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Observation of Ultrafine Channels of Solar Corona Heating  

Haisheng Ji   Submitted: 2012-04-04 19:17

We report the first direct observations of dynamical events originating in the sun's 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 meter aperture NST at BBSO, and satellite data from AIA on board SDO. The NST imaging observations in Helium I 10830~AA, 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 sun's ubiquitous, convectively driven granules. In many cases, AIA/SDO detects co-spatial and co-temporal 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 blue-shifted events in simultaneous H_alpha 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 co-spatial mass flows.

Authors: Haisheng Ji, Wenda Cao, Philip R. Goode
Projects: SDO-AIA

Publication Status: ApJ Letter in press
Last Modified: 2012-04-10 11:31
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Converging motion of conjugate flaring kernels during two large solar flares  

Haisheng Ji   Submitted: 2007-05-07 21:36

In this paper, we analyze the footpoint motion of two large solar flares using observations made by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The two flares are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They are both classical two-ribbon flares as observed in TRACE 1600 or 171 Å images and have long-duration conjugate hard X-ray (HXR) footpoint emission. We use the `center-of-mass' method to locate the centroids of the UV/EUV flare ribbons. The results are: 1) The conjugate UV/EUV ribbons and HXR footpoints of the two flares show a converging (inward) motion during the impulsive phase. For the two flares, the converging motion lasts about 3 and 10 minutes respectively. The usual separation (outward) motion for the flare ribbons and footpoints take place only after the converging motion. 2) During the inward and the outward motion, the conjugate ribbons and footpoints of the two events exhibit a strong unshear motion. In obtaining above results, TRACE UV/EUV and RHESSI HXR data show an overall agreement. The two events demonstrate that the magnetic reconnection for the flares occurs in highly sheared magnetic field. Furthermore, the results support the magnetic model constructed by Ji et al. (2007), who proposed that the contracting motion of flaring loops is the signature of the relaxation of sheared magnetic fields.

Authors: T. Zhou, H. Ji, and G. Huang
Projects: RHESSI

Publication Status: Advances in Space Research, in press
Last Modified: 2007-05-08 07:15
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The relaxation of sheared magnetic fields - a contracting process  

Haisheng Ji   Submitted: 2007-01-31 22:26

In recent years, several authors have reported a contracting motion for solar flaring loops in many flares. That is, during the early impulsive phase of solar flares, hard X-ray (HXR) looptop sources or radio/extreme-ultraviolet (EUV) flaring loops have a descending or shrinking motion and, at the same time, Hα ribbons or HXR footpoints (FPs) are converging. Only after the impulsive phase does there begin to appear an upward motion for the looptop sources and flaring loops and a corresponding outward motion (the usual separation motion) for the flare ribbons or FPs. So far, the contracting motion cannot be fully explained in a 2D flare model. The recent high-cadence Hα observation of an M-class flare made by Ji et al. (2006) at GanYu Solar Station of Purple Mountain Observatory may provide a clue leading to a proper understanding. The observation shows that the shear angle of the flare decreases steadily during both the contraction and expansion phases of the flaring loops. In this paper, we provide supporting observational evidence by giving a detailed analysis of the M6.8 class flare of June 17, 2003. For this flare, the EUV flaring loops, Hα ribbons, and HXR looptop source show well-correlated contraction and subsequent expansion. The flare shear of this event, indicated by Hα ribbons, HXR FPs, and EUV flaring loops have a steady decrease throughout. This new solar flare phenomenon implies magnetic reconnection in a sheared magnetic field. Therefore, 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.

Authors: Haisheng Ji, Guangli Huang, and Haimin Wang
Projects: RHESSI

Publication Status: ApJ accepted
Last Modified: 2007-02-01 09:30
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Fast relaxation of a sheared magnetic field during converging motion of Hα and HXR conjugate kernels  

Haisheng Ji   Submitted: 2005-12-04 19:00

In this Letter, we present the results from high-cadence (~ 40 ms) Hα blue wing observation of an M1.1 class solar flare, which occurred in NOAA Active Region 10687 on 2004 November 1. In collaboration with RHESSI, the observation was made with the Hα Fine Structure Telescope at GanYu Solar Station of Purple Mountain Observatory. For this flare, a pair of conjugate Hα kernels shows the kind of converging motion during the impulsive phase. After the impulsive phase, there appears normal separation motion. The motion of one Hα kernel is perpendicular to the magnetic neutral line (NL), while another kernel's 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 above features are conirmed with 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 Hα observations. Both kinds of time profile show a good correlation with RHESSI HXR light curve in the higher energy range (>= 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 second cadence, we obtained the time pro le of the radio source's velocity using the same method as we used with Hα images. The velocity-time curve of the microwave source shows a good correlation with that obtained from the two Hα kernels. {If you have problem to download the preprint, please contact Haisheng Ji: jihs@mail.pmo.ac.cn, he will send you a pdf file.}

Authors: H. Ji, G. Huang, H. Wang, T. Zhou, Y. Li, Y. Zhang, M. Song
Projects: RHESSI

Publication Status: ApJ Lett, accepted
Last Modified: 2005-12-04 19:00
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Abstracts by Author
Observation of Ultrafine Channels of Solar Corona Heating
Converging motion of conjugate flaring kernels during two large solar flares
The relaxation of sheared magnetic fields - a contracting process
Fast relaxation of a sheared magnetic field during converging motion of H-alpha and HXR conjugate kernels

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