by Dr Daniel Gerber

Science Motivation

The outer frontier of our Earth, where the atmosphere gradually transitions to space, is the least well know region of the planet. The altitude range between 50km and 250km – i.e. the upper mesosphere and lower thermosphere / ionosphere (MLT) – is too high up to be reached by balloons or aircraft, but too low for persistent satellite orbits due to residual drag, primarily due to the most abundant species at this height: atomic oxygen. Atomic oxygen also drives the chemistry and photochemistry of the MLT, but despite its importance it is poorly known. The reason for this is that existing optical remote sensing techniques in the infrared (IR) and at visible and ultraviolet light (UV-Vis) only offer a very indirect method to estimate the amount of atomic oxygen in the MLT. There is no global and long-term dataset for atomic oxygen so far.
The Keystone mission is built around a heterodyne radiometer at THz frequencies, which are wavelengths
between the IR and the UV-Vis. Heterodyne instruments offer a very high spectral resolution, which allows
the abundance of a gas to be directly retrieved from the shape of the spectral emission lines by its molecules.

Such instruments have existed at longer wavelengths for a long time, but because of technological limitations we were never able to build them at the THz frequencies that are required to measure atomic oxygen, and other key gases in the upper atmosphere (e.g. NO, OH, and HO2). Recent progress in quantum cascade laser (QCL) technology has since opened up the possibility to build a highly integrated THz remote sensing radiometer. A QCL-pumped THz radiometer is compact, simple (compared to conventional technologies), and thus uniquely suited for deployment in space on a satellite platform.

Keystone not only closes the THz gap in heterodyne remote sensing, but by measuring atomic oxygen – which is considered the “holy grail” of upper atmospheric science – also closes a long-standing gap in the
understanding of our planet. Combined with co-located IR and UV-Vis measurements, the Keystone mission provides the missing keystone to an overarching understanding of the least well-known region of the planet.

Mission Objectives & Benefits

Thermosphere Ionosphere Interactions: Both the neutral upper atmosphere (thermosphere) and the
ionosphere are in a constant state of flux, subject to natural fluctuation in incoming energy (from breaking atmospheric gravity waves, as well as from solar radiation). The thermosphere and ionosphere are linked in their response to external stimuli, but these links are poorly understood because the main physical parameters of the atmosphere – density and temperature – are not routinely measured, due to technological limitations. By filling this gap, Keystone will revolutionise our understanding of the chemical and photochemical processes that govern the upper atmosphere.

Thermal Balance and Climate: Direct abundance measurement of atomic oxygen [O] will allow us to
understand the physical processes behind the observed upper-atmospheric cooling rates and help us understand if and how they are linked to anthropogenically increased levels in [CO2] in the atmosphere. Crucially, when combined with infrared heat flux measurements, abundance measurements of atomic oxygen will finally reveal the true O-CO2 quenching rates, thus resolving an old conundrum and adding significant new value to decades of existing infrared measurements.

Space Weather: Abundance measurements of [NO] and [OH], [H2O] during space weather events will help understand the impact that space weather has on the physics and chemistry of the upper- and middle atmosphere, and thus on our climate. Combined with next generation solar weather missions this provides a powerful package to study the sun-atmosphere system.

Improved NWP and climate models: Atmospheric models have become crucial to our society, with the
economy and transport heavily depending on reliable weather forecast, and government policies depending on the predictions of climate change from global climate models. The performance of climate and weather prediction models is significantly improved by including the MLT region, but measurements are needed to validate them. Keystone provides missing composition measurements to be assimilate into atmospheric models. Keystone also allows mesospheric winds to be retrieved from the Doppler shift in the narrow upper atmospheric THz lines (as demonstrated by MLS and SMILES).

Space Situational Awareness: The models that compute orbit trajectories of LEO satellites (and space
debris!) are crucially dependant on the getting atmospheric density and temperature right at the mesopause (90-100km). Keystone measurements provide more accurate input for this than the current model assumptions.

Mission Implementation Concept

Keystone 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. It is accompanied by an IR filter radiometer, as well as an UV-Vis spectrometer. Keystone will be deployed in a polar, sun synchronous, low Earth orbit, scanning the atmospheric limb between 50 km and 250 km at ≤ 2 km vertical sampling and a longitudinal spacing of ≤ 1000 km (9°). Atmospheric emission spectra are simultaneously recorded in five THz bands, five to seven IR channels. The measured THz spectra and IR/UV-Vis radiances will be downloaded to ground stations each orbit and converted to temperature and mixing ratio profiles by an optimal estimation retrieval scheme. The nominal mode of operation is daily global sampling. Alternative modes are foreseen for e.g., space weather event campaigns (finer vertical sampling of [NO] over a reduced altitude range). A mission-duration of two years will enable inter-annual variability to be observed as well as seasonal and shorter-term variability. The mission is compatible with a small satellite platform and a large selection of launchers, including Vega.,

Data Products

THz instrument: The THz radiometer measures vertical abundance profiles up to 250km (depending on
species) of key upper atmospheric gases: [O], [OH], [NO], [CO], [H2O], [O3], [HO2] and temperature. The
comprehensive composition mapping will allow us to study the photochemistry and thermal structure of the thermosphere and ionosphere, and how it reacts to external forcing, e.g. from climate change, space weather or gravity waves from large scale weather cells.

IR Instrument: The IR radiometer will measure the heat emitted by greenhouse gases from collisions with
[O]. Combining this with [O] abundance measurements from the THz instrument will establish the kinetic
quenching rates, and result in more accurate upper atmospheric temperatures (climate change record!). It also provides abundances of the greenhouse gas [CO2] which – because [O] is known from the THz measurements – will be more accurate than from IR alone. This will show the impact of anthropogenic climate change on the upper atmosphere.

UV-Vis Instrument: The UV-Vis spectrometer measures direct and indirect airglow emissions from oxygen
species ([O], [O+], [O2], [O3]) as well as several metals and metal ions. Combining this with [O] abundance
measurements from the THz instrument will improve and validate our photochemical models. A large number of metals and metal ions to study ionospheric processes are also being measured.