SWIFT-DASH: Spatial Heterodyne Spectroscopy Approach to Stratospheric Wind and Ozone Measurement

Abstract

Passive wind measurements using Doppler shifts from atmospheric emissions were well demonstrated by the Wind Imaging Interferometer (WINDII) and the High Resolution Doppler Imager (HRDI) instruments on the National Aeronautics and Space Administration's (NASA's) Upper Atmosphere Research Satellite, operated from 1991 to 2005. For WINDII these emissions were from visible region upper atmospheric airglow in the altitude range from 80 to 300?km. Application of the same technique in the stratosphere requires using thermal emission from a minor constituent, and an ozone line near 1133?cm^?1 (about 8.8?μm) has been identified as a suitable target line. The WINDII method employed a Doppler Michelson Interferometer, in which the wind is measured from phase shifts of a single spectral line. Isolating a single ozone spectral line is a major challenge but using Spatial Heterodyne Spectroscopy (SHS) offers a way to resolve a number of interferogram spectral components (fringes) within a narrow spectral range. The instrument is a Michelson interferometer similar to WINDII but one in which the two mirrors are replaced by diffraction gratings. A developmental instrument capable of measuring the phase shifts from several ozone lines within a spectral range of 4?cm^?1 has been designed, built, and operated in the laboratory. Simulated retrievals using the measurement parameters of this instrument demonstrate the capability of wind measurement with an accuracy better than 3?m?s^?1 over an altitude range of 24 to 60?km. The retrieval employs four spectral lines for wind and three fringe frequencies for ozone concentration (of about 30 possible), each of which provides an optimal measurement for a particular altitude range. Ozone concentrations are also provided with an accuracy better than 10% from 20 to 50?km. Further detailed tests of this instrument are planned for the future. This work is supported by the Canadian Space Agency.     

The solar terrestrial event of 14–21 December 1971: The pattern of 6300 Å emission over the polar cap

An exceptional solar terrestrial event was initiated by the ejection of plasma from the Sun on 14 December 1971 and was followed by a spectacular pattern of soft particle precipitation into the polar cap, which evolved in a slowly changing sequence until 21 December. The storm was characterized by high proton fluxes in space and near the Earth, an extended interval of northward B_z, a highly developed ring current and widespread polar cap particle fluxes. These varied from a dayside “butterfly pattern” early in the event, to highly structured Sun-aligned polar cap patterns late in recovery. A number of polar cap ground-based measurements were compiled and are reported upon. The storm seems reminiscent of the great red auroras of the IGY and some common features are noted. The 6300 Å emission is shown to result largely from direct excitation by low energy electrons, of a few hundred eV. Since the emission covered the Earth's high-latitude dayside region nearly to the invariant pole it indicates a magnetospheric topology that permits entry of low energy plasma over this region or of mechanisms which allow the generation or penetration of the plasma.     

Twelve years of ground-tilt measurements on the Soufriere of St. Vincent, 1977–1989

Five ground-tilt stations were established on the flanks of the Soufriere of St. Vincent; two in 1977, two in 1980, and one in 1983. Four of these stations have survived; two consist of lines oriented radially to the volcano, and the other two consist of cross-shaped arrays. Collectively, this network showed that the volcano inflated gradually before the eruption of 1979 and then deflated rapidly during the eruption and for about a year after it had ended. The volcano then reinflated during much of 1981 and inflated only slightly, if at all, during the 7-year period 1982–1989. The measured amount of ground tilt from 1977 to 1989 was relatively small; the maximum recorded change of radial tilt at a station 6.5 km from the summit of the volcano totaled about 20 rad. Local seismicity correlates well with measured ground deformation: numerous earthquakes accompanied the 1979–1981 deflation/inflation cycle; relatively few earthquakes occurred during the 1982–1989 period of little or no ground deformation. In the hope that the experience we have gained might be of use to others who are considering a program of ground-tilt monitoring on volcanoes elsewhere in the humid tropics, we describe many practical aspects of our program that have evolved over the years. For example: (1) stainless steel bench marks cemented into solid bedrock appear to be stable for at least 12 years, (2) bench marks can be located in concrete-filled drums where bedrock is absent, (3) bench marks should be concealed to protect them from vandalism, (4) care must be exercised where sight lines graze the ground (<0.5 m), and (5) automatic levels are preferable because of their efficiency of operation, especially with inexperienced personnel.     

Observations of trace gases and aerosols over the Indian Ocean during the monsoon transition period

Characteristics of trace gases (O₃, CO, CO₂, CH₄ and N₂O) and aerosols (particle size of 2.5 micron) were studied over the Arabian Sea, equatorial Indian Ocean and southwest part of the Bay of Bengal during the monsoon transition period (October–November, 2004). Flow of pollutants is expected from south and southeast Asia during the monsoonal transition period due to the patterns of wind flow which are different from the monsoon period. This is the first detailed report on aerosols and trace gases during the sampled period as the earlier Bay of Bengal Experiment (BOBMEX), Arabian Sea Monsoon Experiment (ARMEX) and Indian Ocean Experiments (INDOEX) were during monsoon seasons. The significant observations during the transition period include: (i) low ozone concentration of the order of 5 ppbv around the equator, (ii) high concentrations of CO₂, CH₄ and N₂O and (iii) variations in PM2.5 of 5–20μg/m³.     

Inter-hemispheric mesospheric coupling in a comprehensive middle atmosphere model

Observations of noctilucent clouds have revealed a surprising coupling between the winter stratosphere and the summer polar mesopause region. In spite of the great distance involved, this inter-hemispheric link has been suggested to be the principal reason for both the year-to-year variability and the hemispheric differences in the frequency of occurrence of these high-altitude clouds. In this study, we investigate the dynamical influence of the winter stratosphere on the summer mesosphere using simulations from the vertically extended version of the Canadian Middle Atmosphere Model (CMAM). We find that for both Northern and Southern Hemispheres, variability in the summer polar mesopause region from one year to another can be traced back to the planetary-wave flux entering the winter stratosphere. The teleconnection pattern is the same for both positive and negative wave-flux anomalies. Using a composite analysis to isolate the events, it is argued that the mechanism for inter-hemispheric coupling is a feedback between summer mesosphere gravity-wave drag (GWD) and zonal wind, which is induced by an anomaly in mesospheric cross-equatorial flow, the latter arising from the anomaly in winter hemisphere GWD induced by the anomaly in stratospheric conditions.     

Energy transfer from excited N2 and O2 as a source of O(1S) in the Aurora

It is proposed that energy transfer from excited O₂ contributes to the production of O(¹S) in aurora. An analysis is presented of the OI5577 Å emission in an IBC II⁺ aurora between 90 and 130 km. The volume emission rate of the emission at these altitudes is consistent with the production rate of O(¹S) by energy transfer to O(³P) from N₂ in the A³Σ₂⁺ state and O₂ in the A³Σ_u⁺, C³Δc¹Σ_u^? states, the N₂A state being populated by direct electron impact excitation and B → A cascade and the excited O₂ states by direct excitation. Above the peak emission altitude (~105 km), energy transfer from N₂A is the predominant production mechanism for O(¹S). Below it, the contribution from quenching of the O₂ states becomes significant.     

Validation of a region-wide model of landslide susceptibility in the Manawatu–Wanganui region of New Zealand

Since European settlement 160 years ago, much of the indigenous forest in New Zealand hill country has been cleared for pastoral agriculture, resulting in increased erosion and sedimentation. To prioritise soil conservation work in the Manawatu–Wanganui region, we developed a model of landslide susceptibility. It assigns high susceptibility to steep land not protected by woody vegetation and low susceptibility everywhere else, following the commonly used approach for identifying inappropriate land use. A major storm on 15–16 February 2004 that produced many landslides was used to validate the model. The model predicted hills at risk to landsliding with moderate accuracy: 58% of erosion scars in the February storm occurred on hillsides considered to be susceptible. The model concept of slope thresholds, above which the probability of landsliding is high and below which the probability is low, is not adequate because below 30° the probability of landsliding is approximately linearly related to slope. Thus, reforestation of steep slopes will need to be combined with improved vegetation management for soil conservation on moderate slopes to significantly reduce future landsliding.     

Observations of horizontal transport effects on high latitude metastable O(1D), N(2D) auroral emissions

An analysis is presented of photometric measurements of the NI (λ = 520nm),OI(λ = 630nm)and other emissions made at Nord, where the invariant latitude is Λ = 80°4. The time variations of the intensities are interpreted in the following way by comparison with simultaneous ground based or satellite measurements.The N(²D) atoms formed in the dayside cleft are carried by the neutral wind in a plume across the polar cap, so that the ratio of λ(630 nm) to λ(520 nm) intensities decreases along the plume with increasing distance from the source region.In the polar cap, but outside the plume region, 630 nm emission is produced by electron impact of polar rain and by substorms that reach high latitudes. Ionization produced at the same time, especially by the substorms, will produce further 630 nm emission through dissociative recombination. In any case, the region outside the plume may be regarded as a source region, with a high value of the ratio $\text{I(630)}\text{I(520)}$. This explains in part the diurnal variations, since this ratio is depressed as Nord crosses the dayside plume.The electron energy along the oval increases progressively from the dayside to the nightside. The intensity ratio increases with increasing electron energy because N(²D) is quenched more rapidly than O(¹D). Thus the ratio rises progressively from noon to midnight.An effect of the interplanetary magnetic field is superimposed on this pattern : as its North-South component B_z increases, the oval contracts so that Nord becomes nearer from the cleft source and the intensity ratio increases on the dayside. The inverse effect is also observed. On the nightside, negative B_z is associated with substorms that produce poleward expansions of the poleward oval boundary, that brings more energetic precipitation to Nord. This causes the intensity ratio to increase with decreasing B_z in a way that is opposite to that for the dayside.     

Photometric observations of the dayside cleft emissions from Cambridge Bay,December 1976

Dayside cleft emissions were observed from Cambridge Bay, N.W.T. during quiet, very quiet and disturbed periods from 10–17 December 1976. Good agreement is found in comparisons of the ground based photometric data with ISIS satellite data for the cleft region. A marked asymmetry around noon was found. Dynamical changes, observed especially in the poleward emission boundary, appear to have no counterpart in the nightside magnetometer data.