Radio‐Frequency Ion Mass Spectrometer Measurements of Ion Composition, Velocity and Temperature: the EXOS‐D Suprathermal Mass Spectrometer

The suprathermal mass spectrometer (SMS) on EXOS-D is a 3-stage, folded Bennett-type radio-frequency mass spectrometer designed to measure thermal (0.1-25 eV/q) and suprathermal (<4 keV/q) ions in the 0.9-67 amu/q mass-per-charge range. In continuous operation since 1989, it has three dynamically selectable entrance apertures which have a geometric factor ratio of 10 5 , a mass resolution (Δm/m) that is independent of ion mass and sufficient for resolving a minor ion species from an adjacent major species (e.g. N + adjacent to O + ), and a Gaussian angular response to a drifting Maxwellian. In its fast scan mode, it measures the spin angle distributions of different ion species at several retarding potential settings. These distributions are used to derive estimates of bulk plasma parameters, including the spacecraft potential, the perpendicular ion convection velocity, and the mass-dependent parallel ion velocities and ion temperatures, using an iterative procedure to correct for the undetected component of the measured distribution in the case of positive spacecraft potential.