Tracing Electron Beams with Radio Dynamic Imaging Spectroscopy
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Solar flares involve production of energetic electron beams. They carry a significant amount of magnetic energy released and therefore play a key role in flare physics. One method to probe these beams is through their radio signature: type III radio bursts. Type III radio bursts are emitted at the fundamental or harmonic of the local electron plasma frequency, fpe~9ne1/2 kHz, where ne is the electron number density. Since ne varies with height in the corona radio emission from an electron beam rapidly drifts from high to low frequencies for upward-propagating beams, and in the opposite sense for downward-propagating beams. Type III radio bursts have been observed for decades in dynamic spectra – records of radio flux density as a function of time and frequency. Recently, it has become possible to perform dynamic imaging spectroscopy of these, and other, types of solar radio bursts using the recently upgraded Karl G. Jansky Very Large Array (Perley et al. 2011). That is, an image of the radio emission can be produced at each frequency and time over a substantial frequency range. Here we report the first use of this technique to trace electron beams in the low corona through observations of type III bursts at decimeter wavelengths (Chen et al. 2013).
Trajectories of electron beams and their place of origin
Type IIIdm bursts were observed between 1.0-1.5 GHz (λ=20-30 cm) in association with a coronal jet during the aftermath of a GOES class M1.8 SXR flare (Figure 1B) on 2011 November 5 by the VLA. Figure 1 shows the jet (panel A) and the cross-power dynamic spectra of the type IIIdm bursts (panel C-D). Each bright vertical or nearly-vertical feature in the dynamic spectra represents an individual type IIIdm burst. Each pixel of the bursts in the dynamic spectrum was imaged and the emission centroid location was fit with high accuracy owing to the high signal-to-noise of these bursts. At any given time, type-IIIdm-burst centroids as a function of frequency manifest as a well-defined trajectory, showing the passage of an electron beam in the corona during the 100-ms integration (Figure 2B-G). Figure 2A shows a superposition of all the type-IIIdm-burst centroids observed during the ~2-min bursting period. They all fall within a narrow envelope with the high frequency ends (high density, thus low coronal heights) originating near the location of the EUV jet and the HXR foot-points. For bursts that are temporally-resolved (Figure 2B-G), the emission drifts from high to low frequencies with a rate of 0.3-1 GHz/s, indicating that the electron beams were propagating upward in the corona with a speed of ~0.3c.
The close spatiotemporal association of the type-IIIdm-burst trajectories and the HXR footpoints (Figure 1E, 2B-G) suggests that the X-ray-producing downward-propagating electron beams and the type-IIIdm-emitting upward-propagating electron beams originate from a common energy release site, which is located in the low corona between the HXR footpoints and the highest-frequency type-IIIdm-burst sources, at a height of a few to ~15 Mm.
Ultra-thin flux tubes and nature of the reconnection region
The electron number density ne and its variation along the type-IIIdm-emitting loops (or flux tubes) is immediately known since it is directly related to the plasma frequency or its harmonic, which is 3.3-7x109 cm-3 assuming harmonic emission. The best-fit density scale height Ln=ne(-dne/dh)-1 of the flux tubes is ~40 Mm, corresponding to a temperature of 0.8 MK under the assumption of hydrostatic equilibrium.
Surprisingly, no trace of loop-like structures can be found along the type-IIIdm-burst trajectories against the background in any SDO/AIA EUV filters, implying that the column emission measure of the type-IIIdm-emitting loops is too small to result in detectable emission or absorption relative to the background. With the knowledge of the plasma density inside the flux tubes, the upper-limit of the diameter of each electron-beam-conducting tube can be constrained to be only a few tens of kilometers. We conclude that these tubes are ultra-thin and occupied with over-dense cool plasmas.
Furthermore, the multitudes of discrete electrons beams linking to the reconnection region are observed to have access to spatially distinct flux tubes in < 1 s, which indicates that the reconnection region likely consists of a large number of discrete reconnection sites in a localized spatial volume. Our observations rather directly suggest a bursty reconnection scenario involving a localized reconnection region containing a distribution of many small-scale dynamically-evolving structures.
Dynamic imaging spectroscopy of type IIIdm bursts with the VLA has allowed us to map the trajectories of electron beams produced by magnetic energy release during a coronal jet event. Electrons escaped along discrete, ultra-fine magnetic tubes into the upper atmosphere, producing the observed type IIIdm bursts. Downward propagating electrons resulted in the observed HXR footpoint emission, which is closely associated with the type IIIdm bursts both temporally and spatially. Properties of the beam-conducting tubes have been deduced from the observations, including the plasma density and its variation over height, temperature, and the upper-limit of their diameter. The spatial scales in the reconnection region are likely 10s of km or less. We conclude that the magnetic energy release process is highly fragmentary and that the surrounding coronal medium is fibrous in nature.
Chen, B., Bastian T. S., White, S. M., Gary, D. E., Perley, R., Rupen, M., Carlson, B. 2013, ApJL, 763, 21
Perley, R. A., Chandler, C. J., Butler, B. J., & Wrobel, J. M. 2011, ApJL, 739, 1