Characteristics of the flare acceleration region derived from simultaneous hard X-ray and radio observations

arXiv:1102.2342v2 [astro-ph.SR] 17 Feb 2011 Astronomy & Astrophysics manuscript no. type3start˙xray c⃝ESO 2018 October 23, 2018 Characteristics of the flare acceleration region derived from simultaneous hard X-ray and radio observations Hamish A. S. Reid1,2, Nicole Vilmer2, and Eduard P. Kontar1 1Department of Physics and Astronomy, University of Glasgow, G12 8QQ, United Kingdom 2 LESIA, Observatoire de Paris, CNRS, UPMC, Universit´e Paris-Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France Received 22 Nov 2010 / Accepted 10 Feb 2011 ABSTRACT We investigate the type III radio bursts and X-ray signatures of accelerated electrons in a well observed solar flare in order to find the spatial properties of the acceleration region. Combining simultaneous RHESSI hard X-ray flare data and radio data from Phoenix-2 and the Nanc¸ay radioheliograph, the outward transport of flare accelerated electrons is analysed. The observations show that the start- ing frequencies of type III bursts are anti-correlated with the HXR spectral index of solar flare accelerated electrons. We demonstrate both analytically and numerically that the type III burst starting location is dependent upon the accelerated electron spectral index and the spatial acceleration region size, but weakly dependent on the density of energetic electrons for relatively intense electron beams. Using this relationship and the observed anti-correlation, we estimate the height and vertical extent of the acceleration region, giving values of around 50 Mm and 10 Mm, respectively. The inferred acceleration height and size suggest that electrons are accelerated well above the soft X-ray loop-top, which could be consistent with the electron acceleration between 40 Mm and 60 Mm above the flaring loop. Key words. Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays — Sun: particle emission 1. Introduction Accelerated electron beams are believed to be responsible for both hard X-ray (HXR) and coherent radio emission during solar flares. Upwards travelling electron beams propagate through the coronal plasma and sometimes escape into interplanetary space. Emission from such beams is often observed as coronal and interplanetary type III radio bursts. Electron beams travelling downwards with small pitch angles enter the dense plasma of the chromosphere and are generally seen through bremsstrahlung emission in HXR. Before entering the chromosphere, down- wards propagating electron beams may also produce reverse type III bursts. Despite this wealth of electromagnetic beam emission from accelerated electrons propagating in plasma, the location of the electron acceleration site and its spatial characteristics are poorly known. Indirect evidence of electron acceleration sites first came from broad band radio spectral observations of pairs of type III and reverse type III bursts (e.g. Aschwanden et al. 1995a; Aschwanden & Benz 1997). The starting frequencies of these burst pairs are found between 220 −910 MHz, implying a range of electron densities in the acceleration region between 6×108 − 1010 cm−3 for fundamental emission or 1.5 × 108 −3 × 109 cm−3 for harmonic emission. These densities are lower than ones ob- served in bright soft X-ray loops (2 × 1010 −2 × 1011 cm−3) suggesting that the acceleration region lies above the soft X- ray loops, being located for example in a cusp reconnection site. HXR observations have also been independently used to provide insight into typical electron acceleration region heights above the photosphere. Through electron time-of-flight analysis us- ing HXR emission in the range 20-200 keV (Aschwanden et al. 1998), height estimates have been found in the range 20-50 Mm. The spatial size of the acceleration region still remains largely unknown. The simultaneous observation of HXR and metric/decimetric radio emission is commonplace during flares and the relation- ship between type III bursts and hard X-ray emissions has been studied for many years (see for example Pick & Vilmer 2008, for a review). The first studies performed by Kane (1972) found a good similarity between HXR and type III ra- dio emission, suggesting the two emissions are produced by electrons originating from a common acceleration site. Many subsequent studies have specifically dealt with the associa- tion of coherent type III radio emission and HXR bursts, both statistically (e.g. Kane 1972, 1981; Hamilton et al. 1990; Aschwanden et al. 1995a; Arzner & Benz 2005) and for individ- ual events (e.g. Kane et al. 1982; Benz et al. 1983; Dennis et al. 1984; Raoult et al. 1985; Aschwanden et al. 1995b; Raulin et al. 2000; Vilmer et al. 2002). A more recent statistical study of 201 flares above GOES class C5 (Benz et al. 2005) reports an 83% association rate with coherent radio emission, within the range between 4 GHz and 100 MHz. These results suggest that practi- cally all flares with HXR GOES class > C5 are associated with some form of coherent radio emission. An in depth statistical

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