Science Study for a LOw Cost Upper Atmosphere Sounder (LOCUS)
Gerber, Daniel1; Swinyard, Bruce2; Ellison, Brian1; Plane, John3; Feng, Wuhu3; Nocerino, Theresa4; Bird, Rachel4
1RAL Space, Harwell Oxford, UNITED KINGDOM; 2RAL Space, Harwell Oxford & UCL, London, UNITED KINGDOM; 3School of Chemistry, University of Leeds, UNITED KINGDOM; 4SSTL, Guildford, UNITED KINGDOM

We report on the results of a mission study for LOCUS, a low cost sounder for the upper atmosphere. The project aims to assess the requirements and feasibility of providing a low cost space mission to observe terahertz frequency (THz) atomic and molecular transitions to trace and monitor important chemical species in the Mesosphere (55 - 90 km) and lower thermosphere (90 - 120 km), a region known as the MLT. Observations show that in some places the mesosphere is cooling up to 30 times faster than the troposphere is warming. Although not fully understood, this can partly be attributed to processes which are linked to climate change either directly (increased greenhouse gas concentrations in the middle atmosphere, stratospheric ozone depletion) or indirectly (changes in meridional circulation patterns, gravity wave spectrum). Despite the uncertainty, it therefore provides a highly geared indicator of global climate change. Given its importance, it is surprising that the chemistry of the MLT has not been well studied to date by space based platforms. The reason for this is that the best probes of the important chemical species (atomic O, OH, HO2, NO etc.) lie in the THz range for the MLT and the corresponding detection technology is relatively immature and perceived as expensive to deploy in comparison to other wavebands. However, recent technical advances in heterodyne receivers, especially in the area of Quantum Cascade Lasers (QCLs) used as local oscillators and planar Schottky frequency mixers and high-speed digital signal processing, mean that payload resource demands (mass power and volume) are being significantly reduced and are evolving towards compliance with smaller space borne missions. Within our study, radiative transfer simulations have been performed with the reference forward model (RFM) to assess the line strengths of the species O, OH, HO2, NO, CO and O2 in frequency regimes open to exploitation by QCL and Schottky mixer technology. For an input to these simulations, we use a set of dedicated atmospheric distribution profiles calculated by the Whole-Atmosphere Community Climate Model (WACCM) model from 40 km - 140 km and, where applicable, the Mass Spectrometer Incoherent Scatter (MSIS) Model to extend the top-of-atmosphere up to 500 km. Based on this work a selection of micro windows that meet both the scientific and technological requirements was made. Here we present the results of the atmospheric model study, as well as the radiative transfer calculations that have been used to perform a conceptual instrument definition.