Assessing Coulomb Collision Contributions to Photoelectron Deceleration above Earth’s Polar Caps

NASA’s Endurance rocket mission resolved a subtle energy shift in photoelectrons escaping from Earth’s polar caps, from which the first successful measurement of Earth’s ambipolar electrostatic field was made. Preflight models predicted that coulomb collisions with thermal electrons could add an extra energy shift of up to 1 eV. However, no such additional shift was reported in the initial analysis. Here, we identify a previously unreported effect related to the spacecraft wake, which introduced a systematic +0.25eV energy shift that countered and masked the shift from coulomb collisions. After applying an in-flight calibration for this wake effect, we find a total photopeak energy shift consistent with the combined influence of the ambipolar field and the subtler -0.37eV coulomb shift, predicted by the fully kinetic STET model. Further analysis of these recalibrated measurements yields an ambipolar field estimate in close agreement with the initial analysis.