Ground-based measurements of hydrogen Balmer-alpha emission for night-side exospheric density sensing

The Carruthers Geocorona Observatory, planned to launch in late September 2025, will obtain unprecedented wide-field images of Earth’s exospheric Lyman-alpha emission, which will be used to retrieve global, 3D distributions of exospheric hydrogen (H) density at high temporal cadence. However, from its vantage near the Earth-Sun L1 Lagrange point, the Carruthers cameras do not directly observe Earth’s nightside exobase region, and retrieval of the H density distribution in the low altitude shadow zone relies on model-supported extrapolation of dayside and twilight observations. Coincident, ground-based photometric observations of visible exospheric emission (at 656.3 nm, the Balmer-alpha line) provide independent constraints on H density across Earth’s nightside that, when coupled with Carruthers observations of UV exospheric emission, would improve the accuracy of the density retrieval in this region. We present a new system, the Photometer Array for Tomographic Hydrogen Sensing (PATHS), that is designed for nightside Balmer-alpha photometry from multiple ground-based vantages simultaneously and thus avoids the spatiotemporal aliasing of exospheric structure and variability associated with historical single vantage systems. In this poster, we describe the PATHS instrumentation, first-light observations, and a deployment strategy that optimizes synergy with the Carruthers mission.