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Particle Acceleration by a Solar Flare Termination Shock  

Bin Chen   Submitted: 2015-12-08 18:53

Solar flares - the most powerful explosions in the solar system - are also efficient particle accelerators, capable of energizing a large number of charged particles to relativistic speeds. A termination shock is often invoked in the standard model of solar flares as a possible driver for particle acceleration, yet its existence and role have remained controversial. We present observations of a solar flare termination shock and trace its morphology and dynamics using high-cadence radio imaging spectroscopy. We show that a disruption of the shock coincides with an abrupt reduction of the energetic electron population. The observed properties of the shock are well-reproduced by simulations. These results strongly suggest that a termination shock is responsible, at least in part, for accelerating energetic electrons in solar flares.

Authors: Bin Chen, Timothy S. Bastian, Chengcai Shen, Dale E. Gary, Säm Krucker, Lindsay Glesener
Projects: GOES X-rays ,Hinode/XRT,RHESSI,SDO-AIA,SoHO-LASCO,Very Large Array

Publication Status: Published on Science on December 4, 2015. 28 pages, 10 figures.
Last Modified: 2015-12-09 12:21
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Direct Evidence of an Eruptive, Filament-Hosting Magnetic Flux Rope Leading to a Fast Solar Coronal Mass Ejection  

Bin Chen   Submitted: 2014-09-02 07:36

Magnetic flux ropes (MFRs) are believed to be at the heart of solar coronal mass ejections (CMEs). A well-known example is the prominence cavity in the low corona that sometimes makes up a three-part white-light CME upon its eruption. Such a system, which is usually observed in quiet-Sun regions, has long been suggested to be the manifestation of an MFR with relatively cool filament material collecting near its bottom. However, observational evidence of eruptive, filament-hosting MFR systems has been elusive for those originating in active regions. By utilizing multi-passband extreme-ultra-violet (EUV) observations from SDO/AIA, we present direct evidence of an eruptive MFR in the low corona that exhibits a hot envelope and a cooler core; the latter is likely the upper part of a filament that undergoes a partial eruption, which is later observed in the upper corona as the coiled kernel of a fast, white-light CME. This MFR-like structure exists more than one hour prior to its eruption, and displays successive stages of dynamical evolution, in which both ideal and non-ideal physical processes may be involved. The timing of the MFR kinematics is found to be well correlated with the energy release of the associated long-duration C1.9 flare. We suggest that the long-duration flare is the result of prolonged energy release associated with the vertical current sheet induced by the erupting MFR.

Authors: Bin Chen, Tim Bastian, Dale Gary
Projects: RHESSI,SDO-AIA,SoHO-LASCO,STEREO

Publication Status: ApJ (in press)
Last Modified: 2014-09-03 13:11
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Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy  

Bin Chen   Submitted: 2012-12-26 09:40

We report observations of type III radio bursts at decimeter wavelengths (type IIIdm bursts) - signatures of suprathermal electron beams propagating in the low corona - using the new technique of radio dynamic imaging spectroscopy provided by the recently upgraded Karl G. Jansky Very Large Array (VLA). For the first time, type IIIdm bursts were imaged with high time and frequency resolution over a broad frequency band, allowing electron beam trajectories in the corona to be deduced. Together with simultaneous hard X-ray (HXR) and extreme ultraviolet (EUV) observations, we show these beams emanate from an energy release site located in the low corona at a height below ~15 Mm, and propagate along a bundle of discrete magnetic loops upward into the corona. Our observations enable direct measurements of the plasma density along the magnetic loops, and allow us to constrain the diameter of these loops to be less than 100 km. These over-dense and ultra-thin loops reveal the fundamentally fibrous structure of the Sun's corona. The impulsive nature of the electron beams, their accessibility to different magnetic field lines, and the detailed structure of the magnetic release site revealed by the radio observations indicate that the localized energy release is highly fragmentary in time and space, supporting a bursty reconnection model that involves secondary magnetic structures for magnetic energy release and particle acceleration.

Authors: Bin Chen, Timothy S. Bastian, Stephen M. White, Dale E. Gary, Richard A. Perley, Michael P. Rupen, Brent R. Carlson
Projects: RHESSI,SDO-AIA,SDO-HMI,STEREO,Very Large Array

Publication Status: Accepted for publication in the Astrophysical Journal Letters. 6 pages, 5 figures
Last Modified: 2012-12-26 12:54
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The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and Gamma-ray Sources  

Bin Chen   Submitted: 2011-08-03 18:09

Coronal hard X-ray (HXR) and continuum gamma-ray sources associated with theimpulsive phase of solar flares have been the subject of renewed interest inrecent years. They have been interpreted in terms of thin-target, nonthermalbremsstrahlung emission. This interpretation has led to rather extreme physicalrequirements in some cases. For example, in one case, essentially all of theelectrons in the source must be accelerated to nonthermal energies to accountfor the coronal HXR source. In other cases, the extremely hard photon spectraof the coronal continuum gamma-ray emission suggest that the low energy cutoffof the electron energy distribution lies in the MeV energy range. Here weconsider the role of inverse Compton scattering (ICS) as an alternate emissionmechanism in both the ultra- and mildly relativistic regimes. It is known thatrelativistic electrons are produced during powerful flares; these are capableof up-scattering soft photospheric photons to HXR and gamma-ray energies.Previously overlooked is the fact that mildly relativistic electrons, generallyproduced in much greater numbers in flares of all sizes, can up-scatter EUV/SXRphotons to HXR energies. We also explore ICS on anisotropic electrondistributions and show that the resulting emission can be significantlyenhanced over an isotropic electron distribution for favorable viewinggeometries. We briefly review results from bremsstrahlung emission andreconsider circumstances under which nonthermal bremsstrahlung or ICS would befavored. Finally, we consider a selection of coronal HXR and gamma-ray eventsand find that in some cases the ICS is a viable alternative emission mechanism.

Authors: Bin Chen, T. S. Bastian
Projects: None

Publication Status: Submitted to ApJ
Last Modified: 2011-08-04 08:32
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Spatially and Spectrally Resolved Observations of a Zebra Pattern in Solar Decimetric Radio Burst  

Bin Chen   Submitted: 2011-05-03 20:33

We present the first interferometric observation of a zebra-pattern radio burst with simultaneous high spectral (~ 1 MHz) and high time (20 ms) resolution. The Frequency-Agile Solar Radiotelescope (FASR) Subsystem Testbed (FST) and the Owens Valley Solar Array (OVSA) were used in parallel to observe the X1.5 flare on 14 December 2006. By using OVSA to calibrate the FST the source position of the zebra pattern can be located on the solar disk. With the help of multi-wavelength observations and a nonlinear force-free field (NLFFF) extrapolation, the zebra source is explored in relation to the magnetic field configuration. New constraints are placed on the source size and position as a function of frequency and time. We conclude that the zebra burst is consistent with a double-plasma resonance (DPR) model in which the radio emission occurs in resonance layers where the upper hybrid frequency is harmonically related to the electron cyclotron frequency in a coronal magnetic loop.

Authors: Bin Chen, Timothy S. Bastian, Dale E. Gary, Ju Jing
Projects: Hinode/SOT,Hinode/XRT,Owens Valley Solar Array

Publication Status: Accepted for publication in ApJ
Last Modified: 2011-05-11 12:15
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Short-Lived Absorptive Type III-Like Microwave Bursts as a Signature of Fragmented Electron Injections  

Bin Chen   Submitted: 2008-11-11 13:30

In this paper, we devoted ourselves to interpreting the short-lived absorptive type III-like microwave bursts in the 2006 December 13 flare event observed with high temporal and spectral resolutions (10 MHz and 8 ms) by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at 2.6-3.8 GHz. In the decimeter-centimeter wavelength range, we first present the observations of short-lived bursts represented as a number of absorptive "spikes" superposed on the type IV continuum that can be connected by fast-drifting lines. The mean drift rate, the instantaneous bandwidth, and the absorption depth of these absorptive ''spikes'' are about -12 GHz/s, 70 MHz, and 40%, respectively. The duration at a single frequency band can be less than the instrument resolution of 8 ms. On the basis of numerical investigations of the loss-cone instability, we suggest that fragmented electron injections with durations of as short as several milliseconds into the loss-cone could be the most appropriate mechanism with which to explain the bursts. The length of an electron beam is estimated to be about 400 km, on the basis of the observational results. These injections may be related to the fragmented energy release processes during the flare. We also observe some absorptive type III-like bursts accompanying ordinary type III bursts with reverse drifts. They start at the same frequency, and the starting frequency slowly drifts to the low-frequency region. This could be a signature of propagating bi-directional electron beams originating near the reconnection region.

Authors: Bin Chen, Yihua Yan
Projects:

Publication Status: Accepted by ApJ; will appear in the 08 Dec 20 issue
Last Modified: 2008-11-12 18:46
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Subject will be restored when possible  

Bin Chen   Submitted: 2007-09-29 02:18

Through the data around 3 GHz from the Radio Spectrometer in Huairou, Beijing, zebra-pattern structures in the April 21, 2002 event have been studied. Zebra stripes consist of periodically pulsating superfine structures in this event. Analysis of temporal profiles of intensities at multiple frequency channels shows that the Gaussian temporal profiles of pulse groups on zebra stripes are caused by drifting zebra stripes with Gaussian spectral profiles. The observed quasi-periodic pulsations with about 30 ms period have a peculiar feature of oscillation near a steady state. It is probably due to relaxation oscillations which modulate electron cyclotron maser emission that forms the zebra stripes during the process of wave-particle interactions. All the main properties of the zebra stripes with pulsating superfine structures indicate that the double plasma resonance model might be the most suitable one, with the relaxation oscillations to form the superfine structures. The model of LaBelle et al. (2003) could not account for the observed properties of zebra-pattern structures in this event as well as most zebra-pattern structures occupying a wide frequency range, mainly because the allowable frequency range of the ZS in their model is too narrow to reproduce the observed zebras.

Authors: Chen, B. and Yan, Y.H.
Projects: None

Publication Status: Solar Phys. (In press)
Last Modified: 2007-09-29 21:41
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Subject will be restored when possible  

Bin Chen   Submitted: 2007-09-29 02:18

Through the data around 3 GHz from the Radio Spectrometer in Huairou, Beijing, zebra-pattern structures in the April 21, 2002 event have been studied. Zebra stripes consist of periodically pulsating superfine structures in this event. Analysis of temporal profiles of intensities at multiple frequency channels shows that the Gaussian temporal profiles of pulse groups on zebra stripes are caused by drifting zebra stripes with Gaussian spectral profiles. The observed quasi-periodic pulsations with about 30 ms period have a peculiar feature of oscillation near a steady state. It is probably due to relaxation oscillations which modulate electron cyclotron maser emission that forms the zebra stripes during the process of wave-particle interactions. All the main properties of the zebra stripes with pulsating superfine structures indicate that the double plasma resonance model might be the most suitable one, with the relaxation oscillations to form the superfine structures. The model of LaBelle et al. (2003) could not account for the observed properties of zebra-pattern structures in this event as well as most zebra-pattern structures occupying a wide frequency range, mainly because the allowable frequency range of the ZS in their model is too narrow to reproduce the observed zebras.

Authors: Chen, B. and Yan, Y.H.
Projects:

Publication Status: Solar Physics (in press)
Last Modified: 2007-09-30 09:09
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Abstracts by Author
Particle Acceleration by a Solar Flare Termination Shock
Direct Evidence of an Eruptive, Filament-Hosting Magnetic Flux Rope Leading to a Fast Solar Coronal Mass Ejection
Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy
The Role of Inverse Compton Scattering in Solar Coronal Hard X-ray and Gamma-ray Sources
Spatially and Spectrally Resolved Observations of a Zebra Pattern in Solar Decimetric Radio Burst
Short-Lived Absorptive Type III-Like Microwave Bursts as a Signature of Fragmented Electron Injections
Subject will be restored when possible
Subject will be restored when possible

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