Subject will be restored when possible |
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Wei Liu Submitted: 2008-05-07 06:37
We present a detailed imaging and spectroscopic study of the conjugate hard X-ray (HXR) footpoints (FPs) observed with RHESSI in the 2003 October 29 X10 flare. The double FPs first move toward and then away from each other, mainly parallel and perpendicular to the magnetic neutral line, respectively. The transition of these two phases of FP unshearing motions coincides with the direction reversal of the motion of the loop-top (LT) source, and with the minima of the estimated loop length and LT height. The FPs show temporal correlations in HXR flux, spectral index, and magnetic field strength. The HXR flux exponentially correlates with the magnetic field strength which also anti-correlates with the spectral index before the second HXR peak's maximum, suggesting that particle acceleration sensitively depends on the magnetic field strength and/or reconnection rate. Asymmetries are observed between the FPs: on average, the eastern FP is 2.2 times brighter in HXR flux and 1.8 times weaker in magnetic field strength, and moves 2.8 times faster away from the neutral line than the western FP; the estimated coronal column density to the eastern FP from the LT source is 1.7 times smaller. The two FPs have marginally different spectral indexes. The eastern-to-western FP HXR flux ratio and magnetic field strength ratio are anti-correlated only before the second HXR peak's maximum. Neither magnetic mirroring nor column density alone can explain these observations when taken together, but their combination, together with other transport effects, may play a role.
Authors: Wei Liu, Vahé Petrosian, Brian R. Dennis, and Gordon D. Holman
Projects: RHESSI
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Publication Status: Submitted to ApJ, available at astro-ph/arXiv:0805.1055
Last Modified: 2008-09-23 21:02
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Subject will be restored when possible |
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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, Vahé Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI
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Publication Status: Submitted to ApJ, available at astro-ph/arXiv:0709.1963
Last Modified: 2007-09-13 12:41
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Subject will be restored when possible |
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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, Vahé Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI
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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 |
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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, Vahé Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI
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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 |
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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 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, Vahé Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI
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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 |
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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 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, Vahé Petrosian, Brian R. Dennis, and Yan Wei Jiang
Projects: RHESSI
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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 |
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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: Vahé Petrosian)
Projects: RHESSI
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Publication Status: 2006, Ph.D. Thesis, Stanford University (10 chapters, 224 pages)
Last Modified: 2006-12-21 09:54
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