Observations of Pc5 field line resonance azimuthal phase speeds: A diagnostic of their excitation mechanism

Abstract

A number of recent publications have suggested that Pc5 field line resonance (FLR) azimuthal phase speeds may provide a diagnostic for determining pulsation excitation mechanisms. We analyze a subset of 27 multiple frequency FLR pulsation events identified from a total of 137 events observed with the IMAGE magnetometer array from March 1994, March 1995, and March 1996 to test the validity of this diagnostic. We show that during intervals of high solar wind speed, the frequency components within a pulsation event display a commonality of azimuthal phase speed, which may be related to the propagation speed of a shear‐flow instability on the magnetopause. In contrast, events observed during lower solar wind speed intervals often result in frequency components which have differing azimuthal phase speeds and may be more closely associated with impulses in the solar wind. Examining the local time dependence of each type of event reveals that the common phase speed events seem to be much more prevalent in the morning sector. We suggest that the local time dependence of the magnetospheric waveguide modes' boundary condition may account for the observed pulsation characteristics, in accordance with the theory of Mann et al. [1999]. Our observations provide some support for the hypothesis that many of the morning sector events are driven by magnetopause shear‐flow instabilities during intervals of enhanced solar wind speed and that these events are superposed on a background of more impulsively driven pulsations which may occur over a wider range of local time in response to solar wind discontinuities and buffeting.