Using Faraday Rotation to Measure TEC and Infer 3d Plasma Density: Lattice test flight on GIRAFF Rocket Mission

Faraday Rotation is a phenomenon that causes the polarization plane of radio waves to rotate when passing through a magnetized plasma. Specifically, a beam of radio waves entering the ionosphere will have its polarization plane rotated by an angle directly proportional to both the Total Electron Content (TEC) along the path and the along-path component of the magnetic field. The GIRAFF rockets, missions 36.380 and 36.381 (Michell), both launched in February 2025. Each carried a linearly polarized, 150 MHz radio beacon, as well as an additional 400 MHz beacon, intended to be used for phase comparisons to estimate TEC from propagation delay. The 150 MHz beacon by itself, being linearly polarized, was able to be used as a test of the eventual Lattice Beacon system for the Winter 2026 GNEISS Rocket mission, which instead uses Faraday rotation to estimate TEC from a single frequency transmission. For the GNEISS mission, 12 ground stations will be deployed and two rockets will be simultaneously launched, resulting in more numerous and spatially distributed TEC cuts. This will allow a tomographic inversion to be conducted, producing a 3-dimensional distribution of plasma density in the ionosphere.
The Lattice Beacon ground receivers consist of two-channel software-defined radios with cross-yagi antennas, and in winter 2025 were deployed to several sites in Alaska (two in Poker Flat, and one each in Toolik Lake and Venetie). These radios can measure the polarization of the incoming 150 MHz transmission, and by doing so estimate TEC from only a single radio frequency channel. The inversion from this test opportunity of the full lattice system is complicated by the fact that the transmitting antenna for GIRAFF is mounted on the rocket motor, decoupled from the main body of the rocket, and therefore no time-varying spin data are available to use as a reference for the expected polarization. Despite this, under the assumption of a constant spin rate we can fit and generate an artificial reference to estimate Faraday rotation angle. We then use the Faraday rotation data, compared with that from the Poker Flat Incoherent Scatter Radar (PFISR), to estimate the line-of-sight TEC over interior Alaska on the nights of the GIRAFF 381 and 380 launches (~10 PM AKST on 1 Feb and ~11:30 PM AKST on 8 Feb, respectively) for the fielded receiver sites. Next winter, we will use tomographic inversion techniques on the larger and more separated set of paths for the GNEISS mission, to estimate E-region plasma densities over interior Alaska in 3 dimensions. This allows the construction of a 3d map of ionospheric conductivity. This conductivity volume will inform the 3-dimensional structure of auroral current closure, as will be quantified by heterogeneously data-driven Gemini modelling.