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Gradual Solar Coronal Dimming and Evolution of Coronal Mass Ejection in the Early Phase  

Jianxia Cheng   Submitted: 2017-04-10 20:40

We report observations of a two-stage coronal dimming in an eruptive event of a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming persists for more than half an hour before the onset of the two-ribbon flare and the fast rise of the CME. It is followed by abrupt rapid dimming. The two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the two flare ribbons, and the flare onset marks the transition between the two stages of dimming. At the onset of the two-ribbon flare, transient brightenings are also observed inside the dimming regions, before rapid dimming occurs at the same places. These observations suggest that the CME structure, most probably anchored at the twin dimming regions, undergoes a slow rise before the flare onset, and its kinematic evolution has significantly changed at the onset of flare reconnection. We explore diagnostics of the CME evolution in the early phase with analysis of the gradual dimming signatures prior to the CME eruption.

Authors: Qiu, Jiong; Cheng, Jianxia
Projects: SDO-AIA

Publication Status: published
Last Modified: 2017-04-11 23:29
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The Nature of CME-Flare Associated Coronal Dimming  

Jianxia Cheng   Submitted: 2016-04-19 22:38

Coronal mass ejections (CMEs) are often accompanied by coronal dimming evident in extreme ultraviolet (EUV) and soft X-ray observations. The locations of dimming are sometimes considered to map footpoints of the erupting flux rope. As the emitting material expands in the corona, the decreased plasma density leads to reduced emission observed in spectral and irradiance measurements. Therefore, signatures of dimming may reflect properties of CMEs in the early phase of its eruption. In this study, we analyze the event of flare, CME, and coronal dimming on December 26, 2011. We use the data from the Atmospheric Imaging Assembly (AIA) on Solar Dynamics Observatories (SDO) for disk observations of the dimming, and analyze images taken by EUVI, COR1, and COR2 onboard the Solar Terrestrial Relations Observatories to obtain the height and velocity of the associated CMEs observed at the limb. We also measure magnetic reconnection rate from flare observations. Dimming occurs in a few locations next to the flare ribbons, and it is observed in multiple EUV passbands. Rapid dimming starts after the onset of fast reconnection and CME acceleration, and its evolution well tracks the CME height and flare reconnection. The spatial distribution of dimming exhibits cores of deep dimming with a rapid growth, and their light curves are approximately linearly scaled with the CME height profile. From the dimming analysis, we infer the process of the CME expansion, and estimate properties of the CME.

Authors: J. X. Cheng, J. Qiu
Projects: SDO-AIA,SDO-HMI,STEREO

Publication Status: accepted by ApJ
Last Modified: 2016-04-20 13:24
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Solar flare hard X-ray spikes observed by RHESSI: a statistical study  

Jianxia Cheng   Submitted: 2012-11-14 18:04

Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions. Aims. We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes. Methods. Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV. Results. The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times. (3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions.

Authors: J. X. Cheng, J. Qiu, M. D. Ding, and H. Wang
Projects: RHESSI

Publication Status: A&A, in press
Last Modified: 2012-11-18 21:23
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Solar flare hard X-ray spikes observed by RHESSI: a case study  

Jianxia Cheng   Submitted: 2012-10-26 06:02

Context. Fast-varying hard X-ray spikes of subsecond time scales were discovered by space telescopes in the 70s and 80s, and are also observed by the Ramaty High Energy S olar S pectroscopic Imager (RHESSI). These events indicate that the flare energy release is fragmented. Aims. In this paper, we analyze hard X-ray spikes observed by RHESSI to understand their temporal, spectral, and spatial properties. Methods. A recently developed demodulation code was applied to hard X-ray light curves in several energy bands observed by RHESSI. Hard X-ray spikes were selected from the demodulated flare light curves. We measured the spike duration, the energydependent time delay, and count spectral index of these spikes. We also located the hard X-ray source emitting these spikes from RHESSI mapping that was coordinated with imaging observations in visible and UV wavelengths. Results. We identify quickly varying structures of  1 s during the rise of hard X-rays in five flares. These hard X-ray spikes can be observed at photon energies over 100 keV. They exhibit sharp rise and decay with a duration (FWHM) of less than 1 s. Energydependent time lags are present in some spikes. It is seen that the spikes exhibit harder spectra than underlying components, typically by 0.5 in the spectral index when they are fitted to power-law distributions. RHESSI clean maps at 25x100 keV with an integration of 2 s centered on the peak of the spikes suggest that hard X-ray spikes are primarily emitted by double foot-point sources in magnetic fields of opposite polarities.With the RHESSI mapping resolution of 4'', the hard X-ray spike maps do not exhibit detectable difference in the spatial structure from sources emitting underlying components. Coordinated high-resolution imaging UV and infrared observations confirm that hard X-ray spikes are produced in magnetic structures embedded in the same magnetic environment of the underlying components. The coordinated high-cadence TRACE UV observations of one event possibly reveal new structures on spatial scales < 1-2''at the time of the spike superposed on the underlying component. They are probably sources of hard X-ray spikes.

Authors: J. Qiu, J. X. Cheng, G. J. Hurford, Y. Xu, and H. Wang
Projects: RHESSI,SoHO-MDI,TRACE

Publication Status: A&A, in press
Last Modified: 2012-10-26 14:53
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RADIATIVE HYDRODYNAMIC SIMULATION OF THE CONTINUUM EMISSION IN SOLAR WHITE-LIGHT  

Jianxia Cheng   Submitted: 2011-10-20 17:36

It is believed that solar white-light flares (WLFs) originate in the lower chromosphere and upper photosphere. In particular, some recently observed WLFs show a large continuum enhancement at 1.56 ?m where the opacity reaches its minimum. Therefore, it is important to clarify how the energy is transferred to the lower layers responsible for the production of WLFs. Based on radiative hydrodynamic simulations, we study the role of non-thermal electron beams in increasing the continuum emission. We vary the parameters of the electron beam and disk positions and compare the results with observations. The electron beam heated model can explain most of the observational white-light enhancements. For the most energetic WLFs observed so far, however, a very large electron beam flux and a high low-energy cutoff, which are possibly beyond the parameter space in our simulations, are required in order to reproduce the observed white-light emission.


Authors: J. X. Cheng, M. D. Ding, and Mats Carlsson
Projects: None

Publication Status: ApJ,published
Last Modified: 2011-10-21 09:09
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HARD X-RAY AND ULTRAVIOLET OBSERVATIONS OF THE 2005 JANUARY 15 TWO-RIBBON FLARE  

Jianxia Cheng   Submitted: 2011-10-19 19:11

In this paper, we present comprehensive analysis of a two-ribbon flareobserved in UV 1600{AA} by Transition Region and Coronal Explorer and in HXRsby Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV)imaging observations show two kernels of size (FWHM) 15?? moving along the twoUV ribbons. We find the following results. (1) UV brightening is substantiallyenhanced wherever and whenever the compact HXR kernel is passing, and duringthe HXR transit across a certain region, the UV count light curve in thatregion is temporally correlated with the HXR total flux light curve. After thepassage of the HXR kernel, the UV light curve exhibits smooth monotonicaldecay. (2)We measure the apparent motion speed of the HXR sources and UV ribbonfronts, and decompose the motion into parallel and perpendicular motions withrespect to the magnetic polarity inversion line (PIL). It is found that HXRkernels and UV fronts exhibit similar apparent motion patterns and speeds. Theparallel motion dominates during the rise of the HXR emission, and theperpendicular motion starts and dominates at the HXR peak, the apparent motionspeed being 10-40 km s-1. (3) We also find that UV emission is characterized bya rapid rise correlated with HXRs, followed by a long decay on timescales of15-30 minutes. The above analysis provides evidence that UV brightening isprimarily caused by beam heating, which also produces thick-target HXRemission. The thermal origin of UV emission cannot be excluded, but wouldproduce weaker heating by one order of magnitude. The extended UV ribbons inthis event are most likely a result of sequential reconnection along the PIL,which produces individual flux tubes (post-flare loops), subsequent non-thermalenergy release and heating in these flux tubes, and then the very long coolingtime of the transition region at the feet of these flux tubes.

Authors: J. X. Cheng, G. Kerr, and J. Qiu
Projects: None

Publication Status: accepted
Last Modified: 2011-10-21 09:09
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Diagnostics of the Heating Processes in Solar Flares Using Chromospheric Spectral Lines  

Jianxia Cheng   Submitted: 2006-09-14 21:29

We have calculated the Hα and Ca {sc ii} 8542 {AA} line profiles based on four different atmospheric models, including the effects of nonthermal electron beams with various energy fluxes. These two lines have different responses to thermal and nonthermal effects, and can be used to diagnose the thermal and nonthermal heating processes. We apply our method to an X-class flare that occurred on 2001 October 19. We are able to identify quantitatively the heating effects during the flare eruption. We find that the nonthermal effects at the outer edge of the flare ribbon are more notable than that at the inner edge, while the temperature at the inner edge seems higher. On the other hand, the results show that nonthermal effects increase rapidly in the rise phase and decrease quickly in the decay phase, but the atmospheric temperature can still keep relatively high for some time after getting to its maximum. For the two kernels that we analyze, the maximum energy fluxes of the electron beams are sim 1010 and 1011 ergs cm-2 s-1, respectively. However, the atmospheric temperatures are not so high, i.e., lower than or slightly higher than that of the weak flare model F1 at the two kernels. We discuss the implications of the results for two-ribbon flare models.

Authors: J. X. Cheng, M. D. Ding, and J. P. Li
Projects:

Publication Status: ApJ (accepted)
Last Modified: 2006-09-25 21:21
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Abstracts by Author
Gradual Solar Coronal Dimming and Evolution of Coronal Mass Ejection in the Early Phase
The Nature of CME-Flare Associated Coronal Dimming
Solar flare hard X-ray spikes observed by RHESSI: a statistical study
Solar flare hard X-ray spikes observed by RHESSI: a case study
RADIATIVE HYDRODYNAMIC SIMULATION OF THE CONTINUUM EMISSION IN SOLAR WHITE-LIGHT
HARD X-RAY AND ULTRAVIOLET OBSERVATIONS OF THE 2005 JANUARY 15 TWO-RIBBON FLARE
Diagnostics of the Heating Processes in Solar Flares Using Chromospheric Spectral Lines

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