Group-Velocity and Long Retardations of Radio Echoes

Van der Pol´s hypothesis that group-velocity may account for the retardation of echoes observed by Stoermer is analyzed. It is shown that only under very special circumstances can the electron-distribution be proper. A favorable condition is obtained if the refractive index decreases exponentially with the height. It is shown that by slightly varying the electron-distribution anomalous results for skip-distances should follow. It is suggested that the echoes observed by Stoermer and van der Pol were splashes of the same echo focussed accidentally on a favorable patch of ground.