Bin's Research

I study the Sun, our own star. Our Sun is a G-type main sequence star. It consists of several very distinct layers: the Core, the Radiative Zone, the Convective Zone, the Photosphere, the Chromosphere, and an outer atmosphere called the Corona that extends all the way to our Earth.

My research interests have been on Solar Flares and the physical environment in which they occur. A solar flare is a sudden and intense activity on the Sun that results in enhanced emission almost across the entire electromagnetic spectrum and affects all layers of the solar atmosphere. A flare can release an enormous amount of energy into the interplanetary space, which can strongly influence the Space Weather in the vicinity of the Earth. An outstanding problem in the physics of solar flares is a detailed understanding of energy release in flares, as well as the associated particle acceleration and transport processes. My research activities have focused on studies of non-thermal emissions in radio and X-ray wavelengths, which are intimately related to the energetic particles produced in flares and are therefore believed to be the key to understanding of those fundamental questions. (Image from www.nswp.gov.)

Coherent radio bursts are intense and short-lived radio emission produced by energetic particles via coherent mechanisms. They have a particular advantage in tracing energetic particles very efficiently, since they arise from a relatively small number of particles. Therefore they can serve as a powerful and independent tool to understand the energy release and the complex coronal structure.

One of the most interesting such burst types is the so-called "type III radio bursts" (upper panel). They are radio signatures of propagating energetic electron beams in the solar corona, which are accelerated by the released magnetic energy in flares. One of my previous publications reported observation of a peculiar type III radio burst in absorption against the background. Numerical investigations were used to explain its dynamic spectral properties in terms of loss-cone plasma instabilities disturbed by the electron beams (Chen & Yan 2008). However, a barrier to exploiting these bursts has been the lack of simultaneous imaging observations. This has now changed with the recently upgraded Karl G. Jansky Very Large Array. Using dynamic imaging spectroscopy we were, for the first time, able to deduce detailed trajectories of propagating energetic electron beams in the solar corona (bottom panel). Combined with observations in other wavelengths (optical, EUV, hard X-ray), we also constrained the site of the flare energy release and deduced the properties of the coronal environment in which the beams propagate (Chen et al. 2013).

I have also studied another type of coherent radio bursts - the zebra-pattern bursts, which appear in dynamic spectra as a number of closely spaced, quasi-parallel bands of emission (left panel). Although known for many years, the precise emission mechanism has been controversial. Another of my publications reported superfine pulsating structures (with a ~30 ms period) in a zebra-pattern burst (Chen & Yan 2007), and interpreted them as wave-particle oscillations from magnetically trapped energetic electrons. More recently Chen et al. 2011 reported observations of a zebra pattern using a three-element interferometer (FST), a sub-system testbed of the next generation solar radio telescope - the Frequency Agile Solar Radiotelescope (FASR). Though it could not form synthesized images, for the first time, we were able to infer spatial information of each pixel in the high-resolution dynamic spectra. As a result, we were able to conclude that a double plasma resonance mechanism is strongly favored, where the local plasma frequency is in resonance with harmonics of the electron cyclotron frequency. With the help of a coronal magnetic field extrapolation method, we further inferred its 3-D source position in the corona and suggested its relation to the magnetic energy release in the flare (right panels).

Another part of my work involves studies of the non-thermal emission from solar flares at the other end of the electromagnetic spectrum: hard X-ray (HXR) wavelengths. HXR emission is generally produced by electron-ion and electron-electron bremsstrahlung radiation from nonthermal electrons, usually dominated by sources at the foot-points of flaring magnetic loops. Sometimes HXR sources can also be found in the corona, which is of particular interest because they are thought to be very close to the magnetic energy release site. However, in the case of some coronal HXR sources in flares, rather extreme conditions are needed in order to understand the emission in terms of bremsstrahlung. In Chen & Bastian (2012) we investigated whether inverse compton scattering (ICS) could be an alternative emission mechanism for these coronal HXR sources, which might open a new window in diagnosing the flare energy release. We suggested that energetic electrons accelerated in the corona are able to up-scatter optical, EUV, and/or soft X-ray photons to HXR wavelengths and produce observable HXR flux. The ICS mechanism may be favored over non-thermal bremsstrahlung when the ambient coronal density is low and/or the energetic electron population has an anisotropic distribution. (Figure from Masuda et al. 1994.)