My Research Activities

My research is about the Sun, our own star. Our Sun is a G-type main sequence star. It consists of several very distinct layers: the Core, Radiative Zone, the Convective Zone, the Photosphere, the Chromosphere, and an outer atmosphere called the Corona that extends all the way to our Earth. This figure shows the structure of the Sun (from Wikipedia):

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. Generally, my study is trying to understand the solar flares and their influences on the space weather.

Sun Structure

Coronal Hard X-Ray Emission

The release of magnetic energy stored in the corona is believed to be the source of energy for particle acceleration. These accelerated particles are responsible for many intense flare radiation and the Solar Energetic Particle (SEP) events received on Earth. However, the details of particle acceleration during solar flare remain unknown until now. The coronal hard x-ray (HXR) emission is thought to be directly related to the acceleration site, as such can serve as the most direct diagnostics of particle acceleration. Currently, the thin-target non-thermal bremsstrahlung emission (which comes from flare-accelerated electrons colliding with thermal ambient protons in the corona) is considered to be the most relevent emission mechanism. Based on this emission machnism, information about the accelerated particles are obtained by performing spectral inversions to the observed HXR spectra. The following figure shows the observed HXR emission during a flare on 1992 January 13 (left), which consists both radiation from foot points of the coronal magenetic loops and from the above-loop-top (coronal) source. The flare model on the right shows that the coronoal HXR source may come from the magnetic release region above the flaring coronal loop (from Masuda et al. 1994).

In our recent work (Chen & Bastian 2011), we found that, in fact, the Inverse Compton Scattering (ICS) can be an alternative emission mechanism for certain coronal HXR sources, which might open a new window in diagnosing particle acceleration in flares. Energetic electrons accelerated in the corona are able to up-scatter photospheric, extreme ultra-violet, and/or soft X-ray photons to HXR wavelengths and produce observable HXR flux. The ICS can win over the thin-target bremsstrahlung especially when the ambient density is too low and/or the energetic electron population has an anisotropic distribution in favor of the ICS emission.
Loop-top Hard X-ray Loop-top Hard X-ray

Coherent Solar Radio Bursts

Coherent radio bursts are frequently seen in solar flares. They are originated from fast particles accelerated by the huge amount of energy released during solar flares. As a result, they can serve as another powerful tool to diagnose the flare energy release region. Coherent solar radio burst are characterized by their very high brightness temperature, small bandwidth and short duration. One of the most interesting type of solar coherent radio bursts is the so-called "Zebra-pattern Bursts". They appear in radio dynamic spectra as a number of closely spaced, quasi-parallel bands of emission (right figure). These structures have been observed for decades. Tens of emission mechanisms have been proposed, but none is broadly accepted.

One of the most important reasons is that most previous observations were detected by total-power radiometers with high time and frequency resolution, but lack spatial information at the same time. Today, inteferometers start to develop the abilities of observing with simultaneous high spatial, time, and frequency resolution. One of the most important telescopes for future solar radio observations is the Expanded Very Large Array (EVLA). I am now involved in the commissioning process to allow the "solar mode" of the EVLA to be available. Once completed, the EVLA is able to observe the Sun with unprecedently large bandwidth, high spatial, spectral and temporal resolution.

Zebra-patthen Bursts
Zebra-patthen Bursts

In one of my recent works (Chen et al. 2011), I used a three-element interferometer, a sub-system testbed of the next generation solar radio telescope - the Frequency Agile Solar Radiotelescope (FASR), called FST, to observe solar coherent radio bursts. Of course, it doesn't have the full ability to make synthesized images (as the FASR and EVLA will do), but it is able to obtain simultaneously high time and frequency resolution dynamic spectra. Using FST, for the first time, we were able to track the actual position for each pixel on the radio dynamic spectra as a function of time and frequency. As a result, we were able to examine the emission models and constrain the plasma parameters in the emission source. We did a careful analysis on a Zebra-pattern observed by FST. We identified a most appropriate emission mechanism, estimated the plasma density, magnetic field strength, and pinpointed its 3-D source position in the solar corona. With the help of a coronal magnetic field extrapolation method, we successfully interpreted most observed properties of the burst and established its relation to the associated flare.

Refereed Publications