Relative intensity of toroidal and compressional Pc 3‐4 wave power in the dayside outer magnetosphere

Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field data are used to study the excitation mechanism of local toroidal harmonic field line resonances (MRs). Enhanced toroidal fluctuations can exhibit either clear harmonic resonance signatures or broadband power enhancements. This may indicate that two different processes are operating. To assess the differences between toroidal harmonic and broadband events statistically, we examined data from 56 dayside orbits and selected intervals that displayed either clear toroidal harmonics (Harmonic) or primarily broadband (Broadband) toroidal power. The resulting database consisted of 878 intervals, each 26.5 min long. Spectral power was integrated from 10 to 50 mHz to obtain radial, PR, toroidal (eastward), PE, and compressional (northward), PN, power levels. Absolute toroidal power levels were comparable for both Harmonic and Broadband events, ranging from 0.1 to 20 nT 2 for both categories. To minimize the influence of activity variations in power level, we examined power ratios, for example, PE/PN, rather than the absolute power levels. The Harmonic events have an average PE/PN of 2.28±0.07, whereas PE/PN averages 1.51±0.05 for the Broadband events. The average radial power is essentially equal to the compressional power: average PR/PN is 1.04±0.04 for Harmonic and 1.00±0.04 for Broadband events. That toroidal power is enhanced for the Broadband as well as Harmonic events suggests that resonance effects occur in the Broadband events but that the resonances are not sufficiently developed to be apparent as harmonics in power spectra. For the Harmonic events PE/PN decreases with L, whereas for Broadband events PE/PN is constant or rises with L, also suggesting that their sources are different. There are three ways to interpret these results. First, one can attribute the Harmonic events to coupling of compressional modes to local MRs. Since the observed MRs occur over a continuous range in frequency, one must assume, however, that the cavity eigenfrequency spectrum is also nearly continuous, that is, the number of eigenmodes is very large. The Broadband events are not readily accounted for in the cavity mode picture, however. Alternately, if the magnetospheric system is considered to be a relatively poor global resonator, low Q, due to Joule dissipation and/or a Poynting flux directed out of the cavity, there are two possible explanations. In the first low-Q scenario, harmonic toroidal fluctuations are driven by the high-latitude entry mechanism of Engebretson et al. (1991), whereas the Broadband events are due to coupling of propagating compressional waves with the shear Alfven mode. The second low-Q scenario attributes both Harmonic and Broadband events to local MR excitation by propagating compressional waves, but the Harmonic events correspond to local MRs excited by longer wavelength perturbations at the magnetopause, yielding better coupling to the local MRs and hence more fully developed resonance signatures