Spacecraft observations of man‐made whistler‐mode signals near the electron gyrofrequency

Abstract

Whistler-mode signals from low-frequency ground-based transmitters in the 60-100 kHz range are observed by the OGO-1 satellite at frequencies unusually close to the local electron gyrofrequency, [t•. The intensity of signals observed on outbound passes near 40 ø geomagnetic latitude remains high until the decrease in [t• causes the normalized frequency [/[t• to approach 0.9. Here the signal disappears abruptly into the background noise, a change of at least 20 dB. Ray-tracing calculations provide a simple explanation for the sudden loss of signal. Rays in this frequency range which are launched just inside the plasmapause reach an altitude where [/[• = 0.9 before abruptly bending inward toward the earth's equator. Rays launched outside the plasmapause are reflected in the form of a "Spitze" at the point where the plasma frequency decreases to the wave frequency. Thus the primary features of the observed LF propagation in the magnetosphere can be explained using classical cold-plasma expressions for the refractive index. The narrow spread of the distribution of observed upper-cutoff frequencies indicates that the cutoff is caused by the loss of ducting at 0.5 [•o as suggested by Smith [1961]. Satellite-borne receivers, which extend the observations to nonducted signals, show the presence of whistler-mode signals at higher frequencies relative to the gyrofrequency. Using the OGO-3 satellite, Angerami [1970] has observed whistlers at frequencies as high as 0.7 •Present address: ESL, Inc., 495 Java Dr., Sunnyvale, California 94086. Copyright (• 1977 by the American Geophysical Union. [a. These whistlers had apparently leaked from ducts as their frequency increased above 0.5 Signals from ground-based transmitters operating near 18 kHz have been studied by Heyborne [ 1966, pp. 111-202] using the OGO-1 satellite. Transmissions were generally observed from low altitudes out to the region where [/[• decreased to •-0.3, which on outbound passes occurred in the vicinity of 30 ø to 40 ø geomagnetic latitude. A slow fadeout rather than a sudden cutoff was commonly observed. After the fadeout there sometimes occurred magnetic equator, J. Geophys. Res., 75, 6115-6135. Angerami, J. J., and D. L. Carpenter (1966), Whistler studies of the plasmapause in the magnetosphere, 2, Electron density and total tube content near the knee in magnetospheric ionization, J. Geophys. Res., 71, 711-725. MAN-MADE WHISTLER-MODE SIGNALS 829 Angerami, J. J., and J. O. Thomas (1964), Studies of planetary atmospheres, 1, The distribution of electrons and ions in the earth's exosphere, J. Geophys. Res., 69, 4537-4560. i3udden, K. G. (1966), Radio Waves in the Ionosphere, p. 269, Cambridge University Press, London. Carpenter, D. L. (1967), Relations between the dawn minimum in the equatorial radius of the plasmapause and Dst, Kp, and local K at Byrd Station, J. Geophys. Res., 72, 2969-2971. Carpenter, D. L. (1968), Ducted whistler-mode propagation in the magnetosphere; A half-gyrofrequency upper intensity cutoff and some associated wave growth phenomena,...

Self‐Consistent Theory of Triggered Whistler Emissions

“…The terms on the right hand side of eq. (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) scale respectively like (dropping the fourth),…”

“…(see also eqs. (7-12) through (7)(8)(9)(10)(11)(12)(13)(14)). The numerical difficulty associated with the phase equation can be seen rather clearly from equation (6-7).…”

“…First we neglect the linear damping and the divergence of the group velocity (as higher order in the E"/ expansion as outlined below). The field amplitude is written a, = ETrAeiw, (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) where ET is the field of the triggering wave, so that A is a dimensionless, order unity, slowly varying amplitude. We then scale the velocity to V, = cno/w 1 Note that the slowly varying wave phase W is included with 0, 4 in computing the nonlinear current, so that frequency shifts, bw = -80/Ot , away from the triggering frequency, w = -Oi/ t, are included in the non-linear current.…”

“…This is a rounded-off square wave pulse with continuous first derivative. At each time step the trapping condition (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), as well as the cut-off condition equation (4-27) (evaluated using the average amplitude over the interaction region) is checked for all perpendicular velocities and grid points. If the particles in question are trapped -r, a, and L are stepped using the above schemes and the real current is accumulated.…”

“…If the width of the signal spectrum exceeds 10 Hz, there is substantial loss of gain by as much as 20 dB. The occurrance of triggering exhibits a sharp threshold behavior with respect to input signal intensity and duration 8 . Onset of triggering appears at signal lengths 3 1 in the range of 50 to 100 ms and intensities of the order of kW, with these values varying somewhat according to magnetospheric conditions.…”