At near-noon local times, at locations in the high arctic near 80 degrees North and South, the magnetic fields which originate in the conducting core of our planet extend upwards and are magnetically connected to the dayside magnetopause. This subsolar magnetopause is the point where the magnetic field of the Earth first touches the highly supersonic solar wind flow, and the interplanetary magnetic field of solar origin which is embedded in it. This creates the magnetospheric cusps, which are the primary entry points for energy of solar wind origin into the regions of space controlled by the terrestrial magnetic field, and the atmospheric regions which underlie them.
This energy transfer occurs through a process called magnetic reconnection. As such, this crucial region of near-Earth space is fundamental to understanding the flow of energy, mass and momentum throughout the Earth’s magnetosphere, ionosphere and upper atmosphere, and hence in our understanding of "space weather". The magnetospheric cusps are longstanding areas of research interest, but their highly variable nature, in both space and time, makes them a highly challenging region to fully understand. Here we describe a multi-instrument research programme based around an exciting new NASA space mission, TRACERS, due for launch in late 2022, on which the proposal PI is a named collaborator. The TRACERS programme relies on coordination with ground-based instrumentation. Of particular interest for TRACERS is the Svalbard region, an area of the high arctic uniquely well instrumented with, for example, numerous optical instruments and the NERC-funded EISCAT Svalbard radar. Around northern winter solstice Svalbard is in darkness at noon, and for ~10 days the moon is below the horizon. Such conditions offer a unique opportunity for multi-instrument cusp experiments involving cusp auroral optical observations. Our multi-instrument research programme requires the construction and deployment of a new state-of-the art digital imaging radar system, the Hankasalmi auroral imaging radar system (HAIRS). HAIRS will look northwards from Hankasalmi in Finland, having a field of view centred over the Svalbard region, revealing the ionospheric cusp region electrodynamics at high spatial and temporal resolution over a ~1 million square kilometre region of the ionosphere. In this programme, low earth orbit measurements of energetic ions precipitating from the cusp region taken by the twin TRACERS spacecraft will provide measurements of the temporal and spatial structuring of the cusp reconnection processes. Magnetically conjugate measurements of the footprint of the reconnection line from HAIRS and associated ground-based instrumentation, will measure the length and the location of the reconnection line. HAIRS will provide an analysis of the boundary motion, and of the convection velocities detected near the boundary, allowing a calculation of the reconnection rate mapped down to the ionosphere. Such a combination of instrumentation will provide an unprecedented opportunity to understand the temporal and spatial behaviour of cusp reconnection and its role in controlling terrestrial space weather. Outside of the science programme described here, HAIRS will offer vital complementary datasets to support the upcoming NERC-funded EISCAT 3D radar system at lower latitudes in Scandinavia, coming on stream in 2021 which will also lie in the HAIRS field of view. HAIRS will also directly complement the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), launching in 2023, a joint mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS). The innovative SMILE wide-field Soft X-ray Imager (SXI), provided by the UK Space Agency and other European institutions, will obtain unique measurements of the regions where the solar wind impacts the magnetosphere, regions which are directly magnetically connected to the area under study in this programme.