The Retarding Potential Analyzer (RPA) 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−1, and ram angle of RPA aperture θ, small or moderately small). The current correction increases with the temperature ratio Te/Ti, and ranges from a 15%–30% reduction at Mθ=0 to a 15%–30% increase at Mθ=2, for typical values of M,Te/Ti 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 quasineutral–ionacoustic waves.

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