Correlation of Microwave and Hard X‐Ray Spectral Parameters

Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.

doi:10.1086/317822

Cited by 93 publications

(79 citation statements)

References 33 publications

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“…We have mentioned that a similar flattening was reported earlier based on the microwave-to-X-ray slope comparisons (Silva et al 2000;White et al 2011). In the previous cases, however, the radio-derived and X-ray-derived spectral indices of nonthermal electrons were obtained using one or another approximation, e.g., from the thick-target fit of the footpoint X-ray emission or a simplified analytical Dulk-Marsh formula for the GS spectrum, which necessarily involves some level of Figure 2); (e): [30,50,70,90]% contours of the OVSA 5.6 GHz emission (white; same as in Figure 2) on top of the computed microwave 5.6 GHz image; (f): model HXR emission at 15 keV (blue) on top of the model microwave image at 5.6 GHz.…”

Section: Discussionsupporting

confidence: 88%

“…However, we quickly find that it is not possible to obtain a good fit to the microwave spectrum with such a steep electron energy spectrum, irrespective of other model assumptions. Unavoidably, we have to assume the existence of a spectral break up above some energy, to a lower spectral index of about δ n2 ≈3.5 to fit the high-frequency microwave spectral slope, which may not be surprising given that a number of previous radio-to-X-ray comparisons (see, e.g., Silva et al 2000;White et al 2011, and references therein) concluded that the radio-derived spectra of the nonthermal electrons are often flatter than the X-rayderived ones.…”

Section: D Modeling: the Rise Phasementioning

confidence: 99%

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Validation of the Coronal Thick Target Source Model

Fleishman

Nita

et al. 2016

ApJ

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We present detailed 3D modeling of a dense, coronal thick-target X-ray flare using the GX Simulator tool, photospheric magnetic measurements, and microwave imaging and spectroscopy data. The developed model offers a remarkable agreement between the synthesized and observed spectra and images in both X-ray and microwave domains, which validates the entire model. The flaring loop parameters are chosen to reproduce the emission measure, temperature, and the nonthermal electron distribution at low energies derived from the X-ray spectral fit, while the remaining parameters, unconstrained by the X-ray data, are selected such as to match the microwave images and total power spectra. The modeling suggests that the accelerated electrons are trapped in the coronal part of the flaring loop, but away from where the magnetic field is minimal, and, thus, demonstrates that the data are clearly inconsistent with electron magnetic trapping in the weak diffusion regime mediated by the Coulomb collisions. Thus, the modeling supports the interpretation of the coronal thick-target sources as sites of electron acceleration in flares and supplies us with a realistic 3D model with physical parameters of the acceleration region and flaring loop.

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Section: Discussionsupporting

confidence: 88%

Section: D Modeling: the Rise Phasementioning

confidence: 99%

Validation of the Coronal Thick Target Source Model

Fleishman

Nita

et al. 2016

ApJ

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“…Electrons with higher energies are trapped for longer periods than the slow electrons and consequently are depleted from the loop-legs (e.g., Lee & Gary 2000). This result is not in disagreement with Silva et al (2000) measurements from X rays and microwave spectra. They found that, in general, the electronic spectrum is harder for the high energy range (as inferred from microwaves) than for the lower energy range (as inferred from hard X rays).…”

Section: Resultsmentioning

confidence: 55%

Microwave emission from the trapped and precipitated electrons in solar bursts

Costa¹,

Rosal²

2005

A&A

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Abstract.We analyzed the microwave spectra of a sample of 13 solar flares out of 40 events observed by the Nobeyama Radio Polarimeter in the period of August 8, 1998 through November 24, 2001. The solar burst time profiles were filtered to separate the slow component from the fast structures which were associated with the emission from trapped particles and to the emission from the minor precipitating population, respectively. The spectra of both components suggest a trap in a region with a mean magnetic field of about 600 G and a minor emission from the loop legs with a magnetic field of 1100 G, resulting in a mirror ratio of about two. The spectral indices of the optically thin emission from these two regions indicate pitch angle diffusion by Coulomb collisions.

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“…Statistical and case studies in the last two decades (Kosugi, Dennis, & Kai 1988 and references therein), and most recently by Silva, Wang, & Gary (2000), revealed other respects in which the impulsive and gradual emissions show contrasting properties. These include (1) the microwaves in the gradual events show greater time delays, by more than a few tens of seconds, with respect to the hard X-rays than in the impulsive events (Silva et al 2000); (2) the gradual events exhibit a harder and hardening (soft-hard-harder, or SHH) spectrum compared with the impulsive events, which exhibit a soft-hard-soft (SHS) spectrum (Silva et al 2000); and (3) the gradual bursts are usually microwave-rich events, and statistically the ratio of the microwave to hard X-ray flux is larger in the gradual events than in the impulsive events by a factor of 5 (Kosugi et al 1988;Silva et al 2000). The most critical issue is whether these contrasting properties between impulsive and gradual emissions are produced by different acceleration mechanisms or merely represent transport effects, such as precipitation versus trapping, due to different properties of the ambient plasmas and magnetic configurations.…”

Section: Introductionmentioning

confidence: 99%

Impulsive and Gradual Nonthermal Emissions in an X‐Class Flare

Qiu

Lee

Gary

2004

ApJ

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In this paper we present a comprehensive case study of an X-class flare observed on 2001 April 6. The flare consists of two episodes, the first characterized by impulsive spiky bursts and the second by gradual smooth emission at hard X-ray and microwave wavelengths. Emissions in the two episodes are regarded as the impulsive and gradual components, respectively. We compare the temporal, spatial, and spectral evolution of the two components in hard X-rays and microwaves. For this event, the most important finding is that both the impulsive and gradual hard X-rays at !50 keV are thick-target emissions at conjugate footpoints. Evolution of hard X-rays and microwaves during the gradual phase exhibits a separation motion between two footpoint sources, which reflects progressive magnetic reconnection. Observations further reveal distinct spectral properties of the gradual component. It is most prominently observed in the high-energy (>20 keV) range, and the gradual hard X-rays have a harder and hardening spectrum compared with the impulsive component. The gradual component is also a microwave-rich event, with the microwaves lagging the hard X-rays by tens of seconds. A correlation analysis of the hard X-ray light curves shows energy-dependent time delays, with the 200 keV hard X-rays lagging the 40 keV emission by 20 s. The observations and analyses suggest that magnetic reconnection occurs during both the impulsive and gradual phases that generate nonthermal electrons, primarily precipitating at the footpoints. However, the temporal and spectral properties of the gradual component must be produced by an acceleration mechanism different from that of the impulsive phase. We propose that the ''collapsing-trap'' effect, as a product of progressive magnetic reconnection in bipolar magnetic fields, is a viable mechanism that continuously accelerates the gradual-phase electrons in a low-density trap before they precipitate into the footpoints.

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Correlation of Microwave and Hard X‐Ray Spectral Parameters

Cited by 93 publications

References 33 publications

Validation of the Coronal Thick Target Source Model

Validation of the Coronal Thick Target Source Model

Microwave emission from the trapped and precipitated electrons in solar bursts

Impulsive and Gradual Nonthermal Emissions in an X‐Class Flare

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