The propagation of low-latitude whistlers: A review

Hayakawa, Masashi; Ohta, Kenji

doi:10.1016/0032-0633(92)90090-b

Cited by 15 publications

(9 citation statements)

References 45 publications

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“…As can be seen in Figures and , and in Figures S1–S3 in the Supporting Information, the very low latitude whistler‐mode signals, and hence their propagation paths, are often continuous for many hours, particularly at Rarotonga where, on 3 and 4 September, in Figures and S3, they last continuously for ≳11 and ~10 hr, respectively. This is in contrast to some studies made using lightning generated natural whistlers at low latitudes, where lifetimes ~1 hr, for the presumed field‐aligned ducts, have been estimated (e.g., Hayakawa et al, ; Hayakawa & Ohta, ; Singh, ). At midlatitudes, whistler duct lifetimes have been estimated theoretically, and found experimentally, to be many hours, up to ~1 day (e.g., Thomson () and references therein).…”

Section: Observationscontrasting

confidence: 70%

“…By monitoring wave‐normal angles on the FR‐1 satellite orbiting at 750 km altitude, Cerisier () was able to detect ducted propagation at midlatitude L ‐values, but could find no ducted propagation for low latitudes where L ≲ 1.7. Any very low latitude ducts would need very high enhancements (~400% even at moderately low latitudes of 25°, Singh & Tantry, ) and might need to be unrealistically narrow, ~10 km (see also Hasegawa et al, ; Hayakawa et al, ; Hayakawa & Ohta, ). Very low latitude whistler echoes had been observed: Hayakawa et al () found up to 10% of very low latitude whistlers showed echoes; Liang et al () even observed the same very low latitude whistler with echo simultaneously at both Zhanjiang (10°N geomagnetic) and Wuchang (19.4°N geomagnetic).…”

Section: Discussionmentioning

confidence: 99%

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Very Low Latitude Whistler‐Mode Signals: Observations at Three Widely Spaced Latitudes

2019

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Very low frequency (VLF) radio signals with travel times~100 ms were observed continuously for up to~11 hr at night on Rarotonga (Cook Islands,~21°S) at 21.4 kHz from U.S. Navy transmitter NPM, Hawaii (~21°N). These signals travelled in the whistler-mode on well-defined paths, though not field-aligned ducts, through the ionospheric F region, and across the equator reaching altitudes~700-1,400 km depending on time of night. These same signals were also observed simultaneously in Dunedin (46°S), New Zealand, with very nearly the same travel times but with somewhat lower amplitudes and occurrence rates, consistent with the whistler-mode part of the propagation being at very low latitudes. Both sets of signals had similar Doppler shifts, typically tens of mHz, but sometimes up to a few hundred mHz, being positive during most of the night, while the whistler-mode group delays decreased due to both the shortening of the path and the decay of the near equatorial ionosphere, but negative near dawn when the Sun's rays start ionizing the F region. The signals are not observable during the day, fading out during dawn, due to increasing attenuation from the increasing electron density, and hence increasing collisions, in both the D and F regions. Similar weaker NPM signals were also seen at Rothera (68°S). In addition, similar 24.8-kHz signals were seen from the more distant NLK (Seattle,~48°N) at Rarotonga, though clearly weaker than from NPM, but not at Dunedin.

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Section: Observationscontrasting

confidence: 70%

Section: Discussionmentioning

confidence: 99%

Very Low Latitude Whistler‐Mode Signals: Observations at Three Widely Spaced Latitudes

2019

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“…The field-analysis direction finding [Okada et at., 1977;Ohta et at., 1984;Hayakawa et at., 1992] based on the simultaneous measurements of three field components, yields us the direction of arrival (incident and azimuthal angles) and polarization. As shown in Fig.…”

Section: Polarization Analysismentioning

confidence: 99%

Characteristics of mid‐latitude whistler ducts as deduced from ground‐based measurements

Ohta

Kitagawa

Shima

et al. 1996

Geophysical Research Letters

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Propagation characteristics of mid‐latitude whistlers, especially whistler duct characteristics, have been investigated based on measurements in August, 1994 at Dunedin, New Zealand(L=2.78) and in August, 1989 at Ceduna, Australia(L=1.93), both during local midnight. Polarization analyses have enabled us to locate whistlers which exited the ionosphere just above the observing station (LDF is defined in this way). The nose extension method was also applied to these whistlers (their Ln is estimated by this method). The following findings have emerged from the analyses; LDF ≃ Ln (at Dunedin) and LDF ≃ Ln‐0.43(at Ceduna). Our analysis suggests that mid‐latitude ducts are likely to extend down to the ionosphere at L ≈ 2.8. Ray tracing studies for realistic density profiles indicates that a whistler duct terminates at an altitude of about 3,500 ∼ 5,500 km at an L value of ∼1.9 with its enhancement factor being a few percent at least. These results may imply a strong variability of the latitudinal or temporal variations of mid‐latitude whistler ducts.

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“…As is summarized in our review papers [Hayakawa and Tanaka, 1978;Hayakawa and Ohta, 1992], the propagation mechanism of whistlers at very low latitudes (geomagnetic latitude, less than 20 ø ) is very controversial, because both ducted and nonducted propagations have been proposed (see the references in the above two reviews). However, in our previous paper [Ohta et al, 1997] we have investigated the nonducted propagation in the three-dimensional (3-D) situation (unlike the previous 2-D) with including both the latitudinal and longitudinal gradients of the ionosphere and also some possible scatter in the initial wave normal angles, and we have succeeded in explaining even the presence of echo train whistlers (observed by Hayakawa et al [1990]) at geomagnetic latitudes 10ø-15 ø by this nonducted propagation.…”

Section: Introductionmentioning

confidence: 99%

Three‐dimensional ray‐tracing for very low latitude whistlers, taking into account the latitudinal and longitudinal gradients of the ionosphere

Ohta¹,

Hayakawa²

2000

J. Geophys. Res.

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Abstract. Propagation mechanism of very low latitude (geomagnetic latitude less than 20 ø ) whistlers is poorly understood, and this paper examines the propagation characteristics of nonducted propagation at very low latitudes by using three-dimensional ray-tracing for realistic ionosphere/magnetosphere models with latitudinal and longitudinal gradients and International Geomagnetic Reference Field (IGRF) magnetic model. By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input Southern Hemisphere as in our previous paper [Ohta et al., 1997], we have found that it is possible for us to detect, at a very low latitude position in the Northern Hemisphere, some whistler rays being able to penetrate through the ionosphere and some others that are subject to total reflection and come back to the magnetosphere. By making full use of systematic analyses of ionospheric parameters (local time (LT) dependence of latitudinal and longitudinal gradients), we conclude that the LT dependence of occurrence rate of whistlers at very low latitudes can be accounted for by means of such LT dependence of the gradient (especially latitudinal), lending to a further support to our previous nonducted propagation mechanism.

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The propagation of low-latitude whistlers: A review

Cited by 15 publications

References 45 publications

Very Low Latitude Whistler‐Mode Signals: Observations at Three Widely Spaced Latitudes

Very Low Latitude Whistler‐Mode Signals: Observations at Three Widely Spaced Latitudes

Characteristics of mid‐latitude whistler ducts as deduced from ground‐based measurements

Three‐dimensional ray‐tracing for very low latitude whistlers, taking into account the latitudinal and longitudinal gradients of the ionosphere

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