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Development of a Sensor to Measure Directly the Ionospheric
Thermal Electron Distribution, 15-9111

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Principal Investigators
Craig J. Pollock
Susan E. Pope
Scott E. Weidner

Inclusive Dates: 01/01/99 - Current

Background - This project was undertaken to fill a significant gap that exists in space plasma measurement capability. Electron distribution function measurements are typically conducted from energies upward of a few eV, with few measurements being made at energies below a few eV. Yet the ionospheric electron temperature is typically less than one eV, when expressed in energy units. That is, the kinetic energy of a typical ionospheric electron is less than one eV. Important electric currents are believed to be carried by ionospheric thermal electrons, and these electrons may drive high-latitude ion outflows by setting up ambipolar electric fields. This research project addresses this issue by developing a sensor specifically geared to measuring electrons in Earth’s ionosphere at low energy.

Approach - Although the research team is investigating several aspects, the team’s approach is centered on the concept of miniaturization. Miniaturization is necessary, because the low-energy electrons gyrate about the geomagnetic field in small circles, and desirable for effective use of spacecraft resources. Electrons gyrate in the geomagnetic field, making their measurement difficult using curved parallel plate electrostatic analyzers. Transmission is cut off if the radius of gyration is not much larger than the radius of analyzer curvature. Therefore, SwRI emphasized development of a very small analyzer, with a six-millimeter central radius of curvature. Therefore, the effort involves electrostatic ray tracing of particle trajectories through such an analyzer in the presence of a background magnetic field. In the past, the team has observed cutoff in particle count rates in space with small analyzers near 0.5 eV. The investigators are studying 1) electron transmission using ray tracing to determine if this cutoff is due to the geomagnetic field and 2) the manufacturability of small electrostatic analyzers, from the point of view of maintaining the tight mechanical tolerances required for uniform transmission characteristics throughout the analyzer. Finally, the team is studying techniques to image the particle arrivals at the exit aperture of small electrostatic analyzers, using microchannel plate detectors and a delay line anode.

Accomplishments - To date the team has conducted ray tracing studies of a numeric model of a small electrostatic analyzer in the presence of a background magnetic field. This model simulates an instrument flown in the Earth’s ionosphere in January 1995 that displayed cutoff in transmitted electrons at an energy near 0.5 eV. Preliminary results of this ray tracing indicate that the cutoff was not magnetic, as cutoff was not found in the ray tracing until the electron energy dropped below 0.1 eV. The ray tracing also revealed an electrostatic analyzer design flaw that significantly reduces electron transmission at all energies. This flaw will be corrected when a test model is built as part of this effort. The team has fabricated a prototype delay line detector and operated it in a vacuum chamber under ultraviolet illumination, with encouraging results. The measured time delay is in fact a monotonic function of the position at which a photon strikes the detector. This result will be directly applicable to electrons when this detector is incorporated into an electron sensor.

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