Determination of the acceleration region size in a loop-structured solar flare

Abstract

Aims. To study the acceleration and propagation of bremsstrahlung-producing electrons in solar flares, we analyze the evolution of the flare loop size with respect to energy at a variety of times. A GOES M3.7 loop-structured flare starting around 23:55 on 2002 April 14 is studied in detail using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations. Methods. We construct photon and mean-electron-flux maps in 2-keV energy bins by processing observationally-deduced photon and electron visibilities, respectively, through several image-processing methods: a visibility-based forward-fit (FWD) algorithm, a maximum entropy (MEM) procedure and the uv-smooth (UVS) approach. We estimate the sizes of elongated flares (i.e., the length and width of flaring loops) by calculating the second normalized moments of the intensity in any given map. Employing a collisional model with an extended acceleration region, we fit the loop lengths as a function of energy in both the photon and electron domains. Results. The resulting fitting parameters allow us to estimate the extent of the acceleration region which is between ∼13 arcsec and ∼19 arcsec. Both forward-fit and uv-smooth algorithms provide substantially similar results with a systematically better fit in the electron domain. Conclusions. The consistency of the estimates from these methods provides strong support that the model can reliably determine geometric parameters of the acceleration region. The acceleration region is estimated to be a substantial fraction (∼1/2) of the loop extent, indicating that this dense flaring loop incorporates both acceleration and transport of electrons, with concurrent thick-target bremsstrahlung emission.

Hard X-ray emitting energetic electrons and photospheric electric currents

“…These values are in good agreement with what has been found for a few events by Kontar et al (2011) and Guo et al (2012). In their work, Xu et al (2008) modelled the transport of electrons on the basis of a standard collisional transport model.…”

Validation of the Coronal Thick Target Source Model

“…These data are needed to understand the process of electron acceleration in flares and to develop theoretical models of the acceleration region. A larger number of flares were analyzed by Guo et al (2012aGuo et al ( , 2012bGuo et al ( , 2013 with the same interpretation of the coronal sources as acceleration regions with a high plasma density. Interestingly, if one takes at face value the mean source size and the ambient plasma density derived from the X-ray image analysis of these coronal thick-target sources, computes the microwave emission, and compares it with the observed microwave spectrum, a substantial mismatch is often found: the low-frequency part of the computed microwave spectrum is predicted to be strongly suppressed by the Razin effect, which is not typically observed.…”

“…For example, Lee et al (2013) found that to roughly match the model and observed microwave spectrum in one of the coronal thick-target flares, one must adopt an ambient plasma density that is almost one order of magnitude smaller than that derived from the fit of the energydependent X-ray source sizes. However, Jeffrey et al (2014) performed a more accurate analysis of the X-ray data and concluded that the actual source density can in fact be a factor of 5 smaller than that derived from the simplified modeling (Xu et al 2008;Guo et al 2012aGuo et al , 2012bGuo et al , 2013. Taking all these different pieces of the puzzle together leaves us with a large uncertainty about the true physical parameters of the dense coronal thick-target sources.…”

Spatially inhomogeneous acceleration of electrons in solar flares

“…Xu et al 2008;Guo et al 2012). In order to examine the effects of a spatially dependent, extended acceleration region in a regime with simultaneous transport, we introduce a spatially non-uniform velocity diffusion coefficient:…”