High‐amplitude VLF transmitter signals and associated sidebands observed near the magnetic equatorial plane on the ISEE 1 satellite

Abstract

On February 16, 1983, during a period of strong magnetic disturbance (Kp ∼ 5), as the ISEE 1 satellite passed near the magnetic meridian of the Omega navigation transmitter (98°W, 46°N geographic, L ∼ 3.4), the Stanford University VLF wave receiver detected the presence of Omega transmitter signals over the range 2 ≤ L ≤ 3.7 at magnetic latitudes (λm) within 10° of the magnetic equator. Over a 500‐km orbital segment near L ∼ 3.4 and λm ∼ 5°S, the electric field amplitude of the Omega signals reached ∼ 0.6 mV/m, a value > 40dB higher than the signal amplitude both preceding (L ∼ 3.7–3.5) and following (L ≤ 3.2) the high‐amplitude period. Over the range 3.3 ≤ L ≤ 3.7 the transmitter pulses at 13.1 and 13.6 kHz were associated with sideband signals spaced in frequency roughly symmetrically about the carrier and generally reduced in amplitude ∼5 dB with respect to the carrier. The strongest sidebands were generally offset in frequency from the carrier by ∼ ±55 Hz. Simultaneous data from the University of Iowa Plasma Wave experiment indicated that the region of high wave amplitude was located just beyond the outer edge of the plasmapause where low values of cold plasma density (∼40 el/cm³) prevailed. High amplitudes were observed only during the time in which the signal frequency lay within the range of a natural noise band of rising center frequency in which bursts of VLF emissions were occasionally triggered by “knee‐trace” whistlers. On the basis of the group time delay and amplitude distribution of the Omega signals and the presence of knee‐trace whistlers, it is concluded that the high amplitude signals originally propagated along the base of the plasmapause surface into the southern hemisphere and subsequently reflected or scattered back up to the satellite. The high‐wave amplitude was presumably produced through the coherent whistler mode instability as the input waves interacted with gyroresonant energetic electrons near the magnetic equatorial plane, and the presence of sideband signals indicates that this interaction had reached a nonlinear stage. The wave magnetic field in the interaction region is estimated to be approximately 20 mγ. The high amplitudes reached by the signals indicates that particle trapping effects could be responsible for the sideband generation.

ELF magnetospheric lines observed by DEMETER

“…The generation mechanism of sidebands may be due to the scattering from ionospheric irregularities which are enhanced by the magnetic activity. All these processes have been already discussed in the past (see, for example, Brinca (1972); Matthews et al (1984); Bell (1985); Nunn (1986); Bell and Ngo (1988); Shklyar et al (1992) and references therein).…”

Physical mechanisms of man-made influences on the magnetosphere

“…Their ionospheric perturbations have mainly been observed in-situ by satellites. It includes: triggering of new waves (Dowden et al, 1978;Bell et al, 1983;Titova et al, 1984;Bell, 1985;Groves et al, 1988;Bell andNgo, 1988, 1990;Sotnikov et al, 1991), ionospheric heating (Inan, 1990;Bell et al, 1991;Dowden and Adams, 1992;Inan et al, 1992;, wave-electron interactions (Inan et al, 1978;Inan et al, 1982Inan et al, , 1984Inan et al, , 1985Dowden, 1985), and particle precipitation (Koons et al, 1981;Vampola, 1987Vampola, , 1990. Moreover, magnetospheric amplification of VLF pulses through wave-electron interaction is enhanced during magnetic storms (Smith and Clilverd, 1991).…”

Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA

“…Within the plasmasphere, discrete VLF emissions are commonly triggered by externally generated discrete whistler mode waves such as lightning generated whistlers and signals from ground based VLF transmitters, with peak emission intensities reaching values as large as 16 pT [Bell, 1985]. The triggered emissions are thought to be generated near the magnetic equator through a gyroresonance interaction between •0 1-20 keV energetic electrons and the triggering wave in which the particle pitch angles are altered and free energy is transferred from the particles to the waves [Hel- The triggered emission process can be studied most simply by focusing on the emissions triggered by signals from known sources such as ground-based VLF transmitters [Bell et al, 1981;Bell, 1985 ].…”

“…The triggered emissions are thought to be generated near the magnetic equator through a gyroresonance interaction between •0 1-20 keV energetic electrons and the triggering wave in which the particle pitch angles are altered and free energy is transferred from the particles to the waves [Hel- The triggered emission process can be studied most simply by focusing on the emissions triggered by signals from known sources such as ground-based VLF transmitters [Bell et al, 1981;Bell, 1985 ]. In the present work we present simultaneous plasma wave and energetic electron data acquired with the PWI and HYDRA instruments on the POLAR spacecraft during periods when VLF emissions were triggered by VLF transmitter signals.…”