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While several observations have been made in recent years of instability features in airglow images of atmospheric gravity waves (AGWs), such measurements are still rare. To date, these features are characterized by appearing to be aligned perpendicular to the AGW wave fronts. Multi-instrument observations confirm the theoretical prediction that such features are caused by convective instabilities where the AGW-induced temperature variation causes the total lapse rate to exceed the adiabatic lapse rate. In February 2000, airglow observations were obtained at Buckland Park, Australia, which showed instability features with a different characteristic. These images showed small-scale (less than 10 km horizontal wavelength) features aligned parallel to the larger scale AGW wave fronts. These features were only seen in OH images, not in O2A images, indicating that they originate below 90 km altitude. Simultaneous MF radar wind data reveal the presence of a mean wind shear which, during the period of the small-scale features, was aligned nearly in the direction of AGW propagation. In addition, the larger scale AGW approached a critical level near 90 km altitude. While the wind shear itself is not large enough to cause an instability, an analysis of the data suggests that the small-scale features are the result of a dynamic (wind shear-induced) instability in the 87–90 km altitude region. The instability was due to a combination of the background wind shear and the large shear induced by the passage of the larger scale AGW as it approached the critical level.

From analyzing nine months of airglow imaging observations of atmospheric gravity waves (AGWs) over Adelaide, Australia (35°S) [Walterscheid et al., 1999] have proposed that many of the quasi-monochromatic waves seen in the images were primarily thermally ducted. Here are presented 15 months of observations, from February 1996 to May 1997, for AGW frequency and propagation direction from a northern latitude site, Urbana Illinois (40°N). As Adelaide, Urbana is geographically distant from large orographic features. Similar to what was found in Adelaide, the AGWs seem to originate from a preferred location during the time period around summer solstice. In conjunction with these airglow data there exists MF radar data to provide winds in the 90 km region and near-simultaneous lidar data which provide a temperature climatology. The temperature data have previously been analyzed by States and Gardner [2000]. The temperature and wind data are used here in a full wave model analysis to determine the characteristics of the wave ducting and wave reflection during the 15 month observation period. This model analysis is applied to this and another existing data set recently described by Nakamura et al. [1999]. It is shown that the existence of a thermal duct around summer solstice can plausibly account for our observations. However, the characteristics of the thermal duct and the ability of waves to be ducted is also greatly dependent on the characteristics of the background wind. A simple model is constructed to simulate the trapping of these waves by such a duct. It is suggested that the waves seen over Urbana originate no more than a few thousand kilometers from the observation site.

To obtain information regarding the spatial scales involved in lower thermospheric composition changes in the auroral zone, simultaneous observations were carried out using photometers at Kangerlussuaq and Narsarsuaq, Greenland, two sites separated by ∼750 km in distance. At night, Kangerlussuaq, located at 67°N geographic is most often at the northern edge of the auroral oval, while Narsarsuaq, at the southern tip of Greenland, is more often in or at the southern edge of the auroral oval. On January 29–30, 1998, moderate auroral activity was recorded from both sites. Besides the photometer systems, observations were obtained during part or all of this period from Polar satellite instruments, from the Greenland magnetometer chain, from the CANOPUS chain of magnetometers and photometers, from DMSP, and from the Sondrestrom incoherent scatter radar. The analysis of these data show the necessity of considering both nonlocal and local sources in modeling aurorally induced changes in lower thermospheric composition.

During the Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign there were several unusual observations of the O 2 Atmospheric (0,1) and OH Meinel (6,2) brightness fluctuations being nearly 180° out of phase. In this paper we discuss data from May 8-9, 1989, during which meteor radar and airglow instrumentations were operating and thus wind data were available. We show that the nearly out-of-phase result on this night is due to the fact that the OH Meinel brightness and temperature are out of phase while the O 2 atmospheric temperature and brightness are nearly in phase. The characteristics of the wave causing the out of phase fluctuations are derived and comparisons are made between results obtained by the Aerospace nightglow camera and those obtained by the York University airglow camera [Zhang et al., 1993]. Although there are differences between them, both of the instrumental data sets are consistent with the presence of a wave with a horizontal wavelength ≥600 km and a period of several hours. An analysis of these data shows that the out-of-phase relationship is most likely due to strong wave reflection in the lower thermosphere caused by thermal gradients and wind shears.

An analysis is reported of near zenith viewing brightness observations of O I 844.6 nm, N2+ ING 391.4 nm, N2 2PG 337.1 nm, O I 555.7 nm, and O I 630.0 nm recorded by photometers onboard the ARIA II rocket launched from Poker Flat, Alaska, on February 12, 1994. The rocket passed through diffuse aurora mixed with intermittent soft precipitation on the upleg followed by much more structured precipitation on the downleg. The most meaningful analysis of the data comes from the upleg portion of the flight and is reported here. The primary objective of the investigation was to quantify possible departures of the density profiles of N2 and O from their representations within the MSIS model. To do so requires knowledge of the electron precipitation responsible for the observed emissions. This was obtained by fitting electron transport results to altitude profiles of in situ electron spectrometer data and 391.4 nm photometer data. Good agreement with profile shapes was achieved with precipitation characterized by a Maxwellian distribution having a characteristic energy E0 of 1.75 keV. The strength of the precipitation was determined by the brightness of the 391.4 nm profile which yielded an energy flux Q of 3.08 ergs cm−2 s−1. Simultaneous good fits to the low-altitude 391.4 nm and O I 844.6 nm data required a scaling of the MSIS-based O density profile by a factor of 0.45. This factor amounts to a significant reduction in O and is close to that obtained from an analysis of coincident ground-based optical data [Christensen et al., 1997]. Perhaps more significant is a clear sign of an abrupt composition change as the rocket approached apogee based on the observed brightness ratios of 844.6/391.4 and 557.7/391.4. Both exhibit the same gross structure as well as fine structure arising from intermittent soft precipitation. Their ratios are observed to drop by more than a factor of 2 along a portion of the trajectory as the rocket passed above 218 km. Upon achieving such heights, a close relationship exists between these ratios and the ratio of O to N2 column densities (O/N2). Full consideration of the relationships between the observed brightnesses of O I 844.6 nm, O I 557.7 nm, and 391.4 nm leads to the conclusion that the rocket entered a region of enhanced N2 density with continued depletion of the O density similar to what was observed prior to entering this region. Relative to MSIS, the N2 densities are at least a factor of 2 higher while the O densities are at least a factor of 2 smaller leading to a reduction in MSIS-based O/N2 of more than a factor of 4.

A Fabry-Perot spectrometer optimized for maximum near infrared (NIR) transmission and featuring a germanium detector with a quantum efficiency of 86% at 10,830 A is used to measure the brightness and spectral width of the metastable helium emission in the upper thermosphere. Observations are made at the optical facility associated with the Millstone Hill Incoherent Scatter Radar site in Massachusetts and at the optical facility associated with the Arecibo Observatory in Arecibo, Puerto Rico. Using a spectral resolution of 0.117 A, these are the first observations to isolate the metastable helium triplet 2p3P0,1,2–2s3S1 from nearby OH airglow contamination. Measured 10,830 A brightness decays with shadow in a manner consistent with an emission that is due to resonant scattering of the solar 10,830 A emission. Although the technique we describe was conceived as a powerful tool to measure upper thermospheric and exospheric temperatures, the data indicate a surprising increase of temperature with shadow height to values much greater than commonly ascribed to the near solar minimum thermosphere. We assert that the broad 10,830 A spectral line widths, retrieved when the shadow heights are above ∼300 km, are due to a nonthermal metastable helium population. This source of hot, metastable helium is very likely He+ recombination in the twilight. That conclusion is contrary to an historical understanding that the dominant source of metastable helium is the impact of photoelectrons with energies in excess of 19.8 eV with ground state helium.

Observations made during the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA-93) campaign during October 1993 revealed the presence of persistent large perturbations of the temperature and density in the 80 to 100 km mesopause region. These perturbations, seen both by passive airglow and active lidar techniques, were identified as due to the presence of diurnal and semidiurnal tides. These results were compared with the predictions of the thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME-GCM). This model was recently tuned to agree with the observations from the Upper Atmosphere Research Satellite (UARS) with respect to mesosphere/lower thermosphere winds (60–200 km). It is found that although the TIME-GCM agrees with UARS winds, it underpredicts the diurnal, and possibly the semidiurnal, tidal temperature perturbations seen during ALOHA-93. Since the TIME-GCM only includes migrating tides, this comparison suggests that nonmigrating tides may have had a significant amplitude during the ALOHA-93 campaign period. UARS observations of the major nonmigrating diurnal mode were made just before and after the ALOHA-93 period. It is found that the zonally symmetric nonmigrating tide can account for the discrepancy between the ALOHA-93 results and the TIME-GCM predictions only for a few periods and only at an altitude around 88 km. If the zonally symmetric nonmigrating tide is to account for the discrepancies between the model and the data, than it has to be highly variable.

The ARIA II experiment carried out on February 12, 1994, provided the neutral wind and electron density measurements needed to calculate the neutral-driven field-aligned current densities. In situ rocket measurements were made approximately 60 min after the onset of an auroral substorm in a moderately disturbed (Kp=5−) postmidnight auroral oval over Alaska. Two chemical release rockets deployed four widely separated TMA trails, and E region neutral wind profiles were obtained from the motion of those trails. An instrumented rocket launched near-simultaneously measured the electric fields, electron densities, and neutral composition. The divergence and the vertical component of the vorticity in the neutral flow were obtained from the neutral wind profiles, and those values, together with the measured electron densities, were used to calculate the neutral-driven field-aligned current densities. Our best estimate is that the current density below 114 km was effectively zero. Above 114 km the field-aligned current was downward with a peak value 0.23 μA m−2. Our results indicate that, although the wind-driven current was a significant contributor to the total field-aligned current in the recovery phase of the substorm, the dominant sources of the field-aligned current were due to conductivity gradients and the divergence in the ionospheric electric field. The measurements also show that the current density profile has vertical structure with a scale of ∼ 10 km due to height variations in the vorticity, divergence, and conductivity profiles.

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6–2) band (hereafter designated OH) and the O2 atmospheric (0–1) band (hereafter designated O2) nightglow emissions; the University of Illinois Na density/temperature lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30- to 50-km horizontal wavelength waves, which had observed periods of about 10–20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the ≈30- to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed small-scale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities.

Measurements of atmospheric structure and dynamics near the mesopause were performed using a sodium lidar, an MF radar, and a night-glow CCD camera during the CORN campaign performed in central Illinois during September 1992. The major features of the observed structure on September 27/28 include a low-frequency, large-scale wave accounting for persistent overturning of the temperature and sodium density fields, superposed higher-frequency motions, small-scale transient ripples in the nightglow images suggestive of instability structures, and large-scale wind shear near the height of apparent instability. We describe four simulations of wave breaking with a three-dimensional model designed to assist in the interpretation of these observations. Two simulations address the instability of a low-frequency wave in a background shear flow with and without higher-frequency modulation. These show higher-frequency motions to be important in assigning the spatial and temporal scales of instability structures. Two other simulations examine the instabilities accompanying a convectively unstable inertia-gravity wave with and without higher-frequency modulation without mean shear. These show the instability structure to remain aligned in the direction of wave propagation, with only weak influences by the high-frequency motion. Our results suggest that instability due to a superposition of waves accounts best for the nightglow features observed during the CORN campaign and that streamwise convective instabilities observed due to wave breaking at higher intrinsic frequencies continue to dominate instability structure for internal waves for which inertial effects are important.

A study is presented that discusses the information content of a zenith viewing brightness ratio for optically thin O and N 2 emissions arising from auroral electron impact excitation. The discussion focuses on the altitude dependence of the ratio and distinguishes between a topside portion and the rest which lies below. The topside is defined as the region in which the auroral electron flux is essentially constant with altitude. The lower boundary to the topside is a function of the average energy of the precipitating electrons. The profile of the brightness ratio is a function of and the ratio of O to N 2 column densities (designated as O/N 2 ). On the topside, the dependence disappears leaving a unique relationship between the brightness ratio and O/N 2 . Thus profiles of both quantities have identical shapes on the topside. The shape depends on the exospheric temperature T exo . Thus a model-based brightness ratio being used to compare with measurements on the topside must take this into account. The temperature can be quantified from measurements having good counting statistics or in their absence from an atmospheric model such as the mass spectrometer/incoherent scatter (MSIS) model. Without knowledge of , data/model comparisons must be restricted to the topside in order to uniquely specify O/N 2 . With such knowledge, however, data/model comparisons may be extended into the region below the topside. The chief advantages are better counting statistics and the decreasing dependence of the brightness ratio on T exo with decreasing altitude. Derived values of O/N 2 may be related to O density profiles throughout the lower thermosphere with the use of a model atmosphere. MSIS-83 is used in this work following the preference of J. H. Hecht and colleagues in analyzing ground-based optical data. The findings discussed above are used to analyze zenith viewing OI 844.6/N 2 + 391.4 ratio data from the Atmospheric Response in Aurora (ARIA) flights I and IV. The ARIA I data support an O concentration that is a factor of ∼1.1 above MSIS-83 (similar to the findings of Hecht et al. [1995]), while the ARIA IV data lead to a factor of ∼0.75 referenced to the ARIA IV MSIS atmosphere. Both atmospheres possess similar O/N 2 values below 120 km. Within relative calibration errors between the experiments, one may conclude that the ARIA IV O concentrations throughout the lower thermosphere (120 km - 200 km) were less than ARIA I by a factor of ∼0.7.

Results are presented for zenith observations of a noctilucent cloud (NLC) display over the Sondrestrom atmospheric research facility near Kangerlussuaq, Greenland, on July 30/31, 1995. The observations were made with a Rayleigh lidar, which measured the NLC particle volume backscatter coefficient, and with a UV spectrograph, which measured the intensity and degree of linear polarization of solar light scattered from the NLC. The intensity and polarization measurements were made at solar depression angles of −1.8° to −4.6°. These data allowed the first simultaneous observation from the ground of the altitude and thickness of the NLC and of the radius of the NLC particles. The NLC was found to be between 86 and 84 km in altitude with a thickness of 1 to 2 km and the NLC particles had a radius at or below 0.07 μm. We also report the first observation of an NLC sublimating due to the passage of an AGW through the 85-km altitude region. These observations are generally in agreement with models of noctilucent clouds.

[1] The Aerospace Corporation's Nightglow Imager observed a large step function change in airglow in the form of a traveling front in the OH Meinel (OHM) and O2atmospheric (O2A) airglow emissions over Alice Springs, Australia, on 2 February 2003. The front exhibited nearly a factor of 2 stepwise increase in the OHM brightness and a stepwise decrease in the O2A brightness. There was significant (∼25 K) cooling behind the airglow fronts. The OHM airglow brightness behind the front was among the brightest for Alice Springs that we have measured in 7 years of observations. The event was associated with a strong phase-locked 2 day wave (PL/TDW). We have analyzed the wave trapping conditions for the upper mesosphere and lower thermosphere using a combination of data and empirical models and found that the airglow layers were located in a region of ducting. The PL/TDW-disturbed wind profile was effective in supporting a high degree of ducting, whereas without the PL/TDW the ducting was minimal or nonexistent. The change in brightness in each layer was associated with a strong leading disturbance followed by a train of weak barely visible waves. In OHM the leading disturbance was an isolated disturbance resembling a solitary wave. The characteristics of the wave train suggest an undular bore with some turbulent dissipation at the leading edge.

[1] Model-derived electron and proton auroral FUV emission efficiencies of relevance to auroral FUV remote sensing methods are evaluated using coincident observations by Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5), both on board the Defense Meteorological Satellite Program (DMSP) satellite F16. This follows earlier work by Knight et al. (2008), which reported higher than expected proton Lyman-Birge-Hopfield (LBH) emission efficiencies based on F16 SSUSI and SSJ/5 comparisons, and Correira et al. (2011), which suggested a downward revision in the proton LBH efficiencies from Knight et al. (2008). These proton efficiency results rely on proton extrapolation methods to supply the unmeasured proton flux above 30 keV (the upper limit of SSJ/5). Correira et al. (2011) determined that there was a bias in the proton extrapolation method used by Knight et al. (2008) that was caused by column emission rate (CER) thresholding in the coincident SSUSI and SSJ/5 sets. In the latest work, a more robust proton flux extrapolation method is introduced which does not have the problem of CER threshold dependence. The new extrapolation method uses coincident SSUSI Lyman alpha observations to constrain the extrapolated proton flux above 30 keV in such a way that unknown Lyman alpha model yield errors and SSUSI and SSJ/5 calibration errors drop out without biasing the extrapolation. With the latest extrapolation method, SSUSI-SSJ/5 comparisons indicate that proton aurora is typically a factor of ∼4.5 more efficient per unit of energy flux in producing LBH than electron aurora.

We test the utility of the OII 83.4 nm emission feature as a measure of ionospheric parameters. Observed with the Remote Atmospheric and Ionospheric Detection System (RAIDS) Extreme Ultraviolet Spectrograph on the International Space Station (ISS), limb profiles of 83.4 nm emissions are compared to predicted dayglow emission profiles from a theoretical model incorporating ground-based electron density profiles measured by the Millstone Hill radar and parameterized by a best-fit Chapman-{\alpha} function. Observations and models are compared for periods of conjunction between Millstone Hill and the RAIDS fields-of-view. These RAIDS observations show distinct differences in topside morphology between two days, 15 January and 10 March 2010, closely matching the forward model morphology and demonstrating that 83.4 nm emission is sensitive to changes in the ionospheric density profile from the 340 km altitude of the ISS during solar minimum. We find no significant difference between 83.4 nm emission profiles modeled assuming a constant scale height Chapman-{\alpha} best-fit to the ISR measurements and those assuming varying scale height.

[1] Observations of airglow emission using the RAIDS (Remote Atmospheric and Ionospheric Detection System) instruments on the International Space Station Kibo module are reported and compared to a photochemical model of the emission process. Launched in Sept. 2009, RAIDS performed routine observations of the O2(b1Σ → X3Σ) Atmospheric band (O2 A-band) transition during solar minimum conditions from October 2009 to December 2010. Limb brightness of the (0,0), (0,1) and (1,1) vibration band emissions were measured in the altitude range 80 to 180 km with the Near Infrared Spectrometer (NIRS) instrument, one of eight limb viewing instruments in the RAIDS experiment. Comparison of observed brightness profiles with the model shows very good agreement for the (0,0) and (0,1) bands. The model underestimates the (1,1) brightness profiles throughout the region, especially near the peak. Reasonable variations of composition and selected rate constants do not account for the underestimation of (1,1) band brightness. A contributing factor could be in the assumption of detailed balance and the accepted energy transfer pathways that redistribute energy between the v = 0 and v = 1 states.

[1] The Mass Spectrometer and Incoherent Scatter (MSIS) series of models has been a standard for specification and prediction of conditions in the Earth's thermosphere. One aspect of the model that has not been well validated is its predictions of the lower thermospheric compositional response to varying periodic and transient solar and geomagnetic forcings. Here we compare the predictions by the NRLMSIS model over the years 2002 to 2006 for column O/N2 with GUVI satellite (daytime) observations at a fixed longitude of the Poker Flat Research Range in central Alaska. NRLMSIS predictions are also compared with nightside auroral O/N2 observations obtained from ground-based photometric measurements from central Alaska. While there is often good agreement between the GUVI and NRLMSIS O/N2 at high latitudes, there are also times during large geomagnetic storms when there is some disagreement with the largest occurring at midlatitudes. The 9-day periodicity, first described in 2007 and attributed to high-speed solar wind streams, is present in the model at all latitudes but is quite weak in the observations below 55N latitude. Away from high-latitudes, these differences may exist because the NRLMSIS O/N2 dependence on ap forces nonperiodic contributions contained in the NRLMSIS database to give a periodic response when ap is varied periodically. While the nighttime observations show considerable agreement with NRLMSIS they are much more variable than predicted by the model. This variability suggests that there are significant local effects in the auroral zone that are not captured by the model.

[1] We present the first published measurement of an altitude profile of the O II 61.7 nm emission, a dayglow feature that can be used to monitor photoionization of O in the lower thermosphere. This photoionization process also results in the O II 83.4 nm emission that, unlike 61.7 nm, is resonantly scattered by ionospheric O+. Although ionospheric characteristics can be inferred from the shape and intensity of 83.4 nm altitude profiles, the interpretation can result in nonunique ion density profiles if the intensity of this source of photons that illuminates the ionosphere from below is unknown. The 61.7 nm emission provides a means to test the accuracy of current models used to calculate the intensity of that source. The data presented here were collected by the Remote Atmospheric and Ionospheric Detection System from the International Space Station on 29 October 2009. The measured 61.7 nm profiles show a steeper drop in intensity below 260 km, where the emission peaks, compared to our model calculations. While the current analysis cannot resolve if the discrepancy is caused by inaccuracies in our model thermospheric composition, photoabsorption cross sections, or both, a 15%–20% increase in the effective O2 photoabsorption at 61.7 nm produces the best qualitative match to the measured profile. Ostensibly, 61.7 nm measurements could replace these model calculations as a more direct measure of the intensity of the 83.4 nm photon source region. In either case, accurate specification of local thermospheric neutral species remains an important component of daytime ionospheric remote sensing.

[1] A significantly higher N2 Lyman-Birge-Hopfield (LBH) emission efficiency for auroral proton precipitation compared to model calculations was reported by Knight et al. (2008) based on a statistical study utilizing coincident far ultraviolet and particle data from the sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5) on board the DMSP satellite F16. Here, the quantity of interest from that study is the median ratio of LBH column emission rates (CERs) from SSUSI and derived from SSJ/5 spectra using monoenergetic emission yields. The median ratio was found to be 2.83 for proton aurora, suggesting the need for significant increases in currently used LBH proton/H-atom impact cross sections. A key step in their analysis was extrapolation of SSJ/5 spectra above 30 keV. Limited testing of this algorithm using NOAA Polar Orbiting Environmental Satellites Total Energy Detector and Medium Energy Proton and Electron Detector (TM) data found no significant bias. This work reports on a more detailed investigation of the algorithm's performance, also using TM data, and has uncovered a bias that reduces the median column emission rates (CER) ratio to 1.75. Within expected uncertainties, including calibration, this still calls for cross section increases but to a lesser extent. The discovered bias becomes apparent with CER thresholding that was overlooked during testing by Knight et al. (2008). Thresholding at 400 Rayleighs (R) is necessary since Knight et al. excluded CERs < 400 R in deriving their median ratio. We show that the algorithm's performance degrades with increasing energy flux of the precipitation. A method is reported for eliminating most of the bias which utilizes auroral Ly α, whose emission strength is closely coupled to spectral hardness.

[1] The quasi 2 day wave, with a nominal mean period just above 50 h, is a significant feature of the 80–100 km altitude region in both hemispheres. It becomes particularly prominent in the Southern Hemisphere summer at midlatitudes where, a short time after summer solstice, its amplitude rapidly increases and its mean period is found to be approximately 48 h, producing an oscillation phase locked in local time. This lasts for a few weeks. Presented here are observations of the meridional winds and airglow over two sites in Australia, for 4 years during the austral summers of 2003–2006. We show that during those times when the large-amplitude phase-locked 2 day wave (PL-TDW) is present the diurnal tide greatly decreases. This is consistent with the Walterscheid and Vincent (1996) model in which the PL-TDW derives its energy from a parametric excitation by the diurnal tide. These data also show that the diurnal tide is more suppressed and the PL-TDW amplitude is larger in odd-numbered years, suggesting a biannual effect. The airglow data indicated that, for the PL-TDW, the winds and temperature are nearly out of phase. When the PL-TDW is present airglow amplitudes can become quite large, a result dependent on the local time of the PL-TDW maximum. The airglow intensity response was, in general, much larger than what would be expected from the airglow temperature response, suggesting that the PL-TDW is causing a significant composition change possibly due to minor constituent transport.

[1] During the Tropical Warm Pool International Cloud Experiment (TWPICE) an intense tropical low was situated between Darwin and Alice Springs, Australia. Observations made on 31 January 2006 by the Atmospheric Infrared Sounder instrument on the NASA Aqua satellite imaged the presence of atmospheric gravity waves (AGWs), at approximately 40 km altitude, with horizontal wavelengths between 200 and 400 km that were originating from the region of the storm. Airglow images obtained from Alice Springs (about 600 km from the center of the low) showed the presence of similar waves with observed periods of 1 to 2 h. The images also revealed the presence of 30- to 45-km-horizontal-wavelength AGWs with shorter observed periods of near 15 to 25 min. Ray tracing calculations show that (1) some of the long wavelength waves traveled on rays, without ducting, to the altitudes where the observations were obtained, and (2) shorter-period waves rapidly reached 85 km altitude at a horizontal distance close to the storm, thus occurring over Alice Springs only if they were trapped or ducted. The mesospheric inversion layer seen in the measured temperature data almost forms such a trapped region. The winds therefore critically control the formation of the trapped region. Wind profiles deduced from the available data show the plausibility for the formation of such a trapped region. Variations in the wind, however, would make ideal trapped region conditions short-lived, and this may account for the sporadic nature of the short-period wave observations.

[1] The launch of the Defense Meteorological Satellite Program (DMSP) satellite F16 in 2003 provided the first opportunity to analyze extensive sets of high-quality coincident auroral particle and FUV data obtained by the onboard sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor Auroral Particle Sensor (SSJ/5). Features of interest are Ly α (121.6 nm), Lyman-Birge-Hopfield short (LBHS, the SSUSI 140–150 nm channel), and Lyman-Birge-Hopfield long (LBHL, 165–180 nm). We report on comparisons of column emission rates (CERs) by deriving simulated SSUSI values using SSJ/5 electron and ion (treated as proton) spectra. Field-line tracing is performed to determine the locations of coincidences. CERs are obtained by integrating the products of particle spectra and monoenergetic emission yields. A technique is reported for deriving these yields from nonmonoenergetic CERs obtained by our particle transport model. SSJ/5 ion spectra are extrapolated above 30 keV using a statistical representation based on Polar Orbiting Environmental Satellites particle data. Key quantities of interest are ratios of SSUSI to SSJ/5-based CERs (S-S ratios) and corresponding ratios of proton-produced to total emission (unity for Ly α and from 0 to 1 for LBHS and LBHL). SSJ/5-based CERs are used to derive the latter ratios. Median ratio values are determined in order to reduce the error budget to primarily calibration and model errors. The median LBH S-S ratios increase by a factor of ∼2.5 from electron to proton aurora and support significantly higher proton LBH emission efficiencies (3 times the electron efficiencies) assuming reported calibration uncertainties. This calls for significant increases in proton and/or H-atom LBH cross sections. In turn, FUV auroral remote-sensing algorithms must explicitly address both electron and proton aurora.

[1] Aerospace imagers operating at Alice Springs (23°42′S, 133°53′E) and Adelaide (34°55′S, 138°36′E) have collected more than 4-year of OH and O2 atmospheric emission data. Images taken over the course of each moonless night at 5-min intervals were used to determine OH Meinel (6, 2) and O2 Atmospheric (0, 1) band emission intensities and temperatures, as well as atmospheric gravity wave parameters. The NCAR general circulation model TIME-GCM was run for years 2002–2005 for comparison with these data. The data presented here show the interannual variability of OH and O2A emissions at two sites, Alice Springs and Adelaide, over a 4-a period. It was found that the TIME-GCM successfully reproduces many observed features of the data, including equinoctial maxima associated with the diurnal tide, a 6-h phase shift between OH temperature and intensity maxima, and springtime OH intensity enhancements at Alice Springs. However, the model tends to underestimate the depth of the summertime temperature minimum at both sites, possibly due to inadequate specification of the seasonal variation of gravity waves in the model. The model does, however, successfully describe many of the mesospheric changes observed during the 2002 stratospheric warming event.

[1] Small-scale (less than 15 km horizontal wavelength) wavelike structures known as ripples are a common occurrence in OH airglow images. Recent case studies attribute their origin to the presence of either convective or dynamical instabilities. However, little is known about their frequency of occurrence and period. The Maui-MALT Observatory, located at Mt. Haleakala, is instrumented with a Na wind/temperature lidar, which allows the determination of whether the atmosphere is dynamically or convectively unstable, and a fast OH airglow camera which takes images every 3 s with a sensitivity high enough to see the ripples. This study reports on 2 months of observations in October/November 2003 and in August 2004, eight nights of which also included Na lidar measurements. The imager results suggest that instability features occur in the 85- to 90-km region of the atmosphere for around 20% of the time. The nominal observed period for the ripples is between 2 and 4 min. While there are clear night-to-night variations, the average observed period is similar for both the 2003 and 2004 observations. In addition, a few of the small-scale structures are not ripples caused by instabilities but rather have features consistent with their being short horizontal wavelength evanescent waves. Their fractional intensity fluctuations are as large or larger than those of the ripple instabilities. Unlike the instabilities, the origin of the evanescent waves is not determined.

[1] Small-scale (less than 15 km horizontal wavelength) structures known as ripples have been seen in OH airglow images for nearly 30 years. The structures have been attributed to either convective or dynamical instabilities; the latter are mainly due to large wind shears, while the former are produced by superadiabatic temperature gradients. Dynamical instabilities produce Kelvin-Helmholtz (KH) billows, which have been known for many years. However, models and laboratory experiments suggest that these billows often spawn a secondary instability that is convective in nature. While laboratory investigations see evidence of such structures, the evolution of these instabilities in the atmosphere has not been well documented. The Maui Mesosphere and Lower Thermosphere (Maui MALT) Observatory, located on Mt. Haleakala, is instrumented with a Na wind/temperature lidar that can detect dynamic or convective instabilities with 1 km vertical resolution over the altitude region from about 85 to 100 km. The observatory also includes a fast OH airglow camera, sensitive to emissions coming from approximately 82 to 92 km altitude, which obtains images every 3 s at sufficient resolution and signal to noise to see the ripples. On 15 July 2002, ripples were observed moving at an angle to their phase fronts. After a few minutes, structures appeared to form approximately perpendicular to the main ripple phase fronts. The lidar data showed that a region of dynamical instability existed from approximately 85.5 to 87 km and that the direction of the wind shear in this region was consistent with the phase fronts of the ripple features. The motion of the ripples themselves was consistent with the wind velocity at 85.9 km. Thus in this case the observed ripple motion was the advection of KH billows by the wind. The perpendicular structures were seen to be associated with the KH billows: they formed at the time when the atmosphere briefly became convectively unstable within the region where the KH billows most likely formed. Because of this and because the ripples were oriented approximately perpendicular to and moved with the billows, we speculate that they are the secondary instabilities predicted by models of KH evolution. The primary and perpendicular features were seen to decay into unstructured regions suggestive of turbulence. While the formation and decay time appear consistent with models, the horizontal wavelength of the perpendicular structures seems to be larger than models predict for the secondary instability features.

[1] The breakdown of a high-frequency quasi-monochromatic gravity wave into small-scale ripples in OH airglow was observed on the night of 28 October 2003 at Maui, Hawaii (20.7°N, 156.3°W). The ripples lasted ∼20 min. The phase fronts of the ripples were parallel to the phase fronts of the breaking wave. The mechanism for the ripple generation is investigated using simultaneous wind and temperature measurements made by a sodium (Na) lidar. The observations suggest that the wave breaking and the subsequent appearance of ripples were related to dynamical (or Kelvin-Helmholtz) instabilities. The characteristics of the ripples, including the alignment of the phase fronts with respect to the wind shear, the motion of the ripples, and the horizontal separation of the ripple fronts were consistent with their attribution to Kelvin-Helmholtz billows. It is likely that the dynamical instability was initiated by the superposition of the background wind shear and the shear induced by the wave. The wind shear, the mean wind acceleration, and the propagation of the breaking wave were found to be in the same direction, suggesting that wave-mean flow interactions contributed significantly to the generation of the strong (>40 m/s/km) wind shear and instability.

[1] A number of observations of the sodium density primarily in the mesosphere and lower thermosphere (but also of the electron density structure) have shown what appears to be overturning or convective roll cells near the transition from the mesosphere to the lower thermosphere. The cells are found in the region between 95 and 105 km and occur near the boundary between the region of lower stability in the mesosphere and the region of higher stability in the lower thermosphere. The vertical scale for the rolls is ∼5–6 km, and the timescale is ∼1–3 hours. The rolls occur in a region characterized by large shear in the neutral winds, but the timescales are too long to be explained by simple Kelvin-Helmholtz instability. We present observations from the University of Illinois lidar facility located at the Starfire Optical Range near Albuquerque, New Mexico, from the Arecibo Observatory in Puerto Rico, and from the Maui/MALT Lidar Facility located on Maui in Hawaii that show the overturning structure. possible mechanisms for generating the convective rolls are discussed, Including the relationship of the observed features to the characteristics expected for an inflection point instability.

[1] The Darwin Area Wave Experiment occurred in Australia from October to December 2001. An objective was to characterize the atmospheric gravity wave field produced from intense convective activity that is routinely observed around Darwin during November and December. Two airglow imagers were sited at Adelaide and at Alice Springs, each located over 1000 km south of Darwin. Waves were observed at the mesopause region propagating predominantly toward the southeast, with some going to the northwest but with none observed going from east to west. The lack of waves propagating toward the west suggests some wind filtering mechanism below 80 km altitude. Waves observed over Alice Springs were analyzed in detail on three nights. On 16 November they were seen propagating toward the northwest. It is proposed that they were generated by dynamical events associated with a cutoff low-pressure system present over southwest Australia. On 17 and 19 November the observations are consistent with wave generation by convective activity present in the Darwin area. Thus as proposed by Walterscheid et al. [1999] and Hecht et al. [2001a], the ducting of waves from distant sources is shown to be a viable explanation for the quasi-monochromatic waves frequently observed in airglow observations. Walterscheid et al. [1999] suggested that ducting of waves from the extensive region of deep cumulus convection over northern Australia explained the strong poleward directionality seen in the summer months. The present study suggests that propagation from northern Australia is selective, and ducted waves from this region may not be the primary source of waves over Adelaide when convection is occurring over central Australia.

[1] The Turbulent Oxygen Mixing Experiment (TOMEX) combined Na lidar measurements from Starfire Optical Range in Albuquerque, New Mexico, with a launch of a payload from White Sands Missile Range (WSMR), located a little over 100 km from Starfire. The payload included a trmethyl aluminum release to measure winds and diffusion, a 5-channel ionization gauge to measure neutral densities, and a 3-channel photometer experiment to measure atomic oxygen related airglow. The payload was launched at 0957 UT on 26 October 2000 and successfully obtained data from all the experiments. The photometer experiment consisted of three liquid nitrogen cooled filter photometers which measured emission from the O2 atmospheric band (0, 0) emission, the OH Meinel (9, 4) band, and the OI(557.7 nm) greenline. Measurements were made as the rocket went from 80 to 110 km on the upleg. The pointing of the photometers was within a few degrees of zenith. Differentiating these data allowed volume emission rates to be derived which can be inverted to form atomic oxygen density profiles. The interpretation of the data made use of simultaneous atmospheric temperature data from the Na lidar. The airglow data showed lower brightness values and lower peak altitudes for the O2 atmospheric (0, 0) band and OI(557.7 nm) emissions than predicted by the thermosphere/ionosphere/mesopshere/electrodynamics general circulation (TIME-GCM) model. The peak altitude of the OH Meinel emission seemed nominal. Inverting the O2 atmospheric (0, 0) and OI(557.7 nm) data following McDade et al. [1986] produced O density profiles whose peak densities and peak altitudes are lower than the model values. The shape of the O density profile is also more constant with altitude than model predictions. The O mixing ratio shows a more altitude-independent profile than given by the model, especially between 85 and 95 km. Significant deviations in the measured shape of the mixing ratio also occur at 90, 97, and 102 km. The interpretation of these data is that the O mixing ratio was significantly perturbed by the passage of an atmospheric gravity wave or tide and the subsequent convective or dynamical instabilities produced by that wave. Dynamically or convectively unstable layers at 90, 97, and 102 km at the time of the launch also appear to be reflected in the mixing ratio data.

[1] The Na wind/temperature lidar located at Starfire Optical Range near Albuquerque, New Mexico, provided real time measurements of wind, temperature, and Na density in the mesopause region during the TOMEX rocket campaign in October 2000. The state of the atmosphere in which the rocket was launched into was examined using the lidar measurements. Both convectively and dynamically unstable layers were observed at various times and altitudes during the night. The low convective stability region below 90 km was found to be associated with the diurnal tide. The unstable layers are the combined results of wave and tidal perturbations. Comparison with the thermosphere/ionosphere/mesopshere/electrodynamics general circulation model (TIME-GCM) simulation showed that the model can produce the general feature of the observed atmospheric structure (but with a much smaller diurnal amplitude in temperature), which likely leads to underestimate of instability and gravity wave effects.

[1] The Turbulent Oxygen Mixing Experiment (TOMEX), which was carried out at White Sands Missile Range in New Mexico on 26 October 2000, included a rocketborne trimethyl aluminum (TMA) chemical tracer experiment. The subsequent TMA trails provided detailed information about the horizontal neutral wind, turbulence, and diffusivity properties of the atmosphere between approximately 85 and 140 km altitude. Measurements with the University of Illinois Na wind/temperature lidar located at the Starfire Optical Range, NM, provided a detailed time history of the stability properties between 85 and 105-km altitude, including high-resolution wind and temperature measurements prior to and during the chemical tracer measurements. The diffusivities estimated from the trail expansion rates have values consistent with the values expected for molecular diffusion above 110-km altitude and values that are larger than those for molecular diffusion at most altitudes below. Below 103 km, both regions of dynamic and convective instability were found, and the diffusivities are strongly controlled by the instabilities. The unstable regions are well mixed, but the intermediate regions, in some cases, have very small eddy energy dissipation rates. The nearly instantaneous measurements also suggest that eddy diffusion is still important in the height range between 103 km, the nominal turbopause height, and 110 km.

[1] The Turbulent Oxygen Mixing Experiment (TOMEX) was designed to measure the atmospheric response to the existence of unstable layers as determined by wind and temperature measurements from 80 to 105 km. TOMEX combined Na lidar measurements, from Starfire Optical Range in Albuquerque, New Mexico, with a launch of a payload from White Sands Missile Range, located between 100 and 150 km south of Starfire. The payload included a trimethyl aluminum chemical release to measure winds and diffusion, a 5-channel ionization gauge to measure neutral density fluctuations at high vertical resolution, and a 3-channel photometer experiment to measure atomic oxygen related airglow. The rocket was launched when the lidar data indicated the presence of convectively and dynamically unstable regions between 80 and 100 km altitude. For several hours prior to the launch, there had existed a large amplitude atmospheric gravity wave or tide which brought the background atmosphere into being nearly convectively unstable over the 85 to 95 km region. In addition a large overturning in Na density, possibly due to a convective roll, existed at altitudes around 100 km. This type of instability had not been previously seen and identified in this altitude region. The TOMEX payload measured the existence of Kelvin-Helmholz billows, enhanced neutral density fluctuations, enhanced energy dissipation, and well-mixed regions. These features were associated with convectively unstable regions, dynamically unstable regions, convective rolls, and the presence of this large wave. The unstable regions were persistent and covered large vertical (and horizontal regions) of the atmosphere. The atmospheric mixing and energy dissipation appeared to be largely determined by the presence and nature of these instabilities.

[1] Acoustic-gravity wave relations indicate that when wave frequency and horizontal wave number approach the characteristic curve-delineating gravity and acoustic solutions, the horizontal group velocity and Eckart's characteristic impedance become infinite and wave energy E vanishes. It is shown that this behavior is equivalent to assuming incorrectly that wave energy is the same function of vertical wave number for internal and evanescent waves. When the correct form of E for evanescent waves is used, the energy flow velocity U defined in terms of the energy flux Fx = EU is bounded by the sound speed, impedance is bounded by values near unity, and E does not vanish. For waves near the Lamb-wave solution, the dominant dynamical control of the vertically integrated airglow response is the horizontal divergence. The only significant long-lived evanescent response following an excitation event should be near the Lamb curve, and here the horizontal divergence is largest for small-scale high-frequency waves. The transient response is strongest for those waves that disperse least: waves with long-horizontal wavelengths and waves not too far from the Lamb and Brunt-Vaisala (BV) curves. Wave properties are well behaved near the characteristic curve and solutions are linked at the common intersection of the Lamb and BV curves implying that waves should be able to transfer energy from one regime to the other in response to background variations. Nonisothermal calculations show that when lapse rates are large enough waves that are at least partially internal can exist at all frequencies.

[1] In November 1999 a new near-IR airglow imaging system was deployed at the Starfire Optical Range outside of Albuquerque, New Mexico. This system allowed wide angle images of the airglow to be collected, with high signal to noise, every 3 seconds with a one second integration time. At approximately 1000 UT on November 17, 1999, a fast wavelike disturbance was seen propagating through the OH Meinel airglow layer. This wave had an observed period of ≈215 seconds, an observed phase velocity of ≈160 m/s and a horizontal wavelength of ≈35 km. This phase velocity is among the fastest yet reported using an imager viewing the OH Meinel bands, while the wave period is among the shortest. Simultaneous Na lidar wind and temperature data from 80 to nearly 110 km altitude allow the intrinsic properties of the wave to be calculated. The Einaudi and Hines [1970] WKB approximation for the acoustic-gravity wave dispersion relation was used to calculate the wave's intrinsic properties. Using this approach indicates that the observed disturbance was an external acoustic wave in the 90 to 107 km altitude region and an external gravity wave at other altitudes between 80 and 90 km. Using model atmospheric data for altitudes below and above this altitude regime indicates that the wave is essentially external everywhere except perhaps in narrow regions around 80 and 105–110 km. This is confirmed using a more exact full-wave model analysis. The observations and model results suggest that this wave was not generated in the troposphere and propagated up to the mesosphere, but rather near 100 km altitude where it was possibly generated by a Leonids meteor.

The Atmospheric Response in Aurora (ARIA) I rocket experiment was designed to measure the energy and momentum forcing of the atmosphere during auroral disturbances and the resultant compositional and dynamical changes. It consisted of one instrumented rocket, three trimethyl aluminum chemical release rockets, and various ground-based optical instruments. The rockets were launched from Poker Flat Research Range, Alaska, in March 1992. The instrumented payload included a set of eight instruments for measuring various atmospheric and ionospheric quantities. This paper describes the contents of the program and the results of electrodynamic modeling and measurements. A substorm onset occurred approximately 4 hours before launch of the instrumented payload, giving rise to both particle and Joule heating in the vicinity of Poker Flat. By launch time, the substorm was well into recovery. We used optical measurements, electron density measurements from the Langmuir probe instrument, and model results from the Strickland electron transport code to specify latitudinal profiles of the height-integrated Pedersen conductivity. Comparison with assimilated mapping of ionospheric electrodynamics (AMIE) calculations of the Pedersen conductivities for this event indicated that AMIE located the enhanced auroral conductivity region well. However, the magnitudes of the AMIE conductivities in the enhanced region were considerably less than the measurements due to localized substorm-related particle precipitation enhancements not accounted for by AMIE. Our conductivity profiles were used in conjunction with electric field values produced by the AMIE routine to examine the atmospheric heating rates associated with the substorm. The latitudinally integrated Joule heating rate was initially less than the particle heating rate, but rapidly increased to its maximum value at the time of the substorm maximum while the particle heating rate peaked prior to substorm maximum. The particle and Joule heating were collocated during the expansion and maximum phase, but as the substorm recovered, the Joule heating moved to higher latitudes, so that by the time of launch, the two heating regions were completely separated by several degrees. The analysis indicates that the rocket was launched directly into the atmospheric region where the maximum heating had occurred.

The goal of the Atmospheric Response in Aurora (ARIA) experiment carried out at Poker Flat, Alaska, on March 3, 1992, was to determine the response of the neutral atmosphere to the long-lived, large-scale forcing that is characteristic of the diffuse aurora in the post midnight sector. A combination of chemical release rocket wind measurements, instrumented rocket composition measurements, and ground-based optical measurements were used to characterize the response of the neutral atmosphere. The rocket measurements were made at the end of a 90-min period of strong Joule heating. We focus on the neutral wind measurements made with the rocket. The forcing was determined by running the assimilated mapping of ionospheric electrodynamics (AMIE) analysis procedure developed at the National Center for Atmospheric Research. The winds expected at the latitude and longitude of the experiment were calculated using the spectral thermospheric general circulation model developed at the Danish Meteorological Institute. Comparisons of the observations and the model suggest that the neutral winds responded strongly in two height ranges. An eastward wind perturbation of approximately 100 m/s developed between 140 and 200 km altitude with a peak near 160 km. A southwestward wind with peak magnitude of approximately 150 m/s developed near 115 km altitude. The large amplitude winds at the lower altitude are particularly surprising. They appear to be associated with the upward propagating semidiurnal tide. However, the amplitude is much larger than predicted by any of the tidal models, and the shear found just below the peak in the winds was nominally unstable with a Richardson number of approximately 0.08.

A narrow wavelength band CCD camera, built at Aerospace, has been used to obtain images of the OH Meinel (6,2) band and the O2 atmospheric (0,1) band nightglow. The field of view of the camera is approximately 100 by 100 km at an altitude of 90 km, the nominal height of the nightglow. It is shown how Fourier techniques can be applied to these data to optimally smooth the images, to identify the presence of monochromatic waves, and to obtain both the one-dimensional and, for the first time, the two-dimensional horizontal wave number spectrum of gravity waves passing through the emission layers. Both measures of the spectrum depend, to a certain extent, on a technique which makes use of Krassovsky's η ratio. These techniques are applied to sample data taken from May 9, 1989, during the Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign in Puerto Rico, and from the recent Collaborative Observations Regarding the Nightglow (CORN) campaign in Illinois. While future papers will describe these data in more detail, a brief comparison is made with recent models of the two-dimensional horizontal wave number spectra presented by Gardner et al. (1993) and Gardner (1994).

The incoherent scatter radar at Sondre Stromfjord, Greenland, measured electron density profiles from 90 to 500 km during four auroral events over a 3-hour period on February 28, 1987. The profiles were obtained with the radar pointed along the magnetic field near zenith at 15-s intervals. Under the assumption that proton/H atom precipitation was unimportant during these events a representation of the incident electron flux was obtained by fitting calculated profiles with measured profiles in the vicinity of their peaks (lower E region). Maxwellian and Gaussian electron distributions with high- and low-energy tails were used to generate the calculated profiles. The distributions were specified in terms of average energy 〈E〉 and energy flux Q. We find that we can clearly distinguish between profiles that result from a Maxwellian incident electron spectrum and those that result from a Gaussian spectrum. Interpreting Gaussian and Maxwellian spectra as representative of discrete and diffuse aurora, respectively, the measurements indicated good correlation between 〈E〉 and Q for discrete aurora, while essentially no correlation was observed for diffuse aurora. This is consistent with current understanding that discrete auroras are produced by electrons accelerated by magnetic field-aligned potential drops whereas diffuse auroras are produced by pitch angle diffusion of plasma sheet electrons into the loss cone.

Optical emission ratios of the N2 Second Positive Group 0,0 band to N2+ First Negative Group 0,0 band were observed in the magnetic zenith at Sondre Stromfjord in parallel with radar observations of the electron density height profile. The dependence of the optical ratios on the inferred incident electron energies was investigated to assess whether the ratios could be used as an electron energy monitor. Analysis procedures were developed to minimize the effects of atmospheric scattering as well as spectral blending with other emission features. Two independent computational techniques for inferring the electron energies from the electron density profiles were compared and found to agree sufficiently well for the current study. The observed optical emission ratios were found to vary by less than 15% over the incident electron energy range from 2 to 12 keV. Comparisons were made with the ratios predicted by several auroral electron deposition models.

Several nights of optical and radar data were obtained from February 3 to 10, 1986 at Sondre Stromfjord, Greenland during a great geomagnetic storm when Ap reached 202. These data, excluding O I (630.0 nm), were used to monitor changes in O and O{sub 2} densities in the region from 100 to 200 km and are consistent with results of Hecht et al. (1989) wherein the O I (630.0 nm) emission was used. That is, there were large depletions of O in the E region during the peak of the storm. Using 24-hour Ap values both MSIS-83 and MSIS-86 underestimated the depletion of O during the storm. MSIS-86 did noticeably worse than did MSIS-83, probably a result of the low exospheric temperature predicted by MSIS-86. However, when 3-hour ap values were used both models did better in predicting the measured O depletions, although MSIS-86 again was less successful than was MSIS-83. The data results also show that the enchancements in O{sub 2} were less than predicted by MSIS models in either mode. From 0200-0230 UT on February 9, 1986, a time just after the peak of geomagnetic activity as measured by Kp, the O mixing ratio near 135 km wasmore » found to increase by nearly a factor of 2. This increase is consistent with the passage of an atmospheric gravity wave through this altitude region providing that the scale height of O above 140 km is greater than predicted by the model.« less

A theoretical method is proposed to obtain thermal structural information about the mesopause using multiple rotational lines from ground-based observations of a single vibration-rotation nightglow Meinel band of the electronic ground state of OH. The method determines the mean temperature of the emission region, the temperature difference across the region, and the magnitude of the temperature gradient for physically realistic assumptions about emission region thickness. It is applied to the OH(6, 2) Meinel band spectra of the nightglow obtained during periods of low geomagnetic activity during January 1986 at Longyearbyen, Spitzbergen. The results of the analyses indicate that during the periods of observation the emission region of the nightglow OH Meinel bands is significantly nonisothermal, with temperature gradient magnitudes of about 10 K km{sup {minus}1} (for an emission region thickness of 8 km). Such large temperature gradients are consistent with other independent measurements of a highly structured mesopause reported in the literature. Large-amplitude gravity waves propagating through the mesopause are probably responsible for the large temperature gradients.

On day 59 of 1987 a well calibrated set of photometric observations of auroral emissions (427.8, 630.0, 844.6, and 871.0 nm) was obtained at Sondre Stromfjord, Greenland coincident with incoherent scatter radar measurements of electron density. Ratios of these optical emissions were used to correct for atmospheric extinction and to infer the average energy of the precipitating electrons and the deviations in the atmospheric composition induced by auroral heating. Previously, the radar data taken alone had been used to infer the average energy of the auroral particles. For the first time a comparison is made of the results from this optical ratio technique with those obtained by combining the optical data with the radar data and with results obtained using the radar data alone. Specifically, we compare the average energy inferred by these techniques as well as the changes in the atomic oxygen to molecular nitrogen ratio in the lower thermosphere. This study shows the critical importance of characterizing the atmospheric scattering if reliable results are to be derived. In particular, it is shown that on occasion the optical technique may be even more reliable than using radar and optics together since, because of light scattering, the radar and the optics are sometimes observing different parts of the aurora. Magnetometer data are used to estimate local Joule heating. The combined data are also used to address the question of whether observed composition changes were produced locally.

The long-recognized association of brighter aurora with more deeply penetrating, and hence more energetic, electrons is examined. Using the Knight (1973) relation between the magnetic-field-aligned current density and potential drop (derived from the theory of single-particle motion in the presence of a magnetic-field-aligned electric field), an approximate expression relating the energy flux of the precipitating electrons over discrete aurora and the mean particle energy is derived. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red/blue ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora.

OH nightglow emission from the (6,2) band were observed during February 12-14, 1986, at Sondre Stromfjord, Greenland. The data were analyzed using time series analysis techniques to determine {eta} = ({Delta}I/{anti I})/({Delta}T/{anti T}), where {Delta}I and {Delta}T represent correlated fluctuations from the mean OH intensity ({anti I}) and temperature ({anti T}) in various frequency bands. For correlated fluctuations due to measurement error, {vert bar}{eta}{vert bar} 1.5. The 4-hour (2-hour) period wave has an {vert bar}{eta}{vert bar} equal to 3.1 {plus minus} 1.1 (1.6 {plus minus} 0.5) and a phase angle, between the intensity and temperature components, of 5{degree} {plus minus} 23{degree} (40{degree} {plus minus} 20{degree}). These results are consistent with a recent theoretical model for the passage of AGWs through an OH emission layer. Agreement is closest for the 4-hour (2-hour) period wave whenmore » the layer is peaked at 87 km (83 km) and the O scale height is {minus}2.0 ({minus}4.0km).« less

We conclude from a study of the production and loss of O(¹D) in auroras that the “traditional” sources, direct electron inpact excitation of atomic oxygen and dissociative recombination of molecular oxygen ions, can account for most of the O I 6300-A emission rate. In a specific application of the model to the comprehensive observation of an auroral event by Sharp et al. (1979), we show that there is no compelling need for the reaction N(²D) + O2 → NO + O(¹D). We also present a study of the sensitivity of the red line emission rate to a wide variety of input conditions.

We examine the problem of monitoring composition and the behavior of precipitating electron spectra in auroras using N2+ 4278 A (blue), O I 6300 A (red), O I 7774 A (narrow; e+O), and O I 7774 A (broad; e+O2) as observed from the ground. Calculated column emission rates for these features as well as those of narrow O I 8446 A and broad O I 8446 A are presented as functions of the hardness of the incident electron spectrum and the concentrations of O and O2. Selected ratios of these rates such as narrow/broad (for 7774 A) and red/blue are also presented. Incident spectra are characterized by Maxwellian energy distributions with characteristic energies ranging from 0.1 to 8 keV. Composition is modeled using a Jacchia (1977) model where scaling factors are applied to the O and O2 number densities. Scaling factors for O range from 0.1 to 1, and factors of 1 and 1.5 are considered for O2. As a group, the above rates and ratios show considerable variation over the just described parameter ranges, making them attractive for monitoring incident electron energy flux, its mean energy, and the concentrations of O and O2 relative to N2. The red line is a key feature of the above group which is sensitive to the low-energy portion (subkilovolt) of the incident electron spectrum. Since this can vary considerably from one aurora to another having the same approximate mean energy, it becomes an added parameter to be considered within any algorithm using the red line. Paper 2 (Meier et al., this issue) discusses this problem as part of a detailed investigation of the red line. In the current paper, one particular representation of a low-energy component is considered for making the red line calculations. A final subject of this paper is the use of temperatures deduced from measurements of rotational line distributions and atomic line Doppler widths to infer mean energies of precipitating electrons. Calculated effective temperatures versus hardness of the incident electron spectrum are presented and discussed in the context of more precise techniques for relating measurements of rotational line distributions and Doppler widths to electron spectral hardness.

Ground-based optical observations have been made of the N{sup +}{sub 2} (4278 A), O I (6300 A), O I (7774 A), and O I (8446 A) auroral emissions. Data were collected on 12 nights from 1984 to 1986 at observatories in the auroral zone and were analyzed using the Strickland et al. (this issue) aurroal electron impact excitation model results. It was found that during quiet geomagnetic conditions these data were consistent with O densities, at altitudes from 110 to 200 km, less than one-half the densities given in the Jacchia 1977 or MSIS 1983 models. During highly disturbed periods such as during the great February 1986 storm, where {ital Ap} reached 202, the O density decreased an additional factor of 8. During the same storm the O{sub 2} density increased by less than a factor of 2 from its quiet value. These data also imply that the decrease in the O density is larger at higher altitudes during disturbed periods, in qualitative agreement with the heating model of Hays et al. (1973). {copyright} American Geophysical Union 1989

Temporal variations in the absolute intensity (I) and the rotational temperature (T) of the polar winter airglow OH(8, 3) bands were observed over Longyearbyen, Spitsbergen (78.2°N), in December 1984. The high latitude of the observatory places it in a continuous polar night around the winter solstice period, thereby permitting 24-hour optical measurements. Spectral analyses of the I and T time series reveal a dominant semidiurnal component and a wave with a period of 3.7 hours. Other waves with shorter periods may be present for part of the total observing period, but the signal-to-noise ratios of their amplitudes averaged over a 24-hour interval are too low to permit definitive identification. The ratio η of the correlated Fourier components of (I − Ī)/Ī to that of (an overbar signifies time average) has amplitude 3.2 ± 0.9 and 4.3 ± 1.3 for the 12- and 3.7-hour waves, respectively, while the phase ϕ of η for these periods is 6° ± 16° and −12° ± 12°. The value of η for the 3.7-hour wave can be compared with the theoretical prediction of η for gravity wave modulation of the OH emission. For an emission height of 87 km and an atomic oxygen scale height ℋ(O) of −2 km, theory gives |η| = 4.2 and ϕ = 0° for the 3.7-hour wave, in good agreement with the observation. The theoretical interpretation of η at the 12-hour period is complicated by the likelihood that the observed semidiurnal oscillation is not a simple tide. For tidal modulation of the OH nightglow, theory gives |η| between 2.6 and 0.9 and ϕ between 0° and 10° for high-order semidiurnal modes and for the same values of emission height and ℋ(O) as above.

A CEDAR campaign organized to observe Sun-aligned arcs from Sondre Stromfjord, Greenland, obtained a coordinated satellite- and ground-based observation of the development and subsequent zenithal passage of a Sun-aligned transpolar arc on February 26, 1987. A broad array of ground-based optical instruments plus the Sondre Stromfjord incoherent scatter radar characterized the optical and ionospheric parameters of the arc as it passed through the geomagnetic zenith. Optical instrumentation obtained detailed intensity measurements in the geomagnetic zenith for several auroral emissions and recorded the spatial and temporal evolution of the arc. During the transit of the arc the incoherent scatter radar was pointed in the geomagnetic zenith direction, providing electron density height profiles. The DMSP F6 satellite imaged a segment of the Sun-aligned arc and measured the precipitating flux along its path at the same time that ground-based measurements were being taken. All-sky images in N2+ 1 NG and O I (6300 A) emissions reveal the progression of the arc from an F region feature to an E region feature and its resulting stability for the following 20 min as it drifted out of the zenith. The maximum emission intensity of O I (6300 A) within the arc was 1.1 kR. The observed λ6300/λ4278 spectral ratio was 1.3. The altitude of peak electron density changed from 170 to 250 km preceding the zenithal transit of the arc and then shifted to 120 km as the brightest structure moved into the radar field of view. Model calculations based on the observed electron density height profile implied a peak energy of 3.5 keV for the precipitating electrons with a total electron energy flux of 2.1 ergs/cm² s assuming a Gaussian incident electron distribution with a low-energy tail included to match the observed emissions. The low Balmer Hα emission intensity implied a proton energy flux of less than 0.04 erg/cm² s. Matching model results with high-resolution auroral O I (7772 A) measurements implies scaling the MSIS model atomic O densities by 0.4 for this event. Optical spectral ratios predicted by the model were near observed values. Our results show that this event is best described by a soft electron precipitation with a very low upper limit imposed on the proton contribution.