Atomic Oxygen Number Density Research Articles

Nitric Oxide Concentration: A New Data Set Derived From SABER Measurements

AbstractVertical profiles of nitric oxide (NO) concentration are derived between 120 and 250 km using updated NO emission rates measured by Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. The Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar (MSIS) 2.1 model is used to provide the required parameters of temperature, atomic oxygen number density, and molecule oxygen number density needed to derive the NO concentrations using a non‐local thermodynamic equilibrium (non‐LTE) model. The SABER NO concentration shows a significant correlation with solar activity with larger peak NO concentrations and higher altitude extent during solar maximum years compared to those during the solar minimum years. The SABER NO agrees well with the MSIS 2.1 NO at altitudes above 120 km for all latitudes, while the pronounced SABER‐MSIS NO discrepancy below 120 km is likely due to the temperature underestimation by MSIS 2.1. A detailed error analysis is presented and considers systematic and random errors in all the terms in the non‐LTE model used to derive the NO concentration. Random error in MSIS 2.1 temperature and atomic oxygen dominates the uncertainty in single NO profiles above 120 km. We estimated a systematic error up to ∼36% between 120 and 250 km during solar maximum years.

Spectral insight into the interface evolution of a carbon/carbon composite under high-enthalpy, nonequilibrium flow

Gas–surface interactions between thermal protection materials and high-enthalpy nonequilibrium flow are one of the greatest challenges in accurately predicting aerodynamic heating during supersonic flights. Finer microscopic details of flow properties are required for elaborate simulation of these interactions. Spectral insight, with quantum-state-specific characteristics, is provided in this work to investigate the physico-chemical processes in high temperature interface of a carbon/carbon (C/C) composite. The nonequilibrium air flow is produced by a 1.2 MW inductively coupled plasma wind tunnel at an enthalpy of 20.08 MJ/kg. The duration of each test is up to 100 s, and quartz is also tested for comparison. Spectral insights into the reaction mechanisms of the gas–surface interactions are acquired by the optical emission spectroscopy and laser absorption spectroscopy. Dynamic evolution of the chemical reaction pathways and thermal nonequilibrium are discussed based on the results of optical emission spectroscopy. Temporally and spatially resolved results of the translational temperature and number density of atomic oxygen are quantified by laser absorption spectroscopy. Controlling mechanisms in the surface chemistry are further analyzed in conjunction with the surface temperature, scanning electron microscopy, and energy dispersive spectroscopy. Reaction mechanisms on the C/C composite surface sequentially experience an oxidation-dominant, an intense competitive, a nitridation-dominant, and a recession dominant period. Distributions in the axial direction and dynamic characteristics of the translational temperature and number density of atomic oxygen are found closely related with surface swelling, recession, and chemical reactions. The results herein are consistent with each other and are instructive to further investigate the interface evolution on C/C composites.

Atomic-Oxygen Number Densities in Ar-O2 DBDs and Post-discharges with Small Initial O2 Fractions: Plug-Flow Model and Experiments

Number densities of oxygen atoms, nO, in Ar-O2 mixtures with small initial O2 fractions, {x}_{{O}_{2}} < 1%, flowing through a dielectric-barrier discharge (DBD), are calculated using a plug-flow reactor model, presuming that dissociation and excitation of oxygen species are solely driven by energy-transfer from long-lived excited Ar species, collectively denoted as Ar*. The rate by which Ar* species are generated is calculated from the volume density of power dissipated in the DBD. To obtain extended post-discharge (PD) regions with large nO, experiments were performed with {x}_{{O}_{2}} = 100 ppm. For such low O2 fractions, the time-dependence of nO in the DBD and the early PD can be calculated by a closed equation. Calculations are compared with optical emission spectroscopic (OES) results, utilizing the proportionality of O-atom emission intensity at 777.4 nm to nO. O-atom densities in the PD are made accessible to OES using a tandem setup with a second DBD as sensing discharge. Model testing by experiment is based on the functional dependence of nO on DBD-residence time and PD-delay time, respectively. Wall losses of O atoms in asymmetrical DBD reactors are calculated by an alternative to Chantry’s equation. The agreement between O-atom densities attained at the DBD exit and experimental results is generally good while the speed of rise of nO in the discharge is overestimated, due to the assumption of a constant wall-loss frequency, kW. Compared with literature data, kW is orders of magnitude higher in the DBD and at least one order of magnitude lower in the PD.

Gas–surface interactions in a large-scale inductively coupled plasma wind tunnel investigated by emission/absorption spectroscopy

Precise prediction of aerothermal loads is significantly limited by the unclear interactions between the thermal protection system surface and the surrounding high-enthalpy gas. To address this, we propose an optical diagnostic method based on optical emission spectroscopy and laser absorption spectroscopy to investigate the gas–surface interactions within the boundary layer. Experiments are conducted in an air plasma flow produced by the 1.2 MW inductively coupled plasma wind tunnel at the China Academy of Aerospace Aerodynamics with an enthalpy of 20 MJ/kg and a heating time of 100 s. The cylindrical samples made of pure silicon carbide are tested, and quartz samples with the same exposed geometry are tested in parallel as a reference material. The optical emission spectroscopy system has four spectrometers to cover the wide wavelength range of 200–1100 nm, providing qualitative, spatially, and spectrally resolved measurements of the multi-species radiative emission adjacent to the sample surface. Laser absorption spectroscopy is deployed at different axial locations to quantify the number density and translational temperature of OI (3s5S) with a 500 Hz scanning rate and 200 kHz acquisition rate. Additionally, the surface temperature of each sample is detected by an infrared pyrometer. Scanning electron microscopy and energy dispersive spectrometry are performed before and after the plasma heating. Our measurement results provide valuable information on surface reaction pathways and catalytic recombination effects on atomic oxygen number density distributions. Finally, these self-consistent results show that the proposed method is reliable to deeply investigate gas–surface interactions within boundary layer in harsh aerothermal environment.

A Simulation Study on the Variation of Thermospheric O/N2 With Solar Activity

AbstractThe ratio of number density of atomic oxygen (O) to that of molecular nitrogen (N2) in the thermosphere (O/N2) on the constant pressure surface, which has complex temporal and spatial characteristics, is widely regarded as an important parameter connecting the terrestrial thermosphere and daytime ionosphere. Previous studies demonstrated that the thermospheric O/N2 increases with increasing solar activity, and the changes in O/N2 with solar activity show significant difference between winter and summer hemispheres. However, the root causes, which are responsible for the solar activity variation of O/N2, are not fully understood. In this study, the contributions of various physical and chemical processes on the response of O/N2 to the solar radiation change were quantitatively investigated through a series of controlled simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model. The simulation results suggested that the chemical processes lead to the increase of thermospheric O/N2 over the globe with increasing solar activity. The increase of O/N2 with solar activity is dominated by the enrichment of O abundance and the loss of N2 abundance in the lower and upper thermosphere, respectively. Moreover, the simulation results suggested that the stronger hemispheric asymmetry is attributed to the stronger thermospheric circulation, which changes the vertical advection of O/N2 through both direct and indirect effects.

Radiative cooling due to NO at 5.3 [formula omitted]m emission as observed by TIMED/SABER over Asian sector

The effect of geomagnetic storms on the peak emission of NO Volume Emission Rate (NO VER) at 5.3 μm, in mesosphere and lower thermosphere (MLT) region, is studied over the Asian sector during 26–29 September 2011 (storm 1) and 18–21 February 2014 (storm 2). The data for peak emission of NO VER is obtained from SABER instrument onboard the NASA’s TIMED satellite. The SABER retrieved data along with the neutral densities obtained from NRLMSISE-00 model have been used to study the latitudinal and longitudinal variation of peak NO VER during the storm period. The variations induced in the peak emission of NO VER is understood with the help of fluctuations in neutral species and the resulting changes in chemistry. It has been found that the peak emission of NO VER is strongly influenced by the storm conditions. The peak emission of NO VER at 5.3 μm is found to be maximum at higher latitudes during the storms. However, the magnitude of peak NO VER gradually decreases towards the equator during the storms. The modeled atomic oxygen number density shows depletion at the higher latitudes corresponding to peak altitude of NO VER. There is a negative correlation between the peak emission of NO VER and Dst index during the main phase of the storm. The peak emission of NO VER and modeled atomic oxygen number density shows the positive correlation at the equator region, while negative correlation at the higher latitudes. At higher latitudes modeled atomic oxygen number density shows positive correlation with Dst index, while negative correlation at the equator. The correlation factors obtained between various parameters related to the storm time radiative cooling strongly support the existing understanding of the variation of NO VER during extreme space weather events.

Atomic oxygen number densities in the mesosphere–lower thermosphere region measured by solid electrolyte sensors on WADIS-2

Abstract. Absolute profiles of atomic oxygen number densities with high vertical resolution have been determined in the mesosphere–lower thermosphere (MLT) region from in situ measurements by several rocket-borne solid electrolyte sensors. The amperometric sensors were operated in both controlled and uncontrolled modes and with various orientations on the foredeck and aft deck of the payload. Calibration was based on mass spectrometry in a molecular beam containing atomic oxygen produced in a microwave discharge. The sensor signal is proportional to the number flux onto the electrodes, and the mass flow rate in the molecular beam was additionally measured to derive this quantity from the spectrometer reading. Numerical simulations provided aerodynamic correction factors to derive the atmospheric number density of atomic oxygen from the sensor data. The flight results indicate a preferable orientation of the electrode surface perpendicular to the rocket axis. While unstable during the upleg, the density profiles measured by these sensors show an excellent agreement with the atmospheric models and photometer results during the downleg of the trajectory. The high spatial resolution of the measurements allows for the identification of small-scale variations in the atomic oxygen concentration.

Model results of OH airglow considering four different wavelength regions to derive night-time atomic oxygen and atomic hydrogen in the mesopause region

Abstract. Based on the zero-dimensional box model Module Efficiently Calculating the Chemistry of the Atmosphere/Chemistry As A Box model Application (CAABA/MECCA-3.72f), an OH airglow model was developed to derive night-time number densities of atomic oxygen ([O(3P)]) and atomic hydrogen ([H]) in the mesopause region (∼75–100 km). The profiles of [O(3P)] and [H] were calculated from OH airglow emissions measured at 2.0 µm by the Sounding of the Atmosphere using Broadband Emission Radiography (SABER) instrument on board NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The two target species were used to initialize the OH airglow model, which was empirically adjusted to fit four different OH airglow emissions observed by the satellite/instrument configuration TIMED/SABER at 2.0 µm and at 1.6 µm as well as measurements by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument on board the Environmental Satellite (ENVISAT) of the transitions OH(6-2) and OH(3-1). Comparisons between the “best-fit model” obtained here and the satellite measurements suggest that deactivation of vibrationally excited OH(ν) via OH(ν≥7)+O2 might favour relaxation to OH(ν′≤5)+O2 by multi-quantum quenching. It is further indicated that the deactivation pathway to OH(ν′=ν-5)+O2 dominates. The results also provide general support of the recently proposed mechanism OH(ν)+O(3P)→OH(0≤ν′≤ν-5)+O(1D) but suggest slower rates of OH(ν=8,7,6,5)+O(3P), partly disagreeing with laboratory experiments. Additionally, deactivation to OH(ν′=ν-5)+O(1D) might be preferred. The profiles of [O(3P)] and [H] derived here are plausible between 80 and 95 km but should be regarded as an upper limit. The values of [O(3P)] obtained in this study agree with the corresponding TIMED/SABER values between 80 and 85 km but are larger from 85 to 95 km due to different relaxation assumptions of OH(ν)+O(3P). The [H] profile found here is generally larger than TIMED/SABER [H] by about 50 % from 80 to 95 km, which is primarily attributed to our faster OH(ν=8)+O2 rate.

Spatial Distribution of Io's Neutral Oxygen Cloud Observed by Hisaki

AbstractWe report on the spatial distribution of a neutral oxygen cloud surrounding Jupiter's moon Io and along Io's orbit observed by the Hisaki satellite. Atomic oxygen and sulfur in Io's atmosphere escape from the exosphere mainly through atmospheric sputtering. Some of the neutral atoms escape from Io's gravitational sphere and form neutral clouds around Jupiter. The extreme ultraviolet spectrograph called EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) installed on the Japan Aerospace Exploration Agency's Hisaki satellite observed the Io plasma torus continuously in 2014–2015, and we derived the spatial distribution of atomic oxygen emissions at 130.4 nm. The results show that Io's oxygen cloud is composed of two regions, namely, a dense region near Io and a diffuse region with a longitudinally homogeneous distribution along Io's orbit. The dense region mainly extends on the leading side of Io and inside of Io's orbit. The emissions spread out to 7.6 Jupiter radii (RJ). Based on Hisaki observations, we estimated the radial distribution of the atomic oxygen number density and oxygen ion source rate. The peak atomic oxygen number density is 80 cm−3, which is spread 1.2 RJ in the north‐south direction. We found more oxygen atoms inside Io's orbit than a previous study. We estimated the total oxygen ion source rate to be 410 kg/s, which is consistent with the value derived from a previous study that used a physical chemistry model based on Hisaki observations of ultraviolet emission ions in the Io plasma torus.

Two-photon absorption laser induced fluorescence measurement of atomic oxygen density in an atmospheric pressure air plasma jet

Atomic oxygen number density [O] is measured in an air atmospheric pressure plasma jet (APPJ) using two-photon absorption laser induced fluorescence (TALIF). Gas flow is fixed at 8 slpm, the RF power coupled into the plasma jet varied between 5 W and 20 W, and the resulting changes in atomic oxygen density measured. Photolysis of molecular oxygen is employed to allow in situ calibration of the TALIF system. During calibration, O2 photo-dissociation and two-photon excitation of the resulting oxygen atoms are achieved within the same laser pulse. The atomic oxygen density produced by photolysis is time varying and spatially non-uniform which needs to be corrected for to calibrate the TALIF system for measurement of atomic oxygen density in plasma. Knowledge of the laser pulse intensity I0(t), wavelength, and focal spot size allows correction factors to be determined using a rate equation model. Atomic oxygen is used for calibration and measurement, so the laser intensity can be increased outside the TALIF quadratic laser power dependence region without affecting the calibration reliability as the laser power dependence will still be the same for both. The atomic O density results obtained are not directly benchmarked against other known density measurement techniques. The results show that the plasma jet atomic oxygen content increases as the RF power coupled into the plasma increases.

A study of OI 844.6 nm dayglow emission under geomagnetic storm conditions

A comprehensive model is developed to study 844.6nm dayglow emission. The Solar2000 EUV (extreme ultraviolet) flux model, neutral atmosphere model (NRLMSISE-00) and latest available cross-sections are incorporated in this model. The present model is used to study the effects of geomagnetic storm on the 844.6nm dayglow emission at a low latitude station Tirunelveli (8.7°N, 77.8°E). Three geomagnetic storms which occurred during 23rd–27th August 2005, 13th–17th April 2006 and 1st–5th February 2008 are chosen in the present study. It is found that the volume emission rate (VER) shows a negative correlation with the Dst index for all the three geomagnetic storms. The present study also shows that the altitude of the peak emission rate does not vary with the activity of geomagnetic storm. The model predicts a positive correlation between the zenith intensity of 844.6nm dayglow emission and atomic oxygen number density. The consistency of atomic oxygen number density obtained from the NRLMSISE-00 model during a geomagnetic storm is checked using the satellite measurements of Earle et al. (2013). It is found that the atomic oxygen number density given by NRLMSISE-00 model is significantly lower than the measured values. Consequently, the effect of atomic oxygen number density abundance on 844.6nm dayglow emission is further studied by treating the atomic oxygen number density as a variable parameter in the present model. An increase of more than 50% in the zenith intensity above the normal level (before the onset of the storm) is found when the atomic oxygen number density which is obtained from NRLMSISE-00 model is doubled (under the limits of measurements).

A study of 732.0 nm dayglow emission at the equator under varying atomic oxygen density conditions for equinox and solstice cases

AbstractA comprehensive model is developed to compute the volume emission rate of O+(2P) 732.0 nm dayglow emission. The modeled volume emission rates are compared with the measurements as provided by Atmosphere Explorer C satellite and Dynamics Explorer 2 spacecraft. It is found that the model explains quite well the measured emission profiles. The present model is used to study the effect of atomic oxygen abundance on the volume emission rate of 732.0 nm dayglow emission at the equator for equinox and solstice. To study the effect of atomic oxygen abundance on 732.0 nm dayglow emission, the atomic oxygen number densities obtained from the NRLMSISE‐00 model are increased (or decreased) in an increment (or decrement) of 20% and are incorporated into the model to compute volume emission rate profiles. The present study shows that the peak emission rate (PER) varies linearly below the reference level of atomic oxygen number density and does not vary linearly above the reference level of atomic oxygen number density. The atomic oxygen number density at reference level corresponds to that value which is obtained from the NRLMSISE‐00 model. It is found that the altitude of peak emission rate moves upward as the F10.7 solar index increases. On average the upward movement of altitude of PER is about 9 km for both the equinox and solstice cases. The upward movement of the altitude of peak emission rate is due to the enhancement in atomic oxygen number density with increase in F10.7 solar index.

Two-Photon Absorption Laser-Induced Fluorescence Investigation of Plasmas for Mars Entry

Two-photon absorption laser-induced fluorescence measurements have been conducted to investigate atomic oxygen temperature and number density in a plasma relevant to Martian entry. The high-enthalpy plasma flow has been produced by an inductively heated plasma generator. This paper describes the experimental setup and shows the applicability of laser diagnostics to Mars-relevant plasmas. Particular emphasis is put on the use of the generated data for the numerical simulation of the aerothermal chemistry of a Martian entry. Relative fluorescence signals are calibrated by a procedure using xenon as a reference. This calibration method allows the in situ experimental determination of the instrumental broadening of the spectroscopic setup and thus an accurate temperature measurement. The acquired, highly resolved line profiles of the fluorescence signal yield a translational temperature of and a mean number density of .

Use of particle-in-cell simulations to improve the actinometry technique for determination of absolute atomic oxygen density

Actinometry is a non-invasive optical technique that can be used to quantitatively monitor atomic oxygen number densities [O] in gas discharges under certain operating conditions. However, careless application of the technique can lead to erroneous conclusions regarding the behaviour of atomic oxygen in plasma. One limitation on this technique is an accurate knowledge of the various rate constants required, which in turn is hampered by an insufficiently precise knowledge of the electron energy distribution function (EEDF) in the plasma. In this work, particle-in-cell (PIC) simulations are used to generate theoretical EEDFs. To validate a simulation the electron density ne produced by the PIC code is compared with experimental ne values measured using a hairpin probe. The PIC input parameters are adjusted to optimize agreement between the PIC and experimental ne results. This approach should in principle yield an EEDF that more accurately reflects the true EEDF in the plasma. The PIC EEDF is then used to generate rate constants for the actinometry model which should improve the accuracy of the quantitative [O] result for that particular set of plasma conditions. The actinometry [O] results are then compared with [O] results obtained using two-photon absorption laser-induced fluorescence (TALIF) to validate the approach.

Satellite observations of thermospheric wave 4 at high latitudes

Satellite observations of the ionosphere and thermosphere routinely reveal a zonal wavenumber 4 pattern in longitude. In the equatorial region, this is attributed primarily to the eastward propagating non-migrating diurnal tide of zonal wavenumber 3 known as DE3. In this report, the wave 4 is identified using daytime thermospheric airglow emission of atomic oxygen at 557.7 nm emitted from the O(1S) level, using the WIND imaging interferometer (WINDII), launched on NASA’s upper atmosphere research satellite (UARS) in 1991. Waves in the airglow emission rate are attributed to variations in the number density of atomic oxygen, but the WINDII interferometer also sees wave 4 in thermospheric winds from the phase shifts of the emission. What is new in this report is the observation of the wave 4 at high latitudes, where it has not been detected before. The characteristics are described and the possible influences discussed.

Global thermospheric atomic oxygen variations observed with the WIND Imaging Interferometer (WINDII): Wave 4 at low and high latitudes

[1] The O(1S) 557.7 nm atomic oxygen emission as observed in the daytime at 250 km altitude by the WINDII instrument on the UARS satellite is presented as a means of determining atomic oxygen concentrations there. This offers to fill a gap lying between the well-studied MLT region and the upper thermosphere at 400 km. At 250 km the O(1S) emission is dominantly produced by the impact of photoelectrons on atomic oxygen, so that atomic oxygen number densities may be determined simply by dividing the observed volume emission rate by the solar flux. Day-by-day plots of the derived atomic oxygen concentrations are seen to track both the solar flux and Kp, showing reasonable consistency with NRLMSISE-00 values. One of the important issues of this region is the influence on it of the DE3 nonmigrating tide produced by deep tropical convection in the troposphere. At 250 km the resulting wave 4 pattern is seen at the equator in this emission, apparently the result of bunching of smaller-scale N-S elongated features, oriented in the NE to SW direction. Although absent at midlatitudes, the wave 4 appears strongly at high latitudes; the first time this has been reported. As this is not predicted by existing models it presents a challenge for further study. In a polar map of the emission rate the primary features are a wave 1 pattern on which large scale spiral structures are superposed, suggesting transport of atomic oxygen out of the polar cap.

O and N2disturbances in theFregion during the 20 November 2003 storm seen from TIMED/GUVI

[1] The ratio of height-integrated atomic oxygen number density, [O], to molecular nitrogen number density, [N2], defined as Σ[O]/[N2], has been used as an indicator of the thermosphere-ionosphere coupling during geomagnetic storms. However, the disturbance in [O]/[N2] in the F region can be different from the disturbance in Σ[O]/[N2], and knowledge of the storm-time behaviors of [O] and [N2] is necessary to precisely describe the thermospheric neutral composition disturbance and its effect on the ionosphere. In this study, we examine the separate roles of [O] and [N2] in modifying the F region [O]/[N2] and Σ[O]/[N2] by analyzing far ultraviolet images made by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED)/Global Ultraviolet Imager (GUVI) during the severe 20 November 2003 geomagnetic storm. The GUVI observation shows that the fractional storm-time change in [N2] is greater than that in [O] in the F region in areas where Σ[O]/[N2] is both decreased and increased. Therefore the disturbance in [O]/[N2] in the F region is primarily determined by the change in [N2]. The reduction in [O]/[N2] in the F region during the storm period is consistent with the reduction in Σ[O]/[N2]. However, only a minor change of the F region [O]/[N2] is observed in the region where the increase in Σ[O]/[N2] is observed. The TEC increase observed at the locations of the Σ[O]/[N2] increase in the American-Atlantic sectors is associated with the poleward extension of the equatorial ionization anomaly rather than with the neutral composition change in the F region.

Record‐low thermospheric density during the 2008 solar minimum

We use global‐average thermospheric total mass density, derived from the drag effect on the orbits of many space objects, to study the behavior of the thermosphere during the prolonged minimum in solar activity between cycles 23 and 24. During 2007–2009 thermospheric densities at a fiducial altitude of 400 km were the lowest observed in the 43‐year database, and were anomalously low, by 10–30%, compared with climatologically expected levels. The density anomalies appear to have commenced before 2006, well before the cycle 23/24 minimum, and are larger than expected from enhanced thermospheric cooling by increasing concentrations of CO2. The height dependence of the mass density anomalies suggests that they are attributable to a combination of lower‐than‐expected exospheric temperature (−14 K) and reductions in the number density of atomic oxygen (−12%) and other species (−3%) near the base of the diffusive portion of the thermosphere.

Use of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; airglow for calibrating direct atomic oxygen measurements from sounding rockets

Abstract. Accurate knowledge about the distribution of atomic oxygen is crucial for many studies of the mesosphere and lower thermosphere. Direct measurements of atomic oxygen by the resonance fluorescence technique at 130 nm have been made from many sounding rocket payloads in the past. This measurement technique yields atomic oxygen profiles with good sensitivity and altitude resolution. However, accuracy is a problem as calibration and aerodynamics make the quantitative analysis challenging. Most often, accuracies better than a factor 2 are not to be expected from direct atomic oxygen measurements. As an example, we present results from the NLTE (Non Local Thermodynamic Equilibrium) sounding rocket campaign at Esrange, Sweden, in 1998, with simultaneous O2 airglow and O resonance fluorescence measurements. O number densities are found to be consistent with the nightglow analysis, but only within the uncertainty limits of the resonance fluorescence technique. Based on these results, we here describe how better atomic oxygen number densities can be obtained by calibrating direct techniques with complementary airglow photometer measurements and detailed aerodynamic analysis. Night-time direct O measurements can be complemented by photometric detection of the O2 (b1∑g+−X3∑g-) Atmospheric Band at 762 nm, while during daytime the O2 (a1Δg−X3∑g-) Infrared Atmospheric Band at 1.27 μm can be used. The combination of a photometer and a rather simple resonance fluorescence probe can provide atomic oxygen profiles with both good accuracy and good height resolution.

Energy coupling to the plasma in repetitive nanosecond pulse discharges

A new analytic quasi-one-dimensional model of energy coupling to nanosecond pulse discharge plasmas in plane-to-plane geometry has been developed. The use of a one-dimensional approach is based on images of repetitively pulsed nanosecond discharge plasmas in dry air demonstrating that the plasma remains diffuse and uniform on a nanosecond time scale over a wide range of pressures. The model provides analytic expressions for the time-dependent electric field and electron density in the plasma, electric field in the sheath, sheath boundary location, and coupled pulse energy. The analytic model predictions are in very good agreement with numerical calculations. The model demonstrates that (i) the energy coupled to the plasma during an individual nanosecond discharge pulse is controlled primarily by the capacitance of the dielectric layers and by the breakdown voltage and (ii) the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst. This occurs primarily because of plasma temperature rise and resultant reduction in breakdown voltage, such that the coupled pulse energy varies approximately proportionally to the number density. Analytic expression for coupled pulse energy scaling has been incorporated into the air plasma chemistry model, validated previously by comparing with atomic oxygen number density measurements in nanosecond pulse discharges. The results of kinetic modeling using the modified air plasma chemistry model are compared with time-resolved temperature measurements in a repetitively pulsed nanosecond discharge in air, by emission spectroscopy, and purely rotational coherent anti-Stokes Raman spectroscopy showing good agreement.