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Wei Liu   Submitted: 2007-09-12 15:56

We present data analysis and interpretation of an M1.4-class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}) on April 30, 2002. This event, with its footpoints occulted by the solar limb, exhibits a rarely observed, but theoretically expected, double-source structure in the corona. The two coronal sources, observed over the 6-30 keV range, appear at different altitudes and show energy-dependent structures with the higher-energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly {it nonthermal}, while the emission at lower energies in the outer region is primarily {it thermal}. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra above about 20 keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bi-directional outflows of the released energy in the form of turbulence and/or particle beams away from the reconnection site can be the source of the observed radiation. The spatially resolved thermal emission below about 15 keV, on the other hand, indicates that the lower source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the differences in the magnetic topologies and densities of particles and turbulence between the two sources.

Authors: Wei Liu, Vahe' Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI

Publication Status: Submitted to ApJ, available at astro-ph/arXiv:0709.1963
Last Modified: 2007-09-13 20:53
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Subject will be restored when possible  

Wei Liu   Submitted: 2007-09-12 15:56

We present data analysis and interpretation of an M1.4-class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}) on April 30, 2002. This event, with its footpoints occulted by the solar limb, exhibits a rarely observed, but theoretically expected, double-source structure in the corona. The two coronal sources, observed over the 6-30 keV range, appear at different altitudes and show energy-dependent structures with the higher-energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly {it nonthermal}, while the emission at lower energies in the outer region is primarily {it thermal}. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra above about 20 keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bi-directional outflows of the released energy in the form of turbulence and/or particle beams away from the reconnection site can be the source of the observed radiation. The spatially resolved thermal emission below about 15 keV, on the other hand, indicates that the lower source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the differences in the magnetic topologies and densities of particles and turbulence between the two sources.

Authors: Wei Liu, Vahe' Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI

Publication Status: Submitted to ApJ, available at ADS or astro-ph/arXiv:0709.1963
Last Modified: 2007-10-04 08:30
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Subject will be restored when possible  

Wei Liu   Submitted: 2007-09-12 15:56

We present data analysis and interpretation of an M1.4-class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}) on April 30, 2002. This event, with its footpoints occulted by the solar limb, exhibits a rarely observed, but theoretically expected, double-source structure in the corona. The two coronal sources, observed over the 6-30 keV range, appear at different altitudes and show energy-dependent structures with the higher-energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly {it nonthermal}, while the emission at lower energies in the outer region is primarily {it thermal}. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra above about 20 keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bi-directional outflows of the released energy in the form of turbulence and/or particles from the reconnection site can be the source of the observed radiation. The spatially resolved thermal emission below about 15 keV, on the other hand, indicates that the lower source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the differences in the magnetic field and plasma density of the two sources.

Authors: Wei Liu, Vahe' Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI

Publication Status: Revised version submitted to ApJ, available at ADS or astro-ph/arXiv:0709.1963
Last Modified: 2007-10-31 20:52
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Subject will be restored when possible  

Wei Liu   Submitted: 2007-09-12 15:56

We present data analysis and interpretation of an M1.4-class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}) on April 30, 2002. This event, with its footpoints occulted by the solar limb, exhibits a rarely observed, but theoretically expected, double-source structure in the corona. The two coronal sources, observed over the 6-30 keV range, appear at different altitudes and show energy-dependent structures with the higher-energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly {it nonthermal}, while the emission at lower energies in the outer region is primarily {it thermal}. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra above about 20 keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bi-directional outflows of the released energy in the form of turbulence and/or particles from the reconnection site can be the source of the observed radiation. The spatially resolved thermal emission below about 15 keV, on the other hand, indicates that the lower source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the differences in the magnetic field and plasma density of the two sources.

Authors: Wei Liu, Vahe' Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI

Publication Status: Accepted by ApJ (Dec. 06, 2007), available at ADS or astro-ph/arXiv:0709.1963
Last Modified: 2007-12-07 12:19
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Characteristics of Solar Flare Hard X-ray Emissions: Observations and Models  

Wei Liu   Submitted: 2006-12-21 01:12

The main theme of this thesis is the investigation of the physics of acceleration and transport of particles in solar flares, and their thermal and nonthermal radiative signatures. The observational studies, using hard X-rays (HXRs) observed by the RHESSI mission, concentrate on four flares, which support the classical magnetic reconnection model of solar flares in various ways. In the X3.9 flare occurring on 11/03/2003, there is a monotonic upward motion of the loop top (LT) source accompanied by a systematic increase in the separation of the footpoint (FP) sources at a comparable speed. This is consistent with the reconnection model with an inverted-Y geometry. The 04/30/2002 event exhibits rarely observed two coronal sources. The two sources (with almost identical spectra) show energy-dependent structures, with higher-energy emission being close together. This suggests that reconnection takes place within the region between the sources. In the 10/29/2003 X10 flare, the logarithmic total HXR flux of the FPs correlates with the mean magnetic field. The two FPs show asymmetric HXR fluxes, which is qualitatively consistent with the magnetic mirroring effect. The M1.7 flare on 11/13/2003 reveals evidence of evaporation directly imaged by RHESSI for the first time, in which emission from the legs of the loop appears at intermediate energies. The emission centroid moves toward the LT as time proceeds, indicating an increase of density in the loop. The theoretical modeling of this work combines the stochastic acceleration model with the NRL hydrodynamic model to study the interplay of the particle acceleration, transport, and radiation effects and the atmospheric response to the energy deposition by nonthermal electrons. We find that low-energy electrons in the quasi-thermal portion of the spectrum affects the hydrodynamics by producing more heating in the corona than the previous models that used a power-law spectrum with a low-energy cutoff. The Neupert effect is found to be present and effects of suppression of conduction are tested in the presence of hydrodynamic flows.

Authors: Wei Liu (Adviser: Vahe' Petrosian)
Projects: RHESSI

Publication Status: 2006, Ph.D. Thesis, Stanford University (10 chapters, 224 pages)
Last Modified: 2006-12-21 09:54
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RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003  

Wei Liu   Submitted: 2004-01-20 01:28

We present data analysis and interpretation of a simple X-class flare observed with RHESSI on November 3, 2003. In contrast to other X-class flares observed previously, this flare shows a very simple morphology with well defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward motion of the LT source and increase in separation of the two FP sources are consistent with magnetic reconnection models proposed for solar flares. In addition, we find that the source motions are relatively slower during the more active phases of hard X-ray emission; the emission centroid of the LT source shifts toward higher altitudes with the increase of energy; the separation between the LT emission centroids at two different photon energies is anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic fields could be a possible explanation of these features.

Authors: Wei Liu, Yan Wei Jiang, Siming Liu, Vahe' Petrosian
Projects: RHESSI

Publication Status: ApJL (in press)
Last Modified: 2004-06-16 12:56
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Abstracts by Author
Quasi-periodic Fast-mode Magnetosonic Wave Trains Within Coronal Waveguides Associated with Flares and CMEs
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First SDO AIA Observations of a Global Coronal EUV ''Wave'': Multiple Components and ''Ripples''
An Intriguing Chromospheric Jet Observed by Hinode: I. Fine Structure Kinematics and Evidence of Unwinding Twists
Combined Modeling of Acceleration, Transport, and Hydrodynamic Response in Solar Flares: I. The Numerical Model
Episodic X-ray Emission Accompanying the Activation of an Eruptive Prominence: Evidence of Episodic Magnetic Reconnection
Conjugate Hard X-ray Footpoints in the 2003 October 29 X10 Flare: Unshearing Motions, Correlations, and Asymmetries
Solar Flares as Natural Particle Accelerators: A High-energy View from X-ray Observations and Theoretical Models
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Characteristics of Solar Flare Hard X-ray Emissions: Observations and Models
RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003

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