Observations on the GEOS 1 satellite of whistler mode signals transmitted by the Omega Navigation System Transmitter in northern Norway

ISEE 1 satellite observations of nonducted whistler mode signals from the Siple Station VLF transmitter show that all well‐defined pulses (S/N ≈ 20 dB) are elongated by 20 ms to 200 ms. This elongation is attributed to closely spaced multiple paths of propagation between the ground and the satellite. The presence of multiple paths is further confirmed by the observed amplitude fading pattern where fading occurs not only at half the spin period (1.52 s) of the rotating antenna but also at time scales of the order of 1 s. Electric field measurements show a 2 to 10‐dB amplitude variation, which is again consistent with direct multiple path propagation. We illustrate the results with two cases: one on October 29, 1977, inside the plasmapause and the other on May 7, 1979, outside the plasmapause. Our results establish that, in general, at any point in the magnetosphere the direct signals transmitted from the ground arrive almost simultaneously along two or more closely spaced direct ray paths. It is shown that multiple paths can be explained by assuming field‐aligned irregularities of 1 to 10‐km horizontal scale in the ionosphere with a few percent enhancement or depletion in the plasma density. We discuss the implications of our results for nonducted wave‐particle interaction in the earth's magnetosphere. We show that for reasonable parameters of nonducted multiple path propagation, a cyclotron resonant electron will experience a wave doppler broadening of a few tens of hertz to a few hundreds of hertz.

supporting

confidence: 93%

Direct multiple path magnetospheric propagation: A fundamental property of nonducted VLF waves

Sonwalkar¹,

Bell²,

Helliwell³

et al. 1984

“…Paper number 94JA00866. 0148-0227/94/94JA-00866505.00 1981; Neubert et al, 1983;Sonwalkar and Inan, 1986;Draganov et al, 1993]. An increasing amount of evidence also suggests that nonducted waves, propagating obliquely to the geomagnetic field, can play a significant role in magnetospheric wave particle interactions.…”

Section: Introductionmentioning

confidence: 99%

“…A comparison of the calculated ratio with the measured value leads to the determination of the wave normal direction.The assumption of whistler-mode propagation is justified because for 15-kHz signal frequency and for the plasma density and geomagnetic field values at the altitudes of the COSMOS 1809 and DE 1 satellites, this is the only allowed cold plasma propagation mode. The assumption about propagation in the magnetic merid-ional plane is justified based on previous wave normal direction measurements of ground transmitter signals in the magnetosphere[Neubert et al, 1983; Sonwalkar and lnan, 1986]. Assuming a right-handed coordinate system with the z axis along the geomagnetic field and the :r axis in the magnetic meridional planeframe.…”

mentioning

confidence: 99%

Simultaneous observations of VLF ground transmitter signals on the DE 1 and COSMOS 1809 satellites: Detection of a magnetospheric caustic and a duct

Sonwalkar¹,

Inan²,

Bell³

et al. 1994

Khabarovsk transmitter signals (15.0 kHz, 48°N, 135°E) were observed on the high‐altitude (∼15000 km) Dynamic Explorer 1 (DE 1) and the low‐altitude (∼960) km COSMOS 1809 satellites during a 9‐day period in August 1989. On 7 out of 9 days the linear wave receiver (LWR) on the DE 1 satellite detected signals from the Khabarovsk transmitter. In addition, the DE 1 satellite also detected signals from the Alpha transmitter (11.9‐15.6 kHz) in Russia and an Omega transmitter (10.2‐13.6 kHz) in Australia, as well as natural VLF emissions such as hiss, chorus, whistlers, and wideband impulsive signals. On two days, August 23 and 27, 1989, observations of the Khabarovsk transmitter signals were simultaneously carried out at high altitude on the DE 1 satellite and at low altitude on the COSMOS 1809 satellite. Analysis of data from these 2 days has led to several new results on the propagation of whistler mode signals in the Earth’s magnetosphere. New evidence was found of previously reported propagation phenomena, such as (1) confinement of transmitter signals in the conjugate hemisphere at ionospheric heights (∼1000 km), (2) observation of direct multipath propagation on both DE 1 and COSMOS 1809, (3) detection of ionospheric irregularities of ≤ 100 km scale size with a few percent enhancement in electron density, believed to be responsible for the observed multipath propagation. We report the first detection of an exterior caustic surface near L ∼ 3.5 for VLF ground transmitter signals injected into the magnetosphere; the location of the caustic surface depended on the signal frequency, and the electric and magnetic fields decreased by several hundred decibels per L shell in the dark (shadow) side of the caustic. We also report the first direct detection of a magnetospheric duct at L = 2.94 which was believed to be responsible for the ducted propagation of Khabarovsk signals observed on the COSMOS 1809 satellite; the measured duct parameters were: ΔL ∼ 0.06 and ΔNe, ∼ 10 ‐ 13%. The duct width at the equator was ∼367 km. Our study also indicates that duct end points can extend down to at least ∼1000 km. The peak electric and magnetic fields of ducted Khabarovsk transmitter signals at ∼1000 km were 520 µV/m and 36 pT respectively. Estimated field strengths of these signals inside the duct at the geomagnetic equator were 57 µV/m and 12 pT for electric and magnetic field respectively. The results of two‐dimensional ray tracing simulations were consistent with the observations of the nonducted whistler‐mode propagation of Khabarovsk (15 kHz) and Alpha (11.9 kHz) signals from the transmitter location to the DE 1 and COSMOS 1809 satellites. Our results have direct implications for the question of accessibility of waves injected from the ground to various regions of the ionosphere and the magnetosphere. In situ measurements of electric and magnetic fields of Khabarovsk transmitter signals inside a duct may well prove to be the critical measurements needed to differentiate between the small signal and large signal theories of wav...

“…Our explanation of the Omega signal amplitude modulation observed in the upper ionosphere is in line with, but not the same as this explanation. The difference is that Neubert et al [1983] give only a qualitative hint to understanding amplitude modulation, based on geometrical optics, while we develop a quantitative description of the phenomenon, based on the solution of the wave equations. In section 5, we discuss the effect of amplitude modulation in comparison with spectral broadening of VLF transmitter signals, reported and discussed by a number of authors [Bell et al, 1983 / dp v2(p)lf2(p, z --,.…”

Section: Obviously the Modulation Frequency Of Oe0(t) And •(T) Is Ofmentioning

confidence: 99%

On VLF wave scattering in plasma with density irregularities

Shklyar¹,

Nagano²

1998

Abstract. A systematic approach to the problem of wave scattering on plasma density irregularities is developed. The scattering process is treated as a wave field spatial evolution while crossing the scattering region, without dividing the wave field into a given incident wave and the field of the scattered waves. In such an approach, the wave energy conservation in the scattering process naturally follows from general formulas. We show that a spatial modulation of the wave packet amplitude caused by spectral broadening in k space is a general consequence of wave scattering. As an example of applications of theoretical consideration in this paper, we suggest an explanation of the amplitude modulation of VLF signals observed by the Akebono satellite in the upper ionosphere. We discuss the relation between amplitude modulation and the apparent spectral broadening of VLF transmitter signals. Both effects are described by the present theory in a similar manner.