G. Witt

ETON 2: Quenching parameters for the proposed precursors of O2(b1Σg+) and O(1S) in the terrestrial nightglow

Abstract Volume emission profiles of the O 2 ( b 1 Σ g + − X 3 Σ g t - )( O - O ) Atmospheric Band and the O( 1 S- 1 D) green line are used together with coordinated measurements of the atomic oxygen concentrations to test the hypothesis that both emissions are excited by Barth type mechanisms. By considering O 2 ( b 1 Σ g + ) to be produced from an excited O 2 precursor, with O 2 as transfer agent, and O( 1 S) to be formed from a similar precursor with atomic oxygen as the transfer agent, precursor quenching rates are obtained as a function of altitude. These quenching profiles can be well resolved into components corresponding to collisional deactivation by O( 3 P) and O 2 (or N 2 ), and support the suggestion that Barth type mechanisms are involved. Minimum efficiencies for the production of the two precursors in oxygen atom recombination and ratios for the quenching of each by O( 3 P) and O 2 (or N 2 ) are deduced. Differences in the quenching coefficients for the two precursors are discussed.

A measurement of the O2(b1Σg+−X3Σg−) atmospheric band and the OI(1S) green line in the nightglow

Abstract Simultaneous measurements of the nightglow profiles of the O2(b1Σg+−X3Σg−) A-band, the atomic oxygen green line and the OH (8−3) Meinel band are presented. The altitude profiles are used to determine both the excitation mechanisms for the oxygen emissions and the atomic oxygen altitude distribution. It is shown that the measurements are consistent with a green line excitation through the Barth mechanism and that the molecular oxygen emission is excited through oxygen recombination and the reaction between OH∗ and atomic oxygen. The derived atomic oxygen concentrations,6.2 × 1011cm−3at 98km, are consistent with the Jacchia (1971) model.

Interannual variations of the quasi‐16‐day oscillation in the polar summer mesospheric temperature

Nighttime measurements of the hydroxyl Meinel (4,2) rotational band have been used to infer the mesospheric temperature over Scandinavia from June to August during the years 1992–1995. While the nightly averaged temperatures show a statistically significant, quasi-16-day oscillation in the 1992 and 1994 summer data, none is observed during 1993 and 1995. When present, the period, amplitude, and temporal behavior of this oscillation agree with both model predictions and previous wind measurements of the (1,3) Rossby normal mode in the summer mesosphere. Thus this temperature oscillation appears to correspond to the thermal signature of the 16-day Rossby mode. Its appearance in the summer mesosphere is shown to occur when the oscillating zonal flows in the upper stratosphere near the equator are in an eastward phase, while it appears to be blocked during the westward phases. This correspondence of the 16-day wave in the summer mesosphere with the eastward equatorial wind would favor the explanation that it is generated in the winter hemisphere and propagates vertically and toward the summer pole following the westerly mean winds.

An assessment of proposed O(1S) and O2(b1Σg+) nightglow excitation parameters

Abstract A database consisting of a number of simultaneously measured O( 1 D- 1 S) green line and O 2 ( b 1 Σ g + ) − X 3 Σ g )(0,0) atmospheric band nightglow emission profiles is examined to assess the general validity of the airglow excitation parameters recently proposed by McDade et al . (1986, Planet. Space Sci ., 34 , 789). The measured profiles were obtained under quite diverse atmospheric conditions and should, therefore, allow a critical assessment of the proposed parameters. Model green line emission profiles, calculated from the measured O 2 atmospheric band emission profiles using the proposed parameters, are compared with the green line profiles actually measured on each occasion. The measured and modelled green line profiles are found to be in good agreement under most conditions. The cases for which discrepancies exist are discussed in terms of inadequacies in either the background atmosphere adopted for the analyses or in the parameters themselves.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

Rocket-borne measurements of scattered sunlight in the mesosphere☆

Abstract Two photo-electric polarimeters with transmission bands centered at 256 nm and 536 nm were flown on ESRO Centaure rocket C-32/2 from Kiruna, Sweden at 2101 Z on 8 June 1968 to investigate molecular scattering in the mesosphere and possible deviations from molecular scattering which existed at the time of flight. Both photometers detected increased scattering in a region between 85.5 and 89 km on both ascent and descent. This excess scattering is interpreted as being due to a layer of particulate matter. The high degree of polarization measured in both-wavelength regions suggests that if the particles are of uniform radius, this radius is about 0.05 microns or less depending on the refractive index assumed. This conclusion is supported by the observed contrast between the layer and molecular background at both wavelengths. The ratio of the aerosol to the molecular components of the radiance at 85.5 km and 536 nm wavelength was about 0.5. The 256 nm photometer also observed fluorescent emission from the nitric oxide-bands. From the measured radiance, the observed depolarization and an assumed pressure at 90 km altitude the column density of nitric oxide has been estimated. The derived NO profile is in qualitative agreement with earlier observations, however, the emission rate (3.2 kr at 90 km) was higher than that observed by others at lower latitudes. An unpolarized radiance component above 95 km at 536 nm has been identified as a chemiluminescent emission arising from the raection NO + O → NO2 + hv.

Simulation of rocket‐borne particle measurements in the mesosphere

Nanometer-sized meteoric smoke particles and ice condensates are thought to influence the chemistry in the 80–120 km altitude region and to play an important role in the evolution of Polar Mesosphere Summer Echoes and Noctlucent Clouds. In this paper we show that aerodynamic perturbations introduced by a rocket payload complicate the analysis of dust measurements in this region. We analyze the flow of particles by applying a combined numerical simulation of flight aerodynamics and particle evolution. We show that for typical velocities of 500–1000 ms−1, the detection efficiency drops below 50% for smoke particles with radii 0.8–1.4 nm and for ice clusters with radii 2–5 nm, depending on the rockets angle of attack. The particles are exposed to heating in the shock region, resulting in significant mass loss for ice condensates due to sublimation. Our simulations indicate that a substantial fraction of the expected nm sized meteoric smoke particles could be detected with refined instrumentation.

The excitation of O2(b1Σg+) in the nightglow

Abstract It is proposed that the available measurements of the O 2 ( b 1 Σ g + − X 3 Σ g − ) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.

In situ measurements of the vertical structure of a noctilucent cloud

During the NLC-93 rocket campaign at Esrange, Sweden, the vertical structure of a noctilucent cloud layer has been investigated in situ. As in earlier rocket flights, combinations of scattered light detectors and electrostatic impact probes have been applied. While the photometric measurement provides the total downward radiance scattered from the cloud particles, the impact probe yields local information about the particle properties. The responses of both techniques scale differently with the particle size. This feature is utilized to derive information about the height dependence of the particle population. The analysis of three NLC passages indicates little vertical variation of the population throughout most of the layer. The lower part of the cloud is characterized by an increase in particle size and a decrease in particle density towards the cloud base. This is to be expected for an NLC brightness peak caused by large particles sedimenting out of the cloud. Implications for the dynamical structure of the cloud are discussed.

Scattering phase functions and particle sizes in noctilucent clouds

The MIDAS-DROPPS campaign conducted in Norway in 1999 provided a comprehensive study of the high-latitude summer mesopause region with rocket-borne and ground-based instrumentation. Optical photometers were flown on two rocket payloads through substantially different mesospheric conditions. On both flights, distinct noctilucent cloud (NLC) layers were detected. We present the first analysis of NLC scattering phase functions observed from sounding rockets. Applying Mie calculations, the angular dependence of the scattering is used to extract information about particle sizes. The first flight featured a weak NLC with small particles (r ≤ 20 nm) located below the core of a strong polar mesosphere radar echo (PMSE). The second flight took place in the absence of any detectable PMSE and probed a bright NLC optically dominated by particles in the size range 40–50 nm.