The Low-High-Low Starting Frequency Trend in Groups of Type III Bursts
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We have tested the connection between X-ray spectral index and the starting frequency of groups of type III bursts using medium strength solar flares \citep{Reid:2014aa}. The many forms of radio emission in large flares obscure the type III starting frequencies whilst small flares do not have reliable estimates of the X-ray spectral index. We also required images to check that the emission originated from the same place on the Sun. We looked for flares simultaneously observed by RHESSI, the PHOENIX radiospectrometers and the Nan\c{c}ay Radioheliograph. | We have tested the connection between X-ray spectral index and the starting frequency of groups of type III bursts using medium strength solar flares \citep{Reid:2014aa}. The many forms of radio emission in large flares obscure the type III starting frequencies whilst small flares do not have reliable estimates of the X-ray spectral index. We also required images to check that the emission originated from the same place on the Sun. We looked for flares simultaneously observed by RHESSI, the PHOENIX radiospectrometers and the Nan\c{c}ay Radioheliograph. | ||
+ | [Image:242f2.png|600px|thumb|center| | ||
+ | '''Figure 2''': | ||
+ | Groups of type III bursts using a time cadence of 2 seconds. The background is radio observations from Phoenix 2. Red stars are the starting frequencies found above the background. White bars are the X-ray spectral index derived from RHESSI with the associated 1-sigma errors. The red stars can be observed to follow by eye the starting frequency of the radio bursts. | ||
+ | ]] | ||
From the respective event catalogues we found 30 suitable candidates to test the hypothesis. 17 out of the 30 events showed a good anti-correlation between the X-ray spectral index and the starting frequency of type III bursts (see Figures \ref{fig:sfsi}, \ref{fig:dsi}). The lower spectral indicies deduced from X-rays during the impulsive phase matched the type III bursts with the highest starting frequencies. Moreover, events without a good anti-correlation still showed an anti-correlation between type III starting frequency and X-ray spectral index, just not for the entire duration of the event. | From the respective event catalogues we found 30 suitable candidates to test the hypothesis. 17 out of the 30 events showed a good anti-correlation between the X-ray spectral index and the starting frequency of type III bursts (see Figures \ref{fig:sfsi}, \ref{fig:dsi}). The lower spectral indicies deduced from X-rays during the impulsive phase matched the type III bursts with the highest starting frequencies. Moreover, events without a good anti-correlation still showed an anti-correlation between type III starting frequency and X-ray spectral index, just not for the entire duration of the event. |
Revision as of 22:27, 19 December 2014
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Number: | 242 |
1st Author: | Hamish Reid |
2nd Author: | |
Published: | December 22, 2014 |
Next Nugget: | Type III burst starting frequencies |
Previous Nugget: | The Solar X-ray Limb II |
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Introduction
Flares accelerate electrons. We know this as we detect them in-situ near the Earth. But disentangling transport effects to understand their acceleration can be tricky. We can instead analyse flare accelerated electron beams by comparing two of their electromagnetic signatures; type III radio bursts and hard X-rays (see Figure \ref{fig:morph}). These two wavelength ranges have been known for decades to be statistically correlated. When we observe type IIIs during the impulsive phase of large flares we often observe them at high frequencies $>100$~MHz. However, at the start or the end of such flares the type III emission typically starts at lower frequencies. So what is the physical reason for this behaviour?
The electron beam bump-in-tail instability that creates Langmuir waves, ultimately responsible for type III radio emission, is a kinetic instability. The spectral index of injected electron beams is one of the key parameters that governs how quickly this instability forms. Moreover, collisions make it harder to create radio emission in the deep atmosphere. So we have observationally checked if there is any connection between the detected spectral index of X-rays and the starting frequency of the groups of type III bursts that are observed during some flares.
![](/~tohban/wiki/images/thumb/a/a4/242f1.png/600px-242f1.png)
Simultaneous Radio and X-ray Observations
We have tested the connection between X-ray spectral index and the starting frequency of groups of type III bursts using medium strength solar flares \citep{Reid:2014aa}. The many forms of radio emission in large flares obscure the type III starting frequencies whilst small flares do not have reliable estimates of the X-ray spectral index. We also required images to check that the emission originated from the same place on the Sun. We looked for flares simultaneously observed by RHESSI, the PHOENIX radiospectrometers and the Nan\c{c}ay Radioheliograph.
[Image:242f2.png|600px|thumb|center| Figure 2: Groups of type III bursts using a time cadence of 2 seconds. The background is radio observations from Phoenix 2. Red stars are the starting frequencies found above the background. White bars are the X-ray spectral index derived from RHESSI with the associated 1-sigma errors. The red stars can be observed to follow by eye the starting frequency of the radio bursts. ]]
From the respective event catalogues we found 30 suitable candidates to test the hypothesis. 17 out of the 30 events showed a good anti-correlation between the X-ray spectral index and the starting frequency of type III bursts (see Figures \ref{fig:sfsi}, \ref{fig:dsi}). The lower spectral indicies deduced from X-rays during the impulsive phase matched the type III bursts with the highest starting frequencies. Moreover, events without a good anti-correlation still showed an anti-correlation between type III starting frequency and X-ray spectral index, just not for the entire duration of the event.
Conclusions
We show the well known soft-hard-soft pattern of the X-ray spectrum results in a low-high-low trend for the type III burst starting frequency. Less intense (soft spectra) electron beams produce radio emission with lower starting frequencies. The bump-in-tail instability takes longer to form and the electron beams travel farther from the acceleration region, producing radio emission in the higher corona at lower densities that corresponds to lower frequencies. Conversely, more intense (hard spectra) electron beams become unstable much faster, producing radio emission closer to the acceleration region in higher density plasma and hence at higher frequencies.
We found that 57\% of our flares showed an anti-correlation between X-ray spectral index and type III starting frequency. For the other flares conditions can hide or complicate the anti-correlation, such as multiple acceleration regions at different altitudes or the coronal density profile changing dramatically during the flare. However, when we do observe such a trend we have used the anti-correlation to estimate spatial characteristics of the flare acceleration regions that energised the electron beams.
RHESSI Nugget Date | 22 December 2014 + |
RHESSI Nugget First Author | Hamish Reid + |
RHESSI Nugget Index | 242 + |