In situ measurement of thermal electrons on the SIERRA nightside auroral sounding rocket

Abstract

[1] 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.

Auroral ion outflow: low altitude energization

“…The flows equatorward of the inverted V region are westward, as expected during this period of westward-travelling surges seen in the camera data. The transitional region between the inverted-Vs and the polar cap shows eastward flows, with an additional northward component after T+600 s. Panel (d) shows the various calculated conductivities (Klatt, 2005;MacDonald, 2004); these are discussed later in the paper.…”

“…The details of the SIERRA (Sounding of the Ion Energization Region: Resolving Ambiguities) sounding rocket mission have been presented in various publications (Klatt et al, 2005;MacDonald et al, 2006;MacDonald, 2004;Samara, 2005); here we recapitulate the points of interest to our ion outflow study.…”

“…1 shows various measures of (inverse) conductivity in the context of the particle data. These conductivities were calculated by Klatt (2005) and MacDonald (2004), and are reproduced here to show the relationship to the upflowing ions. We compare the Alfvén velocity calculated from the measured density (red trace); the E/B velocity from the measured field variations (black trace), and the (inverse) Pedersen conductivity caused by the measured electron precipitation (Reiff, 1984).…”

“…The plasma flow velocity is measured by the electric field instruments. The payload potential is calculated from the electron temperature measured by the onboard TED instrument (MacDonald, 2004;MacDonald, 2006), and from the electron density measured by the HF wave experiment. At these energies, the observed ion population is no longer gyrotropic in the detector frame, as can be seen in Figs.…”

SERSIO: Svalbard EISCAT Rocket Study of Ion Outflows

“…[10] The top hat detectors and the TED [MacDonald et al, 2006] were placed on 1 m long booms to avoid effects of the payload sheath. Particle instrumentation on this flight was optimized to study the thermal core of the ionosphere, providing a touchstone for comparison with the EISCAT radars in Tromso and Longyearbyen which simultaneously provided bulk line-of-sight ionospheric parameters.…”