4686A Line Research Articles

Solar line profiles of He I 584 A and He II 304 A

A grazing incidence spectrograph launched in a Black Brant rocket August 30, 1973, at White Sands Missile Range obtained high spectral resolution data of the solar radiation between 200 and 700 A. Absolute intensities and profiles of the brighter lines in this spectral region were measured. Results for the He I 584-A and He II 304-A lines are presented. For the He I 584-A line the full width at half intensity is 0.08 A, and the intensity for the solar disk is 1.3 × 109 photons cm−2 s−1. For the 304-A He II line these values are 0.12 A and 6.3 × 109 photons cm−2 s−1, respectively.

Polarization of the 584- and 304-Angstrom emissions of helium in the geocorona and interplanetary medium

The polarization of the 584- and 304-A emissions of helium that have been resonantly scattered in the earth's atmosphere and in the interplanetary medium is discussed. It is shown that in the geocorona the simultaneous measurement of altitude profiles of the 584-A He dayglow intensity and polarization will test the hypothesis that the 584-A dayglow is excited by resonance scattering of sunlight from geocoronal helium. If this is indeed the major source of the 584-A dayglow emission, a simultaneous measurement of the polarization and the altitude profile of the intensity in the exosphere will uniquely determine the helium number density distribution as a function of altitude, and in the thermosphere it will alow a study of nondiffusive equilibrium processes. An attractive feature of this method is that the interpretation does not require a knowledge of the solar 584-A line center flux or line profile.

Photoabsorption cross sections of H2, D2, N2, O2, Ar, Kr, and Xe at the 584-A line of neutral helium

A Phillips ionization gage helium discharge has been used to provide a stable source of vacuum ultraviolet photons. Filtration of the ultraviolet beam with tandem tin films reduced the spectrum to substantially the 584-A line of helium. Absorption by target gases of the 584-A radiation was observed in a windowless cell with a sodium salicylate detector. Precise absorber gas pressure measurement was achieved with an MKS Baratron. Photoabsorption cross sections, which were determined in this experiment with an accuracy of about 2%, are in essential agreement with values recently reported by others.

Observations of the helium II 304-A and helium I 584-A atmospheric dayglow radiation

Two photometers with bandpasses of 170–500 and 170–800 A were employed to observe dayglow emissions in the extreme ultraviolet (EUV) over an altitude range of 90–186 km. We have identified the emissions observed with these photometers as resonantly scattered He I 584-A and He II 304-A radiations. At 186 km, 209 ± 70 rayleighs of 584-A and 9.3 ± 3.1 rayleighs of 304-A radiation were measured. We have compared these observations to theoretical calculations of resonance scattering of solar emissions from geocoronal He and He+. Using the Jacchia (1971) atmospheric model for He, we find that the observed brightness of the 584-A emission requires that the solar 584-A line width be 0.014 ± 0.004 A. In this model the maximum overhead brightness of 584-A dayglow would occur at 900 km, and its magnitude would be 1.9 ± 0.6 kR. Our observation of 304-A brightness requires that the overhead column density of the He+ ions be 4.2 × 1011 ions/cm² column. This value is consistent with a constant-density plasmasphere model with a He+ ion density of 320 ions/cm³ in the plasmasphere. There is evidence of additional EUV dayglow emission in the 500- to 800-A band near 140 km that is still unidentified.

Geomagnetic effect on the neutral temperature of theFregion during the magnetic storm of September 1969

A spherical Fabry-Perot interferometer on Ogo 6 (launched June 5, 1969) gives the profile of the 6300-A oxygen line in the dayglow at different altitudes between 200 and 320 km. The temperature of the neutral components of the atmosphere is deduced from these measurements with an accuracy of ±65°K. The results presented in this paper correspond to the storm period September 25 to October 5, 1969. During this period a severe geomagnetic storm occurred. The temperature measurements show a very strong effect in the two polar regions, while the equatorial regions present a smooth temperature variation.

Twilight and nighttime ionospheric temperatures from oxygen λ6300 and λ5577 spectral-line profiles

Use of Fabry-Perot interferometer measurements of atomic-oxygen 6300- and 5577-A line profiles from twilight and nightglow to determine the neutral temperatures in the F2 and E regions of the earth's ionosphere. The exospheric temperatures determined from the 6300-A profiles are usually somewhat higher than the temperatures calculated from Jacchia's model, and differences as large as about 300 K are noted when the exospheric temperature equals 1500 to 1600 K. The postsunset and predawn rate of change of the exospheric temperature is often substantially larger than the Jacchia prediction. The 5577-A (E region) measured temperatures range from 200 to 220 K on quiet nights to 500 to 600 K during geomagnetic storms.

Day airglow emissions of 5577 A and 6300 OI lines In Martian atmosphere

In this paper emissions of 5577 A and 6300 A lines of OI in Martian atmosphere have been investigated. It has been found that for the 5577A line there are two peaks at 90 and 120 km and its total intensity is about 3.5 kR. For the 6300A line, since the rate of quenching by CO2 molecules is not known, its emission profile is obtained for two values of quenching coefficient, namely, 1 X 10-13 cm3 sec-1 and 1 X 10-12 cm-3 sec-1. It has been found to have a broad peak at 100-.140 km. Its intensity has been found to be 3 and 1 kR for the two values of rate coefficient.

Detection of conjugate photoelectrons at Millstone Hill

Photoelectrons arriving in the F region from the conjugate point have been detected at Millstone Hill during the winter night, by means of Thomson scatter observations of the plasma lines that appear in the signal spectrum. During winter, when the solar zenith angle χc at the conjugate point is less than 100° throughout the night, the plasma lines are continuously present, provided that the local plasma frequency exceeds 4.0 MHz. For a few days in the spring the lines disappear for a period around midnight when χc≥106°. These observations confirm that photoelectrons can escape from one hemisphere into the other for L values as large as 3.2. The arriving flux appears to be too weak, however, to give rise to significant excitation of the 6300-A line of atomic oxygen (J. F. Noxon, personal communication), which suggests that direct excitation may not be the main cause of the large predawn enhancements seen elsewhere.

A Least-Squares Deconvolution Technique for the Photoelectric Fabry-Perot Spectrometer

A method is described for deconvolving into Voigt profiles a spectrum obtained with a Fabry-Perot spectrometer. This technique removes many of the limitations of some recently discussed schemes for analytically describing such spectroscopic data and reducing it to Voigt profiles. The use of a digital computer is assumed for all computations. The analytical description of the spectrometer includes all the important instrumental parameters as well as four adjustable profile parameters (position, amplitude, Lorentzian width, and Gaussian width) for each line present in the source spectrum. A least-squares adjustment of all nonfixed profile parameters is performed such that the sum of the individually fitted lines reproduces the recorded spectrum. This technique has been successfully applied to the thirteen fine structure components of the 4686-A line of He ii obtained with a double etalon Fabry-Perot spectrometer.

Some properties of the luminous aurora as measured by a photoelectric photometer

Intensity of the auroral radiation from the entire sky as a function of time was monitored photoelectrically at College, Alaska. Measurements extended over two years. Very good correlation was found between the 3914A band of N2+ and the 5577A line of OI. An intensity-time curve of a typical auroral display is shown. A diurnal variation curve is obtained which is very consistent, giving the same form for weak, moderate, and strong aurorae, and showing no evidence of seasonal variation. A rapid fluctuation in the intensity of radiation is reported.

The altitude of the (OI) 5577A line in the night airglow measured from a rocket

For many years, experiments have been conducted in an attempt to determine the altitude in the upper atmosphere from which the forbidden λ 5577 line OI and other night airglow radiations originate. In the usual method, the altitude is derived from observations of the airglow luminosity as a function of zenith angle, using the formula due to Van Rhijn. Recently, there have been attempts to triangulate on patches or other features of the airglow. Neither method has produced,results that are in any sense conclusive.1

Photometry of the aurora

Simultaneous measurements of the 5577A line and the 3914A line in the aurorae are described. The measurements were made with a scanning photoelectric photometer. The 3914A line was present toward the north during the entire period of observation. The 3914A line usually showed a larger increase relative to the night-sky background than did the 5577A line. In a pulsating aurora, the 3914A line showed many small pulsations, whose period was 1 to 2 seconds, in addition to the large pulsations.

Absorption and Resonance of the Helium Infrared Lines

The absorption of helium excited at low voltages for its own lines, the $2^{3}S\ensuremath{-}2^{3}P$ at 10830A and the $2^{1}S\ensuremath{-}2^{1}P$ at 20582A, is determined. The percentage absorption of the $2^{3}S\ensuremath{-}2^{3}P$ line increases when either the exciting current or the pressure in the absorbing tube is increased, and also when either the current or the pressure in the source tube is decreased. When the light to be absorbed is taken end on from a Geissler tube source, the percentage absorption is much less than when the light is taken off at right angles to the capillary. The changes with source tube conditions are ascribed to the corresponding variations of the shape and width of the emission line. Observable absorption begins when the exciting voltage is 25 volts. No absorption is detected at 19.8 volts, the excitation potential of the $2^{3}S$ state. The 10830A line shows strong resonance which is directly proportional to the percentage absorption. The absorption for the 20582A line is very small. It shows no detectable resonance.