Haisheng Ji

RAPID CHANGES OF MAGNETIC FIELDS ASSOCIATED WITH SIX X-CLASS FLARES

In this paper, we present the results of the study of six X-class flares. We found significant changes in the photospheric magnetic fields associated with all of the events. For the five events in 2001, when coronagraph data were available, all were associated with halo coronal mass ejections. Based on the analyses of the line-of-sight magnetograms, all six events had an increase in the magnetic flux of the leading polarity of order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive because the changeover time, which we defined as the time to change from preflare to postflare state, ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear, and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops, (2) a change in the magnetic field direction, and (3) the expansion of the sunspot, which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference; i.e., the flux of leading polarity tends to increase while the flux of following polarity tends to decrease slightly.

The Relaxation of Sheared Magnetic Fields: A Contracting Process

In recent years, several authors have reported a contracting motion for solar flaring loops. That is, during the rising phase of solar flares, hard X-ray (HXR) loop-top sources or radio/extreme-ultraviolet (EUV) flaring loops have a descending motion and, at the same time, H alpha ribbons or HXR footpoints (FPs) are converging. The usual expansion motion of flaring loops occurs only after the contraction. So far, the contracting motion cannot be fully explained in a two-dimensional flare model. The recent high-cadence H alpha observation of an M-class flare made by Ji et al. at GanYu Solar Station of Purple Mountain Observatory may provide a clue leading to a proper understanding. The observations show that the flare shear decreases steadily during both the contraction and expansion phases. In this paper, we provide supporting observational evidence by giving a detailed analysis of the M6.8-class flare of 2003 June 17. For this flare, the EUV flaring loops, H alpha ribbons, and HXR loop-top source show well-correlated contraction and subsequent expansion. The flare shear of this event, indicated by H alpha ribbons, HXR FPs, and EUV flaring loops, decreases steadily throughout. The observations apparently imply that the contracting motion of flaring loops may be the result of the relaxation of the sheared magnetic field. In the framework of sheared linear force-free arcades, we establish a quantitative model to show that the release of magnetic energy will reduce magnetic shear of the arcades and less sheared arcades will have smaller height and span.

Converging Motion of Hα Conjugate Kernels: The Signature of Fast Relaxation of a Sheared Magnetic Field

In this Letter, we present the results from a high-cadence (similar to 40 ms) Ha blue-wing observation of an M1.1-class solar flare, which occurred in NOAA AR 10687 on 2004 November 1. In collaboration with RHESSI, the observation was made with the Ha Fine Structure Telescope at the GanYu Solar Station of the Purple Mountain Observatory. For this flare, a pair of conjugate Ha kernels shows a kind of converging motion during the impulsive phase. After the impulsive phase, there appears a normal separation motion. The motion of one Ha kernel is perpendicular to the magnetic neutral line, while another kernels converging shows both perpendicular and parallel components. Nevertheless, the shear angle decreases during the converging motion, clearly showing the relaxation of a sheared magnetic field. All of the above features are confirmed with hard X-ray (HXR) footpoints observed by RHESSI. We also obtained the time profiles of the rate of change of the shear angle and the relative velocity of the two kernels with Ha observations. Both of these time profiles show a good correlation with RHESSI HXR light curves in the higher energy range (greater than or similar to 50 keV). This indicates that, during the peak times of the flare, the relaxation process may have occurred rapidly. This event was also observed by the Nobeyama Radio Heliograph (NoRH), showing a single microwave source. Using NoRH maps at 17 GHz with 1 s cadence, we obtained the time profile of the radio sources velocity using the same method that we used with Ha images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Ha kernels.

Evidence of Rapid Flux Emergence Associated with the M8.7 Flare on 2002 July 26

In this paper, we present a detailed study of the M8.7 flare that occurred on 2002 July 26 using data from the Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the Transition Region and Coronal Explorer (TRACE), and the Solar and Heliospheric Observatory (SOHO). This flare has interesting properties similar to a number of flares that we studied previously, such as a rapid increase of magnetic flux in one polarity and an increase in transverse fields and magnetic shear associated with the flare. However, this event had the most comprehensive observations; in particular, the high-resolution high-cadence BBSO vector magnetograph observations. At the time of the flare, across the flare neutral line, there was a sudden emergence of magnetic flux at the rate of 1020 Mx hr-1 in both the longitudinal and transverse components. The emerging flux mostly occurred at the sites of the flare. It was very inclined and led to impulsively enhanced shear in the magnetic fields. We discuss these observations in the context of magnetic reconnection triggered by rapid flux emergence. It is also possible that the new flux signifies flare-related change in the field line inclination.

OBSERVATIONS AND NONLINEAR FORCE-FREE FIELD MODELING OF ACTIVE REGION 10953

We present multiwavelength observations of a simple bipolar active region (NOAA 10953), which produced several small flares ( mostly B class and one C8.5 class) and filament activations from April 30 to May 3 in 2007. We also explore nonlinear force-free field (NLFFF) modeling of this region prior to the C8.5 flare on May 2, using magnetograph data from SOHO/MDI and Hinode/SOT. A series of NLFFF models are constructed using the flux-rope insertion method. By comparing the modeled field lines with multiple X-ray loops observed by Hinode/XRT, we find that the axial flux of the flux rope in the best-fit models is ( 7 +/- 2) x 10(20) Mx, while the poloidal flux has a wider range of (0.1-10) x 10(10) Mx cm(-1). The axial flux in the best-fit model is well below the upper limit (similar to 15 x 10(20) Mx) for stable force-free configurations, which is consistent with the fact that no successful full filament eruption occurred in this active region. From multiwavelength observations of the C8.5 flare, we find that the X-ray brightenings ( in both RHESSI and XRT) appeared about 20 minutes earlier than the EUV brightenings seen in TRACE 171 angstrom images and filament activations seen in MLSO H alpha images. This is interpreted as an indication that the X-ray emission may be caused by direct coronal heating due to reconnection, and the energy transported down to the chromosphere may be too low to produce EUV brightenings. This flare started from nearly unsheared flare loop, unlike most two-ribbon flares that begin with highly sheared footpoint brightenings. By comparing with our NLFFF model, we find that the early flare loop is located above the flux rope that has a sharp boundary. We suggest that this flare started near the outer edge of the flux rope, not at the inner side or at the bottom as in the standard two-ribbon flare model.

Two Successive Coronal Mass Ejections Driven by the Kink and Drainage Instabilities of an Eruptive Prominence

We describe a clear case of the initiation of a propagating bright arc and a CME on 2002 December 28, which were associated with an eruptive prominence. In EIT 304 and 195 8 images, a very long filament showed evidence of severe twisting in one of its fragments, which appeared as a prominence on December 26; then, the prominence showed the conversion of its twist into writhe. Two days later, the prominence displayed a slow rising motion for hours. Internal twisting and mass motion took place before the rapid acceleration and final eruption. The propagating bright arc and the following CME corresponded to the early rising and the subsequently eruptive phases of the prominence, respectively. Signatures of magnetic reconnection, i.e., a cusp structure and postflare loops in EUV wave bands and hard X-ray sources in the corona, were observed after the prominence eruption. It appears that the kink instability and the mass drainage in the prominence played key roles in triggering the initiation of the CME. However, the rather impulsive acceleration of the CME resulted from magnetic reconnection beneath the filament.

Observation of Ultrafine Channels of Solar Corona Heating

We report the first direct observations of dynamical events originating in the Suns photosphere and subsequently lighting up the corona. Continuous small-scale, impulsive events have been tracked from their origin in the photosphere on through to their brightening of the local corona. We achieve this by combining high-resolution ground-based data from the 1.6 m aperture New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), and satellite data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The NST imaging observations in helium I 10830 angstrom reveal unexpected complexes of ultrafine, hot magnetic loops seen to be reaching from the photosphere to the base of the corona. Most of these ultrafine loops are characterized by an apparently constant, but surprisingly narrow diameter of about 100 km all along each loop, and the loops originate on the solar surface from intense, compact magnetic field elements. The NST observations detect the signature of upward injections of hot plasma that excite the ultrafine loops from the photosphere to the base of the corona. The ejecta have their individual footpoints in the intergranular lanes between the Suns ubiquitous, convectively driven granules. In many cases, AIA/SDO detects cospatial and cotemporal brightenings in the overlying, million degree coronal loops in conjunction with the upward injections along the ultrafine loops. Segments of some of the more intense upward injections are seen as rapid blueshifted events in simultaneous Ha blue wing images observed at BBSO. In sum, the observations unambiguously show impulsive coronal heating events from upward energy flows originating from intergranular lanes on the solar surface accompanied by cospatial mass flows.

A Hard X-Ray Sigmoidal Structure during the Initial Phase of the 2003 October 29 X10 Flare

We find a hard X-ray (HXR) sigmoidal ( S-shaped) structure observed by RHESSI between 6 and 150 keV during the initial phase of the X10 flare of 2003 October 29. Its counterparts are seen with the Solar X-Ray Imager and TRACE. The flare evolves from a sigmoid to an arcade phase as observed in TRACE 195 8 images. According to the spatial structure of HXR emission, the flare process can be divided into two different phases. During the first phase, HXR emission in different energy ranges shares a similar sigmoidal evolving structure. The structure appears to contract initially as shown by the time profile of the separation between the two footpoints ( FPs) at the ends of the HXR sigmoid. During the second phase, HXR emission in the lower energy range (less than or similar to 30 keV) evolves into two sources located along the neutral line. Meanwhile, the FPs in the higher energies (greater than or similar to 30 keV) move apart as usual. During the whole flaring process, the value of the flare shear, defined as the angle between the line connecting two FPs and the line perpendicular to the neutral line, decreases steadily. We conclude that the flare was triggered by magnetic reconnection near the center of the sigmoid. The converging and unshearing motion during the sigmoid period is explained by continuing magnetic reconnection driven by the erupting sigmoidal flux rope. The reconnection progresses from highly sheared magnetic field lines to less sheared field lines surrounding the flux rope.

Witnessing magnetic twist with high-resolution observation from the 1.6-m New Solar Telescope

Magnetic flux ropes are highly twisted, current-carrying magnetic fields. They are crucial for the instability of plasma involved in solar eruptions, which may lead to adverse space weather effects. Here we present observations of a flaring using the highest resolution chromospheric images from the 1.6-m New Solar Telescope at Big Bear Solar Observatory, supplemented by a magnetic field extrapolation model. A set of loops initially appear to peel off from an overall inverse S-shaped flux bundle, and then develop into a multi-stranded twisted flux rope, producing a two-ribbon flare. We show evidence that the flux rope is embedded in sheared arcades and becomes unstable following the enhancement of its twists. The subsequent motion of the flux rope is confined due to the strong strapping effect of the overlying field. These results provide a first opportunity to witness the detailed structure and evolution of flux ropes in the low solar atmosphere.

Blobs in recurring extreme-ultraviolet jets

In this paper, we report our discovery of blobs in the recurrent and homologous jets that occurred at the western edge of NOAA active region 11259 on 2011 July 22. The jets were observed in the seven extreme-ultraviolet (EUV) filters of the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). Using the base-difference images of the six filters (94, 131, 171, 211, 193, and 335 {\AA}), we carried out the differential emission measure (DEM) analyses to explore the thermodynamic evolutions of the jets. The jets were accompanied by cool surges observed in the H