to criterion 3, whereas the horizontal dashed blue line depicts the limit for criterion 4.
• Figure 3: Visualization of the location of ELP flares on the solar surface, as viewed from a terrestrial perspective. C-class flares are marked with small green circles, M-class flares with medium-sized blue circles, and X-class flares with large magenta circles.
• Figure 4: AIA images as well as GOES and EVE light curves for the confined C6.5 flare on March 23, 2012. Top two rows: Still images taken by AIA in the 13.1 nm (left column), 33.5 nm (middle column), and 17.1 nm (right column) filter. Middle panel: GOES 0.1--0.8 nm irradiance. The start and end of the pre-flare window used to calculate the PFS irradiances are shown by the two vertical dashed lines. Bottom panel: PFS irradiances of selected EVE emission lines: Fe XX 13.29 nm (blue), Fe XVI 33.54 nm (green), Fe IX 17.11 nm (yellow), and He II 30.4 nm (red). The PFS irradiance values for the He II 30.4 nm line were divided by a factor of 3. The horizontal dotted line represents the zero level. The main-flare maximum and late-phase maximum are marked with a black cross. The vertical solid lines mark the beginning and end of the EUV late phase.
• Figure 5: Monthly number of flares from May 2010 to May 2014. Top panel: Non-ELP flares $\geq$C3.0 (green) and ELP flares (orange). Middle panel: ELP flares. Bottom panel: Percentage of ELP flares.