Received v.l.f. signals during sunrise and sunset times over the propagation path are used to deduce information about the attenuation coefficients and phase velocities of the day and night waveguide modes, as well as the conversion factors between these modes at the day/night transition. Unlike any previous method used to obtain modal propagation constants, the present method does not need a mobile receiver, since it directly makes use of the moving secondary source formed by the mode conversion that takes place at the shadow line, or transition. A two-night-modes/two-day-modes model is discussed in detail. It is seen that it has a wider range of validity, with regard to both the path length and the frequency, than both the two-night-modes/one-day-mode model suggested by Crombie in 1964, and the one-night-mode/two-daymodes model used for the early part of the sunrise period by Kaiser.