Properties of the magnetospheric hot plasma distribution deduced from whistler mode wave injection at 2400 Hz: Ground‐based detection of azimuthal structure in magnetospheric hot plasmas

Abstract

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 l 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 α = 40°, to ∼ 5° or ∼2800 km for electrons with energy E = 11 keV and pitch angle α = 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 α = 40°, to ∼ 45° or ∼25,000 km for electrons with energy E = 11 keV and pitch angle α = 80°. We conclude that ground‐based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.

An explanation of non-observation of transmitted VLF signals on ground

“…Thus there is a limit of pitch angles beyond which no cyclotron resonance will be possible. As Sonwalkar et al (1997) consider that electrons with pitch angle aX401 were playing their role in wave-particle interaction in this case we restrict our study to a ¼ 4012801:…”

“…This method is also adopted by Sonwalkar et al (1997) for computing pitch angle dependence of drift/ duct periods. Since total electron energy is now pitch angle dependent we write E as EðaÞ.…”

“…Their central properties are used for diagnostics of both the thermal plasma (Helliwell, 1965) and the energetic electrons that are trapped in the radiation belts (Carlson et al, 1985;Singh, 1994). A lot of work has been done in this field by Stanford group using not only natural whistlers (generated by lightning discharges), ELF/VLF/plasmaspheric hiss but also man-made signals (Helliwell et al, 1986, Sonwalkar et al, 1997. Siple station transmitted signals have been utilized since 1973 to probe the hot plasma of the earth's magnetosphere.…”

“…Received signals bandwidth remained almost constant at 20 Hz. Assuming gyroresonant interaction responsible for observed wave growth and saturation level, Sonwalkar et al (1997) computed longitudinal structures which were found to be between 100-2800 km (for 5-15 min time scale) and 1100-25000 km (for 1-2 h time scale) and found that though signals were transmitted at 1.9-2.9 kHz frequencies, the receiver did not record all the frequencies. Whereas 1.9 kHz waves were received throughout the experiment, other waves in general had an upper cut-off at 2.4 kHz (in some cases the upper cut-off occurred at 2500-2550 Hz but that is not our point to study).…”

“…During 1985-88 the Siple station transmitted signals which were received at Mitsissini, Canada (as received signals) were used to investigate the spatial structure and other properties of the earth's magnetospheric hot plasma. Such an experiment (Sonwalkar et al, 1997) was conducted on January 23-24, 1988 for a 9 h period between 1705 and 0210 UT. In this experiment transmitted frequencies were in the range 1.9-2.9 kHz centred at 2400 Hz.…”