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Observation of An Evolving Magnetic Flux Rope Prior To and During A Solar Eruption  

Jie Zhang   Submitted: 2012-03-21 01:51

Explosive energy release is a common phenomenon occurring in magnetized plasma systems ranging from laboratories, Earth's magnetosphere, the solar corona and astrophysical environments. Its physical explanation is usually attributed to magnetic reconnection in a thin current sheet. Here we report the important role of magnetic flux rope structure, a volumetric current channel, in producing explosive events. The flux rope is observed as a hot channel prior to and during a solar eruption from the Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamic Observatory (SDO). It initially appears as a twisted and writhed sigmoidal structure with a temperature as high as 10 MK and then transforms toward a semi-circular shape during a slow rise phase, which is followed by fast acceleration and onset of a flare. The observations suggest that the instability of the magnetic flux rope trigger the eruption, thus making a major addition to the traditional magnetic-reconnection paradigm.

Authors: J. Zhang, X. Cheng, and M. D. Ding
Projects: SDO-AIA

Publication Status: Nature Communications www.nature.com/naturecommunications
Last Modified: 2012-03-21 13:00
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Identification of Solar Sources of Major Geomagnetic Storms Between 1996 and 2000  

Jie Zhang   Submitted: 2002-11-22 09:38

This paper presents identification of solar coronal mass ejection (CME) sources for 27 major geomagnetic storms (defined by Dst leq -100 nT) occurring between 1996 and 2000. Observations of CMEs and their solar surface origins are obtained from the Large-Angle and Spectrometric Coronagraph (LASCO) and the Extreme-ultraviolet Imaging Telescope (EIT) instruments on the SOHO spacecraft. Our identification has two steps. The first step is to select candidate front-side halo CMEs (FSH CMEs) using a fixed 120-hour time window. The second step is to use solar wind data to provide further constraints, e.g., an adaptive time window defined based on the solar wind speed of the corresponding Interplanetary CMEs (ICMEs). We finally find that 16 of the 27 (59%) major geomagnetic storms are identified with unique FSH CMEs. Six of the 27 events (22%) are associated with multiple FSH CMEs. These six events show complex solar wind flows and complex geomagnetic activity, which are probably the result of multiple halo CMEs interacting in interplanetary space. A complex event occurs when multiple FSH CMEs are produced within a short period. Four (15%) of the 27 events are associated with partial halo gradual CMEs emerging from the east limb. The surface origin of these events are not known because of a lack of any EIT signature. We believe that they are longitudinally extended CMEs having a component moving along the Sun-Earth connection line. One of the 27 major geomagnetic storms is caused by a Co-rotating Interaction Region. We find an asymmetry in the longitudinal distribution of solar source region for the CMEs responsible for major geomagnetic storms. They are more likely to originate from the western hemisphere than from the eastern hemisphere. In terms of latitude, most geo-effective CMEs originate within a latitude strip of pm 30 degree. The average transit time for a solar CME to arrive at the near-Earth space is found to be 64 hours, while it takes 78 hours on average to reach the peak of the geomagnetic storm. There is a correlation between CME transit time from the Sun to the near-Earth space (T in unit of hour) and CME initial velocity (V in unit of km { m s-1}) at the Sun, which can be simply described as T=96-V/21. We also find that, while these geo-effective CMEs are either full halo CMEs (67%) or partial halo CMEs (30%), there is no preference for them to be fast CMEs, or to be associated with major flares and erupting filaments.

Authors: J. Zhang, K.P. Dere, R.A. Howard, V. Bothmer
Projects:

Publication Status: Appear in Astrophysical Journal, Vol 582, Jan., 2003
Last Modified: 2002-11-25 10:43
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On the Temporal Relationship Between Coronal Mass Ejections and Flares  

Jie Zhang   Submitted: 2000-12-21 14:26

The temporal relationship between coronal mass ejections (CMEs) and associated solar flares is of great importance to understand the origin of CMEs, but has been difficult to study due to the nature of CME detection. In this paper, we investigate this issue using LASCO (Large-Angle and Spectrometric Coronagraph) and EIT (Extreme-ultraviolet Imaging Telescope) observations combined with GOES (Geosynchronous Operational Environmental Satellites) soft X-ray observations. We present four well-observed events whose source regions are close to the limb such that we are able to directly measure the CMEs' initial evolution in the low coronal (sim 1-3 Rsun); this height range was not available in previous space-based coronagraph observations. The velocity profiles of the CMEs show that the kinematic evolution of a typical CME can be described in a three-phase scenario: the initiation phase, impulsive acceleration phase and propagation phase. The initiation phase is characterized by a slow ascension with a speed less than 80 km s-1 for a period of tens of minutes. The initiation phase always occurs before the onset of the associated flare. Following the initiation phase, the CME display an impulsive acceleration phase which coincides very well with the rise phase of the flare lasting for a few to tens of minutes. The acceleration of CMEs ceases near the time of the soft X-ray flare peak. The CMEs then undergo a propagation phase which The CMEs then undergo a propagation phase which is characterized by a constant speed or slowly decreasing in speed. Three of the four CMEs studied have an initiation phase. The acceleration rate in the impulsive phase of these three CMEs is in the range of 100 - 500 { m m s-2}. One CME (on Nov. 6, 1997, associated with an X9.4 flare) does not show an initiation phase. It has an extremely large acceleration rate of 7300 { m m s-2}. The possible causes of CME initiation and acceleration in connection with flares are explored.

Authors: Zhang, J., Dere, K.P., Howard, R.A., Kundu, M.R, White, S.M.
Projects:

Publication Status: ApJ, submitted, Dec . 2000
Last Modified: 2000-12-21 15:50
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Abstracts by Author
Observation of An Evolving Magnetic Flux Rope Prior To and During A Solar Eruption
Identification of Solar Sources of Major Geomagnetic Storms Between 1996 and 2000
On the Temporal Relationship Between Coronal Mass Ejections and Flares

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