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First Evidence of Coexisting EIT Wave and Coronal Moreton Wave from SDO/AIA Observations  

P. F. Chen   Submitted: 2011-04-28 04:32

'EIT waves' are a globally propagating wavelike phenomenon. They were ofteninterpreted as a fast-mode magnetoacoustic wave in the corona, despite variousdiscrepancies between the fast-mode wave model and observations. To reconcilethese discrepancies, we once proposed that 'EIT waves' are apparent propagationof the plasma compression due to successive stretching of the magnetic fieldlines pushed by the erupting flux rope. According to this model, an 'EIT wave'should be preceded by a fast-mode wave, which however was rarely observed. Withthe unprecedented high cadence and sensitivity of the {em Solar DynamicsObservatory} ({em SDO}) observations, we discern a fast-moving wave front witha speed of 560 km s-1, ahead of an 'EIT wave', which had a velocity ofsim 190 km s-1, in the 'EIT wave' event on 2010 July 27. The results,suggesting that 'EIT waves' are not fast-mode waves, confirm the prediction ofour fieldline stretching model for 'EIT wave'. In particular, it is found thatthe coronal Moreton wave was sim 3 times faster than the 'EIT wave' aspredicted.

Authors: Chen, P. F.; Wu, Y.
Projects: None

Publication Status: 2011, ApJ, 732, L20 (http://iopscience.iop.org/2041-8205/732/2/L20/ )
Last Modified: 2011-04-28 06:13
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Why Are Halo Coronal Mass Ejections Faster?  

P. F. Chen   Submitted: 2010-02-23 18:29

Halo coronal mass ejections (CMEs) were found to be significantly faster than normal CMEs, which was a long-standing puzzle. In order to solve the puzzle, we first investigate the observed properties of 31 limb CMEs that display clearly loop-shaped frontal loops. The observational results show a strong tendency that slower CMEs are weaker in the white-light intensity. Then, we perform a Monte Carlo simulation of 20000 artificial limb CMEs that have average velocity of ~523 km s-1. The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events, respectively. It is found that the white-light intensity of many slow CMEs becomes remarkably reduced as they turn from being viewed as a limb event to as a halo event. When the intensity is below the background solar wind fluctuation, it is assumed that they would be missed by coronagraphs. The average velocity of ``detectable'' halo CMEs is ~922 km s-1, very close to the observed value. It also indicates that wider events are more likely to be recorded. The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and a part of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations, and therefore are not observed.

Authors: Q. M. Zhang, Y. Guo, P. F. Chen, M. D. Ding, C. Fang
Projects: None

Publication Status: 2010, accepted for publication in Research in Astron. & Astrophys.
Last Modified: 2010-02-24 08:09
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The dependence of the EIT wave velocity on the magnetic field strength  

P. F. Chen   Submitted: 2010-02-23 18:24

``EIT waves'' are a wavelike phenomenon propagating in the corona, which were initially observed in the extreme ultraviolet (EUV) wavelength by the EUV Imaging Telescope (EIT). Their nature is still elusive, with the debate between fast-mode wave model and non-wave model. In order to distinguish between these models, we investigate the relation between the EIT wave velocity and the local magnetic field in the corona. It is found that the two parameters show significant negative correlation in most of the EIT wave fronts, i.e., EIT wave propagates more slowly in the regions of stronger magnetic field. Such a result poses a big challenge to the fast-mode wave model, which would predict a strong positive correlation between the two parameters. However, it is demonstrated that such a result can be explained by the fieldline stretching model, i.e., that ``EIT waves'' are apparently-propagating brightenings, which are generated by successive stretching of closed magnetic field lines pushed by the erupting flux rope during coronal mass ejections (CMEs).

Authors: H. Q. Yang, P. F. Chen
Projects: None

Publication Status: 2010, accepted for publication in Solar Phys.
Last Modified: 2010-02-24 08:09
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EIT waves and coronal magnetic field diagnostics  

P. F. Chen   Submitted: 2010-02-23 18:21

Magnetic field in the solar lower atmosphere can be measured by the use of the Zeeman and Hanle effects. In contrast, the coronal magnetic field well above the solar surface, which directly controls various eruptive phenomena, can not be precisely obtained with the traditional techniques. Several attempts are being made to probe the coronal magnetic field, such as force-free extrapolation based on the photospheric magnetograms, gyroresonance radio emissions, and coronal seismology based on MHD waves in the corona. Compared to the waves trapped in the localized coronal loops, EIT waves are the only global-scale wave phenomenon, and thus are the ideal tool for the coronal global seismology. In this paper, we review the observations and modelings of EIT waves, and illustrate how they can be applied to probe the global magnetic field in the corona.

Authors: P. F. Chen
Projects: None

Publication Status: 2009, Science in China (G), vol. 52, p. 1785
Last Modified: 2010-02-24 08:08
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The Relation between EIT Waves and Coronal Mass Ejections  

P. F. Chen   Submitted: 2009-05-21 07:42

More and more evidence indicates that ``EIT waves'' are strongly related to coronal mass ejections (CMEs). However, it is still not clear how the two phenomena are related to each other. We investigate a CME event on 1997 September 9, which was well observed by both EUV imaging telescope (EIT) and the high-cadence MK3 coronagraph at Mauna Loa Solar Observatory, and compare the spatial relation between the ``EIT wave'' fronts and the CME leading loops. It is found that ``EIT wave'' fronts are co-spatial with the CME leading loops, and the expanding EUV dimmings are co-spatial with the CME cavity. It is also found that the CME stopped near the boundary of a coronal hole, a feature common to observations of ``EIT waves''. It is suggested that ``EIT waves''/dimmings are the EUV counterparts of the CME leading loop/cavity, based on which we propose that, as in the case of ``EIT waves'', CME leading loops are apparently-moving density enhancements that are generated by successive stretching (or opening-up) of magnetic loops.

Authors: P. F. Chen
Projects: None

Publication Status: accepted for publication in ApJ Letters
Last Modified: 2009-05-21 09:12
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Energy spectrum of the electrons accelerated by reconnection electric field: exponential or power-law?  

P. F. Chen   Submitted: 2008-09-16 07:55

The direct current (DC) electric field near the reconnection region has been proposed as an effective mechanism to accelerate protons and electrons in solar flares. A power-law energy spectrum was generally claimed in the simulations of electron acceleration by the reconnection electric field. However, in most of the literature, the electric and magnetic fields were chosen independently. In this paper, we perform test particle simulations of electron acceleration in reconnecting magnetic field, where both the electric and magnetic fields are adopted from numerical simulations of the MHD equations. It is found that the accelerated electrons present a truncated power-law energy spectrum with an exponential tail at high energies, which is analogous to the case of diffusive shock acceleration. The influences of the reconnection parameters on the spectral feature are also investigated, such as the longitudinal and transverse components of the magnetic field and the size of the current sheet. It is suggested that the DC electric field alone might not be able to reproduce the observed single or double power-law distributions.

Authors: W. J. Liu, P. F. Chen, M. D. Ding, & C. Fang
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2008-09-16 10:52
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Subject will be restored when possible  

P. F. Chen   Submitted: 2008-02-14 21:21

Coronal mass ejections (CMEs), as a large-scale eruptive phenomenon, often reveal some precursors in the initiation phase, e.g., X-ray brightening, filament darkening, etc, which are useful for CME modeling and space weather forecast. With the SOHO/SUMER spectroscopic observations of the 2000 September 26 event, we propose another precursor for CME eruptions, namely, long-time prominence oscillations. The prominence oscillation-and-eruption event was observed by ground-based Hα telescopes and space-borne white-light, EUV imaging and spectroscopic instruments. In particular, the SUMER slit was observing the prominence in a sit-and-stare mode. The observations indicate that a siphon flow was moving from the proximity of the prominence to a site at a projected distance of 270'', which was followed by repetitive Hα surges and continual prominence oscillations. The oscillation lasted 4 hours before the prominence erupted as a blob-like CME. The analysis of the multiwavelength data indicates that the whole series of processes fits well into the emerging flux trigger mechanism for CMEs. In this mechanism, emerging magnetic flux drives a siphon flow due to increased gas pressure where the background polarity emerges. It also drives Hα surges through magnetic reconnection where the opposite polarity emerges. The magnetic reconnection triggers the prominence oscillations, as well as its loss of equilibrium, which finally leads to the eruption of the prominence. It is also found that the reconnection between the emerging flux and the pre-existing magnetic loop proceeds in an intermittent, probably quasi-periodic, way.

Authors: P. F. Chen, D. E. Innes, & S. K. Solanki
Projects: None

Publication Status: 2008, submitted for publication in A&A
Last Modified: 2008-02-15 08:57
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Subject will be restored when possible  

P. F. Chen   Submitted: 2008-02-14 21:18

This paper reviews recent progress in the research on the initiation and propagation of CMEs. In the initiation part, several trigger mechanisms are discussed; In the propagation part, the observations and modelings of EIT waves/dimmings, as the EUV counterparts of CMEs, are described.

Authors: Chen, P. F.
Projects: None

Publication Status: 2007, J. Astron. & Astrophys., in press
Last Modified: 2008-02-15 08:57
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Subject will be restored when possible  

P. F. Chen   Submitted: 2008-02-14 21:12

Numerical simulations of two types of flares indicate that magnetic reconnection can provide environments favorable for various particle acceleration mechanisms to work. This paper reviews recent test particle simulations of DC electric field mechanism, and discusses how the flare particles can escape into the interplanetary space under different magnetic configurations.

Authors: P. F. Chen, W. J. Liu, & C. Fang
Projects: None

Publication Status: 2007, Adv. Space Res., 39, 1421
Last Modified: 2008-02-15 08:57
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Spectral Analysis of Ellerman Bombs  

P. F. Chen   Submitted: 2006-07-11 02:16

By use of the high-resolution spectral data obtained with THEMIS on 2002 September 5, the characteristics of 14 well-observed Ellerman bombs (EBs) have been analyzed. Our results indicate that 9 of the 14 EBs are located near the longitudinal magnetic polarity inversion lines.Most of the EBs are accompanied by mass motions. The most obvious characteristic of the EB spectra is the two emission bumps at the two wings of both Ha and Ca ii 8542. For the first time both thermal and nonthermal semiempirical atmospheric models for the conspicuous and faint EBs are computed. In computing the nonthermal models, we assume that the electron beam resulting from magnetic reconnection is produced in the lower chromosphere. The reasons are that it requires much lower energies for the injected particles and that it gives rise to a more profound absorption at the Ha line center, in agreement with our observations. The common characteristic is the heating in the lower chromosphere and the upper photosphere. The temperature enhancement is about 600?1300 K in the thermal models. If the nonthermal effects are included, then the required temperature increase can be reduced by 100?300 K. These imply that the EBs could probably be produced by the magnetic reconnection in the solar lower atmosphere. The radiative and kinetic energies of the EBs are estimated, and the total energy is found to be 1026 to 5*1027 ergs. According to the characteristics of EBs, we tentatively suggest that EBs could be called ??submicroflares.??

Authors: C. Fang, Y. H. Tang, Z. Xu, M. D. Ding, & P. F. Chen
Projects: None

Publication Status: 2006, ApJ, 643, 1325
Last Modified: 2006-07-11 10:23
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Transition-region Explosive Events: Reconnection Modulated by P-mode Waves  

P. F. Chen   Submitted: 2006-07-09 21:53

Transition-region explosive events (TREEs) have long been proposed as a consequence of magnetic reconnection. However, several critical issues have not been well addressed, such as the location of the reconnection site, their unusually short lifetime (~1 min), and the recently discovered repetitive behaviour with a period of 3-5 min. In this paper we perform MHD numerical simulations of magnetic reconnection, where the effect of 5-min solar p-mode oscillations is examined. UV emission lines are synthesised on the basis of the numerical results in order to compare with observations directly. It is found that several typical and puzzling features of the TREEs with impulsive bursty behaviour can only be explained if there exist the p-mode oscillations and the reconnection site is located in the upper chromosphere at a height range of around 1900 km < h < 2150 km above the solar surface. Furthermore, the lack of proper motions of the high velocity ejection may be due to a rapid change of temperature along the reconnection ejecta.

Authors: P. F. Chen & E. R. Priest
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2006-08-02 10:14
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The Relation Between EIT Waves and Solar Flares  

P. F. Chen   Submitted: 2006-03-26 12:17

In order to determine whether EIT waves are generated by coronal mass ejections (CMEs) or pressure pulses in solar flares, 14 non-CME–associated energetic flares, which should possess strong pressure pulses in their loops, are studied. They are selected near solar minimum, as this favors the detection of EIT waves. It is found that none of these flares are associated with EIT waves. Particular attention is paid to AR 0720, which hosted both CME-associated and non-CME types of flares. The SOHO/EIT images convincingly indicate that EIT waves and expanding dimmings appear only when CMEs are present. Therefore, it is unlikely that pressure pulses from flares generate EIT waves.

Authors: P. F. Chen
Projects: SoHO-EIT

Publication Status: ApJ Letters (in press)
Last Modified: 2006-03-26 17:04
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A FULL VIEW OF EIT WAVES  

P. F. Chen   Submitted: 2005-04-13 08:02

Early observations by the EUV Imaging Telescope ( EIT) on the Solar and Heliospheric Observatory indicated that propagating diffuse wave fronts, now conventionally referred to as ''EIT waves'', can often be seen on the solar disk with a propagation velocity several times smaller than that of Ha Moreton waves. They are almost always associated with coronal mass ejections. We have previously confirmed the existence of such a wave phenomenon with numerical simulations, which indicate that there does exist a slower moving ''wave'' much behind the coronal counterpart of the Ha Moreton wave. Further observations have disclosed many new features of the EIT waves: the waves stop near the separatrix between active regions, sometimes they experience acceleration from the active region to the quiet region, and so on. Here we report on MHD simulations performed to demonstrate how the typical features of EIT waves can all be accounted for within our theoretical model, in which the EIT waves are thought to be formed by successive stretching or opening of closed field lines driven by an erupting flux rope. The relationship between EIT waves, Ha Moreton waves, and type II radio bursts is discussed, with an emphasis on reconciling the discrepancies among different views of the ''EIT wave'' phenomenon.

Authors: P. F. Chen, C. Fang, & K. Shibata
Projects: None

Publication Status: 2005, ApJ, 622, 1202
Last Modified: 2005-04-13 08:02
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Evidence of EIT and Moreton Waves in Numerical Simulations  

P. F. Chen   Submitted: 2002-05-17 08:47

Solar coronal mass ejections (CMEs) are associated with many dynamical phenomena, among which EIT waves have always been a puzzle. In this Letter MHD processes of CME-induced wave phenomena are numerically simulated. It is shown that as the flux rope rises, a piston-driven shock is formed along the envelope of the expanding CME, which sweeps the solar surface as it propagates. We propose that the legs of the shock produce Moreton waves. Simultaneously, a slower moving wavelike structure, with an enhanced plasma region ahead, is discerned, which we propose corresponds to the observed EIT waves. The mechanism for EIT waves is therefore suggested, and their relation with Moreton waves and radio bursts is discussed.

Authors: Chen, P. F., Wu, S. T., Shibata, K. and Fang, C.
Projects:

Publication Status: To be published in ApJL Vol. 572, L99 on June 10, 2002
Last Modified: 2002-05-27 07:34
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Abstracts by Author
First Evidence of Coexisting EIT Wave and Coronal Moreton Wave from SDO/AIA Observations
Why Are Halo Coronal Mass Ejections Faster?
The dependence of the EIT wave velocity on the magnetic field strength
EIT waves and coronal magnetic field diagnostics
The Relation between EIT Waves and Coronal Mass Ejections
Energy spectrum of the electrons accelerated by reconnection electric field: exponential or power-law?
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Spectral Analysis of Ellerman Bombs
Transition-region Explosive Events: Reconnection Modulated by P-mode Waves
The Relation Between EIT Waves and Solar Flares
A FULL VIEW OF EIT WAVES
Evidence of EIT and Moreton Waves in Numerical Simulations

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