Measuring the Auroral Ionosphere: Evaluation of Petite-Ion Probe RPA Performance on Multiple Rocket Missions

Observations of ambient ions in the auroral ionosphere can provide a more complete understanding of the underlying physics. In-situ observations of the aurora are often conducted with sounding rockets, which require an instrument package that can fit a small platform and be replicated for multiple subpayloads. Petite-Ion Probes (PIPs) are small retarding potential analyzers (RPAs) that determine thermal ion plasma parameters by using a swept screen voltage to select energies of ions entering the detector and reaching the anode. Thus, the PIP’s data consists of a series of current vs voltage curves (IV curves) over time. Using the known PIP geometry and trajectory, PIP voltage sweep and payload attitude solution, we can model IV curves for a PIP on a payload charged to an assumed potential (Vs) in an assumed Maxwellian plasma, with some assumed ion temperature (Ti) and density (ni) [Fraunberger et al. RSI, 2020]. We use the Levenberg-Marquardt nonlinear least-squares minimization algorithm in the LMFit python library to find the values of ni, Ti and Vs that minimize the difference between the modeled and measured IV curves at a given time for an assumed relative velocity. Additionally, (near-)simultaneous IV curve measurements from two PIPs, separated azimuthally by a fixed angle, can be used to solve for the flow vector, given constraints from the scalar parameters. Thus, scalar and flow vector plasma parameters (ion density, ion temperature and payload potential) can be derived from PIP measurements.
Three recent sounding rocket missions carried PIPs: the May 2021 Kinetic-scale energy & momentum transport experiment (KiNET-X) launched from Wallops, the December 2021 Cusp Region Experiment 2 (C-REX-2) launched from Andoya, and the March 2022 Loss through Auroral Microburst Pulsations (LAMP) mission launched from Poker Flat. Each rocket carried eight main-payload-mounted PIPs onboard. KiNET-X also had two small, deployable subpayload instrument packages (Bobs) with two pips each.
Here we present the preliminary results from these three rocket missions as well as demonstrate the process of determining scalar and flow vector plasma parameters by fitting IV curves to combined simultaneous measured PIP IV curves with the KiNET-X data. Finally, we will discuss how to get the most information out of the PIPs measurements while minimizing the uncertainty in the optimized fitting process. This is considerably more difficult for PIPs on Bobs than for main-payload-mounted PIPs as a Bob’s attitude must be determined separately and only two PIPs are carried on a Bob. However, the usefulness of multi-point measurements for many missions encourages us to reduce the uncertainty in PIPs measured plasma parameters so that we can more accurately interpret future Bob-carried PIPs’ results, and support possible future multi-point sounding rocket missions.