ALBERT

Notes: Abstract We examine some of the consequences of an electrostatically unstable return current associated with heat conduction during a solar flare. We note that an electrostatically unstable return current will lead to strong hydrodynamic effects and more rapid magnetic field thermalization, if reconnection is the source of primary energy release during a solar flare.

Notes: Abstract We discuss the claim frequently found in the solar physics literature that line tying in the photosphere is capable of stabilizing certain classes of both ideal and resistive MHD instabilities. Our approach is to present a picture of the physical origins of line tying and how resistive effects will affect it.

Notes: Abstract We consider a beam driven unstable plasma and estimate the turbulent electric fields which may be excited by this beam. We then estimate the Stark broadening due to such fields.

Notes: Abstract There are two parts to this paper. In the first we calculate the hydrodynamic response of the solar atmosphere to the injection of an intense beam of electrons in a numerical simulation of a solar flare. In the second we predict the spectroscopic consequences of the hydrodynamic behaviour calculated in the first part. The hydrodynamics is predicted by solving the equations of conservation of mass, momentum, and energy. The latter is expressed as two temperature equations; one for the electrons and the other for the neutral atoms and positive ions of hydrogen. The equations are solved in one dimension and the geometric form is of a semi-circular loop having its ends in the photosphere. The results show how the loop is filled at supersonic speed with plasma at temperatures characteristic of flares. At the same time a compression wave is predicted to propagate down towards the photosphere. After the heating pulse stops, the plasma that has risen into the loop, starts to decay and return to the condition it was in before the pulse started. In predicting the spectrum that would be emitted by such a plasma calcium was chosen for illustration. The first and main part of this calculation was setting up and solving the time-dependent equations of ionization/recombination. In order to provide a standard for comparison the same ionization and recombination rate coefficients are used to predict the steady-state distribution of populations of ionization stages. This is then compared with the distribution found from the time-dependent solution and shows that there is a negligibly small time lag predicted by the time-dependent result. However the more significant comparisons to make are between the temperatures of the peak abundances of the various ions under the assumptions of steady-state and time-dependent ionization. For the particular circumstances chosen here the temperature differences are predicted to be in the neighbourhood of 10% or less and in view of the overall accuracy of the atomic data are not significant. It would appear therefore that the much simpler assumption of steady-state ionization balance leads to results of acceptable accuracy for the particular case considered.

Notes: Abstract In this note we clarify the conditions in which charged particles may be accelerated by a DC electric field in the presence of plasma micro-turbulence.

Notes: Abstract We examine some of the possible consequences associated with the steep temperature gradients believed to exist in the transition zone. It is shown that the heat flux in the transition zone may be weakly electrostatically unstable and that the thermal electric field associated with the same step temperature gradients will lead to the generation of copious numbers of non-thermal electrons ‘boiling off’ into the corona.

Notes: Abstract A possible diagnostic for differentiating between a slow and a transient buildup of current prior to a flare is developed. It is noted that if this diagnostic can be realized in practice, it will yield information not only on the rate the current is built up, but also on the magnitude of the electric field that is precipitating the buildup.

Notes: Abstract We present a classification scheme for solar flare models that utilize magnetic free energycurrents. The classification scheme is geometry independent and delineates models into two categories: those models utilizing currents flowing parallel to B and those utilizing currents flowing perpendicular to B. This delineation of drivers allows us to specify what kinds of plasma-magnetic field configurations should be expected for a given current driver. Further, the delineation of drivers will allow us to identify both the strengths and the weaknesses of the various models.