In January 2002 the SIERRA sounding rocket was launched from Alaska into active substorm expansion aurora. Direct measurements of the cold ionospheric population in darkness were made by the Thermal Electron Detector (TED), which was designed to measure thermal electrons that can carry auroral currents coupling the lower ionosphere and the magnetosphere. Measurement of thermal electrons must be accompanied by a careful study of electrostatic potentials forming near conducting bodies in a plasma. The TED instrument measurements show that a nonmonotonic potential barrier can form in the sheath around the attractively biased detector and prevent measurements of the core of the thermal electrons. The TED instrument design and response are discussed along with the current balance conditions which can lead to the formation of a potential barrier. A plasma distribution reconstruction technique enables key measurements of temperature, density, spacecraft potential, and an estimate of field-aligned current flow. Observed thermal electron core temperatures vary greatly, from ∼0.1 eV in the polar cap to ∼0.8 eV in auroral arcs. Outside active precipitation, the electron density agrees with an independent calculation based on measurements from the high-frequency (HF) wave receiver, verifying the method used for estimating the spacecraft potential. In the auroral regions the HF measurement of electron plasma density must be used to extract more accurate results for the spacecraft potential. The thermal electron data indicate that in the dark the nonnegligible auroral and secondary emission currents must be accounted for in order to understand the spacecraft potential and its impact on thermal electron measurements.