We analyze the injections of tailward flowing O+ ions measured by PROGNOZ-8 in the magnetospheric lobes using three-dimensional single-particle codes. These injections are shaped as ''inverted Vs'' and unexpectedly well correlated with AE variations during a time interval of about 7 hours. Such a correlation leads us to consider the effect of transient intensification of the large-scale magnetospheric convection. Using a simple model of time-varying electric field, we show that the source of the O+ lies near the dayside cusp. This source extends over a narrow (a few degrees) latitudinal interval in the vicinity of the last closed field line, likely involving ions expelled into the magnetosphere after being transversely accelerated. We demonstrate that, downstream of the ejection site, impulsive convection leads to prominent structuring of the tailward streaming particles with abrupt energy variations on small length scales. A good agreement is obtained between the numerical results and the observations which supports our impulsive convection based interpretation framework.