Siple station VLF wave injection experiments aimed at finding the properties of the magnetospheric hot plasma were conducted for a 9-hour period between 1705 and 0210 UT on January 23--24, 1988. A special frequency versus time format, lasting 1 min and transmitted every 5 min, consisted of a sequence of pulses, frequency ramps, and parabolas, all in a 1-kHz range centered on 2400 Hz. The transmitted signals, after propagating along a geomagnetic field-aligned duct, were recorded at Lake Mistissini, Canada. At various times during the 9-hour interval the Siple signals showed features characteristic of wave-particle interactions, including wave growth, sidebands, and triggered emissions. Our observations, primarily at 2400 Hz, show that (1) there were no correlations between the initial levels, the growth rates, and the saturation levels of constant-frequency pulses; (2) in general, the values of growth rate and saturation level of two pulses injected within 30 s were nearly the same; (3) the initial level, growth rate, and saturation level showed temporal variations over 5--15 min and 1--2 hour timescales; (4) the leading edges of constant-frequency signals underwent spatial amplification; and (5) under conditions of saturation the received signal bandwidth (~20 Hz) remained constant over a 1-hour period, although the saturation level and growth rate varied during the same period. On the assumption that gyroresonant interactions were responsible for the observed wave growth and saturation, the timescales over which those phenomena varied provide constraints on the possible energetic electron population within the duct. In the reference frame of the duct (L~5.1, Ne~280 cm-3) the particle fluxes showed no variation over a 30-s timescale but varied over 5--15 min and 1--2 hour timescales. The 5--15 min timescale variations indicate longitudinal structures ranging from ~0.2¿ or ~100 km (in the equatorial plane) for electrons with energy E=0.6 keV and pitch angle &agr;=40¿, to ~5¿ or ~2800 km for electrons with energy E=11 keV and pitch angle &agr;=80¿. The hour-long time variations indicate longitudinal structures ranging from ~2¿ or ~1100 km (in the equatorial plane) for electrons with energy E=0.6 keV and pitch angle &agr;=40¿, to ~45¿ or ~25,000 km for electrons with energy E=11 keV and pitch angle &agr;=80¿. We conclude that ground-based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.¿ 1997 American Geophysical Union