Space-Based Observations of Plasma Waves During Conjunctions Between Host Sensors and Space Objects

This study describes exploratory, in-situ experimentation to measure Very Low Frequency (VLF) plasma waves (1-35 kHz) at times when a space physics satellite equipped with a radio plasma wave receiver conjuncts with other space objects. The objective was to learn if a secondary space object's rapid passage near another satellite is detectable. If so, this would offer a new avenue to infer the presence of space debris in Earth orbit. Space objects in Earth's ionosphere develop a region of ion density rarefactions in the wake of their orbital motion which could serve as the basis for object detection. In 2022 the Canadian space physics satellite CASSIOPE used its radio plasma physics package during conjunctions with other satellites and recorded ambient electric field data at times prior to, during and after the time of closest approach of CASSIOPE and the secondary object. CASSIOPE is designed to measure Earth's aurora, particles, fields and has an eccentric 330 × 1200 km orbit which fortuitously samples a variety of plasma regimes in Earth's ionosphere to test this approach. This orbit regularly crosses the altitudes of highly populated orbital shells such as Starlink, Iridium and OneWeb offering regular conjunction opportunities to attempt measurement of plasma oscillations. CASSIOPE collected electric field measurements using its crossed-dipole Radio Receiver Instrument (RRI) which detects plasma electric field oscillations. CASSIOPE sampled 35 conjunctions using the RRI from 4 March to 10 June 2022. It was surmised that if CASSIOPE traversed an ion density rarefaction the RRI should produce broadband noise at times correlating with the time of closest approach. Of the 35 conjunctions sampled, 3 exhibited VLF broadband noise somewhat correlated to the time of closest approach but were difficult to differentiate from background ambient auroral activity. One conjunction showed strong temporal correlation where a conjuncting Starlink appears to have threaded the magnetic field line between itself and CASSIOPE which also traversed the Starlink's wake. All other conjunctions where the secondary object passed behind CASSIOPE or were quite distant (~ 5-10 km) from CASSIOPE did not show wave power exceeding the ambient background. The CASSIOPE findings indicate that sensing of ion density rarefactions in space object wakes does not appear practical to implement and clear, repeatable signatures from known space object conjunctions were not identified during this investigation.