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Ann. Geophys., 23, 3375-3388, 2005
www.ann-geophys.net/23/3375/2005/
© European Geosciences Union 2005
Excitation of field line resonances by sources outside the magnetosphere
A. D. M. Walker
School of Physics, University of KwaZulu-Natal, Durban 4041, South Africa
Abstract. Field line resonances are thought to be excited by sources either at the
magnetopause or outside it. Recent observations suggest that they may be
associated with coherent oscillations or pressure pulses in the solar wind.
In either case the excitation mechanism can be understood by considering the
incidence of a harmonic wave on the magnetopause from outside the
magnetosphere. Calculations are performed in a plane stratified model that
consists of (i) a magnetosheath region streaming tailward at uniform velocity
(ii) a sharp boundary representing the magnetopause, (iii) a magnetosphere
region in which the Alfvén speed increases monotonically with distance from
the magnetopause. The structure implies the existence of a propagating region
within the magnetopause bounded by a reflection level or turning point.
Beyond this is a region in which waves are evanescent and a resonance level.
The reflection and transmission of harmonic waves incident from the
magnetosheath is considered in this model. It is shown that, in most cases,
because of the mismatch between the magnetosphere and the magnetopause, the
wave is reflected from the magnetopause with little penetration. At critical
frequencies corresponding to the natural frequencies of the cavity formed
between the magnetopause and turning point the signal excites the cavity and
may leak evanescently to the resonance. The calculation includes the effect
of the counter-streaming magnetosheath and magnetosphere plasmas on the wave.
This can lead to amplification or attenuation. The nature of the processes
that lead to transmission of the wave from magnetosheath to resonance are
considered by synthesising the signal from plane wave spectra. A number of
mechanisms for exciting cavity modes are reviewed and the relationship of the
calculations to these mechanisms are discussed. Observations needed to
discriminate between the mechanisms are specified.
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