Space-based Ultra-High Energy Cosmic-Ray Experiments

Space-based experiments, either orbiting the Earth or from scientific balloon altitudes, measure high-energy cosmic rays by measuring from above the atmosphere the optical and radio signals generated by extensive air showers (EAS). These experiments are designed to have a large field-of-view (FoV) for observing EAS which translates to monitoring the atmosphere over a large, $\sim 10^6$ km$^2$ area on the ground. Ultra-high energy cosmic rays (UHECRs, $E_{CR} > ~\sim 1$ EeV) are measured by using the isotropic near-UV air fluorescence signal to finely sample the EAS development and to efficiently use the atmosphere as a vast calorimeter. At UHE, these immense EAS particle cascades have sufficient charged particle content to generate the relatively dim fluorescence light that propagates to the space-based instrument. Additionally, the beamed Cherenkov light and geomagnetic radio emission from EAS arrive with $> ~\sim 10$ ns impulse and are measured at small angles away from the cosmic ray trajectories. In particular for optical Cherenkov measurements, the energy thresholds can be $> ~\sim 1$ PeV, i.e. for very-high energy cosmic rays (VHECRs). ... In this chapter, the nature of observing the UHECR-induced shower development from orbiting and balloon-borne experiments is detailed, both for missions that have been flown and those currently in development. This will be accomplished by discussing experimental performance in terms of measuring the UHECR spectrum, UHECR nuclear composition, and UHECR arrival direction. The ability for these experiments to also perform VHECR and VHE and UHE cosmic neutrino measurements will also be discussed.