Low-energy auroral electrons

Sharp, William; Hays, P. B.

doi:10.1029/ja079i028p04319

Cited by 39 publications

(12 citation statements)

References 14 publications

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“…We mention this only because the red line is somewhat sensitive to the spectrum in this region. While the hole has been observed, it is not nearly as deep as theory predicts [e.g., Sharp and Hays, 1974]. The plasma model developed by Basu et al [1982] for the photoelectron spectrum suggests that there may be some smoothing of the electron spectrum due to various instabilities.…”

Section: Discussionmentioning

confidence: 96%

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Strickland¹,

Meier²,

Hecht³

et al. 1989

J. Geophys. Res.

131

106

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We examine the problem of monitoring composition and the behavior of precipitating electron spectra in auroras using N2+ 4278 Å (blue), O I 6300 Å (red), O I 7774 Å (narrow; e+O), and O I 7774 Å (broad; e+O2) as observed from the ground. Calculated column emission rates for these features as well as those of narrow O I 8446 Å and broad O I 8446 Å are presented as functions of the hardness of the incident electron spectrum and the concentrations of O and O2. Selected ratios of these rates such as narrow/broad (for 7774 Å) and red/blue are also presented. Incident spectra are characterized by Maxwellian energy distributions with characteristic energies ranging from 0.1 to 8 keV. Composition is modeled using a Jacchia (1977) model where scaling factors are applied to the O and O2 number densities. Scaling factors for O range from 0.1 to 1, and factors of 1 and 1.5 are considered for O2. As a group, the above rates and ratios show considerable variation over the just described parameter ranges, making them attractive for monitoring incident electron energy flux, its mean energy, and the concentrations of O and O2 relative to N2. The red line is a key feature of the above group which is sensitive to the low‐energy portion (subkilovolt) of the incident electron spectrum. Since this can vary considerably from one aurora to another having the same approximate mean energy, it becomes an added parameter to be considered within any algorithm using the red line. Paper 2 (Meier et al., this issue) discusses this problem as part of a detailed investigation of the red line. In the current paper, one particular representation of a low‐energy component is considered for making the red line calculations. A final subject of this paper is the use of temperatures deduced from measurements of rotational line distributions and atomic line Doppler widths to infer mean energies of precipitating electrons. Calculated effective temperatures versus hardness of the incident electron spectrum are presented and discussed in the context of more precise techniques for relating measurements of rotational line distributions and Doppler widths to electron spectral hardness.

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Section: Discussionmentioning

confidence: 96%

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Strickland¹,

Meier²,

Hecht³

et al. 1989

J. Geophys. Res.

131

106

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“…The auroral secondary electron spectrum has been a subject of considerable interest during the past few years frees and Maeda, 1973; Rees and Jones, 1973;Banks et al, 1974;Sharp and Hays, 1974;reidman and Doering, 1975], and although the results differ somewhat at low energies, they are in reasonable agreement from about 8 to about 100 eV. The measured spectrum of reidman and Doering [1975] was assumed to be the secondary electron spectrum over the energy region from 8 to 100 eV and for altitudes greater than 130 km because it appeared to be more or less representative of an IBC II aurora.…”

Section: Secondary Electron Distributionmentioning

confidence: 99%

Vibrational populations of the excited states of N₂ under auroral conditions

Cartwright¹

1978

J. Geophys. Res.

148

130

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The equations of statistical equilibrium for the population and depopulation of the electronic states of N2 have been solved for steady state normal auroral conditions. All vibrational levels for the seven lowest triplet electronic states (A, B, W, B', C, E, D) and the three lowest excited singlet electronic states (a, a', w) of N2 were included in the calculations. Recently measured auroral secondary electron spectra were employed to calculate the rates for electron impact excitation. The transition probabilities, electron impact excitation cross sections, and vibrationally dependent quenching rates used were a combination of the best available experimental and theoretical data. The results of these calculations are the absolute vibrational populations of all the electronic states. Intrasystem cascading and vibrationally dependent quenching were found to be important in determining the vibrational populations in the lower singlet and triplet excited electronic states. The predicted vibrational populations for the A and B triplet states agree well with the available experimental data. These calculations predict that a fairly large density of N• in excited roetastable states is produced under auroral conditions. Detailed knowledge of the populations processes and emission properties of the valence excited states of Ns is required inorder to obtain a thorough understanding of auroral phenomenology. A significant fraction of the auroral energy incident at the top of the atmosphere appears either as emissions from Ns [Vallance Jones, 1971a, b, 1974] or in the form of metastable Ns electronic states which then participate in energy transfer processes with other atmospheric species. The population processes and emission characteristics for the Cailu state appear to be well understood, since there is now general agreement between auroral measurements of the second positive system and theoretical predictions of the band system intensities. However, no detailed analysis of the population processes for the valence singlet states of Ns under auroral conditions has been given that includes the specific contribu-ments of the population in the lowest levels of the B state under auroral conditions and conclude that their results agree better with the predictions of Cartwright et al. [1971, 1973] than with those of Shemansky et al. [1971]. These recent measurements appear to confirm the earlier predictions that intrasystem cascading is an important mechanism in determining the vibrational populations of certain singlet and triplet electronic states in Ns under auroral conditions. Recent laboratory measurements of the B state vibrational population under discharge conditions [Chen and Anderson, 1975] have also verified the predictions of substantial cascade contributions to the 13 lowest levels of the B state. The purpose of this paper is to present detailed vibrational populations for all the singlet and triplet excited states of Ns, within 12.5 eV of the ground state, excited under auroral tions from the various cascade process...

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“…This measurement is extremely difficult because the negative spacecraft potentials typically observed in these regions are significant in magnitude compared to the low energies of the targeted electron distribution. As early as the 1960s, Hays and Sharp developed an instrument to measure the coldest electrons in the F region and below [ Sharp and Hays , 1974; Hays and Nagy , 1973]. However, early measurements lacked sufficient time and energy resolution for determining the distribution function of the thermal electrons.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2006

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[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.

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Low-energy auroral electrons

Cited by 39 publications

References 14 publications

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Vibrational populations of the excited states of N₂ under auroral conditions

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

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Low-energy auroral electrons

Cited by 39 publications

References 14 publications

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Deducing composition and incident electron spectra from ground‐based auroral optical measurements: Theory and model results

Vibrational populations of the excited states of N2 under auroral conditions

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

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Vibrational populations of the excited states of N₂ under auroral conditions