Statistical study of Type III bursts and associated HXR emissions

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Nugget
Number:	451
1st Author:	Nicole VILMER
2nd Author:	Tomin JAMES
Published:	May 29, 2023
Next Nugget:	TBD
Previous Nugget:	Solar flare hard X-rays from the anchor points of an eruptive filament
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Contents 1 Introduction 2 Data Selection 3 What correlations do we see? 4 Conclusions 5 References

Introduction

This RHESSI Nugget is the continuation of a previous Nugget, completing our Statistical studies. This topic indeed has a long history, since type III emission process (see Figure 1 for a reminder) clearly implicates fast electrons, which at some level must emit bremsstrahlung hard X-rays.

We revisit here the existence of the potential correlations between HXR-emitting electrons and the characteristics of associated radio type III bursts. Such an association may be suggested by the relative locations of type III radio bursts and the hard X-ray (HXR) emission, which we have imaged using RHESSI and the Nançay Radioheliograph (NRH).

In this new study (Ref. [2]), we investigated the potential correlation between characteristics of metric/decimetric radio type III bursts (peak flux, starting and stopping frequencies) and characteristics of HXR producing electrons at the time of the radio peak (spectral index, number of energetic electrons)

Data Selection

The present study is based on 13 years of data (2002-2014). X-ray observations were provided by RHESSI and FERMI/GBM. Solar radio spectra from decimetric to kilometric wavelengths (1 GHz - 0.1 MHz frequency range) were measured with a combination of spectrographs (PHOENIX-2, PHOENIX-3 and ORFEES below 100 MHz, NDA between 80 MHz and 10 MHz and WIND/WAVES below 10 MHz. The Nançay Radioheliograph (NRH) provided information on the spatial localization of the radio type III bursts and was used to measure the radio flux at several frequencies. Events were selected so that impulsive X-ray emission to at least 15 keV be associated with type III emission clearly seen above 100 MHz, in particular in the NRH range for radio flux measurements. Strong flares associated with complex radio emissions were excluded in our sample since we aimed at analysing events with clear and simple association between type III bursts and HXR emissions. Our sample includes 205 events. 89% of these events are associated with GOES C and B class. This agrees with the known low occurrence rate (4%) of type-III-only events associated with flares above C5. Figure 2 shows a representative event, which has a typically close correlation between the timing of the impulsive-phase HXR and the high-frequency (low altitude) onset of the type III emission.

What correlations do we see?

Figure 3 shows correlations between the peak flux of type III bursts at the four Nançay frequencies and the number of energetic electrons above 20 keV, as deduced from RHESSI HXR spectroscopy around the type III peak time (20s interval centered on the type III peak time). A definite correlation exists but with substantial scatter, and it is best for the lowest frequency (highest altitude).

Conclusions

The present observational studies takew a new and important step in understanding and simulations of type III bursts. Our correlations appear to relate escaping electrons and some fraction of the flare electrons seen by RHESSI. This kind of correlation has previously eluded us, presumably because of the non-linear physics relating electrons and their radio emissions. The wavelength dependence of the correlations provides a handle with which to interpret physical models; see Ref. [2] for full details and further confirmation.

The energetic electron characteristics are derived here from HXR emissions clearly detected with instruments such as RHESSI or FERMI/GBM, for which thick-target emissions from downward propagating electron beams dominate the detected signal. Better direct constraints on the energetic electrons in the population actually producing the type III emissions could be obtained if X-ray emission produced by the upward propagating beam could be directly detected via imaging HXR. This will require instruments with higher sensitivities and dynamic ranges using focusing optics - for example, FOXSI.

References

[1] "Emission of Type II Radio Bursts - Single-Beam Versus Two-Beam Scenario"

[2] [https://ui.adsabs.harvard.edu/abs/2023A&A...673A..57J "