Jennifer Nicholls

Surface currents on models of force-free solar magnetic flux tubes

A model of a cylindrically symmetric, force-free magnetic field consisting of a sequence of concentric layers with piecewise-constant a is used to construct models of the surface currents on isolated, force-free magnetic flux tubes. Two boundary conditions are considered: a current-neutralized flux tube (Bq = 0, B, + 0 a t r > r,), and an isolated current-carrying flux tube (Bq f 0, B, = 0 a t r > r,). A single-a model that is current-neutralized is a reverse-field pinch, and is unacceptable as a model for a solar flux tube. Examples of two-a models for a current-neutralized flux tube are presented. The models of the surface currents satisfying either boundary condition are shown to simplify considerably when the surface layer is thin. A model consisting of several layers, with piecewise-constant a, may be used to find an approximate solution for a force-free flux tube with an arbitrarily specified current profile.

A Radio Survey of the SMC at 843 MHz with the MOST: I. The Survey

A radio survey at a frequency of 843 MHz of 36 square degrees containing the Small Magellanic Cloud (SMC) has been made with the Molonglo Observatory Synthesis Telescope (MOST). The angular resolution is around 45 arcsec and the rms noise is about 1 mJy per beam. The radio image of the region is presented showing over a thousand sources with a variety of angular sizes from unresolved to 10 arcmin. Most of the sources are presumed to be background objects but about 70 of the more extended sources are H II regions or supernova remnants within the SMC.

Nonthermal Radio Emission from the Precataclysmic Binary System V471 Tauri

Recent observations have presented strong evidence for the existence of eclipses in the nonthermal radio emission from the precataclysmic binary system V471 Tau. In this paper we calculate the gyrosynchrotron emission from models assuming a dipole-shaped magnetosphere and a magnetized wind around the K dwarf. The model assumes that electrons are accelerated to mildly relativistic energies in the region where the magnetic fields of the two stars interact and become trapped in the K dwarfs magnetosphere. Comparison with the data indicates that azimuthal drift and energy loss lead to a region of enhanced density of mildly relativistic electrons, extending for over 100° in azimuth and preceding the white dwarf in phase. The data is best fitted by models assuming a power-law drop with azimuthal angle of this enhancement in mildly relativistic electron density.

Is Alfvén’s Critical Ionization Velocity Important at the Galactic Centre?

If the relative velocity between a plasma and a molecular cloud impinging on it exceeds the Alfven critical ionization velocity, then the surface of the molecular cloud can be ionized. This phenomenon has been used to model some of the ionized regions at the Galactic Centre. A review of the mechanism behind Alfven’s critical ionization phenomenon is presented here, and its use at the Galactic Centre is discussed.

Energy release at Alfvénic fronts in a force-free magnetic flux tube

Melroses model (1992) for the energy propagation into the energy release site in a solar flare in terms of Alfvenic fronts is extended to a constant-a force-free flux tube, relaxing the constraint of a uniform current profile. The important features of the model depend on α and the flux tube radius, r0, only in the combination αr0.