Magnetospheric plasma just outside the plasmasphere is characterized by an antiloss cone (ALC) distribution function. A new high-frequency electrostatic instability in these ALC plasmas occurs if &rgr;>&rgr;c,&rgr; being the strength of the antiloss cone. The growth rate for electrons (observed by Ogo 3) of energy density of ~10-8 erg cm-3 can be ~0.05&OHgr;e, &OHgr;e being the local electron cycloton frequency. The characteristic frequencies associated with this instability correspond to time periods of ~100--250 s; this may be a possible source for micropulsations of type Pc 5.