A new basic effect in retarding potential analyzers

Abstract

The Retarding Potential Analyzer ͑RPA͒ is the standard instrument for in situ measurement of ion temperature and other ionospheric parameters. The fraction of incoming ions rejected by a RPA produces perturbations that reach well ahead of a thin Debye sheath, a feature common to all collisionless, hypersonic flows past ion-rejecting bodies. This phenomenon is here found to result in a correction to Whipple's classical law for the current characteristic of an ideal RPA ͑sheath thin; inverse ram ion Mach number M Ϫ1 , and ram angle of RPA aperture , small or moderately small͒. The current correction increases with the temperature ratio T e /T i , and ranges from a 15%-30% reduction at M ϭ0 to a 15%-30% increase at M ϭ2, for typical values of M , T e /T i and transparency of aperture grid. Linear analysis of the perturbed plasma beyond the sheath rests on the fact that a Maxwellian undisturbed ion distribution is Vlasov-stable against quasineutralionacoustic waves.