Multipoint Thermal Ion Measurements with Low-Resource Ejectables

Observations of the thermal properties of ionospheric ions can provide a quantitative understanding of the underlying plasma physics. Petite-Ion Probes (PIPs) are small retarding potential analyzers (RPAs) whose data consist of a series of measured anode current vs applied screen voltage (IV) curves over time. Scalar thermal ion properties of the measured plasma can be determined by forward modeling IV curves for a PIP on a (sub-)payload charged to a spacecraft floating potential (Vs) in a drifting Maxwellian plasma, with ion temperature (Ti) and density (ni), to these measured PIP IV curves. Importantly, this forward modeling requires knowledge of a PIP’s orientation (attitude) in the plasma over time, since the thermal flux to the sensor is a function of orientation. For a sounding rocket mission, PIPs can be integrated onto the main payload or into custom ejectables called PIP-Bobs. PIP-Bobs carry two PIPs (in various orientations), a small commercial IMU board and an Arduino-like microprocessor package for data handling. Main payload mounted PIPs’ attitudes can be determined from the rocket’s attitude solution. In contrast, an ejected PIP-Bob’s attitude must be determined from the onboard IMU data and the known attitude at ejection. The precession, cone and spin angles of the PIP-Bob over time, in the frame of the Bob, can be determined by deconvolution of the onboard magnetometer data with the expected magnetic field provided by IGRF. The time-dependent PIP-Bob attitude in the East-North-Up (ENU) frame can then be determined from these angles and knowledge of the initial orientation ENU of the PIP-Bob at ejection in ENU, from the rocket attitude solution. This generalized process for determining a PIP-Bob’s attitude can be applied to PIP-Bobs flown on different sounding rocket missions. We will demonstrate this by presenting PIP-Bob results from PIP-Bobs flown in the KiNET-X and APEP sounding rocket missions. These PIP-Bobs, at a time cadence of about 1s, can give densities to within 10% error in the upper E-region with a separation of up to 2km from the main payload.