The effects of magnetic-field geometry on longitudinal oscillations of solar prominences: Cross-sectional area variation for thin tubes

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: mluna@iac.es
² Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Departament Física, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain
⁴ Institute of Applied Computing & Community Code (IAC 3 ), 07122 Palma de Mallorca, Spain
⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 3 May 2016
Accepted: 10 June 2016

Abstract

Context. Solar prominences are subject to both field-aligned (longitudinal) and transverse oscillatory motions, as evidenced by an increasing number of observations. Large-amplitude longitudinal motions provide valuable information on the geometry of the filament-channel magnetic structure that supports the cool prominence plasma against gravity. Our pendulum model, in which the restoring force is the gravity projected along the dipped field lines of the magnetic structure, best explains these oscillations. However, several factors can influence the longitudinal oscillations, potentially invalidating the pendulum model.

Aims. The aim of this work is to study the influence of large-scale variations in the magnetic field strength along the field lines, i.e., variations of the cross-sectional area along the flux tubes supporting prominence threads.

Methods. We studied the normal modes of several flux tube configurations, using linear perturbation analysis, to assess the influence of different geometrical parameters on the oscillation properties.

Results. We found that the influence of the symmetric and asymmetric expansion factors on longitudinal oscillations is small.

Conclusions. We conclude that the longitudinal oscillations are not significantly influenced by variations of the cross-section of the flux tubes, validating the pendulum model in this context.