Plasma Density Analysis of CubeSat Wakes in the Earth's Ionosphere

Spinning or tumbling CubeSats with Langmuir probes deployed on booms will render spin-modulated plasma densities as the probes move in and out of the spacecraft wake. It is traditionally assumed that the lower-density measurements from the spin cycle are made in the spacecraft wake, and the higher-density measurements are outside the wake. Although this assumption is valid for larger spacecraft in the Earth's ionosphere, this paper scrutinizes its validity for CubeSats in similar conditions. Spacecraft-plasma interactions (surface charging, plasma sheaths, and wakes) are less understood for CubeSats, and the small CubeSat dimensions must be considered with respect to characteristic length scales of the space plasma environment, namely, the Debye length. Spacecraft Plasma Interaction Software, a spacecraft charging analysis tool, is used to investigate CubeSat interactions with the mesothermal plasma environment. For low-density and cold-plasma ionospheric conditions, the CubeSat dimension of 10cm is comparable to the sheath thickness. The simulations show that, under such circumstances, a negatively charged CubeSat in mesothermal ionospheric conditions creates an ion focus region in the far wake. An independently written, first-principles code in MATLAB demonstrates that this feature is a direct result of the CubeSat behaving like a Langmuir probe in the thick-sheath model. The work performed in this paper cautions the community toward assuming CubeSats to have density depletion in their wake and stresses the necessity of having an accurate attitude solution and proper boom length design to derive ambient plasma densities from spin-modulated Langmuir probe measurements on CubeSats.