A comparison of the height distributions of solar flare hard X-rays in thick target and thermal models

The height structure of hard X-ray bremsstrahlung emission in solar flares is computed for two different models of bremsstrahlung production: emission from a descending beam of nonthermal electrons, and thermal emission from a coronally confined hot plasma. It is shown how these models give rise to hard X-ray spatial distributions which are distinguishable by current instrumentation, and that, therefore, the models may be distinguished by such spatially resolved hard X-ray measurements

On the importance of reverse current ohmic losses in electron-heated solar flare atmospheres

The passage of a beam of nonthermal electrons through the flaring solar atmosphere is considered, paying particular attention to the requirement that the beam be stable to the generation of plasma turbulence. The ratio is computed of energy losses due to reverse current ohmic heating, and heating by Coulomb collisions, respectively, for the greatest flux which can pass stably through the atmosphere. It is demonstrated that this ratio is determined by the low energy cutoff of the beam, by the electron temperature of the ambient atmosphere, and by the electron to ion temperature ratio. It is also independent of the atmospheric density

Theoretical motivation for high spatial resolution, hard X-ray observations during solar flares

The important role played by hard X-ray radiation as a diagnostic of impulsive phase energy transport mechanism is reviewed. It is argued that the sub-arc second resolution offered by an instrument such as the Pinhole/Occulter Facility (P/OF) can greatly increase our understanding of such mechanisms

Temperature minimum heating in solar flares by resistive dissipation of Alfven waves

The possibility that the strong heating produced at temperature-minimum levels during solar flares is due to resistive dissipation of Alfven waves generated by the primary energy release process in the corona is studied. It is shown how, for suitable parameters, these waves can carry their energy essentially undamped into the temperature-minimum layers and can then produce a degree of heating consistent with observations

The interpretation of hard X-ray polarization measurements in solar flares

Observations of polarization of moderately hard X-rays in solar flares are reviewed and compared with the predictions of recent detailed modeling of hard X-ray bremsstrahlung production by non-thermal electrons. The recent advances in the complexity of the modeling lead to substantially lower predicted polarizations than in earlier models and more fully highlight how various parameters play a role in determining the polarization of the radiation field. The new predicted polarizations are comparable to those predicted by thermal modeling of solar flare hard X-ray production, and both are in agreement with the observations. In the light of these results, new polarization observations with current generation instruments are proposed which could be used to discriminate between non-thermal and thermal models of hard X-ray production in solar flares

The Crimean Solar Maximum Year Workshop, selected reports

Problems associated with the transport of energy and acceleration of charged particles in solar flares are considered. Existing theories are compared with observation with a view to either discriminating between rival theories (such as whether hard X-rays are emitted by thermal or nonthermal bremsstrahlung), constraining existing theories (such as deduction of the number of nonthermal electrons present from spectroscopic diagnostics in the soft X-ray part of the spectrum), or suggesting theories (such as attempting to explain the observed spatial structure of microwave emission relative to alpha)

The quasi-linear relaxation of thick-target electron beams in solar flares

The effects of quasi-linear interactions on thick-target electron beams in the solar corona are investigated. Coulomb collisions produce regions of positive gradient in electron distributions which are initially monotonic decreasing functions of energy. In the resulting two-stream instability, energy and momentum are transferred from electrons to Langmuir waves and the region of positive slope in the electron distribution is replaced by a plateau. In the corona, the timescale for this quasi-linear relaxation is very short compared to the collision time. It is therefore possible to model the effects of quasi-linear relaxation by replacing any region of positive slop in the distribution by a plateau at each time step, in such a way as to conserve particle number. The X-ray bremsstrahlung and collisional heating rate produced by a relaxed beam are evaluated. Although the analysis is strictly steady state, it is relevant to the theoretical interpretation of hard X-ray bursts with durations of the order of a few seconds (i.e., the majority of such bursts)

How accurately do adult sons and daughters report and perceive parental deaths from coronary disease?

<b>OBJECTIVES</b>: To describe how adult sons and daughters report and perceive parental deaths from heart disease <b>DESIGN</b>: Two generation family study. <b>SETTING</b>: West of Scotland. <b>SUBJECTS</b>: 1040 sons and 1298 daughters aged 30-59 from 1477 families, whose fathers and mothers were aged 45-64 in 1972-76 and have been followed up for mortality over 20 years. <b>OUTCOME</b> : Perception of a "family weakness" attributable to heart disease. RESULTS : 26% of sons and daughters had a parent who had died of coronary heart disease (CHD). The proportion was higher in older offspring (+18% per 10 year age difference) and in manual compared with non-manual groups (+37%). Eighty nine per cent of parental deaths from CHD were correctly reported by offspring. Only 23% of sons and 34% of daughters with at least one parent who had died of CHD considered that they had a family weakness attributable to heart disease. Perceptions of a family weakness were higher when one or both parents had died of CHD, when parental deaths occurred at a younger age, in daughters compared with sons and in offspring in non-manual compared with manual occupations. <b>CONCLUSIONS</b>: Only a minority of sons and daughters with experience of a parent having died from CHD perceive this in terms of a family weakness attributable to heart disease. Although men in manual occupations are most likely to develop CHD, they are least likely to interpret a parental death from CHD in terms of a family weakness. Health professionals giving advice to patients on their familial risks need to be aware of the difference between clinical definitions and lay perceptions of a family history of heart disease

Discrepancies between empirical and theoretical models of the flaring solar chromosphere and their possible resolution

Models of the solar chromosphere during flaring deduced theoretically or empirically are compared. Marked discrepancies are noted and various reasons are offered to explain their existence. A means is presented for testing theoretical heating models (electron heating) by analyzing the net energy loss rates in (observed) empirical atmospheres and inverting the flare energy equation to deduce the parameters of the supposed heating mechanism

Turbulent pitch-angle scattering and diffusive transport of hard-X-ray producing electrons in flaring coronal loops

Recent observations from {\em RHESSI} have revealed that the number of non-thermal electrons in the coronal part of a flaring loop can exceed the number of electrons required to explain the hard X-ray-emitting footpoints of the same flaring loop. Such sources cannot, therefore, be interpreted on the basis of the standard collisional transport model, in which electrons stream along the loop while losing their energy through collisions with the ambient plasma; additional physical processes, to either trap or scatter the energetic electrons, are required. Motivated by this and other observations that suggest that high energy electrons are confined to the coronal region of the source, we consider turbulent pitch angle scattering of fast electrons off low frequency magnetic fluctuations as a confinement mechanism, modeled as a spatial diffusion parallel to the mean magnetic field. In general, turbulent scattering leads to a reduction of the collisional stopping distance of non-thermal electrons along the loop and hence to an enhancement of the coronal HXR source relative to the footpoints. The variation of source size $L$ with electron energy $E$ becomes weaker than the quadratic behavior pertinent to collisional transport, with the slope of $L(E)$ depending directly on the mean free path $\lambda$ again pitch angle scattering. Comparing the predictions of the model with observations, we find that $\lambda \sim$$(10^8-10^9)$ cm for $\sim30$ keV, less than the length of a typical flaring loop and smaller than, or comparable to, the size of the electron acceleration region.Comment: 25 pages, 5 figures, accepted for publication in Astrophysical Journa