VLF duct associated with the lower-hybrid-resonance frequency in a multi-ion upper ionosphere

Abstract

As a result of the presence of two or more ions with different height profiles in the upper F region of the ionosphere, propagation ducts that are associated with the lower‐hybrid‐resonance frequency may form. Such ducts are generally oriented along a horizontal direction. These ducts were originally suggested as a mechanism for the explanation of certain VLF hiss bands observed by a number of satellites with electric antennas. They were questioned, however, as a result of detections of VLF noise components at frequencies well above their trapping bands. The conditions in a diffusive equilibrium region under which such ducts may form are investigated. It is found that the ducts are essentially narrow‐band guides with maximum possible bandwidths of the order of 2.5 to 3.0 kHz; the bandwidths vary with altitude. Duct quality is found to depend on the parameters at the ion density crossover level and at the F region peak. Duct formation is favored by low electron density conditions, thus implying that their existence at high midlatitudes in or near the ionospheric trough is favored. Preliminary examination of data from the VLF receiver connected to the long electric antenna on Alouette 1 revealed a number of hiss events for the year 1963 with bandwidths that fit the duct properties.

LHR band emissions at mid-latitude and their relationship to ionospheric ELF hiss and relativistic electrons

“…2, where the magnetic latitude and magnetic local time are 49 • and 19.2 h, respectively. The result demonstrates well the ionospheric trapping of the LHR waves in the frequency range between the minimum LHR frequency and the smaller of the two maximum frequencies, as proven by Smith et al (1966) and Gross (1970). This trapping corresponds to the frequency range from 5.8 to 7.5 kHz in the altitude range from 250-840 km, as indicated by the hatched area in Fig.…”

“…7. Thus, the horizontally-formed LHR duct is most reasonable to interpret the fundamental features of the mid-latitude LHR band emissions as originally discussed by Smith et al (1966) and Gross (1970). Their speculation that the origin of the ducted LHR is the auroral hiss is, however, not evident, as can clearly be seen in Figs.…”

“…On the other hand, Smith et al (1966) proposed that the observed noise bands are the trapped waves in the ionospheric LHR duct. Gross (1970) extended this work and theoretically showed that an ionospheric duct oriented along a horizontal direction can be formed with a narrow wave guide. They claimed that the propagating auroral hiss through the ionospheric duct is the origin of the mid-latitude LHR emissions (Smith et al, 1966;Gross, 1970).…”

“…Gross (1970) extended this work and theoretically showed that an ionospheric duct oriented along a horizontal direction can be formed with a narrow wave guide. They claimed that the propagating auroral hiss through the ionospheric duct is the origin of the mid-latitude LHR emissions (Smith et al, 1966;Gross, 1970). As for the excitation process of the LHR band emissions, Horita and Watanabe (1969) proposed that a Landau instability caused by an electron stream of a few tens keV can excite continuous LHR band emissions with lower cut-off LHR frequencies.…”

“…Smith et al (1966) suggested that the LHR band waves are the result of the duct propagation of the auroral hiss. To explain the bandwidth of the VLF banded hiss, Gross (1970) calculated the duct properties under the conditions of a diffusive equilibrium ionosphere. The result showed that the VLF ducts can be formed and that the possible bandwidth is of the order of 2.5 to 3 kHz.…”