Observations of Ion Populations from the KiNET-X Sounding Rocket Mission

The Kinetic-scale energy & momentum transport experiment (KiNET-X) investigated kinetic-scale ionospheric plasma transport for a known input energy & momentum by measuring ionospheric perturbations near sounding rocket barium releases. The diagnostic main payload of the rocket, launched May 2021 from Wallops, included an array of eight Petite-Ion-Probes (PIPs), two pairs of orthogonal DC/AC electric field probes and two Electron Retarding Potential Analyzers (ERPAs). Two subpayloads (PIP-Bobs), each carrying two PIPs, were released from the main structure during the initial phase of the flight, forming a line of spatially distributed instruments along the along-ram, perp-B direction. Also, two ground stations and one aircraft made optical measurements of the two barium clouds with video and DSLR cameras.
The PIPs are retarding potential analyzers that measure the thermal ion flux spectrum, combining both oxygen and barium. Extracting scalar plasma parameters from PIP data via forward-modeling requires a reasonable model of the plasma environment. Both the charged payload potential estimated from the ERPAs’ measured electron temperature and the geophysical plasma flow velocity derived from the DC electric field (DCE) probes were incorporated in an improved model of the PIP measured current in a multi-species modified Maxwellian plasma. The resulting oxygen ion temperatures, and ratios of injected (barium) plasma density to ambient (oxygen) plasma density, from this improved PIP data analysis exhibited several features of interest. For this presentation, we will focus on the PIP-measured barium ion densities. The PIPs measured a much higher barium density peak value following the second barium release (which was closer) than following the first one (which was farther, from the main payload). However, ground-based optical data suggested that the net barium ion yields were approximately the same for the two releases. Additionally, the PIPs observed a faster decay of barium density for the second release compared to the first one. In order to determine if these features could be described by a straightforward plasma physics model, we created a simple particle-trace model of the evolution of ion clouds, ionized at different times, in a static geomagnetic field, with only the magnetic force acting on the moving ions. The following assumptions were made for each ion cloud: (1) the neutral gas’s initial expansion velocity directions and thus, spatial distribution, followed a Gaussian distribution; (2) prior to the cloud’s specific ionization time, the neutral cloud expanded with a set expansion velocity as well as with the RAM velocity of the main payload; (3) the number of ions in an ion cloud varied based on the time of ionization. The number of ions produced at each selected ionization time were determined from camera measurements of the number of ions over time following the first barium release. Combining these ion clouds with our knowledge of the positions and look-directions of our PIPs, we estimate the densities that our PIPs would observe in such a cloud. We present comparisons of the measured barium density to this simple-kinetics-model particle-tracer-estimated density, and discuss observed disparities from this simple model.