The heating of the temperature minimum region in solar flares — A reassessment

As a sequel to the work by Machado et al. (1978), we discuss and evaluate the suggestions made by these authors on how to possibly reconcile the observed temperature enhancements at temperature-minimum levels in solar flares with some form of theoretical heating mechanism. After establishing the H^– LTE assumption used by Machado et al., we then consider EUV irradiation, and joule heating by steady currents, as heating mechanisms. We find that, unless there are strong inhomogeneities associated with either mechanism, neither can reasonably be reconciled with observations. It is concluded that detailed, high resolution (both spatial and temporal) measurements are necessary to further our understanding of the flare process at temperature-minimum levels.On leave from: Department of Astronomy, The University, Glasgow G12 8QQ, Scotland, U. K.     

Studies in the application of recurrence relations to special perturbation methods

The procedure of numerical integration of the elliptic three dimensional restricted threebody problem by the use of recurrence relations to evaluate successively higher derivatives of the relative position and velocity vectors of the bodies and of the variational matrix is investigated. A set of recurrence relations is developed which involves the introduction of fewer auxiliary variables than in previous papers of this series, while the recurrence relations themselves are of a simpler form than those in other treatments involving the same number of such auxiliary variables. A technique for automatic adjustment of the integration step-length at each point in the orbit, such that the local truncation error remains close to, but always less than, some specified amount, is incorporated. This technique involves the comparison of pre-integration values with those obtained after consecutive forward and reverse integration steps, and has decided advantages over step-adjustment methods currently in use.Both these modifications to previous techniques are shown, by presentation of sample computational results, to represent considerable savings in machine time for a given calculation and desired accuracy; these savings are generally around a factor of two and become greater as the desired accuracy in the computations increases.     

A case study of impact‐induced hydrothermal activity: The Haughton impact structure,Devon Island,Canadian High Arctic

Abstract— The well‐preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid‐size complex impact craters. A moderate‐ to low‐temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ?100–120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (?250 down to ?90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ?150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton.     

Bayesian calibrated significance levels applied to the spectral tilt and hemispherical asymmetry

Bayesian model selection provides a formal method of determining the level of support for new parameters in a model. However, if there is not a specific enough underlying physical motivation for the new parameters it can be hard to assign them meaningful priors, an essential ingredient of Bayesian model selection. Here we look at methods maximizing the prior so as to work out what is the maximum support the data could give for the new parameters. If the maximum support is not high enough then one can confidently conclude that the new parameters are unnecessary without needing to worry that some other prior may make them significant. We discuss a computationally efficient means of doing this which involves mapping p-values on to upper bounds of the Bayes factor (or odds) for the new parameters. A p-value of 0.05 (1.96σ) corresponds to odds less than or equal to 5:2, which is below the 'weak' support at best threshold. A p-value of 0.0003 (3.6σ) corresponds to odds of less than or equal to 150:1, which is the 'strong' support at best threshold. Applying this method we find that the odds on the scalar spectral index being different from one are 49:1 at best. We also find that the odds that there is primordial hemispherical asymmetry in the cosmic microwave background are 9:1 at best.     

Thermal plasma enhancements in the topside ionosphere and their relationship to the auroral electrojets

Data from the Topside sounder aboard ISIS II are used to calculate the latitudinal distribution of thermal plasma at 1400 km when the satellite moves along a line of constant geographic longitude. The distribution of thermal plasma is related to the latitudinal regime of the auroral electrojets as inferred from data from a meridian line of magnetometers. It is found that thermal plasma at high altitude tends to be found above the poleward portion of the auroral electrojet. This finding is explained in terms of the spectrum of precipitating electrons across the auroral oval. It is found that the thermal plasma distribution in the post-noon sector is distinctly different from that in the premidnight sector despite the fact that both quadrants feature clear latitudinally confined eastward current flow. This difference can be used to define whether or not the polar cleft penetrates into the local time sector traversed by the satellite. On the nightside, the peak in thermal plasma poleward of the ionospheric trough can be used to identify the magnetic field lines which map to the boundary between the tail lobe and the plasma sheet.     

The Future of Radical Geography*

Radical geography has posed new, and refashioned many old, geographical problems. These problems require attention irrespective of one's political leanings. Yet radical geography has been consistently ignored by mainstream geographers since its inception. This essay examines the reasons for this situation, and finds four answers in Peet's recent review of the history of radical geography [1]. It seems likely that radical geography will continue to receive a cold shoulder unless its future exponents take pains to counter this trend. Six conditions are required to ensure progress, including effective communication in established journals, methodological heterodoxy, and a systematic, discipline-wide evaluation of the contribution of radical geography.     

The outflow of solar wind from the active regions

The numerical integration of hydrodynamics equations with an allowance for thermal conductivity was made using the temperature distribution in the corona situated above the active regions obtained from the damping time of solar radio bursts of Types III and V. It is essential that for the integration path serve the magnetic field lines along which exciters of bursts are moving and accelerated coronal plasma can move freely too.The main result is the discovery of such regions, where the high temperature gradient precludes the possibility of a continuous flow of coronal plasma. These regions, where intense heating and rapid acceleration of the coronal plasma take place, were situated at distances of about 2 R from the Sun's center. They probably possess the character of weak detonation waves. The waves of cooling can also be present in these regions of discontinuity of the flow. The observations of bursts of Type V at distances up to 6.3 R gives some evidence that discontinuities of flow of the solar wind of the same nature can possibly arise also in the more remote parts of the solar corona.It is important that the similar jumps of velocity and other parameters of coronal plasma were also discovered earlier in a quite independent way as a result of the interpretation of the solar radio echo data. It can be anticipated that the nonthermal heating of coronal plasma, which was postulated to remove discrepancies between the existing models and observations of solar wind, was localized mainly in these regions thus playing an important role in the formation of the fundamental properties of the interplanetary medium.The obtained results are of preliminary character since there are no reliable and homogeneous data on bursts of Types III and V especially at 20-10 MHz, where the work is difficult due to the man-made interference and also at still lower frequencies, observed by the cosmic probes. We can hope that the filling of this gap allows us to construct a realistic model of outflow of coronal plasma from active regions, which can be successfully compared with the results of direct measurement of parameters of solar wind.     

Radar studies of the non-spherically symmetric solar corona

We review the results of radar studies of the Sun made at El Campo, Texas 1961–69 with particular emphasis on the record of observed solar radar cross sections. Using ray traces which include the effects of refraction, absorption, and scattering in non-spherically symmetric models of the corona, we investigate the role of focusing by large-scale coronal geometries in enhancing the radar cross section. We find that certain coronal geometries (e.g. disk-center coronal holes) can, in principle, significantly increase the radar cross section. However the observations of large cross sections do not correspond very well with periods when such large-scale focusing geometries existed in the corona. We conclude that the present dataset does not support the hypothesis that radar observations of the Sun will be useful in determining the properties of large-scale coronal features.The National Center for Atmospheric Research is sponsored by the National Science Foundation.