Arched Flux Tubes Research Articles

The three-dimensional breakup of a magnetic layer

ABSTRA C T Three-dimensional numerical simulations of the instability of a layer of magnetic field caused by magnetic buoyancy are carried out over a range of parameter values. The layer breaks up into a number of interlocking magnetic flux tubes that become increasingly threedimensional, although strongly arched flux tubes are not observed. The introduction of background rotation has the principal effect of suppressing the instability. The a -effect, which measures the twist of the flux tubes induced by the rotation, is found to be positive (in the northern hemisphere) but small in magnitude.

A siphon-flow model of the photospheric Evershed flow in a sunspot

The Evershed flow at photospheric heights in a sunspot penumbra is modeled theoretically as a siphon flow along individual, arched magnetic flux tubes embedded in an atmosphere permeated by a uniform magnetic field. This approach is suggested by the recent evidence that the penumbra is a deep structure with a significant amount of emerging flux, rather than a shallow structure overlying field-free gas. The model resolves two problems associated with siphon-flow models based on a shallow penumbra: it produces arched flux tubes of sufficient horizontal extent, and it explains how the optically thin flux tubes carrying the Evershed flow can appear dark.

Siphon flows in isolated magnetic flux tubes. V - Radiative flows with variable ionization

Steady siphon flows in arched isolated magnetic flux tubes in the solar atmosphere are calculated here including radiative transfer between the flux tube and its surrounding and variable ionization of the flowing gas. It is shown that the behavior of a siphon flow is strongly determined by the degree of radiative coupling between the flux tube and its surroundings in the superadiabatic layer just below the solar surface. Critical siphon flows with adiabatic tube shocks in the downstream leg are calculated, illustrating the radiative relaxation of the temperature jump downstream of the shock. For flows in arched flux tubes reaching up to the temperature minimum, where the opacity is low, the gas inside the flux tube is much cooler than the surrounding atmosphere at the top of the arch. It is suggested that gas cooled by siphon flows contribute to the cool component of the solar atmosphere at the height of the temperature minimum implied by observations of the infrared CO bands at 4.6 and 2.3 microns.

Siphon flows in isolated magnetic flux tubes. II - Adiabatic flows

This paper extends the study of steady siphon flows in isolated magnetic flux tubes surrounded by field-free gas to the case of adiabatic flows. The basic equations governing steady adiabatic siphon flows in a thin, isolated magnetic flux tube are summarized, and qualitative features of adiabatic flows in elevated, arched flux tubes are discussed. The equations are then cast in nondimensional form and the results of numerical computations of adiabatic siphon flows in arched flux tubes are presented along with comparisons between isothermal and adiabatic flows. The effects of making the interior of the flux tube hotter or colder than the surrounding atmosphere at the upstream footpoint of the arch is considered. In this case, is it found that the adiabatic flows are qualitatively similar to the isothermal flows, with adiabatic cooling producing quantitative differences. Critical flows can produce a bulge point in the rising part of the arch and a concentration of magnetic flux above the bulge point.

Siphon flows in isolated magnetic flux tubes

Previous studies of siphon flows in magnetic flux tubes have been limited to the case of a low-β plasma, appropriate for an embedded flux tube in the solar corona or chromosphere. Here the author studies steady siphon flows in isolated, thin magnetic flux tubes surrounded by field-free gas, with plasma β ≥ 1, appropriate for conditions in the solar photosphere. The author presents the basic equations governing steady siphon flows in a thin, isolated magnetic flux tube, and discusses the case of a purely horizontal flux tube, for which gravitational forces do not come into play. Qualitative features of isothermal flows in elevated, arched flux tubes are then studied. The author presents the results of some numerical computations of isothermal siphon flows in arched flux tubes and discusses the nature of critical flows and the need for standing "tube shocks" in these flows. Finally, he discusses applications to intense magnetic flux tubes in the solar photosphere and considers the possible interpretation of the Evershed flow in the penumbral photosphere of a sunspot as a siphon flow along isolated magnetic flux tubes.