Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft

Kontar, Eduard P.; Brown, J. C.

doi:10.1086/510586

Cited by 60 publications

(57 citation statements)

References 29 publications

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“…In our CSC simulations we find that the electrons move more or less equally up and down the loop axis ( v z+ v Z− ) with v ⊥ / v about 0.05 in the chromosphere and 0.20 in the corona. Unlike the strong downward beaming ( v z+ v z− ) in the basic CTTM (Brown 1972), this distribution is broadly consistent with (Kontar & Brown 2006) albedo mirror diagnostic "near isotropy" results from RHESSI spectra. The v ⊥ /v 1 property of electrons in the CSC LRTTM may, however, still yield enough H α impact polarization to contribute to that observed Kašparová et al 2005) though other mechanisms (e.g.…”

Section: Discussionsupporting

confidence: 78%

“…Brown et al 1990) have reviewed problematic aspects of the standard CTTM model and aspects of recent data (especially from RHESSI - Lin et al (2002)) certainly require modification of the most basic CTTM involving a single monolithic loop. These include: the motion of HXR footpoints (Fletcher et al 2004); the smallness of the albedo component in HXR spectra (Kontar & Brown 2006;Kašparová et al 2007) compared to that expected from the strong downward beaming in the CTTM (Brown 1972); the relative time evolution of the heated soft X-ray (SXR) plasma emission measure EM(t) and temperature T (t), (e.g. Horan 1971;Stoiser et al 2008a).…”

Section: Basic Cttm Properties and Problemsmentioning

confidence: 99%

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Local re-acceleration and a modified thick target model of solar flare electrons

Brown¹,

Turkmani

Kontar³

et al. 2009

A&A

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Context. The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost "standard model" of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves. Aims. We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection. Methods. We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (E) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop. Results. Combined MHD and test particle simulations show that local E fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfvén speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features. Conclusions. Including electron re-acceleration in the HXR source allows an LRTTM modification of the CTTM in which beam density and anisotropy are much reduced, and alleviates theoretical problems with the CTTM, while making it more compatible with radio and interplanetary electron numbers. The LRTTM is, however, different in some respects such as spatial distribution of atmospheric heating by fast electrons.

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Section: Discussionsupporting

confidence: 78%

Section: Basic Cttm Properties and Problemsmentioning

confidence: 99%

Local re-acceleration and a modified thick target model of solar flare electrons

Brown¹,

Turkmani

Kontar³

et al. 2009

A&A

View full text Add to dashboard Cite

show abstract

“…This can be also expressed by a decrease of the directivity of the associated X-ray bremsstrahlung emission. This fact agrees with the statement of Kontar & Brown (2006) that conventional solar flare models with a simple downward beaming should be excluded. Fig.…”

Section: Discussionsupporting

confidence: 91%

“…Nevertheless, the paper by Kontar & Brown (2006) allows us to compare our simulations with their derived ratio of downward-to-upward electron distributions, F d (E)/F u (E). The comparison reveals an agreement between inferred F d (E)/F u (E) and Model F within the confidence interval up to ∼50 keV.…”

Section: Discussionmentioning

confidence: 99%

Electron beam – plasma system with the return current and directivity of its X-ray emission

Karlický¹,

Kašparová²

2009

A&A

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Aims. An evolution of the electron distribution function in the beam-plasma system with the return current is computed numerically for different parameters. The X-ray bremsstrahlung corresponding to such an electron distribution is calculated and the directivity of the X-ray emission is studied. Methods. For computations of the electron distribution functions we used a 3-D particle-in-cell electromagnetic code. The directivity of the X-ray emission was calculated using the angle-dependent electron-ion bremsstrahlung cross-section. Results. It was found that the resulting electron distribution function depends on the magnetic field assumed along the electron beam propagation direction. For small magnetic fields the electron distribution function becomes broad in the direction perpendicular to the beam propagation due to the Weibel instability and the return current is formed by the electrons in a broad and shifted bulk of the distribution. On the other hand, for stronger magnetic fields the distribution is more extended in the beam-propagation direction and the return current is formed by the electrons in the extended distribution tail. In all cases, the anisotropy of the electron distribution decreases rapidly due to fast collisionless processes. However, the magnetic field reduces this anisotropy decrease. The X-ray directivity shows the same trend and it is always closer to the isotropic case than that in a simple beaming model.

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“…However, an electron beam undergoing solely collisional energy loss, as in the standard thick target model, gives up around 10 5 times energy to heat than to bremsstrahlung and demands (Brown 1971) a very high electron production rate to yield observed hard X-ray fluxes. Furthermore the electron beam and hard X-ray source anisotropies in the standard thick target model (Brown 1972) are much higher than inferred from the flare hard X-ray data (Kontar & Brown 2006). Brown et al (2009) have proposed that if fast electrons, on reaching the chromosphere, undergo re-acceleration by current sheets there, their enhanced lifetimes increase the hard X-ray yield per electron, so reducing the injection rate needed for hard X-ray production, while greatly reducing the fast electron anisotropy in the main hard X-ray source.…”

Section: Introductionmentioning

confidence: 70%

Parallel electric field generation by Alfvén wave turbulence

Bian¹,

Kontar²,

Brown³

2010

A&A

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Aims. This work aims to investigate the spectral structure of the parallel electric field generated by strong anisotropic and balanced Alfvénic turbulence in relation with the problem of electron acceleration from the thermal population in solar flare plasma conditions. Methods. We consider anisotropic Alfvénic fluctuations in the presence of a strong background magnetic field. Exploiting this anisotropy, a set of reduced equations governing non-linear, two-fluid plasma dynamics is derived. The low-β limit of this model is used to follow the turbulent cascade of the energy resulting from the non-linear interaction between kinetic Alfvén waves, from the large magnetohydrodynamics (MHD) scales with k ⊥ ρ s 1 down to the small "kinetic" scales with k ⊥ ρ s 1, ρ s being the ion sound gyroradius. Results. Scaling relations are obtained for the magnitude of the turbulent electromagnetic fluctuations, as a function of k ⊥ and k , showing that the electric field develops a component parallel to the magnetic field at large MHD scales. Conclusions. The spectrum we derive for the parallel electric field fluctuations can be effectively used to model stochastic resonant acceleration and heating of electrons by Alfvén waves in solar flare plasma conditions

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Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft

Cited by 60 publications

References 29 publications

Local re-acceleration and a modified thick target model of solar flare electrons

Local re-acceleration and a modified thick target model of solar flare electrons

Electron beam – plasma system with the return current and directivity of its X-ray emission

Parallel electric field generation by Alfvén wave turbulence

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