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

A number of recent publications have suggested that Pc5 field line resonanc e (FLR) azimuthal phase speeds may provide a diagnostic for determining pul sation excitation mechanisms. We analyze a subset of 27 multiple frequency FLR pulsation events identified from a total of 137 events observed with th e IMAGE magnetometer array from March 1994, March 1995, and March 1996 to t est 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 propag ation speed of a shear-flow instability on the magnetopause. In contrast, e vents observed during lower solar wind speed intervals often result in freq uency components which have differing azimuthal phase speeds and may be mor e closely associated with impulses in the solar wind. Examining the local t ime dependence of each type of event reveals that the common phase speed ev ents seem to be much more prevalent in the morning sector. We suggest that the local time dependence of the magnetospheric waveguide modes' boundary c ondition may account for the observed pulsation characteristics, in accorda nce with the theory of Mann et al. [1999]. Our observations provide some su pport 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 im pulsively driven pulsations which may occur over a wider range of local tim e in response to solar wind discontinuities and buffeting.