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

Abstract

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 wave particle interactions.