Introduction Plasmagram records Introduction to the Z-mode Like the whistler mode, the Z mode is a “trapped” mode, with upper and lower limits on its frequency of propagation controlled by the parameters of the local plasma. It has remarkable properties, including a change from superluminous to subluminous phase velocity and from left hand to right hand polarization at a frequency intermediate between its lower propagation cutoff and its upper limit at the local upper hybrid resonance frequency fuh. During the operations of the highly successful ISIS topside sounding satellites in the 1960s and 1970s, Z-mode echoes were often observed but tended to occupy narrow frequency bands in proportion to their center frequencies. Whistler-mode echoes were reported, but only on rare occasions at high latitudes. These results left unanswered many questions about the prospects for successful Z-mode and whistler-mode sounding with RPI. In particular, it was not clear that RPI would be successful as a whistler-mode source. The RPI instrument on IMAGE (Bodo Reinisch, PI) was designed to operate as a radio sounder, launching signals that propagate in the free-space ordinary (O) and extraordinary (X) modes and are received as echoes when certain conditions are met on ray path location and on backscattering or reflection. The RPI sounding frequency range was extended from 3 MHz down to 3 kHz to permit determination of the electron density ne in outer magnetospheric plasmas as tenuous as Ne=0.1 cm-3 and the reception of echoes from remote plasma regions with 10< Ne <105 cm-3. In planning the RPI experiments it was realized that signals transmitted at frequencies below either the local plasma frequency fpe or gyrofrequency fce, whichever is lower, could propagate in the whistler mode, and that Z-mode echoes could be excited in a band below the local upper hybrid frequency fuh. Whistler and Z-mode wave injection experiments were therefore contemplated and their potential was discussed, but they were not advertised in advance as a primary objective of the RPI system. Fortunately, RPI has been highly successful in launching and recording whistler and Z-mode echoes, as indicated by selected plasmagrams that display echo range versus transmitted frequency. Z-mode echoes have been well defined on routine sounding records at a wide range of altitudes, from perigee near 1200 km to several Earth radii. Whistler-mode echoes have been detected most frequently at altitudes below ~4000 km. Our purpose here is to illustrate and briefly discuss the varieties of Z and whistler-mode echoes that have been recorded at various times since the IMAGE launch in March, 2000. ............................................................................................. Records of Z echoes September 2004 October 2004 November 2004 February 2005 March 2005 April 2005 May 2005 June 2005 Distribution of gyrofrequency and plasma frequency Distribution of global parameters