Atomic Nitrogen Density Research Articles

The ultraviolet dayglow 1. Far UV emissions of N and N2

Far ultraviolet rocket spectra of N I and N2 dayglow emissions have been analyzed by using AE‐E photoelectron spectra, laboratory‐measured excitation cross sections, and photochemical models of atomic nitrogen. A self‐consistent picture of both optically thick and thin emission features is found by using a model in which the principal production mechanism for N I 1200‐Å and 1493‐Å photons is photodissociative excitation of N2. The aeronomic data require that ∼50–70% of the excited 4P atoms produced dissociatively have velocities within the Doppler core of the ambient nitrogen atoms, contrary to the expectation that those atoms are produced with large excess kinetic energy. The derived atomic nitrogen density has a maximum density of 2.7 × 107 cm−3 at 170 km, a value that is within 40% of that from recent models of odd nitrogen photochemistry.

High‐altitude atomic nitrogen densities

Theoretical calculations of the seasonal and diurnal variations of atomic nitrogen are compared with measurements made by the open source neutral mass spectrometer on the AE‐C satellite. With the simultaneous measurements of molecular nitrogen and atomic oxygen densities as input, model calculations of odd nitrogen densities predict the same trends in atomic nitrogen as those observed. From these comparisons it is inferred that horizontal transport significantly reduces the diurnal variation of atomic nitrogen. Estimates are given of the sensitivity of atomic nitrogen densities to variations in the photoelectron flux, the neutral temperatures, and the neutral winds.

Far ultraviolet atomic and molecular nitrogen emissions in the dayglow

A scanning spectrophotometer was used to observe the far ultraviolet day airglow between 1130 and 1520 A at 4.4-A spectral resolution. Fourteen bands of the N2 Lyman-Birge-Hopfield (LBH) system are clearly resolved and suggest a total LBH system zenith column emission rate of 3810 plus or minus 520 R extrapolated to the subsolar point. A photoelectron flux model (based on recent photoelectron flux measurements and the observed LBH altitude profile) is used to derive the direct and dissociative excitation contributions to the atomic nitrogen emissions. The calculated atomic nitrogen density agrees with other measurements, although it is an order of magnitude greater than previous photochemical model results.

Oso 7 measurements of atomic oxygen and molecular nitrogen densities in the Earth's upper atmosphere

Densities of molecular nitrogen and monatomic oxygen are presented as a function of altitude for selected intervals of March-May 1972 Oso 7 satellite data. Results on atmospheric attenuation of solar extreme UV radiation at 200-400 A, at heights from 250 to 500 km, are of particular interest. Satellite sunrise measurements above 330 km are not consistent with the Jacchia (1971) atmospheric model, and may constitute evidence for dynamic phenomena occurring in the heated upper levels. Density and temperature variations are considered mainly responsible for the sunrise anomaly observed. Checks were made for possible errors in the satellite pointing system and other instrumental effects.

Diurnal variation of atomic nitrogen

When the perigee of the Atmosphere Explorer C satellite reached the northern latitudes (68°N) by mid-April 1974, descending and ascending portions of the orbits stretched symmetrically across the northern hemisphere. Corresponding local solar times at altitudes above 400 km were 1700 hours and 0200 hours, respectively, thus providing the opportunity to study the diurnal variation of the neutral constituents. Atomic nitrogen densities, measured with the open source neutral mass spectrometer, show a pronounced diurnal variation. At 400 km the density ratios derived from measurements of the descending and ascending portions are 5.2 for N, 4.9 for N2, and 2.0 for O. Absolute densities of atomic nitrogen agree within a factor of 2 or 3 with densities derived from neutral-ion reactions. Measurements taken close to the geomagnetic pole show a decrease in atomic nitrogen densities when an increase in molecular nitrogen is observed.

Atomic nitrogen densities in the thermosphere

Recently atomic nitrogen densities of ∼106cm−3 were measured at 400 km by the open source mass spectrometer on the Atmosphere Explorer‐C satellite (AE‐C). Daytime N densities ∼5 × 107cm−3 at 160 km have also been inferred from airglow and other measurements on AE‐C. We show that atomic nitrogen densities of this magnitude result in significantly lower values for the O2+ concentration than those measured on AE‐C over the altitude range to 160 to 200 km, because of the removal process O2+ + N k3 → NO+ + O. We show that the discrepancy can be explained in terms of latitudinal variations in both the N and O2 densities. We also present evidence which indicates that k3 could be as low as 1 × 10−10cm−3 at ionospheric temperatures.

Effects of atomic nitrogen on the nocturnal ionosphere

Recently, atomic nitrogen densities of ∼5 × 107 to 108cm−3 were inferred in the daytime thermosphere from studies of the NI(²D−4S) 5200Å emission and from the photochemistry of various ion species using data measured by the Atmosphere Explorer‐C satellite. In this paper we use the photochemistry of NO+ and O2+ at night to determine nocturnal N(4S) densities in the thermosphere. We present evidence for a missing source of NO+ and a missing sink for O2+ at night and show that this can be adequately supplied by the reaction O2+ + N → NO+ + O if the N density at 200 km is ∼7 × 106cm−3. The atomic nitrogen has an important effect on studies of the 6300Å airglow. The omission of N in calculations of O(¹D) using ground‐based data results in an overestimate of the rate coefficient for quenching of O(¹D) by N2.

Atomic nitrogen measurements in the upper atmosphere

The open source neutral mass spectrometer (OSS) on the Atmosphere Explorer‐C satellite (AE‐C) measures the neutral constituents of the upper atmosphere. It has been found that atomic nitrogen densities can be determined at altitudes above 380 km. Most of the nitrogen atoms combine with oxygen adsorbed on the walls of the ion source to form NO. The measured net peaks at 14 amu and 30 amu show the scale height expected for atomic nitrogen; both peaks have a pronounced diurnal variation. We computed absolute number densities at 400 km for a time period between February and April 1974 when measurements were taken in the northern hemisphere. Minimum and maximum densities of atomic nitrogen occur between 4 and 6 hr LST in the morning and around 16 hr LST in the afternoon, respectively. At 400 km the minimum particle density is 1.0 × 105 cm−3 and the maximum density 1.5 × 106 cm−3. In contrast to the response of N2 to geomagnetic activity, atomic N shows no appreciable effect.

Satellite observations of the nitric oxide nightglow

The NO δ‐bands have been observed photometrically by the ultraviolet telescope onboard the TD‐1 ESRO satellite. The maximum intensity for the system is about 220 R, in good agreement with a previous rocket measurement. Assuming that the excitation is due to the N + O → NO(C²Π, v=0) preassociation, we used the observed profile to derive the atomic nitrogen density distribution. We found a peak of about 9 × 107 cm−3 at about 180 km. A comparison is made with theoretical predictions, and some implications of such a high N(4S) density are discussed.

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O 2 +, NO + and N 2 + using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [ NO +] [ O 2 +] ratio. In the region of most intense electron fluxes (20 erg cm −2 sec −1) at ∼280 km, the N density is found to be between 2 and 7 × 10 7 cm −3. The resulting N densities are found to account for approx. 60% of the production of N + through electron impact on N and the resonant charge exchange of O +( 2 P) with N( 4 S) . This reaction also provides a significant source of O( 1S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O +( 2 P) .

Surface catalytic efficiency of a sputtered molybdenum layer on quartz and Pyrex for the recombination of nitrogen atoms

The temporal properties of the number density of nitrogen atoms were studied during the decay period of plasmas produced in nitrogen by measuring the intensity of the first positive bands during this period. From the analysis of these data the value of the surface catalytic efficiency γ of a sputtered molybdenum layer on quartz and Pyrex for the recombination of nitrogen atoms was determined for gas pressures varying from 0.1 to 5 Torr and gas temperatures in the range 85–500 °K. A theoretical model explaining the data was derived.

Excitation of atomic nitrogen by electron impact

Absolute cross sections have been measured for the excitation of the NI(λλ1134 1164, 1168, 1200, 1243, and 1743 Å multiplets by electron impact on atomic nitrogen. The λ1134 and λ1200 Å cross sections are large, reaching 2.0× 10−16 and 2.5× 10−16 cm2 at their peaks, respectively. The magnitude of these cross sections is about a factor of two lower than the values reported previously by the authors, due to the discovery of a fault in the apparatus. The presence of vibrationally excited molecular nitrogen in the discharged gas is confirmed and its effect on the measurements is discussed. The ratio of the oscillator strengths of the λ 1200 and λ 1134 Å resonance transitions is measured to be 2.6 ± 0.3. The branching ratio for the NI(λ 1131Å/λ 1164Å multiplets is measured to be 2.8 ± 0.3. Striking differences in the distribution of intensity between the spectra of atomic nitrogen and molecular nitrogen excited by energetic electrons suggest an optical method for measuring the density of atomic nitrogen in the upper atmosphere.

Luminescence of Solids Excited by Surface Recombination of Atoms. V. Quantitative Dependence of Luminescence Response on Oxygen- and Nitrogen-Atom Densities

The quantitative relationship between luminescence response and atom density was investigated for the lumophor CaO:Sb:Cl excited by surface recombination of nitrogen or oxygen atoms. Atom densities were determined by electron spin resonance employing molecular oxygen as a standard of spin density. Luminescence response is essentially linearly proportional to nitrogen-atom density for total nitrogen pressures of 0.007 to 0.29 Torr. Luminescence response to oxygen atoms, determined at a total oxygen pressure of 0.016 Torr, is linearly proportional to oxygen-atom density when account is taken of the decreased response caused by oxygen-atom exposure. However, for certain operating conditions of the discharge, enhanced luminescence response occurred at oxygen-atom densities less than 3×1013 atom/cc; also, no significant decrease of response occurred with exposure to oxygen atoms at densities less than 0.4×1013 atom/cc. The enhanced response and the absence of a decreased response upon continued exposure are attributed to electronically excited oxygen species, probably O(1D) or O(1S). Equal densities of nitrogen and oxygen atoms produced about the same luminescence response.

Atomic oxygen and nitrogen density measurements with e.p.r.

Atomic oxygen and nitrogen density measurements with e.p.r.

Thermal and Radiative Properties of Recombining Atomic Nitrogen

A series of experiments on phenomena associated with the atomic recombination of nitrogen atoms is briefly discussed. The temperature dependence of the three‐body rate coefficient has been found to fit a power law k(T) ∝ T−0.6 over the temperature range of 200°–400°K. The absolute photon production rate accompanying the atomic recombination in the Rayleigh afterglow has been measured and found to be about one photon per 200 recombinations. The experiments rely on an electron spin resonance technique to measure the absolute atomic nitrogen density, and permit a rough determination of the dissociation energy of nitrogen.

ATOMIC REACTIONS IN THE UPPER ATMOSPHERE

The reaction rates of nine upper atmosphere chemical reactions have been analyzed with a digital computer. The dependence of the loss rates of atomic nitrogen and atomic oxygen as well as the relative concentrations of nitric oxide and nitrogen dioxide in the upper atmosphere are given. Using a flow system, nitrogen atoms were produced and then titrated by the addition of NO. The resulting nitrogen and oxygen atom densities were measured with an e.p.r. spectrometer. The dependence of the intensities of the nitrogen afterglow and the NO afterglow were related to the atom densities. An excitation mechanism for OI 5577 has been determined from the relation of the intensity of λ 5577 to the atom densities.