Search

[1] Terrestrial thermospheric dayglow emission from the coupled and overlapping c ' 4 1 E + u (0) and b' 1 Σ + u (1) levels of molecular nitrogen, observed by the Far Ultraviolet Spectroscopic Explorer, is analyzed with the aid of a coupled channels quantum mechanical model of N 2 spectroscopy and predissociation dynamics. Model emission spectra for the mixed c ' 4 1 Σ + u (0) ∼ b' E + u (1) ― X 1 Σ + g (v i = 2,6―9) transitions, calculated for the case of excitation by photoelectron impact, are in excellent agreement with the observations. While the principal excitation mechanism for N 2 in the thermosphere is photoelectron impact, evidence is also found in other transitions of resonant fluorescence, induced by lines in the solar atomic hydrogen Lyman series, atomic oxygen transitions, and other N 2 bands. The observed emission rate of the c ' 4 1 Σ + u (0) ∼ b' Σ + u (1) ― X 1 Σ + g (0) band is ∼1% of that inferred from the emission rates to X 1 Σ + g (v i > 2) levels. A qualitative explanation is given for the drastically reduced intensity and band shape distortion observed in the c ' 4 1 Σ + u (0) ― X 1 Σ + g (0) emission band. Estimates of the total electron excitation rates for the nominal b' 1 Σ + u (1) and c ' 4 1 Σ + u (0) levels are determined from the spectrum by extrapolating the model through regions containing unmeasured and/or resonantly absorbed bands.

[1] Recent Lyman series and Balmer series airglow measurements provide a fresh opportunity to investigate the density distribution and variability of atomic hydrogen in the upper atmosphere. Dedicated nightside Balmer α Fabry-Perot spectrometer measurements at the Pine Bluff Observatory (PBO), University of Wisconsin-Madison, have been acquired since late 1999 taking advantage of several technological advances. Extreme ultraviolet spectral radiance measurements by the Espectrografo Ultravioleta extremo para la Radiacion Difusa (EURD) instrument on the Spanish MINISAT-1 satellite from October 1997 to December 2001 provide extensive sets of geocoronal Lyman β, Lyman γ and He 584 A emission intensities. In this paper, coincident EURD Lyman β and PBO Balmer α radiance measurements from the early March 2000 new moon period are presented. In addition to serving as examples of the data sets now available, the data volume poses an analysis challenge not faced in prior geocoronal studies. A data-model comparison search procedure employing resonance radiation transport results for extensive sets of parametric density distribution models is being developed for use in analyses of multiple large data sets; this is described, and example results for the PBO and EURD March 2000 data sets are presented. The tightness of the constraints obtained for the solar line-center Lyman β irradiance and the atomic hydrogen column abundance is somewhat surprising, given the crudeness of the parameter binning in the search procedure and the fact that a small number of recognized corrections remain to be made to each data set.

Observations of the geocoronal hydrogen absorption feature at the center of the solar Lyman α emission line, together with previously published absorption measurements, are analyzed and compared with the predictions of the Kockarts and Nicolet model and the Liwshitz model of the terrestrial hydrogen distribution. The results show that the hydrogen density at 100 km is between 1.25 and 2×107 cm−3 (for the Kockarts and Nicolet model) for a wide range of exospheric temperatures, that the geocoronal hydrogen abundance decreases with increasing exospheric temperature, that there is reasonable agreement with near-simultaneous satellite Lyman α airglow observations, and that the absorption data are not accurate enough to distinguish between the models. Measurements of atomic hydrogen with mass spectrometers give densities that are larger by 2 orders of magnitude and therefore must be rejected in favor of the optical results.

The spherical Lyman-β radiative transfer problem is solved for several theoretical models of the hydrogen geocorona. Comparisons of the computed intensities with the experimental observations of Lyman β and Balmer α airglows show that the geocoronal Balmer α contribution is superimposed on a uniform background of about 4 R. The 4 R background is probably galactic (or extra-galactic) in origin. The column density of hydrogen required to account for the telluric Balmer α component is about 5 × 1013 atoms per cm² above 650 km for a 950°K exosphere.

Measurements of cross sections are presented for excitation of Balmer alpha and Lyman alpha emissions in collisions of 1.98- to 24.6-keV hydrogen atoms with N2 and O2. Estimates of corrections required for cascade contributions and lack of excited state equilibrium are also given. At 1.87 keV, Lyman alpha and Balmer alpha cross sections reach their largest value, indicating maxima in these cross sections at or below 2 keV. Except for differences in absolute values for Lyman alpha cross sections, the results agree well with the work of others where comparison can be made.

Optical oscillator strengths for the seven most intense dipole-allowed transitions in the 100-eV incident energy electron scattering spectrum of atomic oxygen have been measured from forward scattering spectra. An apparatus was used in which a microwave discharge source, producing atomic oxygen seeded in a helium beam, was coupled to an electron spectrometer. The O ¹D population in the target beam was less than 1.5% of O ³P, and transitions from the O ³P ground electronic state only were observed. The performance of the apparatus was checked by measurement of optical oscillator strengths for helium and atomic hydrogen. Agreement with the National Bureau of Standards (NBS) values for a single spectrum of better than 10% was obtained for three He transitions and 13% for the atomic hydrogen n = 3 transitions when the He 1¹S - 2¹P and H n = 1 - n = 2 Lyman series transitions were normalized to the NBS values. The atomic oxygen 2p4 ³P - 2p³3s ³S° (1304 A) transition was normalized to an average experimental value of f = 0.048. Optical oscillator strengths obtained for the other transitions were 2p4 ³P - 2p³4s ³S° (1040 A), 0.010±0.002; 2p4 ³P - 2p³3d ³D° (1026 A), 0.019±0.001; 2p4 ³P - 2p³3s′ ³D° (989 A), 0.061±0.006; 2p4 ³P - 2p³4d ³D° (972 A), 0.016±0.006; 2p4 ³P - 2p³3s″ ³P° (878 A), 0.086±0.006; 2p4 ³P -; 2s2p5 ³P° (792 A), 0.070±0.004. (All values have ±5% uncertainty in addition to the experimental scatter as indicated.) The electric dipole-forbidden transition 2p4 ³P - 2p³3p ³P was also observed as a weak process.