Error Sources for Velocity Moments Obtained by Imaging Ion Spectrographs

Imaging ion spectrography is a unique method of producing two-dimensional maps of low-energy (<20 eV) ion distribution functions. It was first tested in the Freja Cold Plasma Analyzer, following which a new, CCD- based detector scheme was developed to allow very high resolution in space, time, and velocity space. The Suprathermal Ion Imager has since flown successfully five times including on the GEODESIC, Cusp, JOULE and Japanese S-520-23 sounding rocket missions, demonstrating its ability to make very sensitive measurements of 2-D bulk ion flow (tens of m/s) at rates of over 100 vectors per second. As the technique becomes more widespread, it is necessary that the devices not only be reliable, but also that the data reduction techniques be accurate and robust. We have undertaken a detailed analysis of sources of error in velocity and temperature measurements from CCD imaging spectrographs, specifically the Canadian Electric Field Instrument that will be flown on each of the three Swarm satellites in 2010. The main sources of error come from uncertainties in the instrument transfer functions, the sensor-to-plasma potential difference, particle Poisson noise, and galactic cosmic ray events. We discuss also the challenge of calibrating the instruments with the aid of computer simulations, and the merits and limitations of on-ground and in-flight calibration strategies.