Date of Award

1-2015

Document Type

Thesis - Open Access

Degree Name

Master of Science in Engineering Physics

Department

Physical Sciences

Committee Chair

Dr. Aroh Barjatya

First Committee Member

Dr. Matthew Zettegren

Second Committee Member

Dr. Charles Lee

Abstract

Space exploration is currently being revolutionized by the advent of the CubeSat: 10cm cubed satellites that typically deploy in constellations for less than $100,000. Spacecraft instrumentation design must be redefined to abide by the compact CubeSat payload. Furthermore, the CubeSat dimension must be considered with respect to characteristic length scales of the space plasma environment, namely, the Debye length. Although spacecraft-plasma interactions- surface charging, plasma sheaths and wakes- have been well-studied for larger satellites, they are less understood for CubeSats.

The Dynamic Ionosphere CubeSat Experiment (DICE) is a 1.5U CubeSat which recently ended its mission. DICE carried two fixed-bias Langmuir probes operating in the ion saturation regime that extended 180_ apart from each other on scissor booms. Since the CubeSat was tumbling without proper attitude control, the plasma density measurements rendered by the probes were spin-modulated– as the probes moved in and out of the spacecraft wake, the relative density detected was modulated. Researchers who analyze such spin-modulated data routinely discard the lower density measurements from the spin-cycle attributing it to a density depletion in the spacecraft wake. It is traditionally assumed that the higher density measurement within a spin-cycle is being made outside the wake and thus is a better proxy of the ambient plasma density. While such assumptions might be true for larger spacecraft, this thesis investigates if it holds true for CubeSats in Earth’s ionosphere.

The Spacecraft Plasma Interaction System (SPIS) is a widely recognized and powerful particle-in-cell (PIC) simulation tool. In this thesis, SPIS is employed to investigate the manner that a CubeSat interacts with the plasma environment when electron thermal velocities vastly exceed the spacecraft velocity which, in turn, vastly exceeds ion thermal velocities. These are so-called "meso-thermal" conditions which are typical of plasmas in Earth’s ionosphere. Analysis of surface charging shows that the CubeSat abides by the thick-sheath model of Langmuir probes– the CubeSat dimension of 10 cm is comparable to the sheath thickness. More importantly, it is shown that, contrary to popular belief, there is a density enhancement in the CubeSat wake. SPIS results show that a negatively charged CubeSat in meso-thermal conditions creates an ion focus region in the far-wake. Finally, an independent code, written in MATLAB, is developed which verifies that this feature is a direct result of the CubeSat behaving like a Langmuir probe in the thick-sheath model.

The work performed towards this thesis cautions the community towards assuming Cube- Sats to have density depletions in their wakes, and stresses the necessity of having an accurate attitude solution to derive ambient plasma densities from spin-modulated Langmuir probe measurements. Ultimately, this work may inspire new perspectives in Langmuir probe development and data analysis for CubeSats.

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