In order to confirm the increase of natural ice nucleus count around the 28th day after a meteor shower, measurements of natural ice nuclei were made with an improved filter paper technique at sufficiently separated two or more sites. The measurements were made continuously for the two periods of more than ten days in May and November, 1962 and January-February, 1963. It was found that the concentration of ice nuclei increased at each site around the same day, that is, the 28th day after major meteor showers. Other increase by local source was seldom found during the periods. From 1960 to 1964, the time variation of all the concentration of measured ice nuclei during the several days around the 28th day after a meteor shower was examined by a mixing cloud chamber or filter paper technique. Almost always the ice nuclei showed unusual increase during the 27th to 29th day after the meteor shower. From the statistical examination, it was recognized that the increment of precipitation amount occurs for the period of the ice nucleus increase following the major meteor shower.
Abstract. The propagation mechanism of very low latitude (geomagnetic latitudes of less than 10-15 ø) whistlers is poorly understood. There is a controversy on their propagation; some workers using the observational facts have suggested field-aligned propagation, but some theoretical (ray tracing) works have all indicated nonducted propagation. This paper reexamines the propagation characteristics of nonducted propagation, but we use three-dimensional ray tracing (different from previous works) for realistic ionosphere/magnetosphere models (the electron density profile with latitudinal and longitudinal gradients and the International Geomagnetic Reference Field (IGRF) magnetic field model instead of the conventional dipole model). By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input southern hemisphere, we have found that it is possible for us to detect simultaneously, at a very low latitude position in the northern ionosphere, one-hop whistler rays started from slightly spaced locations in the south with different initial wave normal angles and that some of them can penetrate through the ionosphere, but some others cannot. On the basis of systematic analysis of important parameters, we come to the general conclusion that it is possible for us to find a closely spaced set of paths to reproduce the one-hop and three-hop whistlers in the north and to have the dispersion ratio of 1:3. The echo train whistlers, as were often observed by Hayakawa et al. [1990], are realized also by this nonducted propagation without any serious requirements.
IntroductionGround-based whistlers are generally thought to be attributed to the propagation trapped in field-aligned ducts in the magnetosphere at high, middle [Helliwell, 1965
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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