Search

We present measurements of sunlit aurora during the launch of the Rocket Experiment for Neutral Upwelling 2 (RENU2) on the 13 December 2015, 07:34 UTC. The in situ auroral conditions coincide with those of sunlit aurora and were characterised by the 391.4 and 427.8 nm N2+ emissions. A correlation between several auroral wavelengths, as measured by a meridian-scanning photometer, was used to detect sunlit aurora and indirectly neutral upwelling. These results, based on ground data, agree well with the RENU2 measurements recorded during its pass through the sunlit polar cusp. Using in situ data from RENU2 and the solar photon flux, it was estimated that the sunlit aurora was a major part (≈40 %) of the observed 427.8 nm emission.

Recent attention has been given to mesoscale phenomena across geospace (∼10 s km to 500 km in the ionosphere or ∼0.5 RE to several RE in the magnetosphere), as their contributions to the system global response are important yet remain uncharacterized mostly due to limitations in data resolution and coverage as well as in computational power. As data and models improve, it becomes increasingly valuable to advance understanding of the role of mesoscale phenomena contributions—specifically, in magnetosphere-ionosphere coupling. This paper describes a new method that utilizes the 2D array of Time History of Events and Macroscale Interactions during Substorms (THEMIS) white-light all-sky-imagers (ASI), in conjunction with meridian scanning photometers, to estimate the auroral scale sizes of intense precipitating energy fluxes and the associated Hall conductances. As an example of the technique, we investigated the role of precipitated energy flux and average energy on mesoscales as contrasted to large-scales for two back-to-back substorms, finding that mesoscale aurora contributes up to ∼80% (∼60%) of the total energy flux immediately after onset during the early expansion phase of the first (second) substorm, and continues to contribute ∼30–55% throughout the remainder of the substorm. The average energy estimated from the ASI mosaic field of view also peaked during the initial expansion phase. Using the measured energy flux and tables produced from the Boltzmann Three Constituent (B3C) auroral transport code (Strickland et al., 1976; 1993), we also estimated the 2D Hall conductance and compared it to Poker Flat Incoherent Scatter Radar conductance values, finding good agreement for both discrete and diffuse aurora.

Effects of scattering of electrons from whistler chorus waves and of ions due to field line curvature on diffuse precipitating particle fluxes and ionospheric conductance during the large 17 March 2013 storm are examined using the self-consistent Rice Convection Model Equilibrium (RCM-E) model. Electrons are found to dominate the diffuse precipitating particle integrated energy flux, with large fluxes from ~21:00 magnetic local time (MLT) eastward to ~11:00 MLT during the storm main phase. Simulated proton and oxygen ion precipitation due to field line curvature scattering is sporadic and localized, occurring where model magnetic field lines are significantly stretched on the night side at equatorial geocentric radial distances r(0) ≳8 R(E) and/or at r(0) ~5.5 to 6.5 R(E) from dusk to midnight where the partial ring current field has perturbed the magnetic field. The precipitating protons likewise contribute sporadically to the storm time Hall and Pedersen conductance in localized regions whereas the precipitating electrons are the dominate storm time contributor to enhanced Hall and Pedersen conductance at auroral magnetic latitudes on the night and morning side. The RCM-E model can reproduce general features of the Van Allen Probe/MagEIS observed trapped electron differential flux spectrograms over energies of ~37 to 150 keV. The simulations with a parameterized electron loss model also reproduce reasonably well the storm time Defense Meteorological Satellite Program integrated electron energy flux at 850 km at satellite crossings from predawn to midmorning. However, model-data agreement is not as good from dusk to premidnight where there are large uncertainties in the electron loss model.

Earth’s Magnetosphere-Ionosphere-Thermosphere system is inseparably coupled, with driving from above and below by various terrestrial and space weather phenomena. Global models have done well at capturing large-scale effects, but currently do not capture the meso-scale (~10s-500 km) phenomena which often are locally more intense. As computing power improves, and modeling meso-scales now becomes possible, it is vital to provide data-informed inputs of the relevant drivers. In this presentation, we focus on the energy flux deposited into the ionosphere from the magnetosphere by precipitating particles that result in the aurora, specifically at meso-scales, and the resulting conductance. Thanks to NASA’s THEMIS mission, an array of all-sky-imagers (ASIs) across Canada monitors the majority of the nightside auroral oval at a 3 second cadence, providing a global view at temporal & spatial resolutions required to study the aurora on meso-scales. Thus, we present 2-D maps over time of the energy flux, energy, and conductance that result from the aurora during solar storms and substorms, including those features at meso-scales. We determine conductance using the ASI-determined eflux and energy as inputs to the Boltzman Three Constituent (B3C) auroral transport code, compare values with Poker Flat ISR observations, and find a good comparison. We find that meso-scale aurora contributes at least 60-70% of the total precipitated energy flux during the first ten minutes of a substorm. Our results can be utilized by the broad community, for example, as inputs to atmospheric models or as the resulting conductance from precipitation inferred by magnetospheric models or satellite observations.

The Rocket Experiment for Neutral Upwelling 2 (RENU2) rocket was launched on 13 December 2015 at 07:34 UT. The payload transited the cusp region during a neutral upwelling event, supported by a comprehensive set of onboard and ground-based instrumentation. RENU2 data highlight two important processes. One is that a proper understanding of neutral upwelling by Poleward Moving Auroral Forms (PMAFs) requires a treatment that mimics the quasiperiodic passage of a sequence of PMAFs. As a PMAF reaches a flux tube, its physical consequences must be determined including the residual history of effects from previous passages, implying that understanding such a process requires an accounting of the system hysteresis. Second, RENU2 observations suggest that neutral density enhancements driven by precipitation and/or Joule heating can be highly structured in altitude and latitude. In addition, timescales involving neutral dynamics suggest that the structuring must be slowly changing, for example, over the course of 10 to tens of minutes. publishedVersion

It is important to routinely examine and update models used to predict auroral emissions resulting from precipitating electrons in Earth's magnetotail. These models are commonly used to invert spectral auroral ground-based images to infer characteristics about incident electron populations when in situ measurements are unavailable. In this work, we examine and compare auroral emission intensities predicted by three commonly used electron transport models using varying electron population characteristics. We then compare model predictions to same-volume in situ electron measurements and ground-based imaging to qualitatively examine modeling prediction error. Initial comparisons showed di erences in predictions by the GLobal airglOW (GLOW) model and the other transport models examined. Chemical reaction rates and radiative rates in GLOW were updated using recent publications and predictions showed better agreement with the other models and the samevolume data, stressing that these rates are important to consider when modeling auroral processes. Predictions by each model exhibit similar behavior for varying atmospheric constants, energies, and energy fluxes. Same-volume electron data and images are highly correlated with predictions by each model, showing that these models can be used to accurately derive electron characteristics and ionospheric parameters based solely on multispectral optical imaging data.

The prediction of tropical cyclone rapid intensification is one of the most pressing unsolved problems in hurricane forecasting. The signatures of gravity waves launched by strong convective updrafts are often clearly seen in airglow and carbon dioxide thermal emission spectra under favorable atmospheric conditions. By continuously monitoring the Atlantic hurricane belt from the main development region to the vulnerable sections of the continental United States at high cadence, it will be possible to investigate the utility of storm-induced gravity wave observations for the diagnosis of impending storm intensification. Such a capability would also enable significant improvements in our ability to characterize the 3D transient behavior of upper-atmospheric gravity waves and point the way to future observing strategies that could mitigate the risk to human life caused by severe storms. This paper describes a new mission concept involving a midinfrared imager hosted aboard a geostationary satellite positioned at approximately 80°W longitude. The sensor’s 3-km pixel size ensures that the gravity wave horizontal structure is adequately resolved, while a 30-s refresh rate enables improved definition of the dynamic intensification process. In this way the transient development of gravity wave perturbations caused by both convective and cyclonic storms may be discerned in near–real time.

We present three case studies that examine optical and radar methods for specifying precipitating auroral flux parameters and conductances. Three events were chosen corresponding to moderate nonsubstorm auroral activity with 557.7 nm intensities greater than 1kR. A technique that directly fits the electron number density from a forward electron transport model to alternating code incoherent scatter radar data is presented. A method for determining characteristic energy using neutral temperature observations is compared against estimates from the incoherent scatter radar. These techniques are focused on line-of-sight observations that are aligned with the local geomagnetic field. Good agreement is found between the optical and incoherent scatter radar methods for estimates of the average energy, energy flux, and conductances. The Pedersen conductance predicted by Robinson et al. (1987) is in very good agreement with estimates calculated from the incoherent scatter radar observations. However, we present an updated form of the relation by Robinson et al. (1987), ΣH/ΣP=0.57〈E〉0.53, which was found to be more consistent with the incoherent scatter radar observations. These results are limited to similar auroral configurations as in these case studies. Case studies are presented that quantify auroral electron flux parameters and conductance estimates which can be used to specify the magnitude of energy dissipated within the ionosphere resulting from magnetospheric driving.

The Southern Hemisphere summer 2 day wave (TDW) is the most dramatic large-scale event of the upper mesosphere. The winds accelerate over ~1 week, may attain > 70 m/s, and are often accompanied by a near disappearance of the diurnal tide and stabilization of the period close to 48 h. We denote this as the phase-locked 2 day wave (PL/TDW). We have examined airglow and meteor radar (MR) wind data from the Andes Lidar Observatory (Cerro Pachon, Chile:30°S, 289.3°E), MR data from Darwin (12.5°S, 131°E) and airglow and medium frequency radar data from the University of Adelaide (34.7°S, 138.6°E) for the behavior of the TDW during the austral summers of 2010, 2012, and 2013. The Cerro Pachon and Adelaide sites are located at similar latitudes separated in longitude by about 120°. We find a remarkable coincidence between the TDW oscillations at Chile and Adelaide for the period January–February 2010. The oscillations are nearly in phase in terms of local time and the minima and maxima repeat at nearly the same local time from cycle to cycle consistent with a phase-locked wave number 3 TDW. Data for this and other years (including Darwin) show that the amplitude of the diurnal tide decreases when the TDW is largest and that this occurs when the period is close to 48 h. These observations support the proposal that the PL/TDW is a subharmonic parametric instability wherein the diurnal tide transfers energy to a TDW that is resonant at nearly 48 h.

Observations of molecular oxygen are difficult to make in the Earth's atmosphere between 140 and 200 km altitude. Perhaps the most accurate measurements to date have been obtained from satellite instruments that measure solar occultations of the limb. These do provide height-resolved O2 density measurements, but the nature of this technique is such that the temporal/spatial distribution of the measurements is uneven. Here a new space-based technique is described that utilizes two bright dayglow emissions, the (0,0) transition of the O2 atmospheric band and the O I (630 nm), to derive the height-resolved O2 density from 140 to 180 km. Data from the Remote Atmospheric and Ionospheric Detection System, which was placed on the International Space Station in late 2009, are used to illustrate this technique. The O2 density results for periods in May 2010 that were geomagnetically quiet and disturbed are compared to model predictions.

The Aerospace Corporation's Nightglow Imager (ANI) observes nighttime OH emission (near 1.6 μm) every 2 s over an approximate 73° field of view. ANI had previously been used to study instability features seen over Maui. Here we describe observations of instabilities seen from 5 to 8 UT on 24 March 2012 over Cerro Pachon, Chile, and compare them with previous results from Maui, with theory, and with Direct Numerical Simulations (DNS). The atmosphere had reduced stability because of the large negative temperature gradients measured by a Na lidar. Thus, regions of dynamical and convective instabilities are expected to form, depending on the value of the Richardson number. Bright primary instabilities are formed with a horizontal wavelength near 9 km and showed the subsequent formation of secondary instabilities, rarely seen over Maui, consistent with the primaries being dynamical instabilities. The ratio of the primary to secondary horizontal wavelength was greater over Chile than over Maui. After dissipation of the instabilities, smaller-scale features appeared with sizes in the buoyancy subrange between 1.5 and 6 km. Their size spectra were consistent with the model of Weinstock (1978) if the turbulence is considered to be increasing. The DNS results produce secondary instabilities with sizes comparable to what is seen in the images although their spectra are somewhat steeper than is observed. However, the DNS results also show that after the complete decay of the primary features, scale sizes considerably smaller than 1 km are produced and these cannot be seen by the ANI instrument.

A companion paper describes high-resolution, ground-based imaging of apparent Kelvin-Helmholtz instabilities (KHI) observed in OH airglow at ~87 km over the Andes Lidar Observatory at 30°S. Here we employ direct numerical simulations (DNSs) and large eddy simulations (LESs) of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers of Re ~2500 to 10,000 to illustrate the dependence of primary and secondary KHI on these quantities for the purpose of quantifying KHI dynamics observed by ground-based airglow imagers. Our DNS and LES reveal significant variations of both primary and secondary KHI scales and amplitudes with varying Ri and Re. Lower Ri and higher Re yield stronger and deeper initial 2-D KH billows. Low Re for a given Ri either yield larger-scale 3-D secondary instabilities or suppress them altogether. Secondary instability scales decrease as Re increases for a given Ri. Corresponding variations in implied KHI airglow signatures include (1) stronger airglow intensity variations for larger KHI wavelengths and depths (higher Ri), (2) stronger 2-D and 3-D responses for the initial KHI shear layer displaced somewhat from the airglow layer, and (3) stronger 3-D responses for the lower Ri and intermediate Re yielding the larger secondary instability scales.

[1] The commonly used criteria for shear and convective instabilities were developed for steady horizontally uniform background flows. However, the formalism that rigorously addresses the instability of waves on a basic state modulated by a primary wave is Floquet theory in which the basic state includes a wave. A Floquet system supports parametric instabilities when conventional Richardson number (Ri) criteria indicate that the system is stable. In a study of small-scale instability structures during the Maui MALT campaign, Hecht et al. (2005) noted that there were occurrences of ripple (instability) structure when the conventional criteria indicated stable conditions. We have followed up this work with a detailed survey of the occurrence of ripple structure over Maui during periods that were both stable and unstable according to conventional criteria. Values of Ri were calculated from lidar data. We have found frequent occurrence of ripple structure when Ri > 14. We have focused on a period when there are clear indications of waves and ripple structure exhibiting two-dimensional instability structure when Ri ~ 1 or greater. These results are analyzed in terms of Floquet theory and interpreted as parametric instabilities occurring for modest primary wave amplitudes.

[1] Thermospheric temperatures derived from limb observations of the O2 A-Band (0,0) emission spectrum obtained from January–July 2010, with the Remote Atmospheric and Ionospheric Detection System (RAIDS) Near Infrared Spectrometer (NIRS) aboard the International Space Station, are compared to temperature results from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) experiment. To account for a lack of simultaneous common volume observations, the observed temperatures were scaled by the NRLMSIS-00 model temperatures for comparison. It was found that on average SABER, temperatures are warmer than NIRS at all altitudes between 90 and 140 km. In the altitude range 90–100 km, the SABER temperatures were warmer than NIRS by ~10 K consistent with previous validation experiments and in agreement with Optical Spectrograph and Infrared Imaging System (OSIRIS) O2 A-band comparisons in the polar mesopause region. At higher altitudes, the differences between SABER and NIRS exceed 30 K on average. Thus, the NIRS observations reinforce the idea that the SABER temperatures are too warm below ~110 km; and above that altitude, they are increasingly in error consistent with expectations based on estimated inaccuracies in the retrieval algorithm. Large standard deviations of the SABER and NIRS ratios are reflective of substantial variability of the thermospheric temperatures throughout the region.

Wavelike features suggesting gravity waves were revealed by lidar observations (from El Segundo, California) of smoke layers produced by large wildfires in the Southern California region during a Santa Ana event. Unique features of the observations were multiple thin layers that enabled precise determinations of wave frequency, amplitude, and vertical structure. The data revealed persistent wavelike oscillations that showed no phase variation with altitude, an amplitude of 20 m, and a period near 12 min. The observations were averaged over 1.5 min with a vertical resolution of 3 m and were obtained over a period of 5 h on 25 October and 8 h on 26 October 2007. Vertical profiles of temperature and winds from the Aircraft Communication Addressing and Reporting System (ACARS) of commercial aircraft departing Los Angeles International Airport (LAX) were selected for temporal and spatial coincidence with the lidar observations. In addition, satellite images of the smoke distribution over the Los Angeles Basin and the coastal areas (including coastal waters) to the south were obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) data from overflights 5.5 h prior to the lidar observation window. The images show wavelike features with horizontal wavelengths of ∼10 km or less. The temperature data showed an inversion layer topped at 500-m altitude. The wind data were consistent with a residual sea breeze near the surface and Santa Ana easterlies above. A simple model of wave ducting showed that the observed features were the evanescent extension of resonant waves ducted in the lower stable inversion layer. These are the first detailed observations, including vertical structure, of ducted waves associated with Santa Ana conditions. It is suggested that such waves should be a common feature in conditions occurring during Santa Ana events.

The ability to accurately model precipitating electron distributions is crucial for understanding magnetosphere-ionosphere-thermosphere coupling processes. We use the magnetically and electrically self-consistent Rice Convection Model-Equilibrium (RCM-E) of the inner magnetosphere to assess how well different electron loss models can account for observed electron fluxes during the large 10 August 2000 magnetic storm. The strong pitch angle scattering rate produces excessive loss on the morning and dayside at geosynchronous orbit (GEO) compared to what is observed by a Los Alamos National Laboratory satellite. RCM-E simulations with parameterized scattering due to whistler chorus outside the plasmasphere and hiss inside the plasmasphere are able to account simultaneously for trapped electron fluxes at 1.2 keV to ~100 keV observed at GEO and for precipitating electron fluxes and electron characteristic energies in the ionosphere at 833 km measured by the NOAA 15 satellite.

Passive ground-based optical techniques have been used for over 15 years to study the effects of auroral energy deposition on the composition of the lower thermosphere. These techniques typically involve measuring bright auroral emissions that depend on the number density of atomic oxygen [O] and molecular nitrogen [N 2 ] as well as the energy flux and energy spectrum of the precipitating auroral particles. Here we review the application of these techniques to the composition problem. In particular, we present data obtained from photometric observations in Alaska and Greenland in the 1980s and 1990s. While some of these results are available in the existing literature, some are new and have not been presented before. Recent results from Alaska obtained in support of the TIMED satellite are also presented. The data as a whole suggest that the ratio of column integrated [O] to [N 2 ] above a given site decreases as the total energy fluence from both auroral particle and Joule heating increases. However, the dependence is complicated by effects such as the prior history of disturbance and by horizontal transport due to neutral winds; it is not yet determined if composition responds linearly to energy fluence even in an average sense.

The rarefied, three-dimensional flows experienced during the turbulent oxygen mixing experiment (TOMEX) at altitudes between 85 and 143 km are simulated using the direct simulation Monte Carlo (DSMC) method. The present study focuses on ionization gauge measurements obtained by TOMEX. The payload is, thus, modeled in detail, and the simulations employ complex meshes. The simulations show that a bow shock wave is generated in front of the payload at low altitude that becomes diffusive at higher altitudes. When the altitude increases, the pressure in the channels of the ionization gauge and the pressure variation around the payload are both decreased. The DSMC results agree very well with data predicted by compressible flow theory and free molecular theory when applicable. Comparison between the DSMC results and the TOMEX flight data shows generally good agreement.

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.

Recently, it was shown from a single set of airglow/lidar observations in Urbana, Illinois (40N) that some small-scale wave-like structure seen in OH airglow images can be associated with the breakdown, via a convective instability, of an atmospheric gravity wave. A second set of simultaneous airglow/lidar observations, showing wave breakdown, has been obtained over Kuhlungsborn, Germany (54N) during a period when noctilucent clouds (NLCs) were also observed. This showed that the wave breakdown process can occur under the same cold, low altitude summer mesopause conditions that support the occurrence of NLCs. This new set of data shows that the first observations were not a unique event, and give credence to the suggestion that some of the small scale structure seen in NLC displays may be due to wave breakdown.

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.

On February 20 1998 two rockets were launched from Puerto Rico as part of the Coqui Dos campaign. Data from on-board photometers and ground-based data from a Na lidar and an airglow imager allowed an estimate to be made, using both the TIME-GCM and MSIS models, of the branching coefficient α for the Na airglow emission. Assuming the hydrogen density is within the range predicted by the two models the data are consistent with α being less than 0.05. This value is well below a recent theoretical estimate of 0.67 and also below the nominally accepted value of 0.1 derived from previous aeronomic data.

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.

Observations of wave-driven fluctuations in emissions from the OH Meinel (OHM) and O2 Atmospheric band were made with a narrow-band airglow imager located at Adelaide, Australia (35S, 138E) during the period April 1995 to January 1996. Simultaneous wind measurements in the 80–100 km region were made with a co-located MF radar. The directionality of quasi-monochromatic (QM) waves in the mesopause region is found to be highly anisotropic, especially during the solstices. During the summer, small-scale QM waves in the airglow are predominately poleward propagating, while during winter they are predominately equatorward. The directionality inferred from a Stokes analysis applied to the radar data also indicates a strong N–S anisotropy in summer and winter, but whether propagation is from the north or south cannot be determined from the analysis. The directionality of the total wave field (which contains incoherent as well as coherent features) derived from a spectral analysis of the images shows a strong E–W component, whereas, an E–W component is essentially absent for QM waves. The prevalence of QM waves is also strongly seasonally dependent. The prevalence is greatest in the summer and the least in winter and correlates with the height of the mesopause; whether it is above or below the airglow layers. The height of the mesopause is significant because for nominal thermal structures it is associated with a steep gradient in the Brunt-Vaisala frequency that causes the base of a lower thermospheric thermal duct to be located in the vicinity of the mesopause. We interpret the QM waves as waves trapped in the lower thermosphere thermal duct or between the ground and the layer of evanescence above the duct. Zonal winds can deplete the thermal duct by limiting access to the duct or by negating the thermal trapping. Radar measurements of the prevailing zonal wind are consistent with depletion of zonally propagating waves. During winter, meridional winds in the upper mesophere and lower thermosphere are weak and have no significant effect on meridionally propagating waves. However, during summer the winds in the duct region can significantly enhance ducting of southward propagating waves. The observed directionality of the waves can be explained in terms of the prevailing wind at mesopause altitudes and the seasonal variation of distant sources.

We obtained three high-quality ultraviolet spectra of the hot R Coronae Borealis type star V348 Sgr using the Faint Object Spectrograph of the Hubble Space Telescope (HST). The first of these spectra was obtained when the star was in a shallow minimum (ΔV less than 1 mag), whereas the other two were obtained during a deep minimum (ΔV ≈ 6 mag). While strong carbon and helium emission lines are seen in the shallow minimum, during the deep minimum these lines are apparently eclipsed by a dust cloud. In the first deep minimum spectrum a few weak nebular emission lines are present, but in the second (a month later) these lines have mostly disappeared. This is consistent with a progressive eclipse caused by an expanding dust cloud. Comparison with International Ultraviolet Explorer (IUE) spectra obtained at maximum light allowed three extinction curves to be calculated for the newly formed dust. The extinction curves obtained using the spectra at deep minimum are quite different from those obtained using spectra at shallow minima. Extinction curves from shallow minima (which have been previously calculated using IUE data) show stellar flux attenuated by amorphous carbon dust in front of the star at wavelengths above 2000 A, while at wavelengths below 2000 A they show contributions from light scattered by dust outside the line of sight. During the deep minimum, however, nearly all of the light seen in the ultraviolet below 3200 A is due to scattering into the field of view. The data are consistent with (1) an optically thick cloud of amorphous carbon dust in front of the star and (2) either a few additional optically thick clouds that are outside a direct line from the star to the earth, or an optically thin cloud that surrounds a small fraction of the star and scatters light into the HST field of view. The data taken together are consistent with the dust forming close to the star.

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.

From April, 1995 to January, 1996 a nightglow imager and an airglow photometer were colocated near Adelaide, Australia. The data obtained on more than 50 clear nights revealed seasonal changes in the airglow intensities and temperatures as well as in the gravity wave activity. These temperature data are the first seasonal results from the mid-latitude southern hemisphere mesopause region. The OH Meinel band was observed to have a rotational temperature that was warmer than the O2 Atmospheric band in the winter. There were also summer solstice maxima and winter solstice minima in the O2 Atmospheric band and OI(557.7' airglow intensities. The gravity wave activity, seen in the 50 to 80 km horizontal wavelength waves, was generally greatest in the OH Meinel layer and showed a semiannual variation with a strong summer solstice maximum. The relationship between gravity wave activity and airglow intensities disagrees somewhat with models. Compared to previous studies, the data suggest that there may be a difference in the seasonal variability of short and long period gravity waves.

New International Ultraviolet Explorer (IUE) spectra with high signal-to-noise ratios have been obtained during the recovery of the hot R Coronae Borealis star, V348 Sgr, from its 1992 minimum. These spectra have been combined with spectra from the IUE archives to obtain a definitive extinction curve for dust recently ejected from this star, and make available for the first time an extinction curve of high accuracy in the far-UV for circumstellar dust formed in a hydrogen-poor environment. We find that a Drude profile with a central wavelength of 2425 A and a full width at half-maximum of 1.342 μm-1 is able to satisfactorily represent the circumstellar dust ejected from V348 Sgr, not just during one ejection episode, but in general. We calculated theoretical extinction curves for comparison with the observed data using 400 A graphite grains and 300 A amorphous carbon grains. It is clear that scattering is very important at short wavelengths. Neither type of grain works very well when scattering is not included. The amorphous carbon fits are somewhat better than the graphite fits. The data seem to be more consistent with a cloud covering the entire star, and not just lying in our line of sight. This contradicts other empirical evidence that points toward the formation of patchy clouds around R Coronae Borealis stars.

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.

The He(23S) triplet at 10830 A is observed using a Fabry-Perot interferometer optimized for the near infrared. These observations, made at the zenith at the Millstone Hill Optical Facility during evening twilight, are the first to resolve the individual members of the helium triplet from the surrounding OH(5-2) lines. The introduction of an aeronomically unique germanium detector Noto et al., [1993] was crucial to this achievement. The instrument is configured with a spectral resolution of 1/2 the helium line width at 1000°K, assuring that the line widths accurately reflect the velocity distribution of metastable helium. Although the intensities observed are within a factor of two of helium 10830 A intensities observed by [Tinsley, 1968, Tinsley & Christensen et. al, 1971], the observed temperatures are different from the expected behavior of a neutral species in the upper-thermosphere during twilight.

[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.