Extremely energetic EUV late phase of a pair of C-class flares caused by a non-eruptive sigmoid

TL;DR

This study demonstrates that an extremely energetic EUV late phase can occur in C-class solar flares when a non-eruptive sigmoid forms in a multipolar magnetic configuration and is continually heated by magnetic reconnection. Using multi-instrument data (GOES, RHESSI, SDO/AIA, and GST/BBSO) and advanced analysis (SE-DESAT desaturation, DEM via CHIANTI, NLFFF extrapolation, and QSL/twist diagnostics), the authors show that the late-phase energy in the warm 335 Å channel can exceed the main-phase energy by a factor of about 4, driven by a hot sigmoid with temperatures above $T \gtrsim 10^7$ K. The sigmoid’s formation from reconnection between two J-shaped loops, its non-eruptive confinement (decay index $n < 1.5$ at the sigmoid height, $\sim$11 Mm), and its two-stage cooling dominated by conduction lead to a sustained EUV late-phase emission. The results imply that intense ELPs may significantly affect ionospheric dynamics and underscore continuous reconnection as a key energy source in non-eruptive flare scenarios, motivating broader statistical studies to establish how common energetic ELPs are in non-eruptive events.

Abstract

The EUV late phase is the second increase of the irradiance of the warm coronal lines during solar flares, and has a crucial impact on the Earth's ionosphere. In this paper, we report on the extremely energetic EUV late phase of a pair of C-class flares (SOL2012-06-17T17:26:11) observed on 2012 June 17 in NOAA active region 11504 by the \textit{Atmospheric Imaging Assembly} (AIA) instrument on board the \textit{Solar Dynamics Observatory} (SDO). The light curves integrated over the flaring region show that a factor of 4.2 more energy is released in the ``warm'' (2$-$3$\times 10^6$~K) temperature passbands (e.g. AIA 335 Å) during the late phase than during the main peaks. The origin of the emission in this extremely energetic EUV late phase is a non-eruptive sigmoid situated in a multi-polar magnetic field configuration, which is rapidly energised by C-class flares. The sigmoid plasma appears to reach temperatures in excess of $10^7$~K, before cooling to produce the EUV late-phase emission. This is seen in high-temperature passbands (e.g. AIA 131 Å) and by using differential emission measure analysis. Magnetic extrapolations indicate that the sigmoid is consistent with formation by magnetic reconnection between previously existing J-shaped loops. The sigmoid experienced a fast and a slow cooling stages, both of which were dominated by conductive cooling. The estimated total cooling time of the sigmoid is shorter than the observed value. So, we proposed that the non-eruptive sigmoid, heated by the continuous magnetic reconnection, leads to the extremely energetic EUV late phase.

Figures (10)

• Figure 1: Panel (a): The SXR fluxes observed by GOES in 1$-$8 (blue) and 0.5$-$4.0 Å (purple) passbands for the C1.0 and C3.9 flares on 2012 June 17. Panel (b): The X-ray fluxes (corrected count rates) from RHESSI in various energy ranges from 3 to 25 keV. Panel (c): The normalised intensity in 7 EUV passbands of AIA observed during these two C-class flares. The normalized intensity was calculated for the flaring region within the large white box in panels a1 and b1 of Fig. \ref{['fig:fig2']}. The vertical dashed lines indicate the respective times for the peak fluxes observed by GOES (panel a) and RHESSI (12$-$25 keV, panel b) during these flares.
• Figure 2: The evolution of EUV emission observed in three EUV passbands of AIA at 131, 171, and 335 Å during the peak time of the C1.0 (panels a1-a3) and C3.9 (panels b1-b3) flares. The images in panels c1-c3 and d1-d3 were taken during the EUV late phase. The UV counterparts of these two flares were observed in the AIA 1600 Å passband (panels a4 and b4). The region outlined by the large white box in panels a1 and b1 was used to obtain total intensity in the flaring location, with the resulting profiles shown in panel (c) of Fig. \ref{['fig1:GOES_rhessi_aia']}. Small white boxes in panels a1, c1, a4, and b4 indicate the location where the magnetic reconnection might have occurred, and the red boxes in panels a4, b4, c3-c4, and d3-d4 show the footpoint of the sigmoidal structure that is indicated by FL (left footpoint) and FR (right footpoint). A white diagonal line in panel b1 indicates the position of an artificial slit which was used to create the space-time diagrams in Fig. \ref{['fig:fig3']}. Panel c4: The field-of-view observed by the TiO broadband filter in 7057 Å. Panel d1: The green and red contours represent the X-ray sources at 3$-$6 keV and 6$-$12 keV energy channels of RHESSI. Panel d4: the line-of-sight (LOS) magnetogram observed by HMI. The two footpoints of the sigmoid are rooted in the two sunspots, the leading (positive polarity, P1) and trailing (negative polarity, N1). Between these two sunspots, we observe a pore at the location of a dipolar field labelled as N2 and P2. An animation of the evolution of the flare is available. The animation starts at June 17th, 2012 at 17:20 UT. It ends the same day around 19:45 UT. The real-time duration is 42 seconds. In the animation top row from top left to right is the SDO/HMI LOS, SDO/AIA 94Å, SDO/AIA 131Å, SDO/AIA 171Å. The bottom rows is SDO/AIA 1600Å, SDO/AIA 335Å, SDO/AIA 304Å, and a graph of the GOES lightcurves in 1$-$8 Å (blue) and 0.5$-$4.0 Å (pink) for the C1.0 and C3.9 flares on 2012 June 17. The vertical line indicates the timeline of each snapshot.
• Figure 3: Space-time diagrams in 94 Å, 131 Å and 335 Å passbands along the slit shown as a white line in Fig. \ref{['fig:fig2']} panel b1.
• Figure 4: Background subtracted intensity profiles (in units of energy/s) observed in AIA 335 Å (blue), 211 Å (red), and 193 Å (black) passbands for the large white box region shown in panels a1 and b1 of Fig. \ref{['fig:fig2']}. The background is obtained based on the minimum of the integrated intensity over the time period from 17:00 to 19:00 UT. The time period of the main phase is indicated by dashed lines, starting from 17:25 UT and ending at different turning points of 17:41, 17:43, and 17:48 UT for 335, 211, and 193 Å, respectively. The time period of EUV late phase is shown in solid lines, starting from 17:57 UT for 335 Å and 18:05 UT for 193 and 211 Å, respectively. Solid vertical lines (at 18:30 UT) mark the end of the EUV late phase. The total energy is obtained by integrating over each time period.
• Figure 5: Panels (a) and (b): images at 10830 Å and 131 Å representing the filament and its counterpart in high temperature at $\sim$ 17:26 UT. Panels (c) and (d) display the filament and J-shape structure observed at 10830 and 131 Å respectively, at 17:40 UT. Panel (e) indicates the magnetogram along the line of sight at 17:12:00 UT. The bottom boundary is from the corresponding vector magnetogram. The white lines indicate the region of NLFFF extrapolation. The white portion represents a positive magnetic field, while the black portion represents a negative magnetic field. The red curve represents the polarity inversion line (PIL), and the blue asterisk indicates the location where we measure the decay index. Panel (f) shows magnetic topology obtained from NLFFF extrapolation seen from the Z-direction. The background shows the magnitude of the magnetic field at the plane of Z=0. The pink, cyan and yellow lines represent the magnetic field lines.