Magnetospheric VLF line radiation observed at Halley, Antarctica

Abstract

Spectrograms of broad-band ELF/VLF goniometer data obtained from ground based measurements made at Halley, Antarctica (L 0 =4.3, con jugate near St. Anthony, Newfoundland) have shown the presence of discrete line radiation of magnetospheric origin, in the frequency range 1-4 kHz. The properties of this radiation are broadly similar to Power Line Harmonic Radia tion (PLHR), studied from ground based observations made at Siple, Antarc tica (L =4.1, conjugate-Roberval, Quebec), although there are some interesting differences. Line radiation observed at Halley, is never regularly spaced in frequency by 120 Hz, as one may expect if signals from the Newfoundland power distribution system (60 Hz fundamental) are entering the magneto sphere, and being amplified. Instead, frequency spacings are widely distributed about mean values between 50 to 90 Hz. The lines are observed to trigger emissions and often exhibit 2 hop amplitude modulation, which demonstrates that they are of magnetospheric origin. Events occur mostly in quiet to moderate geomagnetic conditions, and during the late af ternoon period of local time. Arrays of lines are often observed to drift upwards together in fre quency. Line bandwidths are 20-30 Hz-much larger than the bandwidths of locally generated induction lines. We show that the line spacing of ,.., 80 Hz is too large to correspond to sideband separation for waves of equatorial field strength ,.., 10 pT, and we investigate the conditions required for effective particle trapping by the wave array, of the type described by NuNN (J. Plasma Phys., 11, 189, 1974). It is proposed that the line radiation either originates in the signals which enter the magnetosphere from Newfoundland, or is 'naturally' generated, possibly by a linear instability which takes place if the electron distribution is unstable in restricted ranges of wave frequency and wave number.

Physical mechanisms of man-made influences on the magnetosphere

“…The propagation of PLHR through the magnetosphere was first observed on the ground by Helliwell et al (1975). Indirect effects were reported by FraserSmith (1981), Matthews and Yearby (1981), Bullough et al (1985) and Parrot (1990). Direct observations from satellites were given by Koons et al (1978), Park and Helliwell (1981), Bell et al (1982), and Parrot (1994).…”

Magnetospheric line radiation observations at Halley, Antarctica

“…The bandwidths of individual MLR lines is reported to be -20-30 Hz by Helliwell et al [1975] and Matthews and Yearby [1981], but these authors do not provide a definition specifying what they mean. We made simple estimates of the 3 dB bandwidth for the 76 MLR lines which were >3 dB above the background noise, measured from the power spectral density plots (Figure 3).…”

“…Measurements of the frequency, spacing, drift, and intensity of MLR observed at Halley are presented here. The intensity of MLR lines has been previously discussed by Matthews and Yearby [1981], but only for a single MLR example. This is the first report of a large number of measurements of MLR field strengths and frequency drift rates.…”

On the nonlinear triggering of VLF emissions by power line harmonic radiation

“…Most observations of magnetospheric lines reveal spectral broadening (Park et al 1983;Matthews and Yearby, 1981;Yearby et al, 1981;Park and Helliwell, 1981;Helliwell et al, 1975). Line segments with increased bandwidth and amplitude often acquire a periodicity at the two hop wave period.…”