This project will address a major unsolved question in geosciences: how strongly is our planet's atmosphere subjected to forcing from space? The mesosphere–lower-thermosphere–ionosphere (MLTI) is the boundary of the Earth environment, and as such it undergoes forcing from above, in particular at high latitudes. The forcing includes charged particle precipitation from near-Earth space, associated with electric currents producing Joule heating in the upper atmosphere. It also involves trace gas production in the mesosphere which catalytically destroy ozone – including down to the stratosphere when long-lived species descend to lower altitudes – as well as neutral wind changes and atmospheric wave generation. However, these effects in terms of MLTI energetics, chemistry, and dynamics are largely unquantified, leading to unknown uncertainties in lower atmosphere and climate model results. The LOUARN project will bridge this gap by adopting a multidisciplinary approach, combining knowledge and methods from space physics and atmosphere sciences. With LOUARN, I aim at unravelling the MLTI forcing from space by using cutting-edge ground-based instruments measuring both the ionised medium and the neutral gas properties, together with numerical simulations of the upper atmosphere and novel citizen science methods. My objectives are to (1) quantify the energy deposited into the MLTI during geomagnetic disturbances, (2) provide accurate upper-boundary conditions on the ozone balance for atmosphere and climate models, and (3) characterise the wind and atmospheric wave response to this driving from space. My expertise in ionospheric physics and its driving from near-Earth space processes, along with my leadership in auroral citizen science, puts me in an ideal position to address this ambitious challenge. I aim at a breakthrough in atmosphere–ionosphere–space couplings with impacts on atmosphere and climate sciences and on solar–terrestrial physics.