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Two winters of nightglow intensity observations of the atomic oxygen 630-nm redline emission at Bahir Dar ( 11.6 ° N , 37.4 ° E ) were collected from measurements between November 2015 and March 2017 by the selection of 89 nights of quality data. The solar activity trend during the 2015–2017 period was decreasing with the solar flux activity varying within the range of 70 and 140 sfu ( 1 SFU = 10 - 22 W m - 2 Hz - 1 ). The results have shown that for the first winter (2015–2016) for quiet geomagnetic activity and with the solar flux near the higher end of this range, the 630-nm intensity decreased significantly during the post-sunset period of 3–4 h. Also of note was that this trend was generally followed after midnight by a significant short-lived 630-nm brightness enhancement (fivefold). In the subsequent year of low solar fluxes, a similar significant decrease in the 630-nm intensity is seen starting from the early evening but for the remainder of the night the intensity remained generally constant with no significant sign of the post-midnight 630-nm enhancement of the previous year. During disturbed times the study of the temporal variations of the 630-nm intensity found a double peak structure that persisted beyond one day.

The thermospheric winds play an important role in the vertical and horizontal couplings of the upper atmosphere by modulating neutral and plasma dynamics. A large variety of observation techniques and numerical as well as empirical models have been developed to understand the behavior of thermospheric winds. The Fabry-Perot interferometer (FPI) is a widely used ground- and satellite-based optical instrument for the thermospheric winds observations in the upper atmosphere. Due to solar contamination of the fainter airglow emission during the daytime, most of the ground-based interferometric wind measurements are limited to the nighttime period only. Despite these constraints, the Second‐generation, Optimized, Fabry‐Perot Doppler Imager (SOFDI) is designed for both daytime and nighttime measurements of thermospheric winds from OI 630‐nm emission and is currently operating at the Huancayo, Peru, near the geomagnetic equator. In this study, we present a comparative analysis of the observed SOFDI wind climatological data and several other modeled results including, but not limited to, Horizontal Wind Model 2014 (HWM-14), Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model with and without implementing Prompt Penetration Electric Field (PPEF), Whole Atmosphere Model (WAM), SAMI3 model, and Magnetic mEridional NeuTrAl Thermospheric (MENTAT) model. We examine the relative performances of these models in the context of the direct-measured thermospheric winds. The day and nighttime modeled winds show an excellent agreement with the SOFDI wind data at the equatorial latitude, except for the daytime zonal winds. Further, this analysis gives a comprehensive picture of how well the measured winds provided by the SOFDI instrument and various models represent the features of the equatorial thermosphere. We also investigate and give an overview of the sources, drivers, effects, and possible mechanisms of the wind variability in the low-latitude thermosphere.

Periodic waves were observed in the OI6300 airglow images over São João do Cariri (36.5∘ W, 7.4∘ S) from 2012 to 2014 with simultaneous observations of the thermospheric wind using two Fabry–Pérot interferometers (FPIs). The FPIs measurements were carried out at São João do Cariri and Cajazeiras (38.5∘ W, 6.9∘ S). The observed spectral characteristics of these waves (period and wavelength) as well the propagation direction were estimated using two-dimensional Fourier analysis in the airglow images. The horizontal thermospheric wind was calculated from the Doppler shift of the OI6300 data extracted from interference fringes registered by the FPIs. Combining these two techniques, the intrinsic parameters of the periodic waves were estimated and analyzed. The spectral parameters of the periodic waves were quite similar to the previous observations at São João do Cariri. The intrinsic periods for most of the waves were shorter than the observed periods, as a consequence, the intrinsic phase speeds were faster compared to the observed phase speeds. As a consequence, these waves can easily propagate into the thermosphere–ionosphere since the fast gravity waves can skip turning and critical levels. The strength and direction of the wind vector in the thermosphere must be the main cause for the observed anisotropy in the propagation direction of the periodic waves, even if the sources of these waves are assumed to be isotropic. Keywords. Meteorology and atmospheric dynamics (waves and tides)

The midnight temperature maximum (MTM) has been observed in the lower thermosphere by two Fabry–Pérot interferometers (FPIs) at São João do Cariri (7.4° S, 36.5° W) and Cajazeiras (6.9° S, 38.6° W) during 2011, when the solar activity was moderate and the solar flux was between 90 and 155 SFU (1 SFU = 10−22 W m−2 Hz−1). The MTM is studied in detail using measurements of neutral temperature, wind and airglow relative intensity of OI630.0 nm (referred to as OI6300), and ionospheric parameters, such as virtual height (h′F), the peak height of the F2 region (hmF2), and critical frequency of the F region (foF2), which were measured by a Digisonde instrument (DPS) at Eusébio (3.9° S, 38.4° W; geomagnetic coordinates 7.31° S, 32.40° E for 2011). The MTM peak was observed mostly along the year, except in May, June, and August. The amplitudes of the MTM varied from 64 ± 46 K in April up to 144 ± 48 K in October. The monthly temperature average showed a phase shift in the MTM peak around 0.25 h in September to 2.5 h in December before midnight. On the other hand, in February, March, and April the MTM peak occurred around midnight. International Reference Ionosphere 2012 (IRI-2012) model was compared to the neutral temperature observations and the IRI-2012 model failed in reproducing the MTM peaks. The zonal component of neutral wind flowed eastward the whole night; regardless of the month and the magnitude of the zonal wind, it was typically within the range of 50 to 150 m s−1 during the early evening. The meridional component of the neutral wind changed its direction over the months: from November to February, the meridional wind in the early evening flowed equatorward with a magnitude between 25 and 100 m s−1; in contrast, during the winter months, the meridional wind flowed to the pole within the range of 0 to −50 m s−1. Our results indicate that the reversal (changes in equator to poleward flow) or abatement of the meridional winds is an important factor in the MTM generation. From February to April and from September to December, the h′F and the hmF2 showed an increase around 18:00–20:00 LT within a range between 300 and 550 km and reached a minimal height of about 200–300 km close to midnight; then the layer rose again by about 40 km or, sometimes, remained at constant height. Furthermore, during the winter months, the h′F and hmF2 showed a different behavior; the signature of the pre-reversal enhancement did not appear as in other months and the heights did not exceed 260 and 350 km. Our observation indicated that the midnight collapse of the F region was a consequence of the MTM in the meridional wind that was reflected in the height of the F region. Lastly, the behavior of the OI6300 showed, from February to April and from September to December, an increase in intensity around midnight or 1 h before, which was associated with the MTM, whereas, from May to August, the relative intensity was more intense in the early evening and decayed during the night.

Fabry–Perot interferometer (FPI) measurements of thermospheric temperatures and winds show the detection and successful determination of the latitudinal distribution of the midnight temperature maximum (MTM) in the continental mid-eastern United States. These results were obtained through the operation of the five FPI observatories in the North American Thermosphere Ionosphere Observing Network (NATION) located at the Pisgah Astronomic Research Institute (PAR) (35.2∘ N, 82.8∘ W), Virginia Tech (VTI) (37.2∘ N, 80.4∘ W), Eastern Kentucky University (EKU) (37.8∘ N, 84.3∘ W), Urbana-Champaign (UAO) (40.2∘ N, 88.2∘ W), and Ann Arbor (ANN) (42.3∘ N, 83.8∘ W). A new approach for analyzing the MTM phenomenon is developed, which features the combination of a method of harmonic thermal background removal followed by a 2-D inversion algorithm to generate sequential 2-D temperature residual maps at 30 min intervals. The simultaneous study of the temperature data from these FPI stations represents a novel analysis of the MTM and its large-scale latitudinal and longitudinal structure. The major finding in examining these maps is the frequent detection of a secondary MTM peak occurring during the early evening hours, nearly 4.5 h prior to the timing of the primary MTM peak that generally appears after midnight. The analysis of these observations shows a strong night-to-night variability for this double-peaked MTM structure. A statistical study of the behavior of the MTM events was carried out to determine the extent of this variability with regard to the seasonal and latitudinal dependence. The results show the presence of the MTM peak(s) in 106 out of the 472 determinable nights (when the MTM presence, or lack thereof, can be determined with certainty in the data set) selected for analysis (22 %) out of the total of 846 nights available. The MTM feature is seen to appear slightly more often during the summer (27 %), followed by fall (22 %), winter (20 %), and spring (18 %). Also seen is a northwestward propagation of the MTM signature with a latitude-dependent amplitude. This behavior suggests either a latitudinal dependence of thermosphere tidal dissipation or a night-to-night variation of the composition of the higher-order tidal modes that contribute to the production of the MTM peak at mid-latitudes. Also presented in this paper is the perturbation on the divergence of the wind fields, which is associated with the passage of each MTM peak analyzed with the 2-D interpolation. Keywords. Ionosphere (mid-latitude ionosphere)

We present the first ever cross comparisons of F region horizontal neutral wind measurements taken using two different types of optical Doppler spectrometer: all-sky scanning Doppler imagers (SDI) and narrow-field Fabry-Perot interferometers (NFPIs). Horizontal neutral winds were inferred using bistatic observations from three NFPIs, together with monostatic and bistatic observations from two SDIs. All instruments were located in Alaska. Cross comparisons were made for a total of seven nights in January and February 2010. The results show a high degree of correlation between the diurnal behaviors of the line-of-sight (LOS) winds measured by both instruments. The SDI and NFPI LOS wind time series also often contained high-frequency fluctuations with similar overall characteristics, strongly suggesting that these fluctuations were geophysical in origin. However, the amplitude of the high-frequency component was stronger in the NFPI LOS wind than in the SDI data. Even the smallest SDI angular resolution element is much larger than the NFPI field of view, suggesting that its relative insensitivity to high frequencies is because these fluctuations are associated with local-scale structures whose spatial extent is smaller than ∼40 km spanned by the smallest SDI viewing field. Upon fitting vectors to the LOS wind data, close agreement was found between the wind components estimated by the two types of instrument. Discrepancies that did arise occurred most often when the neutral wind speed was weak, suggesting that conditions capable of driving higher neutral wind speeds also suppressed the development of small-scale structures in the thermospheric neutral wind fields.

We present a climatology of quiet time thermospheric winds and temperatures estimated from high-resolution Fabry-Perot interferometer measurements of the 630.0 nm airglow emission spectral line shape. Three locations are examined in this long-term study: northeastern Brazil (August 2009 to August 2014), a midlatitude site in North Carolina, USA (June 2011 to December 2014), and a midlatitude site in Morocco (November 2013 to December 2014). We discuss the day-to-day, seasonal, and solar cycle trends and variations of thermospheric meridional winds, zonal winds, neutral temperatures, and for the first time vertical winds. Observations made from solar minimum to solar maximum (with F10.7 values ranging from ∼70 to ∼159 solar flux units) confirm that neutral temperatures have a strong solar cycle dependence. However, this data set shows that the neutral winds are more closely tied to the seasonal variation, rather than the solar cycle. We also present comparisons between the two midlatitude sites and include neutral wind comparisons to the updated Horizontal Wind Model 14.

United States. National Aeronautics and Space Administration (Living with a Star NNX15AB83G)

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

We introduce a technique for estimating the regional thermospheric wind field from measurements made by a network of interferometers. Unlike previous work, this technique does not make assumptions about the functional form of the wind field and instead uses inverse theory to find the smoothest wind field that agrees with the measurements. This technique is general and applies to any network making radial velocity measurements. We show reconstructions of the thermospheric wind field over the eastern United States and over eastern Brazil, using data from two distinct networks of Fabry-Perot interferometers measuring the Doppler shift of the 630.0 nm airglow emission. In Brazil, we find direct evidence of a convergent wind field during the period of rapid thermospheric temperature increase associated with the equatorial midnight temperature maximum.

An ongoing effort to simulate plasma instability in the equatorial ionosphere leading to equatorial spread F (ESF) in the American sector is described. Ionospheric state parameters including plasma number density and vector drift velocity profiles were measured at the Jicamarca Radio Observatory in the period between 20 September and 3 October 2013. Coherent radar backscatter from plasma irregularities was recorded simultaneously, and images of the irregularities were calculated using aperture synthesis methods. Neutral winds were measured by the red line Fabry-Perot interferometers at Jicamarca and Arequipa, Peru. A fully 3-D numerical simulation of ionospheric irregularities, initialized and forced using parameterizations derived from measurements and empirical models, was used to reproduce the ESF activity observed. Simulations were able to recover many of the features of the irregularities, although some important anomalies can be noted. ESF events in which the first appearance of radar plumes occurred either very early or very late were not reproduced in simulation and may be indicative of nonlocal influence.

Observations of thermospheric neutral winds and temperatures obtained during a geomagnetic storm on 2 October 2013 from a network of six Fabry-Perot interferometers (FPIs) deployed in the Midwest United States are presented. Coincident with the commencement of the storm, the apparent horizontal wind is observed to surge westward and southward (toward the equator). Simultaneous to this surge in the apparent horizontal winds, an apparent downward wind of approximately 100 m/s lasting for 6 h is observed. The apparent neutral temperature is observed to increase by approximately 400 K over all of the sites. Observations from an all-sky imaging system operated at the Millstone Hill observatory indicate the presence of a stable auroral red (SAR) arc and diffuse red aurora during this time. We suggest that the large sustained apparent downward winds arise from contamination of the spectral profile of the nominal thermospheric 630.0 nm emission by 630.0 nm emission from a different (nonthermospheric) source. Modeling demonstrates that the effect of an additional population of 630.0 nm photons, with a distinct velocity and temperature distribution, introduces an apparent Doppler shift when the combined emissions from the two sources are analyzed as a single population. Thus, the apparent Doppler shifts should not be interpreted as the bulk motion of the thermosphere, calling into question results from previous FPI studies of midlatitude storm time thermospheric winds. One possible source of contamination could be fast O related to the infusion of low-energy O+ ions from the magnetosphere. The presence of low-energy O+ is supported by observations made by the Helium, Oxygen, Proton, and Electron spectrometer instruments on the twin Van Allen Probes spacecraft, which show an influx of low-energy ions during this period. These results emphasize the importance of distributed networks of instruments in understanding the complex dynamics that occur in the upper atmosphere during disturbed conditions.

We present measurements of thermospheric neutral winds and temperatures measured by two Fabry–Perot interferometers (FPIs) operating in northeastern Brazil, in the equatorial region, over the period of September 2009 through June 2012. The FPIs observe the Doppler shift and broadening of the nighttime 630.0-nm emission originating at altitudes of approximately 240 km. The temperatures indicate a strong dependence on solar flux conditions, as captured by the F 10.7 index, which ranged from ∼72 to 175 solar flux units over the duration of the study. The signature of the midnight temperature maximum is seen in the data, recurring each year during the local summer months (October through February). Throughout the year, the thermospheric zonal winds show an initial increase in eastward flow after sunset followed by a reduction in the midnight and early morning hours. This reduction in the zonal flow is most rapid in the local summer months. The thermospheric meridional winds show the expected signature of trans-equatorial flow from the summer to winter hemisphere. Superposed on this during the local summer months is a time of equatorward flow during the mid-evening period, 22–23 LT, which is likely a manifestation of the semi-diurnal thermal tidal wave moving upward into the lower thermosphere region from below. The thermospheric neutral winds do not show as much of a dependence on the solar flux as the thermospheric neutral temperatures do, at least for the range of fluxes covered during the study period.

[1] Thermospheric density and neutral velocity perturbations associated with three magnetic storms in the autumn season of 2011 are examined using data from the neutral wind meter (NWM) on the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The data from perigee passes near 400 km altitude show marked increases in neutral density during the storms and associated increases in horizontal neutral flow speeds. These thermospheric responses are characterized by enhanced meridional neutral flows with peak perturbation amplitudes near 100 m/s and relative neutral density enhancements ranging from 50–100%. The increases in the neutral density and meridional flow velocity at equatorial latitudes occur about 5–7 h after the initial perturbations are observed in the z component of the interplanetary magnetic field (IMF), and they persist for 20–30 h. The perturbations in the neutral density are in good agreement with temperature increases predicted by an empirical model that has been validated using data from the CHAMP and Gravity Recovery and Climate Experiment missions, with a maximum lag time of ~1–1.5 h between the model temperature increases and the observed density perturbations. The model temperatures are in excellent agreement with ground-based low-latitude temperature measurements during the storms. Ground-based wind measurements during one of the storms provide additional data for comparison with the perturbation wind amplitudes measured aboard the satellite.

[1] A one-year dataset spanning March 2011 to March 2012 of coincident observations of nighttime thermospheric zonal neutral winds, equatorial plasma bubble (EPB) velocities, and zonal plasma drifts is used to examine the relationship between the thermosphere and the ionosphere near the geomagnetic equator over Peru. Thermospheric neutral winds are determined by using a bistatic Fabry-Perot interferometer (FPI) experiment located at Merihill and Nazca in Peru. The ambient plasma drift velocities were obtained using the incoherent scatter radar at the Jicamarca Radio Observatory in Peru. The EPB zonal velocities were estimated utilizing images of the OI 630.0 nm emission recorded by a narrow-field optical imaging system at the Cerro Tololo Inter-American Observatory in Chile. The joint analysis of these datasets illustrates that the nighttime and night-to-night variations in the zonal neutral winds, EPB velocities, and plasma drifts are well correlated. This consistent result of the local time variations of the neutral winds with that of EPB and plasma drifts illustrates that the F-region dynamo is, in general, fully activated. However, at times, the magnitude of the EPB velocities and the plasma drifts are different from the neutral winds. It is plausible that such a difference is due either to the effect of polarization electric fields developed inside the EPB or due to the latitudinal gradient of the neutral winds and EPB velocity measurements since the EPB velocities are estimated at a higher latitude, corresponding to an apex altitude of ~400 km, than the wind estimates, which derive from an apex altitude of ~250 km.

The thermospheric wind is a critical geophysical parameter for understanding the behavior of the Earth's upper atmosphere. Global-scale characterization of this parameter is needed to enable improved specification and forecasting of the near space environment. Global-scale measurements of horizontal wind vectors versus altitude have been performed from satellites using a variety of techniques, but the available data are still sparse. To address some of the challenges presented by space-based thermospheric wind measurement, the Doppler Asymmetric Spatial Heterodyne (DASH) technique has recently been developed. Here we present results of a ground-based validation of the DASH technique. The successful validation was performed by conducting collocated ground-based measurements with an instrument that uses the well established Fabry–Perot interferometer technique. Due to cloud cover and a limited observation period, data for only one night of simultaneous observations with minor cloud interference were obtained. The wind velocities observed by the two techniques show good overall agreement, but differences larger than the combined uncertainties are present at times. Contributions to these larger disagreements could be due to cloud interference, the minor differences in the observation geometry, or a non-zero vertical wind. A comparison of this single night of data with the Horizontal Wind Model (HWM07) climatology shows differences of up to about 100 m/s on timescales of less than an hour to several hours.

In this paper we present both night and day thermospheric wind observations made with the Second-generation, Optimized, Fabry-Perot Doppler Imager (SOFDI), a novel triple-etalon Fabry-Perot interferometer (FPI) designed to make 24-h measurements of thermospheric winds from OI 630-nm emission. These results were obtained from the northeastern United States and from under the magnetic equator at Huancayo, Peru and demonstrate the current instrument capability for measurements of Doppler shifts for either night or day. We found the uncertainties in the measurements agree with expected values based upon forward modeling calculations; nighttime wind components having an uncertainty of ~20-m s−1 at 30-min resolution and daytime wind components having an uncertainty of ~70-m s−1 at 20-min resolution. The nighttime uncertainties are typically larger than those seen with traditional single-etalon FPIs, which occur at the cost of being able to achieve daytime measurements. The thermospheric wind measurements from Huancayo replicate recently reported CHAMP zonal winds and are in disagreement with current empirical wind climatologies. In addition, we discuss the incorporation of how multiple point heads in the SOFDI instrument will allow for unique studies of gravity wave activity in future measurements.

A new and original stereo imaging method is introduced to measure the altitude of the OH nightglow layer and provide a 3D perspective map of the altitude of the layer centroid. Near-IR photographs of the OH layer are taken at two sites separated by a 645 km distance. Each photograph is processed in order to provide a satellite view of the layer. When superposed, the two views present a common diamond-shaped area. Pairs of matched points that correspond to a physical emissive point in the common area are identified in calculating a normalized cross-correlation coefficient (NCC). This method is suitable for obtaining 3D representations in the case of low-contrast objects. An observational campaign was conducted in July 2006 in Peru. The images were taken simultaneously at Cerro Cosmos (12°09′08.2″ S, 75°33′49.3″ W, altitude 4630 m) close to Huancayo and Cerro Verde Tellolo (16°33′17.6″ S, 71°39′59.4″ W, altitude 2272 m) close to Arequipa. 3D maps of the layer surface were retrieved and compared with pseudo-relief intensity maps of the same region. The mean altitude of the emission barycenter is located at 86.3 km on July 26. Comparable relief wavy features appear in the 3D and intensity maps. It is shown that the vertical amplitude of the wave system varies as exp (Δ z /2 H ) within the altitude range Δ z = 83.5–88.0 km, H being the scale height. The oscillatory kinetic energy at the altitude of the OH layer is comprised between 3 × 10 −4 and 5.4 × 10 −4 J/m 3 , which is 2–3 times smaller than the values derived from partial radio wave at 52°N latitude.

We describe a new suite of instruments planned for deployment to Cape Verde as part of the International Heliospherical Year. The Remote Equatorial Nighttime Observatory of Ionospheric Regions (RENOIR) project consists of a bistatic Fabry–Perot interferometer system, an all-sky imaging system, a dual-frequency Global Positioning System (GPS) receiver, and an array of single-frequency GPS scintillation monitors. This instrumentation will allow for studying the low-latitude thermosphere/ionosphere (TI) system in great detail. Investigations to be conducted using this instrumentation while in Cape Verde include studying equatorial irregularity processes, the effects of neutral winds and gravity waves on irregularity development, the midnight temperature maximum, and ion-neutral coupling in the nighttime TI system. Initial observations from the RENOIR instrumentation during pre-deployment testing at the Urbana Atmospheric Observatory are presented, as is the deployment scenario for the project in Cape Verde.

The hydroxyl nightglow layer is an excellent tracer of the dynamical processes occurring within the mesosphere. A new stereo-imaging method is applied that not only measures the altitude of the airglow layer but also provides a three-dimensional map of the OH-layer centroid heights. A campaign was conducted in July 2006 in Peru to obtain NIR images of the OH nightglow layer which were simultaneously taken for two sites separated by 645 km: Cerro Cosmos (12°09′08.2″S, 75°33′49.3″W, altitude 4630 m) and Cerro Verde Tellolo (16°33′17.6″S, 71°39′59.4″W, altitude 2330 m). Data represented by pairs of images obtained during the nights of July 26–27 and 28–29 are analyzed to yield satellite-type views of the wave field. These are obtained by application of an inversion algorithm. In calculating the normalized cross-correlation parameter for the intensity, three-dimensional maps of the OH nightglow layer surface are retrieved. The mean altitude of the emission profile barycenter is found to be at 87.1 km on July 26 and 89.5 km on July 28. In these two cases the horizontal wavelengths determined are 21.1 and 24.6 km with periods of 18 and 34 min, respectively. A panoramic view of the OH nightglow emission obtained on July 29 at 8 h51–9 h26 UT is presented, in which the overall direction of the waves is found to be N–NW to S–SE, azimuth 150°–330° (counted from South). The wave kinetic energy density at the OH nightglow layer altitude is 3.9×10 −4 W/kg, which is comparable to the values derived from partial reflection radiowave data.

Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.

Advances in Fabry–Perot interferometer (FPI) technology implemented within the past decade have been extensive achieving improvements in sensitivity by one or two orders of magnitude. The measurements of thermospheric winds from such FPI observatories would be extremely valuable for ingestion into Space Weather models for the purpose of forecasting ionospheric variability. However, the current state of FPI networking falls short of its potential for interesting science and significant application. In this paper we review the current state of the global FPI network and explain several new and important science goals.

Presented here are several cases of midnight temperature maximum (MTM) observations using the Millstone Hill incoherent scatter radar (ISR) and Arecibo ISR. The MTM, a temperature enhancement in the upper atmosphere (at ~300 km altitude), is a poorly understood phenomenon as observations are sparse. An upward propagating terdiurnal tide and coupling between atmospheric regions may play a large part in the generation of the MTM, yet this phenomenon and its implications are not fully understood. Two nights (6 March 1989 and 12 July 1988) show clear cases of the MTM occurring between 30 and 34°N with amplitudes of ~100 K and at ~18°N with amplitudes of ~40 K. The MTMs occurred later at the higher latitude. Experiments in 2013 also show a clear MTM at 34° and 36°N from 250 to 350 km altitude. The ionospheric measurements presented here demonstrate a new application of a well-established technique to study atmospheric parameters and allow us to study the latitudinal extent of the MTM. The results provide evidence of the phenomenon occurring at latitudes and altitudes not previously sampled by radar techniques, showing that the MTM is not just an equatorial process, but one that can easily reach midlatitudes. Simultaneous measurements with a Fabry-Perot interferometer allow us to compare the neutral temperatures with the ion temperature. Overall, these are key observations that point to large-scale effects that can help constrain model outputs at different heights and latitudes.

During May, 2002, two Nike Orion sounding rocket payloads were launched from NASA Wallops Flight Facility, Virginia, to explore the effects of gravity waves on the electrodynamics of the mesosphere. This effort extended earlier work that focused on observed correlations between mesospheric temperatures and conductivities. One result of that work was the postulation of subvisible “ice” particles throughout the mesosphere. Because the preferential size of the aerosols depends on the temperature (and therefore also on gravity wave activity), the present effort investigated the influences of thunderstorm-produced gravity waves on the aerosols. Although the extremely small size of these particles makes direct optical observation of them impossible, in situ observations of their ionization by UV radiation was used to show the electrodynamic effects of temperature variations in the lower mesosphere and upper stratosphere. One rocket flight occurred on a clear night with no nearby tropospheric source mechanisms, and the other flight occurred after the passage of a strong thunderstorm system in the vicinity of the rocket range. The effects of gravity waves generatged by the thunderstorms in the lower mesosphere were also observed by an inflatable falling sphere and ground-based observations of the OH layer.

The National Center for Atmospheric Research thermosphere=ionosphere=electrodynamic general circulation model (TIEGCM) is one of the few models that self-consistently solves the coupled equations for the neutral atmosphere and ionosphere. Timely questions are how well the TIEGCM currently simulates the low-latitude ionosphere and what modi9cations might bring about better predictions. Comparisons between data obtained in and around Jicamarca, Peru, near the magnetic equator, and simulations with the TIEGCM indicate good progress has been made but reveal some serious discrepancies. Good-to-excellent agreement is obtained for electron densities, electron and ion temperatures, and nmax. The agreement is fair to poor for hmax, zonal drifts, the 630 nm oxygen nightglow, and the horizontal neutral winds. The most important discrepancy is in the simulated neutral temperature, which is at least 100 K too cold relative to Fabry–Perot interferometer observations. Increasing the EUV ?uxes in the model to improve prediction of the model temperature also improves representation of airglow observations and of the ionosphere, for which the model typically underrepresents the electron densities. The disparity in neutral temperature is also present in comparisons with the empirical model MSIS which represents the largest database of thermospheric temperature measurements. Since the neutral and ionized atmospheres are tightly coupled at low latitudes, simultaneous measurements of neutral and ion parameters, preferably over an extended time period, would be invaluable to further the understanding of the region. Better knowledge of the EUV ?uxes and the high altitude O + ?uxes may also help

A narrow field of view Rayleigh lidar has been constructed at Millstone Hill / MIT Haystack Observatory (42.6°N, 71.5°W) for observations of middle atmospheric temperatures throughout the diurnal cycle. During a 31.5 h measurement on 19–21 March 2001 a mesospheric thermal inversion layer was observed in both the night and day. It developed near 60 km in altitude, progressed downward at 0.40±0.06 km/h, and had an overlying adiabatic lapse rate. The inversion amplitude correlated with the evolution of stratospheric gravity wave activity, although the mesospheric perturbations were too large to be due to conservative gravity-wave growth alone. The 24 h mean temperature profile shows no evidence of a residual inversion layer.

Fabry–Perot interferometric (FPI) measurements of thermospheric zonal neutral winds at Arequipa, Peru ( 16.7° S , 71.5° W , −2.7° dip ), and Carmen Alto, Chile ( 23.1° S , 69.4° W , −10.2° dip ), were collected during the solar minimum periods of September–October 1996 and 1997. The data set included 39 nights from Arequipa and 14 nights for Carmen Alto, with 8 nights of simultaneous observations. Analysis of averaged results found the peak evening zonal neutral wind speed of ∼127±15 m / s eastward for the Arequipa observatory, which is located near the magnetic equator, to occur between 21:30 and 22:30 LT. In contrast, the peak evening zonal winds of ∼100±10 m / s eastward observed from Carmen Alto, which is located near the crest of the equatorial ionization anomaly (EIA), occurred ∼0.5– 1 h later. These measurements represent the first case of groundbased FPI observations of the so called equatorial temperature and wind anomaly (ETWA) over such a small latitude range in the same longitude sector. This reduction in speed of ∼20–25% at Carmen Alto relative to Arequipa is attributed to increased ion drag at Carmen Alto caused by the higher electron density within the EIA region at altitudes of 220– 300 km . Model studies were conducted using electron density and neutral atmosphere parameters form the parameterized ionospheric model (PIM) and the mass spectrometer incoherent scatter (MSIS) models, respectively, to calculate the ratio of ion–neutral collision frequencies at the two sites. We found that the increase in electron density within the EIA was sufficient to account for the observed reduction in the zonal wind. Thus, this analysis confirms the dominant role of ion drag in modulating thermospheric dynamics at equatorial latitudes. A comparison of the FPI results with the predictions by two current neutral wind models, the Horizontal Wind Model-90 and the NCAR Thermospheric Ionosphere Electrodynamics General Circulation Model (TIEGCM), reveals that neither is able to reproduce accurately the latitude dependence reported here. Model refinements for electrodynamics and improved resolution are suggested.

The determination of the physical processes that cause the day-to-day variability of equatorial spread F (ESF) has long been one of the outstanding problems in terrestrial space physics. Within the context of the Rayleigh-Taylor instability model for ESF, mechanisms that either enhance or inhibit the growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric meridional winds play a critical role in suppressing ESF is examined during the Multi-Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New, high-time-resolution Fabry-Perot interferometer (FPI) observations at 6300-A nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hours that had been sampled only sparsely in earlier morphology studies. Evidence of local ESF activity was obtained using GPS-based observations of phase fluctuations (Fp) and 6300-A all-sky optical images from the same site. Additional GPS measurements of Fp and total electron content (TEC) from Bogota (Colombia) and Santiago (Chile) were used to determine the full flux tube development of ESF plumes and to characterize the F region morphology of the interhemispheric Appleton anomaly. Correlative studies between the nightly magnitudes of the meridional winds (Um), ESF activity (Fp), and indices describing the strength (Is) and asymmetry (Ia) of the Appleton anomaly offered no convincing evidence for the wind suppression mechanism. The best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If one assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eastward electric field that causes upward drift is both the necessary and sufficient parameter to forecast ESF with reasonable success. These results reconfirm 60 years of study pointing to the dominance of electrodynamical processes in the onset and growth of plasma instabilities at low latitudes.

An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a ∼10 km wide region of enhanced temperatures (ΔT ∼ 15–50K), is observed with great regularity in both the upper mesosphere (60–70 km) and the mesopause (90–100 km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure.

Thermospheric winds have been systematically determined at Arequipa, Peru, and Arecibo, Puerto Rico, from Fabry-Perot interferometer measurements of Doppler shifts in the nightglow 630 nm line. The wind databases (1983 - 1990 at Arequipa and 1980 - 1990 at Arecibo) have been edited to eliminate measurements during geomagnetically disturbed conditions, then sorted by season and solar flux level. Following this, they were averaged to obtain the climatological behavior of the nighttime wind variations at the two locations. A new averaging technique, multivariate regression analysis, has been applied to the data, and the results compared to our prior binning averages. The observed wind behaviors at the Arequipa and Arecibo Observatories, which are at equal geographic latitudes on opposite sides of the equator, are contrasted to establish the seasonal flow patterns. The regression analysis results have then been compared with the predicted behavior provided by the National Center for Atmospheric Research's Thermosphere-lonosphere-Electrodynamics General Circulation Model. In many cases, qualitative agreement between measurements and predictions is found as to wind directions and temporal variations, with differences in magnitude of - 0-50 m/s. However, some striking differences are found that may arise from ionosphere-thermosphere coupling effects. The overall results provide an important step in establishing the climatology of the thermospheric winds at equatorial and low-latitude sites.

Rayleigh lidar observations of mesosphere temperature profiles obtained from 40 to ∼100 km from Logan, Utah (41.7, 111.8 W, altitude, 1.9 km) over 10 nights in late February, 1995, revealed an interesting development between 60 to 75 km of a winter mesosphere inversion layer with an amplitude of ∼20–30 K and a downward phase progression of ∼1 km/hr. The data also showed two altitude regions exhibiting significant cooling of 10–30 K in extent. These were located below and above the peak of the inversion layer, respectively, at altitudes of ∼50–55 km and ∼70–80 km. When these results were compared with the predictions of a global wave scale model (GSWM), the observed thermal mesosphere structure is similar to the computed composite tidal structure based upon the semi-diurnal and diurnal tides with the exception that observed amplitudes of heating and cooling are ∼10x larger than predicted GSWM values. We suggest that these events over Utah are caused through a localized mechanism involving the coupling of gravity waves to the mesopause tidal structure.

Fabry-Perot interferometric observations at 630 nm of equatorial thermospheric winds and temperatures in the four cardinal directions and zenith from Arequipa, Peru, during local winter for moderate and high solar fluxes showed elevated temperatures over the Andes Mountains that persisted through the night. The difference in temperature between east and west observations was typically ∼100 to 200 K for moderate flux values and as high as 400 K at solar maximum. Correlated with these localized heating regions were differences in the zonal thermospheric wind of 50 to 70 m/s for observations to the west and to the east of the Arequipa observatory. Also noted in these periods for the region over the Andes was the increased variance of the temperature values above the measurement error. These effects of increased variability and localized heating were not observed at solar minimum. The lack of a significant local time dependence in the diurnal variation of the temperature enhancements suggests that the origin of the heating cannot be related to the coupling of the electrodynamics of the ionosphere to the thermosphere. Instead the hypothesis is advanced that gravity wave energy from the surface penetrates into the thermosphere, where viscous dissipation causes the heating. Such wave activity would also explain the increased variability of the temperatures for the thermosphere regions over mountainous terrain.

Abstract. As part of the NSF/CEDAR program (Coupling Energetics, and Dynamics of Atmospheric Regions) in Multi-Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA), an all-sky CCD airglow imaging system has been in operation in Arequipa, Peru, since October 1993. Here we report on the first such use of a wide-field imager to document the optical signature and variability of a brightness feature associated with the so-called midnight temperature maximum (MTM). While the observational driver of this study is a 'rightness wave" (BW) seen in 6300 A and 5577 } airglow images, detailed case studies are conducted during two campaign periods when Fabry-Perot interferometer (FPI) and digital ionosonde data were also available. During the passage of a BW, the FPI observed enhancements in thermospheric temperatures, reversals (from equatorward to poleward) of the meridional neutral winds, and local minima in the zonal neutral winds. The ionosonde recorded decreases in the height of the F- layer during BW events. This lends support to the concept that the poleward winds generated by the MTM pressure bulge cause the lowering of the F- layer to regions of enhanced loss (h < 300 km) and corresponding airglow production. The two- dimensional field -of-view of the imager allows identification of the geographical orientation of the BW pattern. We use the orientation angle of the BW as an indicator of the geographical orientation of the MTM. Significant day-to-day variability in these patterns suggest a complex mix of tidal mode interactions that lead to the overall MTM phenomena.

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day-to-day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi-Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced-antenna satellite scintillation, digisonde, all-sky imager, and Fabry-Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all-sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

Fabry Perot interferometer observations of equatorial 630 nm airglow thermospheric winds and temperatures were made at Arequipa, Peru (16.5 S, 71.5 W) during the solar maximum years of 1989 and 1990. Examination of the thermospheric winds measured from Doppler shifts of the 630 nm airglow emission in the two directions east and west of Arequipa shows that the zonal wind is often significantly weaker over the Andes (east) than over the Pacific (west) in winter. Between 19 LT and 24 LT, the typical reductions in speed found were ∼40 to 70 m/s. In the same time period the temperature results show elevated values of ∼200-500 K for the three directions (north, east, and zenith) over mountainous terrain compared with those of the two directions (west and south) over water. This effect fades near equinoctial periods. Examination of the standard deviation of the temperature ohservations for the five directions showed an increase in this value above the measurement error hy a factor of three for the direction toward the Andes (East) but not for the direction over water (South) ; this elevation is consistent with wave activity as a source of the heating. Viscous dissipation of waves propagating into the thermosphere region from below is suggested as an explanation for the localized regions of heating.

A region of thermal enhancement of the mesosphere has been detected on numerous occasions by in situ measurements, remote sensing from space, and lidar techniques. The source of these 'temperature inversion layers' has been attributed in the literature to the dissipation relating to dynamical forcing by gravity wave or tidal activity. However, evidence that gravity wave breaking can produce the inversion layer with amplitude as large as that observed in lidar measurements has been limited to results of numerical modeling. An alternative source for the production of the thermal inversion layer in the mesosphere is the direct deposition of heat by exothermic chemical reactions. Two-dimensional modeling combining a comprehensive model of the mesosphere photochemistry with the dynamical transport of long-lived species shows that the region from 80 to 95 km may be heated as much as 3 to 10 K/d during the night and half this rate during the day. Given the uncertainties in our understanding of the dynamics and chemistry for the mesopause region, separating the two sources by passive observations of the mesosphere thermal structure looks to be difficult. Therefore we have considered an active means for producing a mesopause thermal layer, namely the release of ozone into the upper mesosphere from a rocket payload. The induced effects would include artificial enhancements of the OH and Na airglow intensities as well as the mesopause thermal structure. The advantages of the rocket release of ozone is that detection of these effects by ground-based imaging, radar, and lidar systems and comparison of these effects with model predictions would help quantify the partition of the artificial inversion layer production into sources of dynamical and chemical forcing.

High-resolution Fabry-Perot interferometric measurements of equatorial 630-nm intensities, thermo spheric winds and temperatures have been obtained at Arequipa, Peru, from 1983 to the present. Scientific results are summarized, and directions for future research are listed.

[1] The neutral vertical circulation in the thermosphere plays an important role in redistributing heat and momentum and strongly affects the neutral composition, as well as plasma and electrodynamic processes. Vertical wind measurements in this altitude range are difficult, however, and the available data give limited temporal, spatial, and vertical coverage. The available ground-based techniques are generally limited to narrow altitude ranges but provide more extended temporal coverage. The in situ techniques provide more extended altitude range coverage but have very limited temporal coverage. The most extensive measurements have come from ground-based optical techniques using the 630.0-nm F-region red-line emission and to a lesser extent the 557.7-nm E-region green-line emission. A general characteristic of these measurements is that they often show large vertical wind speeds that persist for periods of several hours or longer at the same location. The large winds have been difficult to verify because of the difficulties associated with obtaining overlapping measurements and with obtaining a sufficiently comprehensive set of measurements to understand the dynamical drivers for the vertical winds. A surprising conclusion from examining all the available measurements, however, is that there is remarkable consistency between the characteristics of the thermospheric vertical velocities measured by different techniques and at different locations. We give an overview of the available data and find that, when considered together, the available observations all show the following patterns. Vertical velocities with magnitudes of 10–20 m s−1 are common at high latitudes, but also at mid and low latitudes. There is a slight tendency for the winds to increase with height from the E region to the F region, but the increase with height is small, and in particular, is much less than expected from the decrease of background density with altitude. Another surprising characteristic that holds true across altitudes and latitudes is that the large vertical winds frequently persist for extended periods of an hour or longer. The drivers for the observed events of large vertical winds are unknown, but the winds clearly have important implications for the dynamics, electrodynamics, plasma physics, and chemistry of the thermosphere.

[1] We present results from the first extended period of coincident observations of thermospheric zonal neutral winds and equatorial plasma bubble (EPB) zonal drift velocities over northeastern Brazil during the October to December months of 2009 and 2010. The EPB zonal drift velocities are estimated utilizing images of the O I 630.0 nm emissions recorded by a wide-angle imaging system at Cajazeiras. Thermospheric neutral wind estimates are based upon common volume observations made by a bistatic Fabry-Perot interferometer (FPI) experiment using FPIs located at Cajazeiras and Cariri in Brazil observing the Doppler shift of the O I 630.0 nm emission. The results illustrate a similar pattern of nighttime and night-to-night variations in the zonal neutral winds and EPB zonal drift velocities. In general, the geomagnetic zonal neutral winds and the EPB velocities show an excellent agreement illustrating that the F region dynamo is fully developed. However, in the early evening hours the EPB zonal speed is slower than that of the background winds on several occasions. We conclude that this indicates that during the bubble evolution period in the early evening the F region dynamo is not fully activated.

[1] In a companion paper, we derived the high-frequency, compressible, dissipative polarization relations for gravity waves (GWs) propagating in the thermosphere. In this paper, we apply the results to nighttime thermospheric observations of a GW over Alaska on 9–10 January 2010. Using a vertically-pointed Fabry-Perot interferometer (FPI) at Poker Flat that measured vertical wind perturbations (w′) and two FPIs that measured the line-of-sight (LOS) velocities in four common volumes, we inferred a GW ground-based period ∼32.7 ± 0.3 min, horizontal wavelength λH = 1094 ± 408 km, horizontal ground-based phase speed cH ∼ 560 ± 210 m/s, and propagation azimuth θ ∼ 33.5 ± 15.8° east-of-north. We compared the phase shifts and amplitude ratios of this GW with that predicted by the GW dissipative polarization relations derived in the companion paper, enabled by the ability of the FPIs to measure fundamental GW parameters (wind and temperature perturbations). We find that GWs with λH ∼ 700–1100 km, λz ∼ −500 to −350 km, θ ∼ 15 to 50°, and cH ∼ 350–560 m/s agree with the observations if the primary contribution to the 630-nm emission was near the upper portion of that layer. The source of GW was likely thermospheric given the large intrinsic phase speed of the wave. Possible sources are discussed, the most likely of which are related to the onset of auroral activity near the time that the wave was initially observed.

It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U(sub z0) approximately equals (Partial derivative U(sub y)/Partial derivative y)H for the vertical wind U(sub z0) at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and (Partial derivative U(sub y)/Partial derivative y) is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O((sup 1)D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region.

We describe an analysis procedure for estimating the thermospheric winds and temperatures from the multi-order two-dimensional (2D) interferograms produced by an imaging Fabry–Perot interferometer (FPI) as imaged by a CCD detector. We also present a forward model describing the 2D interferograms. To investigate the robustness and accuracy of the analysis, we perform several Monte Carlo simulations using this forward model for an FPI that has recently been developed and deployed to northeastern Brazil. The first simulation shows that a slight cross-contamination at high temperatures exists between neighboring orders in the interferogram, introducing a bias in the estimated temperatures and increasing errors in both the estimated temperatures and winds when each order is analyzed in full. The second simulation investigates how using less than an entire order in the analysis reduces the cross contamination observed in the first set of simulations, improving the accuracy of the estimated temperatures. The last simulation investigates the effect of the signal-to-noise ratio on the errors in the estimated parameters. It is shown that, for the specific FPI simulated in this study, a signal-to-noise ratio of 1.5 is required to obtain thermospheric wind errors of 5 m/s and temperature errors of 20 K.

[1] We report on the climatology of equatorial thermospheric winds and temperatures based on Fabry-Perot interferometer measurements of Doppler shifts and Doppler broadenings of the 630.0 nm spectral emission from the Cajazeiras observatory located in the northeastern part of Brazil (6.89°S, 38.56°W). These results apply to the lower thermosphere region near 240 km and were obtained during a period of weak solar activity with the solar flux typically within the range of 72 and 82 solar flux units. Examination of the monthly averaged meridional thermospheric winds for 1 year of measurement from October 2009 to September 2010 found the wind direction to be equatorward during summer months throughout the early evening hours with maximum speeds reaching ∼65 ms−1. During winter months, the early evening meridional wind direction reversed to poleward with similar speeds. This result is attributed to the cross-hemispheric flow from the summer to winter hemisphere. Superimposed upon this wintertime meridional wind flow was an equatorward surge shortly before midnight. This surge shifted to earlier local times in the transition from the vernal equinox to summer and to later local times between the summer solstice and the autumnal equinox; this flow feature is attributed to tidal wave forcing. The temperature observations exhibited the expected behavior, with the midnight temperature maximum showing a greater amplitude, ∼120 K, in the vernal equinox with somewhat weaker amplitudes, ∼75 K, seen at earlier local times during the summer. Also observed was a phase lag of 60 to 90 min between the appearance of the equatorward meridional wind flow and occurrence of the midnight temperature maximum peak.

[1] The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0–500 km). It represents over 50 years of satellite, rocket, and ground-based wind measurements via a compact Fortran 90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. HWM07 is composed of two components, a quiet time component for the background state described in this paper and a geomagnetic storm time component (DWM07) described in a companion paper.

[1] We present a global empirical disturbance wind model (DWM07) that represents average geospace-storm-induced perturbations of upper thermospheric (200–600 km altitude) neutral winds. DWM07 depends on the following three parameters: magnetic latitude, magnetic local time, and the 3-h Kp geomagnetic activity index. The latitude and local time dependences are represented by vector spherical harmonic functions (up to degree 10 in latitude and order 3 in local time), and the Kp dependence is represented by quadratic B-splines. DWM07 is the storm time thermospheric component of the new Horizontal Wind Model (HWM07), which is described in a companion paper. DWM07 is based on data from the Wind Imaging Interferometer on board the Upper Atmosphere Research Satellite, the Wind and Temperature Spectrometer on board Dynamics Explorer 2, and seven ground-based Fabry-Perot interferometers. The perturbation winds derived from the three data sets are in good mutual agreement under most conditions, and the model captures most of the climatological variations evident in the data.

[1] A Rayleigh-scatter lidar has been operated at the Atmospheric Lidar Observatory (ALO) on the Utah State University (USU) campus (41.7°N, 111.8°W) since August 1993. During the morning of 22 June 1995, lidar returns from a noctilucent cloud (NLC) were observed for approximately 1 hr, well away from the twilight periods when NLCs are visible. This detection of an NLC at this latitude shows that the first reported sighting, in 1999 (Wickwar et al., 2002), was not a unique occurrence. This 1995 observation differs from the 1999 one in that temperatures could be deduced. Near the 83-km NLC altitude the temperatures were found to be up to ∼23 K cooler than the 11-year June climatology for ALO. This analysis shows that these cool temperatures arose, not because the whole profile was cooler, but because of a major temperature oscillation or wave with a 22-km vertical wavelength and a ∼0.9 km/hr downward phase speed. This large-amplitude wave has many of the characteristics of the diurnal tide. However, the amplitude would have to be enhanced considerably. These lidar observations were supplemented by OH rotational temperature observations from approximately 87 km. These NLC observations equatorward of 50° have been suggested to be significant harbingers of global change. However, if that were the case, the mechanism is more complicated than a simple overall cooling or an increase in water vapor. Accordingly, we propose enhanced generation of gravity waves that would interact with the diurnal tide to produce a large-amplitude wave, the cold phase of which would give rise to low enough temperatures to produce the NLC. The gravity wave source might be orographic in the Mountain West or convective far to the east or south.

[1] We analyze ground-based Fabry-Perot interferometer observations of upper thermospheric (∼250 km) horizontal neutral winds derived from Doppler shifts in the 630.0 nm (red line) nightglow. The winds were measured over the following locations: South Pole (90°S), Halley (76°S, 27°W), Arequipa (17°S, 72°W), Arecibo (18°N, 67°W), Millstone Hill (43°N, 72°W), Sondre Stromfjord (67°N, 51°W), and Thule (77°N, 68°W). We derive climatological quiet time (Kp 150). The seasonal dependence of the winds is generally annual, but there are isolated cases in which a semiannual variation is observed. Within the austral winter, winds measured from the South Pole show a substantial intraseasonal variation only along longitudes directed toward the magnetic pole. IMF effects are described in a companion paper.