Gravity waves from O2 nightglow during the AIDA '89 campaign III: effects of gravity wave saturation

The O₂ atmospheric (0–1) night airglow emitted within the gravity wave saturation region at ∼90–100 km can serve as a means of studying the wave activity. In this analysis, the atmospheric motions were described by a mean spectral model and an algorithm was developed to infer the wave kinetic energy density and momentum flux from variations in O₂ (0–1) airglow emission rate and rotational temperature. The method was applied to eight nights of data collected by MORTI, a mesopause oxygen rotational temperature imager, during the AIDA campaign of 1989 in Puerto Rico (18°N, 67°W). The observed r.m.s. fractional fluctuations of airglow emission rate and rotational temperature were of the order of ∼0.07–0.15 and ∼0.02–0.04, respectively, and the characteristic vertical wavelengths were estimated at ∼10 2 -20 km. The inferred r.m.s. horizontal velocities and velocity variances were found to be ∼12–25 m/s and ∼150–600 m²/s², with the majority of the horizontal velocity and its variance associated with low-frequency, large-scale wave motions. The estimated momentum fluxes, mainly contributed by high-frequency, small-scale waves, were ∼2–10 m²/s². These results are in good agreement with those obtained from other measurements using different observational methods at low and mid-latitudes.     

Gravity waves from O2 nightglow during the AIDA '89 campaign II: numerical modeling of the emission rate/temperature ratio, η

The dependence of η, the ratio of emission rate fluctuation to temperature fluctuation, on the gravity wave parameters and chemical parameters is developed using a model for the O₂ airglow's response to a gravity wave. The model is in the Eulerian frame of reference using the traditional linearization and perturbation method. Two photochemical mechanisms, the three-body recombination mechanism and the two-step transfer mechanism and two kinds of waves, evanescent and internal, are separately discussed in detail. Results of the model are compared with other existing models of η(O₂) and found to be in agreement. The predictions of this model agree fairly well with the available gravity wave measurements in the magnitude of η, but the predicted phases are somewhat larger than those normally observed.     

Rotational temperatures for OH and O2 airglow bands measured simultaneously from El Leoncito (31°48′S)

Some results from 54 nights of simultaneous measurements, performed between 1984 and 1987, of rotational temperatures for the OH(6−2) and O₂(¹∑)(0–l) bands are presented. A summer enhancement by 15 K in O₂ temperature has been found that has not formerly been observed in airglow measurements. At least five nights show prominent tide-like temperature oscillations with a phase shift between layers typical of upward wave propagation at about 10 km h⁻¹, with up to 55 K variation. During other nights, similar oscillations are limited to the O₂ layer. Data for different seasons seem to be characterized by different levels of variability. During the one equinox campaign, nocturnal temperature variations show an exceptionally stable pattern of tide-like oscillations.     

Observations of gravity waves from multispectral mesospheric nightglow emissions observed at 23°S

Simultaneous measurements of the 015 57.7 nm, O₂ atmospheric (0,1) band, NaD and OH (9,4) band emissions obtained during the period October November 1989 at Cachoeira Paulista (23°S, 45°W), Brazil, have been analysed to study gravity waves in the mesospheric region at a low-latitude station in the southern hemisphere. It was found that, when these emissions showed large temporal intensity variations, there were also short period quasi-coherent temporal variations superposed on them, suggesting a possible passage of internal gravity waves in the emission layers. Cross-correlation analysis indicates that the time lag between the different emissions is smaller for short period variations compared with the long period variations. The wave parameters, namely a vertical wavelength of 12 km, a horizontal derived wavelength of 200 km with a period of 80 min, estimated from one of the observed short-period coherent oscillations, are typical of the internal gravity waves at the airglow emission height.     

Wave characteristics obtained from OH rotational temperatures and 557.7 nm airglow intensities

Variations of OH rotational temperature and 557.7 nm atomic oxygen intensity have been measured from Calgary, Alberta, Canada (51°10′N, 114°13′W) from 1985 to 1987. For three nights studied in detail the OH rotational temperature wave structure at 85 km was negatively correlated with the green line emission at 95 km, indicating that wave activity linked the two regions. The lower altitude region displayed high and low frequency wave structure, but by 95 km the high frequency component had disappeared. Temperature data from 16 nights during which there was obvious wave activity yielded horizontal wavelengths from about 5–100 km and inferred vertical wavelengths from 0.7 to 8 km. The horizontal and vertical angles of propagation imply a statistical source to the observed waves as being located south of Calgary along the Rocky Mountain range. There appeared to be very few, if any, wave structures propagating towards the southeast indicating a probable filtering mechanism by the background winds between the Earth's surface and 85 km.     

Simultaneous nightglow and Na lidar observations at Arecibo during the AIDA-89 campaign

Simultaneous nightglow and Na lidar observations made at Arecibo during the AIDA-89 campaign are reported from between 0000 and 0530 LT (Local Time) on 6 April 1989 and on 9 April 1989. On 9 April the observations are consistent with the presence of a large amplitude 8.5 km vertical wavelength, 4 h period wave propagating through the 85–91 km region. A lower amplitude 80 min period wave is also observed. The results imply that the O₂ atmospheric band intensity peaked near 91 km while the OH Meinel (6,2) band intensity peaked near 85 km. The OH Meinel temperature and intensity are 160° out of phase which can be explained by low eddy diffusion and high ozone densities near the mesopause. The integrated Na abundance from 87 to 89 (90–92) km correlates well with the OH Meinel (O₂ atmospheric) band intensity. On 5–6 April the OH Meinel and O₂ atmospheric band intensities are not well correlated. The OH Meinel intensity is correlated with the integrated Na abundance from 86 to 88 km. Both the Na and OH measurements reveal the presence of an approximately 1 h period wave. The OH temperature data appear to be consistent with the OH Meinel band originating near 85 km. The O₂ atmospheric band data show the presence of a 2 h period wave. The integrated Na abundance data suggest that the O₂ atmospheric band peaks between 90 and 94 km. A large sporadic Na event which occurs near 6 UT appears related to the presence of a gravity wave near 95 km. In all of the observed waves there is good agreement between the wave parameters derived separately by the optical airglow and Na lidar techniques.     

Mesospheric temperatures and the OH layer height as derived from ground-based lidar and OH1 spectrometry

Night-time mesospheric temperatures were simultaneously determined from the Doppler broadening of the D₂ resonance line of atmospheric sodium excited by a laser and from the rotational distribution of the P₁(1), P₁(3) and P₁(4) lines of the OH(3,1) band by an i.r. spectrometer. Both instruments were located at the Andøya Rocket Range (69°N, 16°E). The mesospheric temperature gradient permits determination of the altitude of the OH¹ emitting layer from a comparison of the equivalent layer temperatures calculated from the height-resolved Na Doppler temperatures with the observed OH¹ rotational temperatures. The altitude of the OH¹ layer maximum is determined with an accuracy of ±4 km. For 3 nights in January 1986 the OH¹ emission layer is found near an altitude of 86 km.     

An assessment of the effect of gravity waves on the width of radar Doppler spectra

Radars can be used to obtain turbulence parameters by measuring the width of the Doppler spectrum of returned power. Other effects can contribute to the Doppler-spectrum width, including short-period gravity waves, making turbulence parameter measurements more difficult. In this paper, an experimental study of the effect of gravity waves on the width of the Doppler spectrum is presented. The data were obtained using the Poker Flat 50 MHz VHP radar. A parameter Y_i is used to describe the discrepancy between the width of spectra sampled over a long period of time and that taken over a shorter time. If Y_i is not significantly different from 1.0, no contamination is present. The experimental data considered were of a time resolution that allowed the period and amplitude of the contaminating gravity wave to be determined. A theoretical expression for gravity wave contamination proposed by Hocking [(1988) J. geophys. Res.93, 2475–2491] was tested and found to agree with measurements. It was also found that non-unity values of Y_i occurred in some cases. This suggests that, at commonly used sampling times, short-period gravity waves can contaminate spectral width estimates.     

Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S

Night-time thermospheric temperatures, T63o, and mesospheric rotational temperatures, T(OH) and T(O₂), have been measured at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), located in both the equatorial ionospheric anomaly and the South Atlantic Geomagnetic Anomaly, with a Fabry-Perot interferometer and a multi-channel tilting filter-type photometer, respectively. The thermospheric temperatures are obtained from the Doppler line broadening of the OI 630.0 nm emission and the mesospheric rotational temperatures from the OH(9,4) and O₂A(0,1) band emissions. Measurements made during three geomagnetic storms showed that the nocturnal mean values of T₆₃₀ during the recovery phase of the storms were lower than those observed during quiet time and from model predictions. Also, the nocturnal mean value of the T₆₃₀ soon after the SSC event on 27 June 1992 was higher than the quiet time and model predictions. The observed mesospheric nocturnal mean rotational temperatures, T(O₂) and T(O₂), were unaffected by the storms. A comparison of the night-time observed temperatures T₆₃₀, T(OH) and T(O₂) with those calculated using the MSIS-86 model is also presented.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

A simple model of gravity-wave momentum and energy fluxes transferred through the middle atmosphere to the upper atmosphere

Vertical fluxes of momentum and energy through the middle atmosphere are calculated by using a simple semi-empirical model of quasi-monochromatic internal gravity waves with dominant vertical wavenumbers. In this model those dominant gravity waves are assumed to saturate and break at each observational altitude by an effective critical-layer mechanism. The dominant value of the vertical wave-number is expressed by an exponential function of altitude, decreasing upward with a scale height of 34 km. This expression gives the momentum and energy flux densities decreasing upward with scale heights of 12 and 18 km, respectively, and typical values at 100 km altitude are estimated as 4 × 10⁻⁵ Pa and 4 × 10⁻³ W/m². A heat flux induced by wavebreaking turbulence also has an order of magnitude similar to that of the wave energy flux. Variabilities around these values and comparisons with other momentum and heat inputs to the upper atmosphere are only briefly discussed.     

Gravity wave activity in the upper mesosphere over Urbana,Illinois: lidar observations and analysis of gravity wave propagation models

We analyze 375 h of Na Wind/Temperature lidar measurements of the mesopause region (≈ 80–105 km) Na density and temperature profiles on 57 nights distributed over 2 yr at Urbana, Illinois. These observations yield a high-resolution seasonal data set of gravity wave activity in the upper mesosphere. From this data, we present measurements of the Brunt-Väisälä period, the relative atmospheric density perturbations and their spectra, and the parameters of 143 quasi-monochromatic gravity waves. The direct measurement of the Brunt-Väisälä period allows accurate calculation of the horizontal velocity perturbations and vertical displacement perturbations from the density measurements. The horizontal velocity and vertical displacement vertical wave number spectrum magnitudes and indices show considerable seasonal and nightly variability. The gravity wave amplitudes, wavelengths, and observed periods exhibit systematic relationships similar to those found in previous studies, and are consistent with the MU radar measurements of intrinsic gravity wave parameters. Here, we present a detailed analysis of the observations in terms of Diffusive-Filtering Theory models of gravity wave propagation. The magnitudes of the vertical wave number spectrum, the form of the joint vertical wave number and frequency spectrum, and the systematic relationships between the monochromatic gravity wave parameters are consistent with the Diffusive-Filtering model. We compare these results with a variety of radar, lidar, and airglow observations from other sites. This observational study suggests that the complex nonlinear interactions of the gravity wave field may be modeled successfully as a diffusive damping process, where the effective diffusivity is a function of the total wave variance.     

Interference of tidal and gravity waves in the ionosphere and an associated sporadic E-layer

Observations made on 10 July 1987 with the EISCAT UHF radar are presented. The F-region measurements of both electron density and field-aligned ion velocity show that an upward propagating gravity wave with a period of about 1 h is present. The origin of the gravity wave is probably auroral. The E-region ion velocities show a tidal wave and both upward and downward propagating gravity waves. The gravity waves have three dominant periods with a possible harmonic relationship and similar vertical wavelengths. These waves are either reflected at a single reflection level, ducted between two levels, or they are generated in a non-linear interaction between gravity and tidal waves. The E-region electron density is dominated by particle precipitation. After a short burst of more intense precipitation, a sporadic E-layer forms at 105km and then disappears 40min later. Within this time, the layer rises and falls by a few kilometres, following closely the motion of a convergent null in the velocity profile. We suggest that the formation and destruction of this layer is controlled by both the precipitation, which indirectly provides a source of metal ions through charge exchange, and the superposition of gravity waves and the tidal wave.     

A study of the meridional structure of the equatorial red arcs in the night-time ionosphere from the ISIS-II satellite

Equatorial 6300 Å arcs observed by the ISIS—II satellite close to the magnetic equator over the African and Asian zones are studied for night-time conditions from 21:00 h to 02:00 h local time in the summer and spring of 1972–1974 and 1976, respectively. Case studies of the arcs have been made for quiet geomagnetic conditions and for minor storms. Sometimes very intense arcs with intensities of 1–2 kR are observed. Arcs of moderate intensities (300–400 R) are observed during geomagnetically disturbed periods. It is confirmed that these intensities can be fully accounted for theoretically by the dissociative recombination of molecular oxygen ions. Since the emission intensities are found to be sensitive to the geomagnetic activity, the influence of the latter has been taken into account and discussed.Equatorial spread-F (ESF)/bubble conditions are usually present at these local times. The data presented here show a correlation between the 6300 Å emission rate at one of the anomaly crests, the gradient in h (the lowest scaled real height from topside ionosonde trace) and the existence of ESF and gravity waves. This correlation is consistent with the scenario put forward by Maruyama and Matuura that the occurrence of ESF requires a symmetrical electron density distribution around the magnetic equator, so that a transequatorial wind causes an asymmetry and inhibits the formation of ESF.For the ISIS data we conclude that where strong transequatorial winds exist the 6300 Å emission rate at one of the anomaly crests is very large and there is a steep gradient in h. When these winds are weak, the 6300 Å emission is low and the gradient in h is also small. In the latter case, gravity waves of wavelength 200–400 km were present as well, which suggests that ESF is promoted by the existence of gravity waves. However, the magnetic disturbance level was higher during these orbits, which offers another source of gravity waves.     

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e-folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.     

Numerical simulations of the saturated gravity wave spectra in the atmosphere

A two dimensional numerical model is used to compute the saturation of small scale gravity waves in the region near the critical level. The vertical wave number spectrum of horizontal velocity fluctuations in the unstable region (USR) where shear instability develops is found to be governed by wave-shear interaction and follows a theoretical saturation spectrum ~ω_b²/2m³. Wave-shear interaction is also found to be responsible for the observed fact that the variance of vertical velocity fluctuations is significantly lower than the level predicted by linear gravity wave theory. On the other hand, the corresponding spectrum in the stable region (SR) following a much shallower spectrum ~m⁻² is found to result from the combined effects of wave-wave interactions and eddy diffusion. The key step in our simulation is the separate parameterization of horizontal and vertical eddy diffusion coefficients instead of a constant molecular viscosity coefficient.     

Positive and negative ion composition measurements in the D- and E-regions during the 26 February 1979 solar eclipse

Two rockets bearing quadrupole mass spectrometers capable of measuring both positive and negative ion composition were launched from Red Lake, Canada, during the solar eclipse. Both instruments had liquid helium cryopumps and shock-attaching conical samplers. The payloads also contained two Gerdien condensers to measure total positive and negative ion concentrations and ion mobilities. Attitude control systems aligned the payloads with the velocity vector throughout ascent and descent. The first rocket was launched so that the D-region was in darkness 35 ± 8 s on the upleg and about 150 ± 15 s on the downleg for the study of ionospheric decay processes. The second rocket was fired after totality into 75% solar illumination for the study of ionospheric recovery. The positive ion composition above 105 km exhibited a strongly increasing NO⁺/O₂⁺ ratio with time after second contact due to O₂⁺ charge transfer with NO and a sharply diminished ionization rate. However, in both nights, the ionization below 105 km was created mainly by energetic particle deposition as exemplified by the increased ion concentrations and the composition signatures of a particle event: asignificant enhancement of O₂⁺ below 105 km and large amounts of H₅O₂⁺ ions in the D-region which result from the O₂⁺ clustering scheme. H₅O₂ was the major ion in the upper D-region while H₇O⁺₃, H₉O₄⁺ and H₅O₂⁺ were dominant ions at lower altitudes. Numerous minor species were also detected. The negative ion distributions in both flights exhibited a distinct shelf at 83 ± 2 km, decreasing by more than an order of magnitude by 90 km and with minima near 75 km. In the 75–90 km range, a significant percentage of the negative ions had masses exceeding 160 a.m.u. Comparisons are made with prior negative ion measurements during similar daytime auroral zone absorption (AZA) events. Two striking characteristics of the precipitating particles were apparent from these and past observations in daytime AZA events: there is a near absence of low energy electrons capable of ionizing above about 105 km and there is'a significant spatial and/or temporal variability in the electron flux. This paper is devoted principally to a presentation of the ion composition measurements and associated uncertainties.     

Solar flux and seasonal variations of the mesopause temperatures at 51°N

Observations of the OH (8-3) band airglow emission, using a multichannel tilting filter type photometer, have been carried out at Calgary (51°N, 114°W), Canada, since 1981. In this paper recent measurements of the nocturnal, seasonal and solar flux variations of the mesopause temperature, obtained from the rotational temperature of the OH (8-3) band observations, are presented. The data presented span the ascending phase of the present solar cycle viz. 1987–1988 (low solar activity) and 1990 (high solar activity). Good correlations (r = 0.73) between the OH (8-3) band rotational temperature and the 10.7 cm solar flux were observed. The mean temperature for the period investigated was about 210 K. The seasonally averaged nocturnal variations show only small irregular excursions, possibly associated with solar tides and the passage of gravity waves in the mesopause region. However, the observed rotational temperatures show considerable night-to-night changes.