Observations Of OI Research Articles

Electron impact dissociative excitation of O2: 2. Absolute emission cross sections of the OI(130.4 nm) and OI(135.6 nm) lines

In this work, we report the OI(135.6 nm) absolute emission cross section resulting from the long‐lived (180 μs) OI(5S → 3P) transition from dissociative excitation of O2. From the ratio of the integrated intensities of the OI(135.6 nm) and OI(130.4 nm) features and from the absolute emission cross section for the OI(130.4 nm) emission feature from electron impact dissociative excitation of O2 at 100 eV, the absolute emission cross section for the OI(135.6 nm) feature was determined to be 6.4 × 10−18 cm2 at 100 eV. Electron impact‐induced optical excitation functions for optically allowed transitions at 115.2 nm and 130.4 nm and for an optically forbidden transition at 135.6 nm were also obtained over the electron impact energy range 0–600 eV. The OI(135.6 nm) emission cross section was measured in the laboratory utilizing a large collision chamber (1.5 m in diameter). Electrons were produced with an electrostatically focusing gun with a large focal length (50 cm). The OI(130.4 nm, 135.6 nm) excitation functions were put on an absolute scale as described in the text, and the OI(135.6 nm)/OI(130.4 nm) ratio was determined for the entire energy range (0–600 eV). The atomic O UV emission cross sections from dissociative excitation of O2 can be used to model the recent Hubble Space Telescope observations of OI(130.4 nm) and OI(135.6 nm) intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995, 1998].

Satellite remote sensing of thermospheric O/N2 and Solar EUV: 2. Data analysis

The companion paper by Strickland et al. (this issue) describes a technique for deriving QEUV and O/N2 from disk observations of OI 135.6‐nm and N2 LBH dayglow emission. QEUV refers to the integrated solar EUV energy flux below 45 nm and is derived from knowledge of the 135.6/LBH ratio or O/N2 in conjunction with either the 135.6 nm or LBH intensity. O/N2 refers to the ratio of the atomic oxygen (O) column density to the molecular nitrogen (N2) column density at a given value of the N2 column density. Strickland et al. show that the least uncertainty in derived values O/N2 occurs in the vicinity of an N2 depth of 1017 cm−2. The O/N2 values presented in this paper are referenced to this depth. While QEUV is obtained from the intensity of either 135.6 nm or LBH, O/N2 is obtained from the intensity ratio designated by 135.6/LBH. The technique has been used to derive O/N2 and QEUV values from nadir‐viewing far ultraviolet dayglow data obtained by the auroral and ionospheric remote sensor instrument on board the Polar BEAR satellite. Data are considered from single passes on July 15, 16, and 21, 1987, spanning a latitude range from 25° to 55°N. The 3‐hour ap index was between 20 and 30 for the first two passes and only about 5 for the third pass. The 135.6‐nm and LBH signals were obtained from spectra recorded at a resolution of 3.6 nm from which a background was subtracted followed by integration over the intervals from 134.5 to 139.0 nm and 155.0 to 170.0 nm. Uncertainties were assigned to the signals and their ratios that took into account statistical uncertainties in the true signal and in the subtracted background signal. Profiles of derived O/N2 with uncertainties reflecting the data uncertainties are shown along with mass spectrometer/incoherent scatter (MSIS) O/N2 profiles over 25°–55°N. More structure is seen in the profiles on July 15 and 16, which are more disturbed days than July 21. In all cases, O/N2 increases from high to low latitudes. MSIS also shows such an increase but is much less structured. The results are shown to agree qualitatively with O/N2 values obtained from a general circulation model. Derived QEUV values are in the range from 1.1 to 1.4 ergs cm−2 s−1 and are in good agreement with values derived from the Hinteregger spectrum for the F10.7 values appropriate to the observations. The results illustrate the potential of the technique for monitoring thermospheric dynamics through latitudinal and temporal variations in O/N2 which are signatures of ascending and descending motion in the thermosphere.

Equatorial atomic oxygen profiles derived from rocket observations of OI 557.7 nm airglow emission

Two rocket observations of the OI λ557.7 nm nightglow emission in the lower thermosphere have been carried out from the equatorial region at Natal (5.8°S, 35.2°W), one on 9 December 1985 and the other on 31 October 1986. The volume emission rate obtained from these observations showed a maximum at a height of 97 ± 1 km with a half-width of 6 km for the first experiment and 98 ± 1 km with a half-width of 7 km for the second experiment. The observed emission profiles showed large variability, suggesting dynamical control of the atomic oxygen. The atomic oxygen density profiles deduced from the present data are compared with atmospheric models and currently available experimental data.

A technique for the analysis of two-dimensional data: application to OI 5577Å airglow

A method to determine whether a two-dimensional data set can be represented by two independent separable functions is demonstrated using both synthetic data and satellite observations of OI 5577Å airglow. For such cases when the functions are separable it is possible to identify the function in the presence of noise using an iterative procedure based on the properties of an equitable matrix. In the specific case of the airglow, the latitudinal and temporal variations are separable for the data set studied. The spatial structure shows a minimum near the equator, with maxima at 35° in the winter hemisphere and 25° in the summer hemisphere. The long-term temporal function shows maxima after the equinoxes, with minima near the solstice. A 12 month component has a maximum near day 300.

Simultaneous observations of OI 7774‐Å and [OI] 6300‐Å emissions and correlative study with ionospheric parameters

Regular measurements of the OI 7774‐Å and [OI] 6300‐Å nightglow emissions were carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brasil, during the period April 1978 to March 1979. An ionosonde is also operated at Cachoeira Paulista. A correlative study of the OI 7774‐Å and [OI] 6300‐Å emission observations, with simultaneous ionosonde measurements, shows good correlations between (J7774)1/2 and F layer peak electron density (nm(e)) and between the ratio (J7774)1/2/(J6300) and the height of the F layer (hmF2), where J7774 and J6300 are the column emission rates for the OI 7774‐Å and [OI] 6300‐Å emissions, respectively. Simultaneous measurements of these two emissions would be a very useful technique for remote sensing of the ionospheric F layer dynamics.

Observations of OI 7774 emission excited by conjugate photoelectrons

Observations and computer calculations of OI 7774 airglow emissions excited by conjugate photoelectrons have been carried out. The observations were made at McDonald Observatory, Texas using a 2m grille spectrometer from December 1972 to June 1973. The zenithal emission intensity during conjugate photoelectron precipitation was fairly constant at 2–4 R until conjugate sunset, after which it diminished steadily and ceased near a conjugate solar zenith angle ( χ c ) of 105 ± 3°. A predawn enhancement in both OI 7774 and [OI] 6300 was observed to commence near χ c ∼ 102°. The computations utilize the two-stream technique of Nagy and Banks (1970) to obtain the escaping photoelectron flux and the local excitation rates of the oxygen emissions. Good agreement with the observations is obtained for the dependence of the emission rate on conjugate solar zenith angle. A lack of agreement in absolute intensity may not be due entirely to uncertainties in the excitation cross section. The discrepancy may indicate significant magnetospheric scattering of photoelectrons with energy greater than 15 eV.

Equatorial airglow and the ionospheric geomagnetic anomaly

OGO D observations of OI (6300A) emissions reveal a global pattern in the equatorial airglow undetected from the ground-based observations. The post sunset emission rate of OI is generally asymmetrical with respect to the geomagnetic equator and shows no apparent correlation with the ultraviolet airglow (OI 1304 and 1356A) and F region electron density measured simultaneously from the same spacecraft. Both the ultraviolet airglow and the ion density measured in the altitude region of 450 km follow similar latitudinal variations and exhibit properties of the equatorial ionospheric anomaly. The asymmetry in OI emission can be attributed to the asymmetry in the height of the F 2 maximum inferred from the height of the maximum emission. From correlative studies of the airglow and the ionospheric measurements, the mechanisms for the ultraviolet and the 6300A emission are discussed in terms of the processes involving radiative and dissociative recombinations. A relationship between molecular oxygen density and the integrated OI emission rate is derived and the feasibility of using this relationship for estimating O2 density is discussed.

International Fishery Investigations 1

THE series of reports now under review on -I the work of the International Council for the Study of the Sea furnishes evidence of continued activity in many branches of the work. One of the most interesting new features is described in the hydrographical bulletin, which contains an account of a series of observations on temperatures, salinities, and direct-current measurements made from ten vessels which were anchored for fourteen days (June 1 to 14, 1911) in a series of positions in the North Sea, selected with the view of studying the principal currents. A repetition oi observations of this character from time to time as opportunity offers cannot fail to give informa tion of the utmost value.