LOCUS is the concept for a new Earth Observation satellite mission to explore the boundary between the Earth's atmosphere and outer space. At altitudes between 50 km - and 150 km above ground massive energy fluxes collide; kinetic energy from breaking atmospheric waves from below, and intense ultraviolet radiation and charged particle streams from the sun. Through these interactions, the Earth - Space interface plays an important role in climate processes
Our lack of knowledge about this particular region of the atmosphere is
rooted in the fact that its dominant species can only be measured by
techniques which would require very complex and expensive
LOCUS presents an alternative approach - a small and inexpensive detector for upper atmospheric science. This is made possible for the first time thanks to novel detector technologies. LOCUS is a candidate mission for the 9th ESA Earth Explorer Call.
The mesosphere – lower thermosphere (MLT) is the transition region between Earth’s atmosphere and space and is influenced strongly both from the atmosphere below and from incoming solar UV, X-rays and charged particles. These drive chemical, radiative and dynamical processes which couple the MLT with the lower atmosphere and with space weather. The detailed processes that govern the MLT are poorly understood, however, because this is the least well-observed region of the atmosphere. For instance, we do not even know if the mesosphere is in global mean radiative balance on monthly or seasonal timescales.
Figure: Schematic of the MLT region at the boundary between atmopshere and outer spaceObserved negative temperature trends indicate that the MLT is a predictive indicator of climate change, but variations in ozone seem to mitigate that trend around the mesopause. That’s why temperature measurements in the MLT will yield information on greenhouse-gas concentrations and global ozone recovery.
The major constituent atomic oxygen (O) dominates the chemistry of the upper MLT. The best current estimate of its global distribution is from an indirect method based on measurements of ozone IR chemiluminescence by SABER (on the TIMED satellite, 2001-current). There are however known systematic biases in the SABER method from assumptions on chemical reaction rates. Furthermore, the infrared method is limited to altitudes below ~95km. The LOCUS mission combines direct measurements of lower thermospheric O emission at THz frequencies with indirect mesospheric observations in the infrared.
Direct measurements at multiple THz frequencies are missing today, because - until today - their technological requirements were prohibitive for space (only large and expensive gas lasers could provide enough power to pump a heterodyne detector). By exploiting newly developed technology, the LOCUS THz limb-sounder overcomes this limitation and will provide for the first time global height-resolved atomic oxygen (O) distributions over several years, enabling examination of inter-annual variability, as well as seasonal and short-term variations. In addition, LOCUS will target nitric oxide (NO), produced by charged particles in the lower thermosphere and an indicator of solar weather, the hydroxyl radical (OH), which is central to mesospheric chemistry, carbon monoxide (CO), an excellent tracer of mesospheric transport, molecular oxygen (O2) and ozone (O3), humidity (H2O) and temperature. Simultaneous measurements of IR emission from vibrationally-excited CO2, NO, O3 and OH will provide complementary information on thermal structure, polar mesospheric clouds (PMC), energetics and chemical processes.
A comprehensive understanding of the composition of the MLT will answer several open scientific questions, i.e. on the abundance of actively radiating species and therefore the source of MLT cooling anti-correlated to tropospheric warming (and the effects of ozone recovery on these trends), the increased occurrence of mesospheric clouds in a changing climate, as well as changes in MLT composition driven by solar proton and EUV events.
Scientific Mission Objectives
Atomic oxygen (O) is crucial to radiative balance in the MLT. It is produced by UV photolysis of O2 in the lower thermosphere, and transported downwards below 100 km where it recombines in the presence of a third body (O+O+M O2). Above 82 km, atmospheric pressure is so low that recombination has a time constant of many hours, so that O concentrations persist during the night. Atomic O therefore represents a significant reservoir of chemical potential energy. Some of this is released as heat through exothermic chemical reactions, but a significant fraction is exported from the MLT through the airglow (e.g. chemiluminescence from vibrationally excited OH). Vibrational excitation through collision with O leads to CO2 emission at 15 m and NO emission at 5.3 μm, which are dominant cooling mechanisms (as is 63 m emission from O itself). Atomic O therefore controls the chemistry and radiative balance of the MLT . These processes need to be understood to interpret the observed cooling trend in the MLT and underlying influences of stratospheric ozone depletion and increasing greenhouse gases . In addition to that, O losses at the mesopause are the major source of O/O2 changes in the thermosphere that directly impact the F2 ionospheric layer. Ionospheric disturbances can interrupt communication and navigation networks around the globe, making a better understanding of this region societally relevant. The ability of LOCUS to measure O directly in the MLT (90 - 150 km) would therefore be a huge advance in understanding this key region.
Noctilucent Clouds (NLC) are composed of water ice particles, which form during summer when the mesopause reaches its lowest temperatures (≤145°K). Observations by SBUV instruments show an increase in PMC occurrence over the last half-century. Two explanations have been proposed: either the mesopause region is cooling or (and) water vapour concentrations there are rising . Water vapour, NLC and temperature measurements by LOCUS will allow us to determine the super-saturation required to nucleate ice particles.
Energetic Particle Precipitation
During a geomagnetic storm when energetic particles (protons and electrons) precipitate into the MLT, the resulting ionisation causes dramatic increases of NOx and HOx species. During the polar night, the lifetime of NO is sufficiently long that it can be transported downwards into the stratosphere, contributing to the catalytic destruction of ozone . This would affect the temperature structure of the stratosphere and is one mechanism, which couples space weather to the climate of the troposphere and lower stratosphere , . Simultaneous measurement of both the NO abundance, through its THz emissions, and its non-thermal IR emission would be a powerful technique in tracing the influence of energetic particle precipitation on the Earth’s atmosphere and climate. LOCUS capability to detect NO+ in highly disturbed conditions would be directly relevant to space weather.
Improved MLT in Climate and Numerical Weather Prediction (NWP) Models
Climate and NWP models are moving towards whole atmosphere models , and eventually full Earth-Sun models. Research indicates that inclusion of the MLT in NWP models can increase their forecast skill, particularly on seasonal to decadal scales . LOCUS will provide global observations of the MLT to validate whole atmosphere models.
LOCUS is a limb-sounding mission comprising a novel terahertz (THz) frequency heterodyne spectrometer, exploiting quantum cascade laser (QCL) local oscillators (LOs) in space for the first time, along with an IR filter radiometer built on the SABER concept, though potentially exploiting new compact imaging IR array technology. LOCUS will be deployed in a sun synchronous, low Earth orbit, scanning the atmospheric limb between 50 km and 150 km at ≤ 2 km vertical sampling and a longitudinal spacing of ≤ 1000 km (9°).
Atmospheric emission spectra are simultaneously recorded in four THz bands and four IR channels. The standard campaign mode will be daily global sampling. Alternative modes are foreseen for e.g. NLC campaigns (finer vertical sampling over a reduced altitude range). A mission-duration of three years will enable inter-annual variability to be observed as well as seasonal and shorter-term variability. A mission concept study in the ESA in-orbit-demonstration (IOD) programme in 2014 included assessments of scan parameters (scan range, view spacing, field of view, scan speed) and orbit parameters (latitude coverage, orbit inclination/spacing, power cycle and downlink requirements).
The THz receivers use Schottky mixer technology. The two lower frequency bands employ semiconductor LOs, while the LOs of the two higher frequency bands are provided by QCL diodes. The QCL LOs are the key enabling technology of LOCUS since they make the 3-5 THz range accessible for the first time. The QCLs are cooled to ≤100K by miniature, closed-cycle coolers.
Figure: Schematic layout of the LOCUS THz receiver stages.
The incident spectrum in each receiver is down-converted in four separate intermediate frequency (IF) chains. The IF signal from each band is spectrally sampled by a digital, wide-band Fourier transform spectrometer (WBS), which provides a power spectrum of the incident THz signal with a 1MHz spectral resolution. A breadboard receiver at 1.14 THz exists in the laboratory and so this receiver and WBS are at TRL 4. A 3.5 THz breadboard with QCL LO is under development and TRL 4 is expected in 2016. The SRL is currently 3 (instrument requirements and mission configuration defined in IOD study) and will rise to 4 by the start of Phase A through UK nationally funded activities.
LOCUS is a joint project between UK universities (Univerity Leeds, University College London, University Oxford), research institutes (STFC Rutherford Appleton Laboratory), industry partners (STAR-Dundee, Surrey Satellite Technologies) and a growing number of international partners.
The development of LOCUS is funded by the Centre for Earth Observation Instrumentation UK, the National Centre for Earth Observation UK, the UK Space Agency, as well as ESA through their in-orbit demonstration programme.