Coherent variable frequency signals (ramps) extending from 1 to 8 kHz, injected into the magnetosphere from Siple Station, Antarctica (L=4.3) exhibit upper and lower cutoffs when received at the conjugate station, Roberval, Quebec. Ramp group delay measurements and ionospheric sounding data are used for the first time to determine the cold plasma density and L shell of the propagation path. Relationships among f, df/dt, and the ''phase equator'' for gyroresonance are calculated using second-order resonance equations generalized to relativistic electrons. Observed upper cutoff characteristics are interpreted in terms of off-equatorial gyroresonant interaction regions and ducted propagation limited to frequencies below half the local gyrofrequency. The observed lower cutoff frequencies varied systematically with transmitted ramp slope, suggesting a threshold in the resonant electron number density above which rapid temporal wave growth and saturation can occur. This concept is used to develop a hot plasma diagnostic technique which, for an assumed g(&agr;)v-n electron distribution, provides an estimate of the energy dependence n. A test of this technique is given using the data and a simplified wave-particle interaction simulation. Additional aspects of the magnetospheric response to ramp injection, including emission triggering, are discussed.