Once ionised, the ions and electrons recombine and react with other gaseous species to produce other ions. At high latitudes, a significant source is energetic particles, in the form of electrons associated with the aurora, and primarily protons emitted from the sun during some solar flares. At middle and low latitudes, the primary source of energy comes from solar radiation at extreme ultra-violet (EUV) and X-ray wavelengths. The ionosphere is part of the upper atmosphere formed by the ionisation of gases including N 2, O 2, and O. The composition of the lower thermosphere is primarily N 2 and O 2, but in the upper thermosphere atomic oxygen (O) becomes the dominant gas, due to the importance of photodissociation (a chemical reaction that degrades compounds by bombarding the molecule with photons) and molecular diffusion at high altitude ( Solomon & Roble, 2015).

Here, mixing is inhibited by the positive temperature gradient, and the various components separate under gravity so that the composition varies with altitude ( Hargreaves, 1992). This is the hottest part of the atmosphere. The thermospheric temperature eventually becomes almost constant at a value that varies with time, but is generally over 1000 K. It is characterized by high temperature and large variability, in response to changes in solar UV radiation and solar-driven geomagnetic activity. The thermosphere is a region of the neutral atmosphere, from ~85 km to ~500 km altitude, heated by solar ultraviolet (UV) radiation, where the temperature gradient remains positive. (2019) provides an overview of the CIRCE mission. The Coordinated Ionospheric Reconstruction Cubesat Experiment (CIRCE) will characterise a region of the space environment, the ionosphere, which is important for a range of defence and civil applications and can impact global navigation satellite system (GNSS), communications and sensing technology. To view this license, visit or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: 1 Introduction

Content includes material licensed under the terms of the Open Government Licence except where otherwise stated. This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. From the US, NRL have provided two 1U Triple Tiny Ionospheric Photometers (Tri-TIPs) on each satellite ( Nicholas et al., 2019), observing the ultraviolet 135.6 nm emission of atomic oxygen at night-time to characterize the two-dimensional distribution of electrons. One IRIS suite will be flown on each satellite, and incorporates an ion/neutral mass spectrometer, a tri-band global positioning system (GPS) receiver for ionospheric remote sensing, and a radiation environment monitor. The UK contribution to CIRCE is the In situ and Remote Ionospheric Sensing (IRIS) suite, complementary to NRL sensors, and comprising three highly miniaturised payloads provided to Dstl by University College London (UCL), University of Bath, and University of Surrey/Surrey Satellite Technology Ltd (SSTL). The Coordinated Ionospheric Reconstruction Cubesat Experiment (CIRCE) comprises two 6U cube-satellites that will be launched into a near-polar low earth orbit (LEO), targeting 500 km altitude, in 2021. * Corresponding author: UK’s Defence Science and Technology Laboratory (Dstl) is partnering with the US Naval Research Laboratory (NRL) on a joint mission to launch miniature sensors that will advance space weather measurement and modelling capabilities. Now at NASA Wallops Space Flight Center, Wallops Island, VA 23337, USAĪthena Space Ltd, Torquay, Devon TQ2 7TD, UK University of Surrey, Stag Hill, University Campus, Guildford GU2 7XH, UK Surrey Satellite Technologies Limited, Tycho House, 20 Stephenson Rd, Guildford GU2 7YE, UK Mullard Space Science Laboratory, University College London, Surrey RH5 6NT, UKĭepartment of Electronic and Electrical Engineering, University of Bath, Bath BA2 7AY, UK Naval Research Laboratory, 4555 Overlook Ave, Washington, DC 20375, USAģ Defence Science and Technology Laboratory, Portsdown West, Portsdown Hill Road, Fareham PO17 6AD, UK Stephan 2, Kevin Wiggins 6 and Craig Underwood 7ĭefence Science and Technology Laboratory, Porton Down, Salisbury, Wiltshire SP4 0JQ, UK Marquis 2 ,8, Sean Murphy 1, Talini Pinto-Jayawardena 5 ,9, Duncan Rust 4, Keith A. Finne 2, Alex Fortnam 3, Bruce Fritz 2, Alex Hands 7, Peter J. Budzien 2, Tobias Carman 3, Rahil Chaudery 4, Kenneth F. Watson 5, James Williams 6, Alex Agathanggelou 3, Charles M.

Nicholas 2, Graham Routledge 3, Junayd A.

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