0
21!.
"iii
5.2 Frequency-Angular Spectrum Evolution in a Current
159
a)
-o.2
.,.,u• --....-·_....._
-1.2
-1.4-1.6 v
Fig. 5.1. Transformation of spectral parameters in increasing counter current (a)
and around blocking point (b) at larger scale (in variables V, {3) with arrow indicating propagation direction: where I- curve corresponding to relation (kC 9 ) = 0;
II - wave packet propagation trajectory; III - curve with Cgx = 0
The determination of straight and reversed waves in a current is presented
by Basovich & Talanov (1977). Wave packets with a positive scalar product of
the wave vector k and the group velocity Cg, (kCg) > 0, are called straight
waves. Correspondingly, the reverse waves are those with C g negative. The
transfer of straight waves to reversed ones takes place at the point with
(kCg) = 0. It is easy to show that this occurs at the trajectory point with
1 + V cos(f3) = 0, rather than at the blocking point, as this was supposed
earlier (Basovich & Talanov, 1977). In order to illustrate this fact, the relation
is shown on the plane { V, {3} as the curve I (see Fig. 5.1a,b). The curve I
touches the curve II at the point B, where VB= -1h./1- (a./4) 2 • Curve I
presents an envelope of the trajectory varieties corresponding to different
values of the parameter a.. The coincidence of the blocking point A with the
point of transfer from straight waves to reversed waves takes place only in