Picked‐up protons near Mars: Phobos observations

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Abstract
The measurements carried out by the plasma spectrometer, ASPERA, onboard the PHOBOS‐2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes ≤7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the MAGMA magnetometer. Analysis of the plasma data shows that weak pitch‐angle diffusion of a ring‐distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived.