The MIGHTI instrument consists of two units with orthogonal fields of view, pointed 45° and 135° from the S/C velocity direction toward the port (northern) side of the spacecraft. With this viewing geometry, MIGHTI makes two perpendicular line-of-sight wind measurements of the same air volume as the spacecraft passes by. These line-of-sight wind measurements are calculated from the Doppler shift of emission lines in the Earth’s upper atmosphere. Two emission lines of oxygen – on green (557.7 nm) and one red (630.0 nm) are used by MIGHTI. When air parcels are moving towards ICON along MIGHTI’s line-of-sight, these emission lines shorten in wavelength (become more blue), and when air parcels are moving away, these emissions lengthen in wavelength (become more red). The amount of this shift is related to the speed of the air parcel, relative to the speed of light. Each measurement represents a set of observations of the Earth’s horizon, covering altitudes between 90 and 300 km. The vector combination from the two perpendicular lines of sight provides an altitude profile of wind vectors.
Temperature measurements are performed coincident with the wind measurements. MIGHTI makes measurements of the 762 nm band of emission from O2. The shape of this emission band changes with temperature. MIGHTI detects this change in shape by measuring the brightness at different parts of the 762 nm band.