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

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo Norway
² Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
³ University of Graz, Institute of Physics, Universitätsplatz 5, 8010 Graz, Austria
⁴ University of Graz, Kanzelhöhe Observatory for Solar and Environmental Research, Kanzelhöhe 19, 9521 Treffen, Austria
⁵ NorthWest Research Associates, 3380 Mitchell Lane, Boulder, 80301 CO, USA

^⋆ Corresponding author: sascha.ornig@astro.uio.no

Received: 3 July 2025
Accepted: 19 August 2025

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 ≥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 (≥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.