On the calibration and use of Dual Ion Sensors for NASA's Magnetospheric MultiScale mission

The scientific target of NASA's Magnetospheric MultiScale (MMS) mission is to study the fundamentally important phenomenon of magnetic reconnection. Theoretical models of this process predict a small size (order of hundred kilometers) for the ion diffusion region where ions are demagnetized at the dayside magnetopause. Yet, the region may typically sweep over the spacecraft at relatively high speeds of 50 km/s. That is why the Fast Plasma Investigation (FPI) instrument suite must have an extremely high time resolution for measurements of the 3D particle distribution functions. The Dual Ion Spectrometers (DISs) provide fast (150 ms) 3D ion velocity distributions, from 10 eV to 30,000 eV, as part of the FPI on NASA's MMS mission. This is accomplished by combining the measurements from eight different spectrometers (packaged in four dual sets) on each MMS spacecraft to produce each full distribution. This approach presents a new and challenging aspect to the calibration and operation of these instruments. The response uniformity among the spectrometer set, the consistency and reliability of their calibration in both sensitivity and their phase space selectivity (energy and angle), and the approach to handling any temporal evolution of these calibrated characteristics all assume enhanced importance in this application. In this paper, we will present brief descriptions of the DIS spectrometers and our approach to their ground calibration, trended results of those calibrations, and our plans to detect, track, and respond to any temporal evolution in instrument performance through the life of the mission.