Fig. 4

Variation of wave normal angle \(\theta\) as a function of \({{\omega_{p} } \mathord{\left/ {\vphantom {{\omega_{p} } {\omega_{c} }}} \right. \kern-0pt} {\omega_{c} }}\) for the incident wave with \({\omega \mathord{\left/ {\vphantom {\omega {\omega_{c} }}} \right. \kern-0pt} {\omega_{c} }} = 2.1\). Blue curves are under the condition that Eq. (4) can be satisfied and red curves are under the condition that Eq. (4) cannot be satisfied. RP indicates the return point where the plasma frequency becomes maximum along the ray-path in the inhomogeneous medium