Magnetospheric cavity modes and field-line resonances: The effect of radial mass density variation

Abstract

The relationship between magnetospheric cavity modes and field-line resonances (FLRs) is examined as a function of the radial plasma mass density distribution in the dayside magnetosphere. Using a numerical model of wave coupling we find that, for monotonie mass densities of the form L − q , the radial position and amplitude of cavity-mode driven FLRs increases as q decreases. This makes the afternoon sector of Magnetic Local Time the most favourable for the detection of such resonances, and suggests that FLRs observed at high magnetic latitudes are not driven by global cavity modes. We also find that the maximum damping of the fundamental cavity mode by coupling to a FLR appears to occur for q ≅ 4 and m ≅ 3, where m is the azimuthai wave number. Using models for morning and afternoon mass densities which include a small plasmapause at L ≈ 4, the structure of wave fields is examined in detail. We find that the base of the plasmapause should be a favoured position for the occurrence of long-period hydromagnetic wave activity. In addition to double FLRs which have been described previously, quasi-FLRs should be present at the plasmapause base. These occur when a cavity mode period (or the period of any other monochromatic driving wave) is less than the minimum field-line eigenperiod at the plasmapause base. Field-lines are driven off-resonance, but a relatively large amplitude peak still results when the driving period is close to the minimum eigenperiod. The distinguishing feature of a quasi-FLR is that there is no polarization reversal across the amplitude peak, and the polarization is not linear at the peak. It is suggested that amplitude peaks with no polarization reversal across the peak may be indicators of the plasmapause position in ground-based observations.