Observation of mesospheric ozone at low latitudes

Stellar ultraviolet light near 2500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Hartley continuum of ozone. The intensity of stars in the Hartley continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the ozone number density profile at the occultation tangent point. The results of approximately 12 stellar occultations, obtained in low latitudes, are presented, giving the nighttime vertical number density profile of ozone in the 60- to 100-km region. The nighttime ozone number density has a bulge in its vertical profile with a peak of 1 to 2×10⁸ cm^?3 at approximately 83 km and a minimum near 75 km. The shape of the bulge in the ozone number density profile shows considerable variability with no apparent seasonal or solar cycle change. The ozone profiles obtained during a geomagnetic storm showed little variation at low latitudes.     

Laboratory measurement of the OI1173989 Å branching ratio

Laboratory measurements of the $\text{OI}\text{1173}\text{989}$ Å (3s' ³D° → 2p⁴¹D, 3s' ³D° → 2p⁴³P) branching ratio have been made with a value of 1.5 × 10^t-4 indicated. This value makes the branching transition at 1173 Å an order of magnitude stronger than the branch at 7990 Å (3s' 3D° → 3p ³P). The 1173 Å branching loss is still too weak a loss process for multiply scattered 989 Å photons to resolve the 989 Å intensity problem in the dayglow.     

A method of calculating 0–20 Å solar X-ray flux and its spectral distribution using 9.1 cm spectroheliograms

1–8 Å, 2–12 Å and 8–20 Å non-flare X-ray flux data and 9.1 cm spectroheliograms for 1237 days during the period July 1966 to June 1970 have been studied to derive physical models of λ < 20 Å X-ray emitting regions on the Sun under quiescent (non-flare) conditions. The preferred regions of emission below 20 Å which coincide with the coronal active regions characterised by enhanced 9.1 cm microwave emission are found to have temperature lying between 1.8 and 3 × 10⁶ K, emission measure 10⁴⁹–10⁵⁰ and electron density 10⁹-10¹⁰ per cc. The average area of an active region is 10²⁰ cm². A slow gradient of temperature and electron density is seen to exist around a region of peak activity, both temperature and electron density decreasing outwards. Based on the derived physical model of the emitting regions a new method is presented for calculating X-ray flux and spectral energy distribution in this wave length region using daily 9.1 cm solar spectroheliograms. The calculated values are in good agreement with the observed values.     

A study of the excitation and radiative decay of the 3s′ 3D0 and 3d 3D0 levels of atomic oxygen

The absolute cross-sections for the excitation of the 989 Å, 1027 Å, 7990 Å, 8446 Å, 1.1287 μm and 1.3164 μm multiplets of atomic oxygen by electron impact dissociation of O₂ are reported. The radiative branching ratios for these transitions are calculated from these results and compared with the NBS compilation of Wiese et al. (1966) and the recent theoretical calculations of Pradhan and Saraph (1977). The cascade models of O⁺ radiative recombination and of electron-impact excitation of the OI(³S) state in the terrestrial airglow are discussed in the light of the laboratory measurements, and the effects of the resonant absorption of components of the λ 989 Å and λ. 1027 Å multiplets by the Birge-Hopfield band system of N₂ are investigated. This process is shown to depend sensitively on the N₂ vibrational temperature and to cause characteristic changes in the OI e.u.v. emission spectrum in auroras and in the sunlit F-region at high exospheric temperatures. It is also suggested that the λ 1027 Å radiation observed in auroral spectra is actually due to molecular nitrogen band emission that has been enhanced by entrapment effects and not to the excitation of the 2p ³P-3d ³D⁰ transition of atomic oxygen as believed previously.     

The Gas Production Rate and Coma Structure of Comet C/1995 O1 (Hale–Bopp)

The University of Wisconsin–Madison and NASA–Goddard conducted acomprehensive multi-wavelength observing campaign of coma emissionsfrom comet Hale–Bopp, including OH 3080 Å, [O I] 6300 Å H₂O⁺ 6158 Å, H Balmer-α 6563 Å, NH₂ 6330 Å, [C I] 9850 ÅCN 3879 Å, C₂ 5141 Å, C₃ 4062 Å,C I 1657 Å, and the UV and optical continua. In thiswork, we concentrate on the results of the H₂O daughter studies.Our wide-field OH 3080 Å measured flux agrees with other, similarobservations and the expected value calculated from published waterproduction rates using standard H₂O and OH photochemistry.However, the total [O I] 6300 Å flux determined spectroscopically overa similar field-of-view was a factor of 3-4 higher than expected.Narrow-band [O I] images show this excess came from beyond theH₂O scale length, suggesting either a previously unknown source of[O I] or an error in the standard OH + ν→ O(¹ D) + H branching ratio. The Hale–Bopp OH and[O I] distributions, both of which were imaged tocometocentric distances >1 × 10⁶ km, were more spatiallyextended than those of comet Halley (after correcting for brightnessdifferences), suggesting a higher bulk outflow velocity. Evidence ofthe driving mechanism for this outflow is found in the Hα lineprofile, which was narrower than in comet Halley (though likelybecause of opacity effects, not as narrow as predicted by Monte-Carlomodels). This is consistent with greater collisional coupling betweenthe suprathermal H photodissociation products and Hale–Bopp's densecoma. Presumably because of mass loading of the solar wind by ionsand ions by the neutrals, the measured acceleration of H₂O⁺ downthe ion tail was much smaller than in comet Halley. Tailwardextensions in the azimuthal distributions of OH 3080 Å,[O I], and [C I] , as well as a Doppler asymmetry in the[O I] line profile, suggest ion-neutral coupling. While thetailward extension in the OH can be explained by increased neutralacceleration, the [O I] 6300 Å and [C I] 9850 Å emissions show 13%and >200% excesses in this direction (respectively), suggesting anon-negligible contribution from dissociative recombination of CO⁺and/or electron collisional excitation. Thus, models including theeffects of photo- and collisional chemistry are necessary for the fullinterpretation of these data.     

First Results from the Soho Ultraviolet Coronagraph Spectrometer

The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is being used to observe the extended solar corona from 1.25 to 10 R⊙ from Sun center. Initial observations of polar coronal holes and equatorial streamers are described. The observations include measurements of spectral line profiles for HI Lα and Lβ, Ovi 1032 Å and 1037 Å, Mgx 625 Å, Fexii 1242 Å and several others. Intensities for Mgx 610 Å, Sixii 499 Å, and 520 Å, Sx 1196 Å, and 22 others have been observed. Preliminary results for derived H⁰, O⁵⁺, Mg₉₊, and Fe¹¹⁺ velocity distributions and initial indications of outflow velocities for O⁵⁺ are described. In streamers, the H⁰ velocity distribution along the line of sight (specified by the value at e^-1, along the line of sight) decreases from a maximum value of about 180 km s^-1 at 2 R⊙ to about 140 km s^-1 at 8 R⊙. The value for O⁵⁺ increases with height reaching a value of 150 km s^-1 at 4.7 R⊙. In polar coronal holes, the O⁵⁺ velocity at e^-1 is about equal to that of H⁰ at 1.7 R⊙ and significantly larger at 2.1 R⊙. The O⁵⁺ in both streamers and coronal holes were found to have anisotropic velocity distributions with the smaller values in the radial direction.     

Dynamics of Coronal Bright Points as Seen by Sun Watcher Using Active Pixel System Detector and Image Processing (SWAP), Atmospheric Imaging Assembly (AIA), and Helioseismic and Magnetic Imager (HMI)

The Sun Watcher using Active Pixel system detector and Image Processing (SWAP) onboard the PRoject for OnBoard Autonomy-2 (PROBA2) spacecraft provides images of the solar corona in EUV channel centered at 174 Å. These data, together with the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO), are used to study the dynamics of coronal bright points. The evolution of the magnetic polarities and associated changes in morphology are studied using magnetograms and multi-wavelength imaging. The morphology of the bright points seen in low-resolution SWAP images and high-resolution AIA images show different structures, whereas the intensity variations with time show similar trends in both SWAP 174 Å and AIA 171 Å channels. We observe that bright points are seen in EUV channels corresponding to a magnetic flux of the order of 10¹⁸ Mx. We find that there exists a good correlation between total emission from the bright point in several UV–EUV channels and total unsigned photospheric magnetic flux above certain thresholds. The bright points also show periodic brightenings, and we have attempted to find the oscillation periods in bright points and their connection to magnetic-flux changes. The observed periods are generally long (10?–?25 minutes) and there is an indication that the intensity oscillations may be generated by repeated magnetic reconnection.     

Characteristics of optical emissions and particle precipitation in polar cap arcs

Simultaneous optical and particle data from the ISIS-2 satellite are used to characterize polar cap arcs. Polar cap arcs are identified from two-dimensional geomagnetic transforms of the optical data along with precipitating electron data for the time at which the satellite is on the field line intersecting the arc. No precipitating protons were detected for any of the arc crossings. The pitch angle. distribution of the precipitating electrons is generally isotropic and the differential electron spectra show enhancements in the flux in the 300–750 eV energy range. The average energy of the precipitating electrons for the different arcs ranges from about 300 to 600 eV. A possible explanation of the observed precipitating particle characteristics is that parallel electric fields are accelerating polar rain type spectra at an altitude of several thousand km. For the arc crossings reported here the equivalent 4278 Å emission rate per unit energy deposition rate has a mean value of 162 R/(erg cm^?2 s^?1). Average 3914 Å intensities are about 0.8 kR while 6300 Å intensities range from 0.5 to 3 kR. Model calculations indicate that direct impact excitation is a minor source for the 5577 Å emission rate, but supplies approx. 40% of the 6300 Å emission.     

Silicon Abundance from RESIK Solar Flare Observations

The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and active-region X-ray spectra in four channels covering the wavelength range 3.8?–?6.1 Å in its operational period between 2001 and 2003. Several highly ionized silicon lines were observed within the range of the long-wavelength channel (5.00?–?6.05 Å). The fluxes of the Si?xiv Ly-β line (5.217 Å) and the Si?xiii 1s ²?–?1s3p line (5.688 Å) during 21 flares with optimized pulse-height analyzer settings on RESIK have been analyzed to obtain the silicon abundance relative to hydrogen in flare plasmas. As in previous work, the emitting plasma for each spectrum is assumed to be characterized by a single temperature and emission measure given by the ratio of emission in the two channels of GOES. The silicon abundance is determined to be A(Si)=7.93±.21 (Si?xiv) and 7.89±.13 (Si?xiii) on a logarithmic scale with H=12. These values, which vary by only very small amounts from flare to flare and times within flares, are 2.6±1.3 and 2.4±0.7 times the photospheric abundance, and are about a factor of three higher than RESIK measurements during a period of very low activity. There is a suggestion that the Si/S abundance ratio increases from active regions to flares.     

Ozone and photochemistry of the Martian lower atmosphere

The calculation of number densities of CO₂, H₂O and N₂ photolysis products was carried out for the Martian atmosphere at heights up to 60 km. The ozone distributed in the atmosphere as a layer of 10 km width with [O₃] max = 2.5 × 10⁹ cm³ at height of 35 km which agree well with the results of u.v. observations on the evening terminator from the Mars-5 satellite. The calculated densities of O₂, CO and H₂O are also in good agreement with the measured data. The eddy diffusion coefficient is equal to 3 × 10⁶ in the troposphere (h ? 30 km) and 10⁸ cm² s^?1 above 40 km. The dependence of the total ozone content on water vapour amount in the atmosphere is considered; the hypothesis about the influence of water ice aerosol on the ozone formation is proposed to explain the high concentrations of ozone in the morning.     

Magnetic ordering of the polar airglow

The visible airglow experiment on the Atmosphere Explorer-C satellite has gathered sufficient data over the Earth's polar regions to allow one to map the geographic distribution of particle precipitation using emissions at 3371 and 5200 Å. Both of these features exhibit large variations in space and time. The 3371 Å emission of N₂(C³π), excited by low energy electrons, indicates substantial energy inputs on the dayside in the vicinity of the polar cusp. More precipitation occurs in the morning than evening for the sample reported here, while the entire night sector between magnetic latitudes 65° and 77.5° is subjected to particle fluxes. Regions of enhanced 5200 Å emission from $\text{N}\text{(}{}^2\text{D)}$ are larger in horizontal extent than those at 3371 Å. This smearing effect is due to ionospheric motions induced by magnetospheric convection.     

A Comparison of Theoretical Si VIII Emission Line Ratios with Observations from Serts

Recent R-matrix calculations of electron impact excitation rates in N-like Si VIII are used to derive theoretical emission line intensity ratios involving 2s ²2p ³–2s2p ⁴ transitions in the 216–320 Å wavelength range. A comparison of these with an extensive dataset of solar active region, quiet-Sun, sub-flare and off-limb observations, obtained during rocket flights of the Solar EUV Research Telescope and Spectrograph (SERTS), indicates that the ratio R ₁= I(216.94 Å)/I(319.84 Å) may provide a usable electron density diagnostic for coronal plasmas. The ratio involves two lines of comparable intensity, and varies by a factor of about 5 over the useful density range of 10⁸–10¹¹ cm^?3. However R ₂= I(276.85 Å)/I(319.84 Å) and R ₃=I(277.05 Å)/I(319.84 Å) show very poor agreement between theory and observation, due to the severe blending of the 276.85 and 277.05 Å lines with Si VII and Mg VII transitions, respectively, making the ratios unsuitable as density diagnostics. The 314.35 Å feature of Si VIII also appears to be blended, with the other species contributing around 20% to the total line flux.

     

On the temperature dependence of the reaction O + NO → NO12

The published data on the temperature dependence of the radiative combination of atomic oxygen with nitric oxide at pressures near 1 torr is examined. Arguments are advanced to suggest that radiation near the cut-off wavelength (～ 3875Å) is coming from the unstabilized activated complex, No¹₂. At 4000Å a positive activation energy of 1 kcal mole^?1 is deduced. Application of this temperature dependence with the rate coefficient at 5200Å is made to airglow measurements in aurora. The deduced NO concentration is about 10⁹ cm^?3, in general agreement with that deduced from the measured NO⁺/O⁺₂ ratio as well as an auroral model prediction.     

The height,spectrum and mechanism of type-B red aurora and its bearing on the excitation of O(1S) in aurora

The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N₂ First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N⁺₂ First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O⁺₂ dissociative recombination and N₂(A)O energy transfer were included. Account was taken of recent laboratory data on O(¹S) quenching. It was concluded that these processes could explain the excitation of O(¹S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(¹S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 10⁹ at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(¹S) for the processes considered and for several other processes which have been suggested recently.     

Impulse response analysis of 5577Å emissions

A method of time series analysis termed impulse response estimation is described. The method is applied to simultaneous ground based photometry measurements of the N₂⁺ ING band at 4278Å and the 5577Å line emission in order to estimate the response in 5577Å to a discrete impulse in excitation rate.From these impulse response estimates the contribution from indirect excitation processes to the O(¹S) state is estimated to be of the order of 80% and the lifetime of the associated intermediate species to be approximately 0.1 s. Effective lifetimes for the O(¹S) excited state are obtained in the range 0.49–0.86 s with a distribution showing a sharp cut-off at 0.8 s and a mean of 0.71 s.     

Photoionization rates in the night-time E- and F-region ionosphere1

Quantitative estimates of ionization sources that maintain the night-time E- and F-region ionosphere are given. Starlight (stellar continuum radiation in the spectral inverval 911–1026 Å) and resonance scattering of solar Ly-β into the night sector are the most important sources in the E-region and are capable of maintaining observable electron densities of order (1–4) × 10³ cm^?3. Starlight ionization rates have substantial variations (factors of 2–4) with latitude and time of year since the brightest stars in the night sky occur in the southern Milky Way and Orion regions. In the lower F-region the major O⁺ source in the equatorial ionosphere is 910 Å radiation from the O⁺ recombination in the F2-region, whereas in the extratropical ionosphere interplanetary 584 Å radiation only exceeds resonance scattering of solar 584 and 304 Å radiation as the dominant O⁺ source during the month of December.     

Equatorial ozone profiles from the solar maximum mission—a comparison with theory

The u.v. spectrometer polarimeter on the Solar Maximum Mission has been utilized to measure mesospheric ozone vs altitude profiles by the technique of solar occultation. Sunset data are presented for 1980, during the fall equinoctal period within ± 20° of the geographic equator. Mean O₃, concentrations are 4.0 × 10¹⁰ cm^?3at 50 km, 1.6 × 10¹⁰ cm^?3 at 55 km. 5.5 × 10⁹ cm^?3 at 60 km and 1.5 × 10⁹ cm^?3 at 65 km. Som profiles exhibit altitude structure which is wavelike. The mean ozone profile is fit best with the results of a time-dependent model if the assumed water vapor mixing ratio employed varies from 6 ppm at 50 km to 2–4 ppm at 65 km.     

Observation of OI 1300 Å radiation in the Martian atmosphere

Measurements from the 1225 to 1340 Å region by the ultraviolet detectors on Mars-3 are presented. Model calculations of the intensity of the OI triplet lines at 1304 Å are compared with the measurements made on December 27, 1971, and February 17, 1972. Agreement is found between experimental data and a model in which the neutral oxygen density at 100 km is 2–8 × 10⁹ cm^?3.     

Some relationships between solar X-ray bursts and SPA's produced on VLF propagation in the lower ionosphere

This paper discusses SPA's measured at long VLF propagation paths in the lower ionosphere and their association with solar X-ray bursts observed by USNRL satellites in the 0–3 Å, 0–8 Å and 8–20 Å bands. Excellent correlations were found between the SPA importances (in degrees per Mm) and the logarithm of the X-ray burst peak intensities. A hardening of the X-ray burst spectra is evident for increasing importance of SPA's; the threshold energy required for the occurrence of such anomalies was estimated, it is 4.3×10^?5 ergs cm^?2 sec^?1 in the main ionizing band of 0–3 Å. It was also possible to derive the effective recombination coefficient at the normal D-region height of 70 km, this beingα _r≈6×10^?6 cm³ sec^?1; furthermore ion production rates were estimated during SPA's at heights below the reference level.     

A determination of the composition of the Saturnian stratosphere using the IUE

We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 Å. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C₂H₂ and C₂H₆ with mixing ratios and distributions of (9 ± 3) × 10^?8 in the top 20 mbar of the atmosphere with none below for C₂H₂ and (6 ± 1) × 10^?6 also in the top 20 mbar with none below for C₂H₆. The C₂H₂ and C₂H₆ distributions and the C₂H₆ mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc.13, 722 (1981), and private communication]. The Voyager IRIS model also includes PH₃, which is not consistent with the uv albedo from 1800 to 2400 Å. Our model requires a previously unidentified absorber to explain the albedo near 1600 Å. After considering several candidates, we find that the best fit to the data is obtained with H₂O, having a column density of (6 ± 1) × 10^?3 cm-am.