M. D. Ding

DIFFERENTIAL EMISSION MEASURE ANALYSIS OF MULTIPLE STRUCTURAL COMPONENTS OF CORONAL MASS EJECTIONS IN THE INNER CORONA

In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis is based on the six-passband EUV observations of solar corona from the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature (>8 MK) and density (~1.0 × 109 cm–3), resulting in an enhanced emission measure over a broad temperature range (3 ≤ T(MK) ≤ 20). On the other hand, the CME LF has a relatively cool temperature (~2 MK) and a narrow temperature distribution similar to the pre-eruption coronal temperature (1 ≤ T(MK) ≤ 3). The density in the LF, however, is increased by 2%-32% compared with that of the pre-eruption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 ≤ T(MK) ≤ 4), but the density decreases by ~35%-~40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection; (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region; and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption.

Simulation of Magnetic Reconnection with Heat Conduction

Magnetohydrodynamic (MHD) equations are numerically solved to study 2.5-dimensional magnetic reconnection with field-aligned heat conduction, which is also compared with the adiabatic case. The dynamical evolution starts after anomalous resistivity is introduced into a hydrostatic solar atmosphere with a force-free current sheet, which might be similar to the configuration before some solar flares. The results show that two jets (i.e., the outflows of the reconnection region) appear. The downward jet collides with the closed line-tied field lines, and a bright loop is formed with a termination shock at the loop top. As the reconnection goes on, the loop rises almost uniformly with a speed of tens of km s−1, and the two footpoints of the loop separate with a speed comparable to the loop rise speed. Besides the apparent loop motion, the magnetic loops below the loop top shrink weakly. Such a picture is consistent with that given by observations of two-ribbon solar flares. Moreover, the results indicate that the slow MHD shock contributes to the bright loop heating. Some detailed structures of the reconnection process are also discussed.

The Driver of Coronal Mass Ejections in the Low Corona: A Flux Rope

Recent Solar Dynamic Observatory observations reveal that coronal mass ejections (CMEs) consist of a multi-temperature structure: a hot flux rope and a cool leading front (LF). The flux rope first appears as a twisted hot channel in the Atmospheric Imaging Assembly (AIA) 94 A and 131 A passbands. The twisted hot channel initially lies along the polarity inversion line and then rises and develops into a semi-circular flux-rope-like structure during the impulsive acceleration phase of CMEs. In the meantime, the rising hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME LF. In this paper, we study in detail two well-observed CMEs that occurred on 2011 March 7 and 2011 March 8, respectively. Each of them is associated with an M-class flare. Through a kinematic analysis we find that (1) the hot channel rises earlier than the first appearance of the CME LF and the onset of the associated flare and (2) the speed of the hot channel is always faster than that of the LF, at least in the field of view of AIA. Thus, the hot channel acts as a continuous driver of the CME formation and eruption in the early acceleration phase. Subsequently, the two CMEs in white-light images can be well reproduced by the graduated cylindrical shell flux rope model. These results suggest that the pre-existing flux rope plays a key role in CME initiation and formation.

A Comparative Study of Confined and Eruptive Flares in NOAA AR 10720

We investigate the distinct properties of two types of flares: eruptive flares associated with coronal mass ejections (CMEs) and confined flares without CMEs. Our study sample includes nine M- and X-class flares, all from the same active region (AR), six of which are confined and three others which are eruptive. The confined flares tend to be more impulsive in the soft X-ray time profiles and show slenderer shapes in the Extreme-ultraviolet Imaging Telescope?195 ? images, while the eruptive ones are long-duration events and show much more extended brightening regions. The location of the confined flares is closer to the center of the AR, while the eruptive flares are at the outskirts. This difference is quantified by the displacement parameter, which is the distance between the AR center and the flare location; the average displacement of the six confined flares is 16?Mm, while that of the eruptive ones is as large?as 39?Mm. Further, through nonlinear force-free field extrapolation, we find that the decay index of the transverse magnetic field in the low corona (~10?Mm) is larger for eruptive flares than for confined ones. In addition, the strength of the transverse magnetic field over the eruptive flare sites is weaker than it is over the confined ones. These results demonstrate that the strength and the decay index of the background magnetic field may determine whether or not a flare is eruptive or confined. The implication of these results on CME models is discussed in the context of torus instability of the flux rope.

Flaring Loop Motion and a Unified Model for Solar Flares

We performed 2.5-dimensional numerical simulations of magnetic reconnection for several models, some with the reconnection point at a high altitude (the X-type point in magnetic reconnection), and one with the reconnection point at a low altitude. In the high-altitude cases, the bright loop appears to rise for a long time, with its two footpoints separating and the —eld lines below the bright loop shrinking, which are all typical features of two-ribbon —ares. The rise speed of the loop and the separation speed of its footpoints depend strongly on the magnetic —eld to a medium extent on the density and B 0 , o 0 , weakly on the temperature the resistivity g, and the length scale by which the size of current T 0 , L 0 , sheet and the height of the X-point are both scaled. The strong dependence means that the Lorentz B 0 force is the dominant factor; the inertia of the plasma may account for the moderate dependence; o 0 and the weak g dependence may imply that ii fast reconnection ˇˇ occurs; the weak dependence implies L 0 that the —aring loop motion has geometrical self-similarity. In the low-altitude case, the bright loops cease rising only a short time after the impulsive phase of the reconnection and then become rather stable, which shows a distinct similarity to the compact —ares. The results imply that the two types of solar —ares, i.e., the two-ribbon —ares and the compact ones, might be uni—ed into the same magnetic reconnection model, where the height of the reconnection point leads to the bifurcation. Subject headings: MHDmethods: numericalSun: —aresSun: magnetic —elds

FORMATION OF A DOUBLE-DECKER MAGNETIC FLUX ROPE IN THE SIGMOIDAL SOLAR ACTIVE REGION 11520

In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line a half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s(-1). The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About 2 hr before the eruption, indications of a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region.

TWIST ACCUMULATION AND TOPOLOGY STRUCTURE OF A SOLAR MAGNETIC FLUX ROPE

To study the buildup of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and topology structure of the three-dimensional (3D) magnetic field, which is derived by the nonlinear force-free field model. The Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (– 16.47 ± 3.52) × 1040 Mx2 hr–1. We find that only about 1.8% of the injected magnetic helicity became the internal helicity of the magnetic flux rope, whose twist increasing rate was –0.18 ± 0.08 Turns hr–1. The quasi-separatrix layers (QSLs) of the 3D magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs.

Recurrent coronal jets induced by repetitively accumulated electric currents

Context. Jets of plasma are frequently observed in the solar corona. A self-similar recurrent behavior is observed in a fraction o f them. Aims. Jets are thought to be a consequence of magnetic reconnection, however, the physics involved is not fully understood. Therefore, we study some jet observations with unprecedented temporal and spatial resolutions. Methods. The extreme-ultraviolet (EUV) jets were observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory(SDO). The Helioseismic and Magnetic Imager (HMI) on board SDO measured the vector magnetic field, from which we derive the magnetic flux evolution, the photosp heric velocity field, and the vertical electric current evol ution. The magnetic configuration before the jets is derived by the nonl inear force-free field (NLFFF) extrapolation. Results. Three EUV jets recurred in about one hour on 2010 September 17 in the following magnetic polarity of active region 11106. We derive that the jets are above a pair of parasitic magnetic bipoles which are continuously driven by photospheric diverging flows. The interaction drove the build up of electric currents that we indeed observed as elongated patterns at the photospheric level. For the first time, the high temporal cadence of HMI allows to follow t he evolution of such small currents. In the jet region, we found that the integrated absolute current peaks repetitively in phas e with the 171 A flux evolution. The current build up and its dec ay are both fast, about 10 minutes each, and the current maximum precedes the 171 A by also about 10 minutes. Then, HMI temporal cadence is marginally fast enough to detect such changes. Conclusions. The photospheric current pattern of the jets is found associ ated to the quasi-separatrix layers deduced from the magnetic extrapolation. From previous theoretical results, the obs erved diverging flows are expected to build continuously suc h currents. We conclude that magnetic reconnection occurs periodically, in the current layer created between the emerging bipoles and the large scale active region field. It induced the observed recurrent coron al jets and the decrease of the vertical electric current mag nitude.

TRACKING THE EVOLUTION OF A COHERENT MAGNETIC FLUX ROPE CONTINUOUSLY FROM THE INNER TO THE OUTER CORONA

The magnetic flux rope (MFR) is believed to be the underlying magnetic structure of coronal mass ejections (CMEs). However, it remains unclear how an MFR evolves into and forms the multi-component structure of a CME. In this paper, we perform a comprehensive study of an extreme-ultraviolet (EUV) MFR eruption on 2013 May 22 by tracking its morphological evolution, studying its kinematics, and quantifying its thermal property. As EUV brightenings begin, the MFR starts to rise slowly and shows helical threads winding around an axis. Meanwhile, cool filamentary materials descend spirally down to the chromosphere. These features provide direct observational evidence of intrinsically helical structure of the MFR. Through detailed kinematical analysis, we find that the MFR evolution has two distinct phases: a slow rise phase and an impulsive acceleration phase. We attribute the first phase to the magnetic reconnection within the quasi-separatrix layers surrounding the MFR, and the much more energetic second phase to the fast magnetic reconnection underneath the MFR. We suggest that the transition between these two phases is caused by the torus instability. Moreover, we identify that the MFR evolves smoothly into the outer corona and appears as a coherent structure within the white-light CME volume. The MFR in the outer corona was enveloped by bright fronts that originated from plasma pile-up in front of the expanding MFR. The fronts are also associated with the preceding sheath region followed by the outmost MFR-driven shock.

IMAGING AND SPECTROSCOPIC DIAGNOSTICS ON THE FORMATION OF TWO MAGNETIC FLUX ROPES REVEALED BY SDO/AIA AND IRIS

Helical magnetic flux rope (MFR) is a fundamental structure of corona mass ejections (CMEs) and has been discovered recently to exist as a sigmoidal channel structure prior to its eruption in the extreme ultraviolet (EUV) high temperature passbands of the Atmospheric Imaging Assembly (AIA). However, when and where the MFR is built up are still elusive. In this paper, we investigate two MFRs (MFR1 and MFR2) in detail, whose eruptions produced two energetic solar flares and CMEs on 2014 April 18 and 2014 September 10, respectively. The AIA EUV images reveal that for a long time prior to their eruption, both MFR1 and MFR2 are under formation, which is probably through magnetic reconnection between two groups of sheared arcades driven by the shearing and converging flows in the photosphere near the polarity inversion line. At the footpoints of the MFR1, the \textit{Interface Region Imaging Spectrograph} Si IV, C II, and Mg II lines exhibit weak to moderate redshifts and a non-thermal broadening in the pre-flare phase. However, a relatively large blueshift and an extremely strong non-thermal broadening are found at the formation site of the MFR2. These spectral features consolidate the proposition that the reconnection plays an important role in the formation of MFRs. For the MFR1, the reconnection outflow may propagate along its legs, penetrating into the transition region and the chromosphere at the footpoints. For the MFR2, the reconnection probably takes place in the lower atmosphere and results in the strong blueshift and non-thermal broadening for the Mg II, C II, and Si IV lines.