Thermodynamic evolution of a sigmoidal active region with associated flares

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-disk sigmoid observed during December 24-28, 2015. For this purpose, we have employed Emission Measure (EM) and filter-ratio techniques on the observations recorded by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and X-ray Telescope (XRT) onboard Hinode. The EM analysis showed multi-thermal 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 {\AA}) and Fe XXI 128.75 {\AA}) lines in the AIA 94 and 131 {\AA} 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 modeling of sigmoidal structures in the core of active regions. Moreover, this study also benchmarks different techniques available for temperature estimation in solar coronal structures.Comment: 19 pages, 13 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Societ

Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H2 at 1333.79 Å, observed at flare ribbons in SOL2014-04-18T13:03. The cool H2 emission is known to be fluorescently excited by Si IV 1402.77 Å UV radiation and provides a unique view of the temperature minimum region (TMR). Strong H2 emission was observed when the Si IV 1402.77 Å emission was bright during the flare impulsive phase and gradual decay phase, but it dimmed during the GOES peak. H2 line broadening showed non-thermal speeds in the range 7-18 km s−1, possibly corresponding to turbulent plasma flows. Small red (blue) shifts, up to 1.8 (4.9) km s−1 were measured. The intensity ratio of Si IV 1393.76 Å and Si IV 1402.77 Å confirmed that plasma was optically thin to Si IV (where the ratio = 2) during the impulsive phase of the flare in locations where strong H2 emission was observed. In contrast, the ratio differs from optically thin value of 2 in parts of ribbons, indicating a role for opacity effects. A strong spatial and temporal correlation between H2 and Si IV emission was evident supporting the notion that fluorescent excitation is responsible

Behaviour of molecular hydrogen emission in three solar flares

We have systematically investigated ultraviolet (UV) emission from molecular hydrogen (H$_{2}$) using the Interface Region Imaging Spectrometer (IRIS), during three X-ray flares of C5.1, C9.7 and X1.0 classes on Oct. 25, 2014. Significant emission from five H$_{2}$ spectral lines appeared in the flare ribbons, interpreted as photo-excitation (fluorescence) due to the absorption of UV radiation from two Si IV spectral lines. The H$_{2}$ profiles were broad and consisted of two non-stationary components in red and in the blue wings of the line in addition to the stationary component. The red (blue) wing components showed small redshifts (blue shifts) of ~5-15 km s$^{-1}$ (~5-10 km s$^{-1}$). The nonthermal velocities were found to be ~5-15 km s$^{-1}$. The interrelation between intensities of H$_{2}$ lines and their branching ratios confirmed that H$_{2}$ emission formed under optically thin plasma conditions. There is a strong spatial and temporal correlation between Si IV and H$_{2}$ emission, but the H$_{2}$ emission is more extended and diffuse, further suggesting H$_{2}$ fluorescence, and - by analogy with flare ''back-warming'' providing a means to estimate the depth from which the H$_{2}$ emission originates. We find that this is 1871$\pm$157 km and 1207$\pm$112 km below the source of the Si IV emission, in two different ribbon locations.Comment: 14 pages, 12 figures, accepted for publication in MNRA

Formation and thermodynamic evolution of plasmoids in active region jets

We have carried out a comprehensive study of the temperature structure of plasmoids, which successively occurred in recurrent active region jets. The multithermal plasmoids were seen to be travelling along the multi-threaded spire as well as at the footpoint region in the EUV/UV images recorded by the Atmospheric Imaging Assembly (AIA). The Differential Emission Measure (DEM) analysis was performed using EUV AIA images, and the high-temperature part of the DEM was constrained by combining X-ray images from the X-ray telescope (XRT/Hinode). We observed a systematic rise and fall in brightness, electron number densities and the peak temperatures of the spire plasmoid during its propagation along the jet. The plasmoids at the footpoint (FPs) (1.0–2.5 MK) and plasmoids at the spire (SPs) (1.0–2.24 MK) were found to have similar peak temperatures, whereas the FPs have higher DEM weighted temperatures (2.2–5.7 MK) than the SPs (1.3–3.0 MK). A lower limit to the electron number densities of plasmoids - SPs (FPs) were obtained that ranged between 3.4–6.1 × 108 (3.3–5.9 × 108) cm−3 whereas for the spire, it ranged from 2.6–3.2 × 108 cm−3. Our analysis shows that the emission of these plasmoids starts close to the base of the jet(s), where we believe that a strong current interface is formed. This suggests that the blobs are plasmoids induced by a tearing-mode instability

Flare-related recurring active region jets: evidence for very hot plasma

We present a study of two active region jets (AR jets) that are associated with two C-class X-ray flares. The recurrent, homologous jets originated from the northern periphery of a sunspot. We confirm flare-like temperatures at the footpoints of these jets using spectroscopic observations of Fe XXIII (263.76 Å) and Fe XXIV (255.11 Å) emission lines. The emission measure loci method was used to obtain an isothermal temperature, and the results show a decrease (17.7 to 13.6 MK) in the temperature during the decay phase of the C 3.0 flare. The electron number densities at the footpoints were found to range from 1.7×1010 to 2.0×1011 cm−3 using the Fe XIV line pair ratio. Nonthermal velocities were found to range from 34 – 100 km/s for Fe XXIV and 51 – 89 km/s for Fe XXIII. The plane-of-sky velocities were calculated to be 462±21 and 228±23 km/s for the two jets using the Atmospheric Imaging Assembly (AIA) 171 Å channel. The AIA light curves of the jet footpoint regions confirmed the temporal and spatial correlation between the two X-ray flares and the jet footpoint emission. The Gamma-ray Burst Monitor (GBM) also confirmed superhot plasma of 27 (25) MK with a nonthermal energy of 2.38×1026 (2.87×1027) ergs−1 in the jet footpoint region during the rise (peak) phase of one of the flares. The temperatures of the jet footpoint regions obtained from EIS agree very well (within an uncertainty of 20%) with temperatures obtained from the Geostationary Environmental Operational Satellite (GOES) flux ratios. These results provide clear evidence for very hot plasma (>10 MK) at the footpoints of the flare-related jets, and they confirm the heating and cooling of the plasma during the flares

Study of the spatial association between an active region jet and a nonthermal type III radio burst

Aims: We aim to investigate the spatial location of the source of an active region (AR) jet and its relation with associated nonthermal type III radio emission. Methods: An emission measure (EM) method was used to study the thermodynamic nature of the AR jet. The nonthermal type III radio burst observed at meterwavelength was studied using the Murchison Widefield Array (MWA) radio imaging and spectroscopic data. The local configuration of the magnetic field and the connectivity of the source region of the jet with open magnetic field structures was studied using a nonlinear force-free field (NLFFF) extrapolation and potential field source surface (PFSS) extrapolation respectively. Results: The plane-of-sky velocity of the AR jet was found to be ∼136 km s−1. The EM analysis confirmed the presence of low temperature 2 MK plasma for the spire, whereas hot plasma, between 5 and 8 MK, was present at the footpoint region which also showed the presence of Fe XVIII emission. A lower limit on the electron number density was found to be 1.4 × 108 cm−3 for the spire and 2.2 × 108 cm−3 for the footpoint. A temporal and spatial correlation between the AR jet and nonthermal type III burst confirmed the presence of open magnetic fields. An NLFFF extrapolation showed that the photospheric footpoints of the null point were anchored at the location of the source brightening of the jet. The spatial location of the radio sources suggests an association with the extrapolated closed and open magnetic fields although strong propagation effects are also present. Conclusions: The multi-scale analysis of the field at local, AR, and solar scales confirms the interlink between different flux bundles involved in the generation of the type III radio signal with flux transferred from a small coronal hole to the periphery of the sunspot via null point reconnection with an emerging structure

Multiwavelength study of 20 jets that emanate from the periphery of active regions

Aims. We present a multiwavelength analysis of 20 EUV jets which occurred at the periphery of active regions close to sunspots. We discuss the physical parameters of the jets and their relation with other phenomena such as Hα surges, nonthermal type-III radio bursts and hard X-ray (HXR) emission. Methods. These jets were observed between August 2010 and June 2013 by the Atmospheric Imaging Assembly (AIA) instrument that is onboard the Solar Dynamic Observatory (SDO). We selected events that were observed on the solar disk within +/–60° latitude. Using AIA wavelength channels that are sensitive to coronal temperatures, we studied the temperature distribution in the jets using the line of sight (LOS) differential emission measure (DEM) technique. We also investigated the role of the photospheric magnetic field using the LOS magnetogram data from the Helioseismic and Magnetic Imager (HMI) onboard SDO. Results. It has been observed that most of the jets originated from the western periphery of active regions. Their lifetimes range from 5 to 39 min with an average of 18 min and their velocities range from 87 to 532 km s-1 with an average of 271 km s-1. All the jets are co-temporally associated with Hα surges. Most of the jets are co-temporal with nonthermal type-III radio bursts observed by the Wind/WAVES spacecraft in the frequency range from 20 kHz to 13 MHz. We confirm the source region of these bursts using the potential field source surface (PFSS) technique. Using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations, we found that half of the jets produced HXR emission and they often shared the same source region as the HXR emission (6−12 keV). Ten out of 20 events showed that the jets originated in a region of flux cancellation and six jets in a region of flux emergence. Four events showed flux emergence and then cancellation during the jet evolution. DEM analyses showed that for most of the spires of the jets, the DEM peaked at around log T [K] = 6.2/6.3 (~2 MK). In addition, we derived an emission measure and a lower limit of electron density at the location of the spire (jet 1: log EM = 28.6, Ne = 1.3 × 1010 cm-3; jet 2: log EM = 28.0, Ne = 8.6 × 109 cm-3) and the footpoint (jet 1 – log EM = 28.6, Ne = 1.1 × 1010 cm-3; jet 2: log EM = 28.1, Ne = 8.4 × 109 cm-3). These results are in agreement with those obtained earlier by studying individual active region jets. Conclusions. The observation of flux cancellation, the association with HXR emission and emission of nonthermal type-III radio bursts, suggest that the initiation and therefore, heating is taking place at the base of the jet. This is also supported by the high temperature plasma revealed by the DEM analysis in the jet footpoint (peak in the DEM at log T [K] = 6.5). Our results provide substantial constraints for theoretical modeling of the jets and their thermodynamic nature