BARREL: A Balloon Array for Monitoring Relativistic Electron Losses during the RBSP Mission

The radiation belts are maintained by competing acceleration and loss, thus understanding radiation belt variability requires a quantitative understanding of the loss processes. BARREL is a multiple-balloon experiment that will (1) determine the total electron loss rate by simultaneously measuring relativistic electron precipitation over a wide range of local times, (2) directly test models of wave-particle interactions by combining precipitation measurements with simultaneous RBSP in situ wave and energetic particle measurements, (3) determine the relative importance of different classes of precipitation and their associated precipitation mechanisms for different magnetic activity levels, and (4) determine the spatial extent and spatial structure of precipitation. To achieve these science objectives, BARREL will consist of three balloon campaigns carried out during the RBSP mission. During each campaign, a total of 15 small (~ 50 lb) balloon payloads will be launched to maintain an array of 5-8 payloads for ~40 days across ~160° of longitude (~8-10 hours of magnetic local time). Each balloon will carry a NaI scintillator to measure the bremsstrahlung X-rays produced by precipitating relativistic electrons as they collide with neutrals in Earth's atmosphere. We present an overview of the BARREL investigation which will provide the first balloon measurements of precipitation while in situ measurements of both waves and particle distributions are available.