HESSI and Type III Radio Bursts

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Number: 2
1st Author: Steven Christe
2nd Author:
Published: 9 April 2005
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The RHESSI hard X-ray spectra and images, which have much greater sensitivity than previously available, may show for the first time the direct X-radiation produced by currently ill-understood coronal particle beams. Type III radio bursts have been observed by radio astronomers with such beams since the 1950s, but never heretofore been identified directly.

A standard RHESSI image of a solar flare usually shows two features: at high energies, one or more compact sources, and at lower energies, a more diffuse structure a shape suggesting a magnetic loop. We identify these, respectively, as the footpoints of flaring loops and by the coronal bodies of these loops. The loops themselves, after cooling down from X-ray temperatures of 10-20 x 10^6 K, are clearly seen by EIT, TRACE, or in ground-based observations such as in those in H-alpha. RHESSI can also distinguish these different sources spectroscopically, for example by use of the spectrograms reported in an earlier RHESSI science nugget. The low-energy sources are caused by violently heated gas. The high-energy footpoint sources result from energy losses by non-thermal electrons. In the standard model, as shown for example in this cartoon, these electrons originate in the solar corona. By a mechanism not currently understood, they gain high energies and become "mildly relativistic" as they follow the magnetic field of the flare loops and crash into the high densities of the chromosphere and photosphere. In addition to emitting the hard (10-100 keV) X-rays of our footpoint sources, these electrons also carry enough energy to cause the heating of the hot material in the coronal loop sources (1-2 keV).

Figure 1 shows RHESSI images across the hard X-ray spectrum that illustrate this source structure:

Figure 1:RHESSI images (2002 feb 20, 11:06:00-11:06:40) of a typical event, with photon energy increasing from left to right as shown. The harder images (at the right) begin to show artifacts (noise) resulting from photon counting statistics. Brighter colors represent larger flux. Image courtesy S. Krucker.

As one can see (click image to enlarge; each image frame is 64 arc sec on a side) a simple two-footpoint structure becomes very clear at higher hard X-ray energies. Because each end of a coronal loop has a footpoint in the lower atmosphere, this double-source pattern is characteristic. Barring some technical questions (such as the all-important one of what accelerated the particles in the first place!), this picture of a solar flare is very robust, yet it is incomplete since it is known that accelerated particles sometimes escape the solar corona instead of going down the loop footpoints.

Radio type III bursts

The presence of escaping particles is known because they emit radio waves in a characteristic pattern: the emission frequency decreases rapidly with time, from for example 1 GHz down to the lowest frequencies observable from the Earth's surface (about 20 MHz). Below this frequency the Earth's ionosphere normally absorbs radio waves, so prior to the space age this was effectively the lowest frequency available for radio astronomers to observe. At still lower frequencies the type III bursts often continue nevertheless, and are sometimes detected at Earth by satellites such as Wind or ACE. Just as RHESSI makes use of spectrograms, so do radio observatories: energy is replaced by frequency. Since the radio waves they emit is related to the density (there is a strong tendency for emission at the plasma frequency), the escaping particles show a particular radio signature where the emission drifts from high frequency (or density) to low frequency (or density). This kind of radio emission is known as a type III radio burst. These same particles should also produce X-rays similar to those seen at the loop footpoints.

Here is a suggestive set of events. Has RHESSI observed the X-rays from the particles which generated the type III emission or are there other accelerated particles which have travelled down to the footpoint?

In this figure please note the event at 14:30 especially: the radio spectrogram (bottom) shows a type III burst, and the X-ray spectrogram (middle panel) shows a hard X-ray spectrogram.

Figure 2:Overview: Panel 1,2-GOES Level, Panel 3 - RHESSI spectrogram, Panel 4 - RHESSI Hard and Soft Channel, Panel 5 - WIND/WAVES Spectrogram


Ideally we can use RHESSI to track non-thermal electrons as they travel around, anywhere in the solar corona; practically we are limited by many factors (such as instrumental and solar background emission). Thus at present we don't know to what extent the above example represents an actual X-ray detection of the type III burst's escaping electrons as they actually escape. But we hope to have clear evidence soon, as RHESSI software and calibrations improve.

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