Abstract
Abstract. A new concept is proposed for the emergence of ULF geomagnetic oscillations with a discrete spectrum of frequencies (0.8, 1.3, 1.9, 2.6 ...mHz) registered in the magnetosphere's midnight-morning sector. The concept relies on the assumption that these oscillations are MHD-resonator eigenmodes in the near-Earth plasma sheet. This magnetospheric area is where conditions are met for fast magnetosonic waves to be confined. The confinement is a result of the velocity values of fast magnetosonic waves in the near-Earth plasma sheet which differ greatly from those in the magnetotail lobes, leading to turning points forming in the tailward direction for the waves under study. To compute the eigenfrequency spectrum of such a resonator, we used a model magnetosphere with parabolic geometry. The fundamental harmonics of this resonator's eigenfrequencies are shown to be capable of being clustered into groups with average frequencies matching, with good accuracy, the frequencies of the observed oscillations. A possible explanation for the stability of the observed oscillation frequencies is that such a resonator might only form when the magnetosphere is in a certain unperturbed state.
Highlights
Since the foundation work by Dungey (1954), the magnetosphere has been regarded as a natural resonator for various types of MHD oscillations
The main results of this work may be summarised as follows: 1. A new concept is proposed for low-frequency oscillations with discrete spectrum which are registered by HF radars and networks of ground-based magnetometers at high latitudes
The quantisation of oscillation frequencies is supposed to be related to their being eigenmodes of the resonator in the near-Earth plasma sheet (NEPS)
Summary
Such oscillations have been registered in reality – both in HF-radar observations (Ruohoniemi et al, 1991; Samson and Harrold, 1992) and by a ground-based magnetometer network (Mathie et al, 1999; Wanliss et al, 2002). The theoretical oscillation frequency considerably exceeds the observed one in this case as well Lateral boundaries of such a waveguide are formed by the magnetopause (a 10 to 100-times Alfven velocity increase in it does produce the effect of reflection of magnetosonic waves). Another version of the model in question exists where the waveguide is located in the plasma sheet (Siscoe, 1969) This case presents the same difficulties with explaining the discrete spectrum and the numerical value of the frequency. Such inspection at a qualitative-analysis level has recently been done by the authors (Leonovich and Mazur, 2005).
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