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Quasi-periodic Fast-mode Magnetosonic Wave Trains Within Coronal Waveguides Associated with Flares and CMEs  

Wei Liu   Submitted: 2015-12-22 22:42

Quasi-periodic, fast-mode, propagating wave trains (QFPs) are a new observational phenomenon recently discovered in the solar corona by the Solar Dynamics Observatory with extreme ultraviolet (EUV) imaging observations. They originate from flares and propagate at speeds up to ~2000 km s-1 within funnel-shaped waveguides in the wakes of coronal mass ejections (CMEs). QFPs can carry sufficient energy fluxes required for coronal heating during their occurrences. They can provide new diagnostics for the solar corona and their associated flares. We present recent observations of QFPs focusing on their spatio-temporal properties, temperature dependence, and statistical correlation with flares and CMEs. Of particular interest is the 2010-Aug-01 C3.2 flare with correlated QFPs and drifting zebra and fiber radio bursts, which might be different manifestations of the same fast-mode wave trains. We also discuss the potential roles of QFPs in accelerating and/or modulating the solar wind.

Authors: Wei Liu, Leon Ofman, Brittany Broder, Marian Karlický, and Cooper Downs
Projects: IRIS,RHESSI,SDO-AIA

Publication Status: Accepted to the Proceedings of the 14th International Solar Wind Conference
Last Modified: 2015-12-23 12:23
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First High-resolution Spectroscopic Observations of an Erupting Prominence Within a Coronal Mass Ejection by the Interface Region Imaging Spectrograph (IRIS)  

Wei Liu   Submitted: 2015-02-18 09:56

Spectroscopic observations of prominence eruptions associated with coronal mass ejections (CMEs), although relatively rare, can provide valuable plasma and 3D geometry diagnostics. We report the first observations by the Interface Region Imaging Spectrograph (IRIS) mission of a spectacular fast CME/prominence eruption associated with an equivalent X1.6 flare on 2014 May 9. The maximum plane-of-sky and Doppler velocities of the eruption are 1200 and 460 km s-1, respectively. There are two eruption components separated by ~200 km s-1 in Doppler velocity: a primary, bright component and a secondary, faint component, suggesting a hollow, rather than solid, cone-shaped distribution of material. The eruption involves a left-handed helical structure undergoing counter-clockwise (viewed top-down) unwinding motion. There is a temporal evolution from upward eruption to downward fallback with less-than-free-fall speeds and decreasing nonthermal line widths. We find a wide range of Mg II k/h line intensity ratios (less than ~2 expected for optically-thin thermal emission): the lowest ever-reported median value of 1.17 found in the fallback material and a comparably high value of 1.63 in nearby coronal rain and intermediate values of 1.53 and 1.41 in the two eruption components. The fallback material exhibits a strong (>5σ) linear correlation between the k/h ratio and the Doppler velocity as well as the line intensity. We demonstrate that Doppler dimming of scattered chromospheric emission by the erupted material can potentially explain such characteristics.

Authors: Wei Liu, Bart De Pontieu, Jean-Claude Vial, Alan M. Title, Mats Carlsson, Han Uitenbroek, Takenori J. Okamoto, Thomas E. Berger, Patrick Antolin
Projects: IRIS,SDO-AIA

Publication Status: Accepted by ApJ (Feb. 15, 2015), on arxiv since 2015/02/17 - http://arxiv.org/abs/1502.04738
Last Modified: 2015-02-18 13:22
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Coronal Condensation in Funnel Prominences as Return Flows of the Chromosphere-Corona Mass Cycle  

Wei Liu   Submitted: 2014-04-03 19:04

We present SDO/AIA observations of a potentially novel type of prominence, called "funnel prominence", that forms out of coronal condensation at magnetic dips. They can drain a large amount of mass (up to 1015 g/day) and may play an important role as return flows of the recently recognized chromosphere-corona mass cycle. We compare funnel prominences with large-scale polar crown prominences, cloud prominences or coronal spiders, as well as coronal rain.

Authors: Wei Liu, Thomas E. Berger, B. C. Low
Projects: SDO-AIA

Publication Status: Published 01/2014, IAU Sympos., Vol. 300, pp. 441, DOI: 10.1017/S1743921313011460
Last Modified: 2014-04-07 10:55
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Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review)  

Wei Liu   Submitted: 2014-04-02 11:21

Global extreme ultraviolet (EUV) waves are spectacular traveling disturbances in the solar corona associated with energetic eruptions such as coronal mass ejections (CMEs) and flares. Over the past 15 years, observations from three generations of space-borne EUV telescopes have shaped our understanding of this phenomenon and at the same time led to controversy about its physical nature. Since its launch in 2010, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) has observed more than 210 global EUV waves in exquisite detail, thanks to its high spatio-temporal resolution and full-disk, wide-temperature coverage. A combination of statistical analysis of this large sample, 30 some detailed case studies, and data-driven MHD modeling, has been leading their physical interpretations to a convergence, favoring a bimodal composition of an outer, fast-mode magnetosonic wave component and an inner, non-wave CME component. Adding to this multifaceted picture, AIA has also discovered new EUV wave and wave-like phenomena associated with various eruptions, including quasi-periodic fast propagating (QFP) wave trains, magnetic Kelvin-Helmholtz instabilities (KHI) in the corona and associated nonlinear waves, and a variety of mini EUV waves. Seismological applications using such waves are now being actively pursued, especially for the global corona. We review such advances in EUV wave research focusing on recent SDO/AIA observations, their seismological applications, related data analysis techniques, and numerical and analytical models.

Authors: Wei Liu, Leon Ofman
Projects: SDO-AIA

Publication Status: Accepted to Solar Physics (Topical Issue, "Exploring the Network of SDO Science")
Last Modified: 2014-04-02 12:11
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Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review)  

Wei Liu   Submitted: 2014-04-02 11:21

Global extreme ultraviolet (EUV) waves are spectacular traveling disturbances in the solar corona associated with energetic eruptions such as coronal mass ejections (CMEs) and flares. Over the past 15 years, observations from three generations of space-borne EUV telescopes have shaped our understanding of this phenomenon and at the same time led to controversy about its physical nature. Since its launch in 2010, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) has observed more than 210 global EUV waves in exquisite detail, thanks to its high spatio-temporal resolution and full-disk, wide-temperature coverage. A combination of statistical analysis of this large sample, 30 some detailed case studies, and data-driven MHD modeling, has been leading their physical interpretations to a convergence, favoring a bimodal composition of an outer, fast-mode magnetosonic wave component and an inner, non-wave CME component. Adding to this multifaceted picture, AIA has also discovered new EUV wave and wave-like phenomena associated with various eruptions, including quasi-periodic fast propagating (QFP) wave trains, magnetic Kelvin-Helmholtz instabilities (KHI) in the corona and associated nonlinear waves, and a variety of mini EUV waves. Seismological applications using such waves are now being actively pursued, especially for the global corona. We review such advances in EUV wave research focusing on recent SDO/AIA observations, their seismological applications, related data analysis techniques, and numerical and analytical models.

Authors: Wei Liu, Leon Ofman
Projects: SDO-AIA

Publication Status: Published in Solar Physics Apr 29, 2014 (Topical Issue, The Many Scales of Solar Activity in Solar Cycle 24 as seen by SDO)
Last Modified: 2014-05-06 15:18
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Plasmoid Ejections and Loop Contractions in an Eruptive M7.7 Solar Flare: Evidence of Particle Acceleration and Heating in Magnetic Reconnection Outflows  

Wei Liu   Submitted: 2013-03-05 20:53

Where particle acceleration and plasma heating take place in relation to magnetic reconnection is a fundamental question for solar flares. We report analysis of an M7.7 flare on 2012 July 19 observed by SDO/AIA and RHESSI. Bi-directional outflows in forms of plasmoid ejections and contracting cusp-shaped loops originate between an erupting flux rope and underlying flare loops at speeds of typically 200-300 km s-1 up to 1050 km s-1. These outflows are associated with spatially separated double coronal X-ray sources with centroid separation decreasing with energy. The highest temperature is located near the nonthermal X-ray loop-top source well below the original heights of contracting cusps near the inferred reconnection site. These observations suggest that the primary loci of particle acceleration and plasma heating are in the reconnection outflow regions, rather than the reconnection site itself. In addition, there is an initial ascent of the X-ray and EUV loop-top source prior to its recently recognized descent, which we ascribe to the interplay among multiple processes including the upward development of reconnection and the downward contractions of reconnected loops. The impulsive phase onset is delayed by 10~minutes from the start of the descent, but coincides with the rapid speed increases of the upward plasmoids, the individual loop shrinkages, and the overall loop-top descent, suggestive of an intimate relation of the energy release rate and reconnection outflow speed.

Authors: Wei Liu, Qingrong Chen, Vahe' Petrosian
Projects: RHESSI,SDO-AIA

Publication Status: Accepted by ApJ, March 04, 2013
Last Modified: 2013-03-18 12:25
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Quasi-periodic Fast-mode Wave Trains Within a Global EUV Wave and Sequential Transverse Oscillations Detected by SDO/AIA  

Wei Liu   Submitted: 2012-04-25 09:50

We present the first unambiguous detection of quasi-periodic wave trains within the broad pulse of a global EUV wave (so-called ''EIT wave'') occurring on the limb. These wave trains, running ahead of the lateral CME front of 2-4 times slower, coherently travel to distances >Rsun/2 along the solar surface, with initial velocities up to 1400 km s-1 decelerating to ~650 km s-1. The rapid expansion of the CME initiated at an elevated height of 110 Mm produces a strong downward and lateral compression, which may play an important role in driving the primary EUV wave and shaping its front forwardly inclined toward the solar surface. The waves have a dominant 2 min periodicity that matches the X-ray flare pulsations, suggesting a causal connection. The arrival of the leading EUV wave front at increasing distances produces an uninterrupted chain sequence of deflections and/or transverse (likely fast kink mode) oscillations of local structures, including a flux-rope coronal cavity and its embedded filament with delayed onsets consistent with the wave travel time at an elevated (by ~50%) velocity within it. This suggests that the EUV wave penetrates through a topological separatrix surface into the cavity, unexpected from CME caused magnetic reconfiguration. These observations, when taken together, provide compelling evidence of the fast-mode MHD wave nature of the {it primary (outer) fast component} of a global EUV wave, running ahead of the {it secondary (inner) slow} component of CME-caused restructuring.

Authors: Wei Liu, Leon Ofman, Nariaki V. Nitta, Markus J. Aschwanden, Carolus J. Schrijver, Alan M. Title, and Theodore D. Tarbell
Projects: SDO-AIA

Publication Status: Accepted by ApJ, Apr 24, 2012
Last Modified: 2012-04-25 12:18
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First SDO/AIA Observation of Solar Prominence Formation Following an Eruption: Magnetic Dips and Sustained Condensation and Drainage  

Wei Liu   Submitted: 2012-01-04 11:43

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Wei Liu, Thomas E. Berger, and B. C. Low
Projects: SDO-AIA

Publication Status: ApJ Letters, Vol. 745, January 2012, in press
Last Modified: 2012-01-04 12:14
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Direct Imaging by SDO/AIA of Quasi-periodic Propagating Fast Mode Magnetosonic Waves Propagating of ~2000 km s-1 in the Low Solar Corona  

Wei Liu   Submitted: 2011-06-15 12:22

Quasi-periodic, propagating fast mode magnetosonic waves in the corona weredifficult to observe in the past due to relatively low instrument cadences. Wereport here evidence of such waves directly imaged in EUV by the new SDO AIAinstrument. In the 2010 August 1 C3.2 flare/CME event, we find arc-shaped wavetrains of 1-5% intensity variations (lifetime ~200 s) that emanate near theflare kernel and propagate outward up to ~400 Mm along a funnel of coronalloops. Sinusoidal fits to a typical wave train indicate a phase velocity of2200 ± 130 km s-1. Similar waves propagating in opposite directions areobserved in closed loops between two flare ribbons. In the k-omega diagramof the Fourier wave power, we find a bright ridge that represents thedispersion relation and can be well fitted with a straight line passing throughthe origin. This k-omega ridge shows a broad frequency distribution withindicative power at 5.5, 14.5, and 25.1 mHz. The strongest signal at 5.5 mHz(period 181 s) temporally coincides with quasi-periodic pulsations of theflare, suggesting a common origin. The instantaneous wave energy flux of(0.1-2.6) imes 107 ergs/cm^2/s estimated at the coronal base is comparableto the steady-state heating requirement of active region loops.

Authors: Wei Liu, Alan M. Title, Junwei Zhao, Leon Ofman, Carolus J. Schrijver, Markus J. Aschwanden, Bart De Pontieu, and Theodore D. Tarbell
Projects: SDO-AIA

Publication Status: Accepted by ApJ Letters (AAS/SPD Press Release, Las Cruces, June 15, http://www.lmsal.com/press/apjl2011_magnetosonic)
Last Modified: 2011-06-18 06:54
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Chromospheric Jet and Growing ''Loop'' Observed by Hinode: New Evidence of Fan-Spine Magnetic Topology Resulting From Flux Emergence  

Wei Liu   Submitted: 2010-12-08 11:25

We present observations of a chromospheric jet and growing ''loop'' system that show new evidence of a fan-spine topology resulting from magnetic flux emergence. This event, occurring in an equatorial coronal hole on 2007 February 9, was observed by the Hinode Solar Optical Telescope in the Ca II H line in unprecedented detail. The predecessor of the jet is a bundle of fine material threads that extend above the chromosphere and appear to rotate about the bundle axis at ~50 km s-1 (period <200 s). These rotations or transverse oscillations propagate upward at velocities up to 786 km s-1. The bundle first slowly and then rapidly swings up, with the transition occurring at the onset of an A4.9 flare. A loop expands simultaneously in these two phases (velocity: 16-135 km s-1). Near the peak of the flare, the loop appears to rupture; simultaneous upward ejecta and mass downflows faster than free-fall appear in one of the loop legs. The material bundle then swings back in a whiplike manner and develops into a collimated jet, which is orientated along the inferred open field lines with transverse oscillations continuing at slower rates. Some material falls back along smooth streamlines, showing no more oscillations. At low altitudes, the streamlines bifurcate at presumably a magnetic null point and bypass an inferred dome, depicting an inverted-Y geometry. These streamlines closely match in space the late Ca II H loop and X-ray flare loop. These observations are consistent with the model that flux emergence in an open-field region leads to magnetic reconnection, forming a jet and fan-spine topology. We propose that the material bundle and collimated jet represent the outer spine in quasi-static and eruptive stages, respectively, and the growing loop is a 2D projection of the 3D fan surface.

Authors: Liu, Wei; Berger, Thomas E.; Title, Alan M.; Tarbell, Theodore D; and Low, B. C.
Projects: Hinode/SOT

Publication Status: Accepted by ApJ, 2010 Dec 8; movie available at http://www.lmsal.com/~weiliu/public/hinode/2007-02-09_sot-jet (360 MB)
Last Modified: 2010-12-10 10:16
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First SDO AIA Observations of a Global Coronal EUV ''Wave'': Multiple Components and ''Ripples''  

Wei Liu   Submitted: 2010-11-03 03:42

We present the first Solar Dynamics Observatory Atmospheric Imaging Assembly (AIA) observations of a global coronal EUV disturbance (so-called ''EIT wave'') revealed in unprecedented detail. The disturbance observed on 2010 April 8 exhibits two components: one diffuse pulse superimposed, on which are multiple sharp fronts that have slow and fast components. The disturbance originates in front of erupting coronal loops and some sharp fronts undergo accelerations, both effects implying that the disturbance is driven by a coronal mass ejection. The diffuse pulse, propagating at a uniform velocity of 204-238 km s-1 with very little angular dependence within its extent in the south, maintains its coherence and stable profile for ~30 minutes. Its arrival at increasing distances coincides with the onsets of loop expansions and the slow sharp front. The fast sharp front overtakes the slow front, producing multiple ''ripples'' and steepening the local pulse, and both fronts propagate independently afterward. This behavior resembles the nature of real waves. Unexpectedly, the amplitude and FWHM of the diffuse pulse decrease linearly with distance. A hybrid model, combining both wave and non-wave components, can explain many, but not all, of the observations. Discoveries of the two-component fronts and multiple ripples were made possible for the first time thanks to AIA's high cadences (<=20 s) and high signal-to-noise ratio.

Authors: Liu, Wei; Nitta, Nariaki V.; Schrijver, Carolus J.; Title, Alan M.; and Tarbell, Theodore D.
Projects: None

Publication Status: Published in ApJ Letters, 11/01/2010, Volume 723, Issue 1, pp. L53-L59
Last Modified: 2010-11-03 08:53
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An Intriguing Chromospheric Jet Observed by Hinode: I. Fine Structure Kinematics and Evidence of Unwinding Twists  

Wei Liu   Submitted: 2009-10-28 08:18

We report a chromospheric jet lasting for more than 1 hr observed by Hinode Solar Optical Telescope in unprecedented detail. The ejection occurred in three episodes separated by 12-14 min, with the amount and velocity of material decreasing with time. The upward velocities range from 438 to 33 km s-1, while the downward velocities of the material falling back have smaller values (mean: -56 km s-1) and a narrower distribution (standard deviation: 14 km s-1). The average acceleration inferred from parabolic space-time tracks is 141 m s-2, a fraction of the solar gravitational acceleration. The jet consists of fine threads (0.5-2.0 arcsec wide), which exhibit coherent, oscillatory transverse motions perpendicular to the jet axis and about a common equilibrium position. These motions propagate upward along the jet, with the maximum phase speed of 744 ± 11 km s-1 at the leading front of the jet. The transverse oscillation velocities range from 151 to 26 km s-1, amplitudes from 6.0 to 1.9 Mm, and periods from 250 to 536 s. The oscillations slow down with time and cease when the material starts to fall back. The falling material travels along almost straight lines in the original direction of ascent, showing no transverse motions. These observations are consistent with the scenario that the jet involves untwisting helical threads, which rotate about the axis of a single large cylinder and shed magnetic helicity into the upper atmosphere.

Authors: Wei Liu, Thomas E. Berger, Alan M. Title, Theodore D. Tarbell
Projects: Hinode/SOT

Publication Status: Accepted by ApJ Letters (2009 Oct 27); movies available online at http://sun.stanford.edu/~weiliu/eprints/2009/2009-05_Ca-jet/paper-I_movies
Last Modified: 2009-11-09 10:29
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Combined Modeling of Acceleration, Transport, and Hydrodynamic Response in Solar Flares: I. The Numerical Model  

Wei Liu   Submitted: 2009-06-13 00:52

Acceleration and transport of high-energy particles and fluid dynamics of atmospheric plasma are interrelated aspects of solar flares, but they were artificially separated for simplicity in the past. We present here self-consistently combined Fokker-Planck modeling of particles and hydrodynamic simulation of flare plasma. Energetic electrons are modeled with the Stanford unified code of acceleration, transport, and radiation, while plasma is modeled with the NRL flux tube code. We calculated the collisional heating rate from the particle transport code, which is more accurate than those based on approximate analytical solutions. We used a realistic spectrum of injected electrons provided by the stochastic acceleration model, which has a smooth transition from a quasi-thermal background at low energies to a nonthermal tail at high energies. The inclusion of low-energy electrons results in relatively more heating in the corona (vs. chromosphere), a larger downward conductive flux, and thus a stronger chromospheric evaporation than obtained in previous studies, which had a deficit in low-energy electrons due to an arbitrarily assumed low-energy cutoff. The energy and spatial distributions of energetic electrons and bremsstrahlung photons bear signatures of the changing density distribution caused by chromospheric evaporation. In particular, the density jump at the evaporation front gives rise to enhanced emission, which, in principle, can be imaged by X-ray telescopes. This model can be applied to investigate a variety of high-energy processes in solar, space, and astrophysical plasmas.

Authors: Wei Liu, Vahé Petrosian, and John T. Mariska
Projects: Hinode/SOT,RHESSI

Publication Status: ApJ (2009 September 10, vol 702, p. 1553-1566, online first August 24)
Last Modified: 2009-08-24 10:32
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Episodic X-ray Emission Accompanying the Activation of an Eruptive Prominence: Evidence of Episodic Magnetic Reconnection  

Wei Liu   Submitted: 2009-02-10 22:52

We present an X-ray imaging and spectroscopic study of a partially occulted (N16W93) C7.7 flare on 2003 April 24 observed by RHESSI that accompanied a prominence eruption observed by TRACE. (1) The activation and rise of the prominence occurs during the preheating phase of the flare. The initial X-ray emission appears as a single coronal source at one leg of the prominence and it then splits into a double source. Such a source splitting happens three times, each coinciding with an increased X-ray flux and plasma temperature, suggestive of fast reconnection in a localized current sheet and an enhanced energy release rate. In the late stage of this phase, the prominence displays a helical structure. These observations are consistent with the tether-cutting and/or kink instability model for triggering solar eruptions. (2) The eruption of the prominence takes place during the flare impulsive phase. Since then, there appear signatures predicted by the classical CSHKP model of two-ribbon flares occurring in a vertical current sheet trailing an eruption. These signatures include an EUV cusp and current-sheet-like feature (or ridge) above it. There is also X-ray emission along the EUV ridge both below and above the cusp, which in both regions appears closer to the cusp at higher energies in the thermal regime (lesssim20 keV). This trend is reversed in the nonthermal regime. (3) Spectral analysis indicates thermal X-rays from all sources throughout the flare, while during the impulsive phase there is additional nonthermal emission which primarily comes from the coronal source below the cusp. This source also has a lower temperature (T=20 pm 1 vs.~25 pm 1 MK), a higher emission measure (EM=[3.3 pm 0.4] vs.~[1.2 pm 0.4] imes 1047 cm-3), and a much harder nonthermal spectrum (electron power-law index delta=5.4 pm 0.4 vs.~8 pm 1) than the upper sources.

Authors: Wei Liu, Tong-Jiang Wang, Brian R. Dennis, Gordon D. Holman
Projects: RHESSI,TRACE

Publication Status: ApJ (2009 June 10, Vol 698, p. 632-?640), also astro-ph/arXiv:0902.1805
Last Modified: 2009-08-24 10:59
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Conjugate Hard X-ray Footpoints in the 2003 October 29 X10 Flare: Unshearing Motions, Correlations, and Asymmetries  

Wei Liu   Submitted: 2008-11-19 08:41

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 between 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 the totality of these observations, but their combination, together with other transport effects, might provide a full explanation. We have also developed novel techniques to remove particle contamination from HXR counts and to estimate effects of pulse pileup in imaging spectroscopy, which can be applied to other RHESSI flares in similar circumstances.

Authors: Wei Liu, Vahe' Petrosian, Brian R. Dennis, Gordon D. Holman
Projects: RHESSI,SoHO-MDI

Publication Status: ApJ, March 2009, vol. 693, in press (also on astroph, http://arxiv.org/abs/0805.1055)
Last Modified: 2008-11-19 13:07
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Solar Flares as Natural Particle Accelerators:A High-energy View from X-ray Observations and Theoretical Models  

Wei Liu   Submitted: 2008-08-07 11:30

Solar flares, which have significant space weather consequences, are natural particle accelerators and one of the most spectacular phenomena of solar activity. RHESSI is the most advanced solar X-ray and gamma-ray mission ever flown and has opened a new era in solar flare research following its launch in 2002. This book offers a glimpse of this active research area from a high-energy perspective and contains a comprehensive guideline for RHESSI data analysis. Its main theme is the investigation of particle acceleration and transport in solar flares. The strength of this book lies in its well-balanced account of the latest X-ray observations and theoretical models. The observational focus is on the morphology and spectra of imaged X-ray sources produced by nonthermal electrons or hot plasma. The modeling takes the novel approach of combining the Fokker-Planck treatment of the accelerated particles with the hydrodynamic treatment of the heated atmosphere. Applications of this modeling technique reach beyond the Sun to other exotic environments in the universe, such as extrasolar planetary auroras, stellar flares, and flares on accretion disks around neutron stars and black holes.

Authors: Wei Liu
Projects: RHESSI

Publication Status: Book published by VDM Verlag Dr. Müller (Saarbrücken, Germany), July 2008; ISBN: 978-3-8364-7432-0, 10 chapters, 252 pages
Last Modified: 2008-09-23 20:52
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Subject will be restored when possible  

Wei Liu   Submitted: 2008-07-30 09:12

Solar flares, which have significant space weather consequences, are natural particle accelerators and one of the most spectacular phenomena of solar activity. RHESSI is the most advanced solar X-ray and gamma-ray mission ever flown and has opened a new era in solar flare research following its launch in 2002. This book offers a glimpse of this active research area from a high-energy perspective and contains a comprehensive guideline for RHESSI data analysis. Its main theme is the investigation of particle acceleration and transport in solar flares. The strength of this book lies in its well-balanced account of the latest X-ray observations and theoretical models. The observational focus is on the morphology and spectra of imaged X-ray sources produced by nonthermal electrons or hot plasma. The modeling takes the novel approach of combining the Fokker-Planck treatment of the accelerated particles with the hydro?dynamic treatment of the heated atmosphere. Applications of this modeling technique reach beyond the Sun to other exotic environ?ments in the universe, such as extrasolar planetary auroras, stellar flares, and flares on accretion disks around neutron stars and black holes.

Authors: Wei Liu
Projects: RHESSI

Publication Status: Monograph published by VDM Verlag Dr. M?ller (Germany), July 2008; ISBN: 978-3-8364-7432-0, 10 chapters, 252 pages
Last Modified: 2008-09-23 20:52
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Subject will be restored when possible  

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, Vahe' Petrosian, Brian R. Dennis, and Gordon D. Holman
Projects: RHESSI

Publication Status: Submitted to ApJ
Last Modified: 2008-05-07 06:41
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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, Vahe' Petrosian, Brian R. Dennis, and Gordon D. Holman
Projects: RHESSI

Publication Status: Submitted to ApJ, available at astro-ph/arXiv:0805.1055
Last Modified: 2008-09-23 21:02
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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 12:41
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Solar Flares as Natural Particle Accelerators: A High-energy View from X-ray Observations and Theoretical Models
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Characteristics of Solar Flare Hard X-ray Emissions: Observations and Models
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