Abstract
ABSTRACT Active regions often show S-shaped structures in the corona called sigmoids. These are highly sheared and twisted loops formed along the polarity inversion line. They are considered to be one of the best pre-eruption signatures for CMEs. Here, we investigate the thermodynamic evolution of an on-disc sigmoid observed during 2015 December 24–28. For this purpose, we have employed Emission Measure (EM) and filter-ratio techniques on the observations recorded by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO) and X-ray Telescope (XRT) on-board Hinode. The EM analysis showed multithermal plasma along the sigmoid and provided a peak temperature of ∼10–12.5 MK for all observed flares. The sigmoidal structure showed emission from Fe xviii (93.93 Å) and Fe xxi (128.75 Å) lines in the AIA 94 and 131 Å channels, respectively. Our results show that the hot plasma is often confined to very hot strands. The temperature obtained from the EM analysis was found to be in good agreement with that obtained using the XRT, AIA, and GOES filter-ratio methods. These results provide important constraints for the thermodynamic modelling of sigmoidal structures in the core of active regions. Moreover, this study also benchmarks different techniques available for temperature estimation in solar coronal structures.
Highlights
On disk observations of the Sun’s corona in extreme ultraviolet (EUV) and X-rays often exhibit S-shaped loops embedded in the core of active regions
By taking advantage of high spatial and temporal resolution observations from the Atmospheric Imaging Assembly (AIA; Lemen et al 2012) and X-ray Telescope (XRT; Golub et al 2007; Kano et al 2008) instruments, we have focused on the thermal structure of an on-disk active region sigmoid and studied its heating/cooling during different phases of solar flares
During the decay phase of the flare, the sigmoid started to cool down from temperature log T [K] = 7.1 to 6.9 (12.5-8 MK), which is in good agreement with the temperatures obtained from AIA Emission Measure (EM) analysis, the EM maps at 02:06:24 UT but slightly higher than those estimated from GOES
Summary
On disk observations of the Sun’s corona in extreme ultraviolet (EUV) and X-rays often exhibit S-shaped (reverse S-shaped and/or two J-shaped) loops embedded in the core of active regions These sheared and twisted coronal structures are known as Sigmoids (Rust & Kumar 1996; Gibson et al 2002; Schwenn et al 2006; Tripathi et al 2006b, 2009) and could be considered as manifestations of twisted flux tubes (Gibson & Fan 2006; Chen 2017; Inoue et al 2018). By taking advantage of high spatial and temporal resolution observations from the Atmospheric Imaging Assembly (AIA; Lemen et al 2012) and X-ray Telescope (XRT; Golub et al 2007; Kano et al 2008) instruments, we have focused on the thermal structure of an on-disk active region sigmoid and studied its heating/cooling during different phases of solar flares.
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