LOCUS Preparatory Activities for EE9

Lead Organisation: STFC RAL Space
Project Lead: Daniel Gerber

The aim of the study was to increase the Science Readiness Level of the ESA Earth Explorer 9 candidate mission LOCUS, a THz limb-sounder for the upper atmosphere. This is achieved by performing linear retrieval simulations to assess the retrieval errors of the mission. Crucially, this includes a first ever retrieval assessment of the key Mesosphere – Lower Thermosphere species atomic oxygen from limb remote sensing measurements. The study shows that it is possible to retrieve the vertical abundance profiles of all the LOCUS targets species, and quantifies the expected measurement precision, the achievable vertical resolution, and the altitude ranges where a retrieval is possible. The core finding is that it is possible to extract some abundance information for atomic oxygen below attitudes of 120km, where the line centre starts to become opaque. It is believed that this information comes from self-absorption of upper mesospheric atomic oxygen against the hot, thermospheric background. This means that abundance profile retrievals of atomic oxygen are possible in an altitude range that overlaps with infrared heat-flux measurements by instrument like SABER. This vindicates the LOCUS science objectives of providing a first 3D abundance map of atomic oxygen from remote sensing measurements, and the quantification of CO2-O quenching rates from co-located THz and infrared measurements.

Mission Requirements Document (MRD) table for the LOCUS mission

The condensed MRD table shown above (full version included in the EE-9 Phase-A proposal) summarises the findings of this CEOI Pathfinder study. It predicts the achievable precision, vertical ranges, and vertical resolutions with which abundance profiles of key geophysical observables can be measured. It presents the missing link between geophysical parameters and measurements that was required to reach the crucial scientific readiness level (SRL) 4 that was required for EE-9.