The 2‐ to 5‐eV energy spectra of thermospheric photoelectrons: Measurements in apparent conflict with theory

Abstract

The existence of marked structure in the 2‐ to 5‐eV energy spectra of near‐earth ionospheric photoelectrons has been predicted by theory and observed by experiment. This structure, arising from resonant vibrational excitation of N2, is seen as a valleylike dip in the spectra near 2.5 eV and is predicted to be most prominent at low altitudes (110 km), then to diminish with increasing altitude until it disappears in the 200‐ to 250‐km region. All measurements to date indicate that the overall magnitude of this valley or dip is significantly less than that predicted by theory, but confirm a trend toward diminishing prominence with increasing altitude above about 150 km. Below 140–150 km, however, the valley is observed in the present data to also diminish with decreasing altitude, a reversal of the predicted trend. The present measurements include the only observations, to our knowledge, of this spectral structure below 140 km, and these indicate that its maximum prominence occurs in the 140‐ to 150‐km altitude region, below which it is consistently observed to decline rapidly in magnitude. Recent refinement of theory may lead, in part, to an explanation of these discrepancies. The experimental results are summarized and the difficulties peculiar to space vehicle measurements of very low energy ambient electrons are examined.

A numerical study of the 30° parallel plate analyzer for ionospheric electron spectroscopy

“…The rocket data of Mukai and Hirao in their paper of 1973 (7) indicated the presence of a broad structure in the energy region 20-30 eV, identified by them as originating from the ionization of atomic oxygen and molecular nitrogen by the HeII 30.4 nm solar emission line. A similar broad feature is found also in the data of McMahon and Heroux (5,6).…”

“…This feature is evident in Doering's data (2,3) and in the sounding rocket investigations of Hays and Sharp (4) and McMahon and Heroux (5,6). This structure reaches a maximum in prominence in the 130-150 km region and gradually diminishes with increasing altitude, disappearing in the 200-250 km region.…”

“…The coefficient m may be considered as a magnification in the image of EA at the focal plane -for this spectrometer, m ≈ 1. In the application contemplated here with ionospheric photoelectrons, the field of view to be used is α = 12 • , consistent with that used previously (5,6) in sounding rocket experiments in order to limit contamination by photoelectrons generated on surfaces near the spectrometer entrance.…”

“…These numbers suggest that this feature could be easily smoothed out if the energy resolution were more than 0.2 eV, as shown by the dashed curve. If the depth of the minimum is important as it is in this case, then an energy resolution of about 0.1 eV is desired to provide the best characterization of the minimum, or the peak to valley ratio as discussed by McMahon and Heroux (6).…”

“…Since the actual experimentally measured photoelectron flux is uncertain, it is useful to indicate the instrument characteristics used previously by McMahon and Heroux (5,6); this will provide a basis for the characteristics of the new instrument. In their 1978 paper, McMahon and Heroux (5) report that their spectrometer scanned the energy in 64 energy steps, each with a dwell time of Downloaded by [Erciyes University] at 01:13 05 January 2015 20 ms over an energy range of approximately 40 eV.…”