Altitude Coverage Research Articles

Frequency Channel Selection and Performance Simulation of a Microwave Radiometer for Temperature and Sulfuric Compound Profiling of the Venusian Lower Atmosphere

AbstractExploring the Venusian lower atmosphere is crucial for studying the atmospheric circulation, surface‐atmosphere interactions, and origin and evolution of the Venusian atmosphere and climate. In this study, we investigate the theoretical capabilities of a downward‐looking passive microwave sounder placed in low Venus orbit to measure the temperature, sulfur dioxide (SO2) and gaseous sulfuric acid (H2SO4(g)) profiles. A nonlinear iterative retrieval algorithm combining a radiative transfer (RT) model and an optimal‐estimation‐based inversion algorithm is established. With the RT model adapted to the Venusian atmosphere, simulations under different atmospheric conditions are performed to optimize the selection of frequency channels. The achievable altitude coverage, vertical resolution and corresponding expected precision of the temperature, SO2 and H2SO4(g) retrievals from the multi‐channel brightness temperature measurements are quantified via retrieval simulations. The temperature can be retrieved from the surface of Venus to an altitude of ∼61 km with a precision of 1–3.5 K and a vertical resolution of 6–15 km. A precision of 10%–35% is expected for SO2 in the ∼12–64 km altitude range and with a vertical resolution of 8–19 km. H2SO4(g) can be retrieved in the ∼36–54 km altitude range with a precision of 10%–30% and a vertical resolution of 6–13 km.

The day-to-day variability in the mesosphere and lower thermosphere in low latitudes: A study using MF radar

This study presents the analysis of planetary waves (PWs) using daily mean wind velocities for four years (August 2013 to July 2017) of continuous measurements using MF radar over the low latitude Indian region Kolhapur (16.8° N; 74.2° E). The MF radar at Kolhapur was upgraded in 2013. These are the first results of PWs after the upgradation of MF radar. The seasonal and intra-seasonal variabilities of East-West (EW) traveling PWs in the MLT region have been studied. In the present work, the data was analyzed to study the waves with various periodicities (e.g. 3–4, 5–8, 15–17, and 30–60 days). The 3.5 day [Ultra-Fast Kelvin (UFK)] wave shows semiannual variability with burst like wave activity observed during the summer months and December solstice. In addition, it is observed to be stronger in the spring equinoctial period. A strong semiannual oscillation (SAO) has been observed in a 6.5-day wave with peaks near the equinoxes. Similar to SAO over the low latitude MLT region, the wave activity is stronger in April/May than in September/October. The 6.5-day waves are observed to be stronger when the background mean wind is westward. From the analysis, it has been seen that the period before and after the equinoctial period is favorable for the 6.5-day wave propagation. The 16-day wave has no significant seasonal dependence; instead, the waves spread to almost all seasons. The Madden-Julian Oscillations (MJOs) have been observed to be propagating with an average wind speed of ∼ 5 m/s when the background mean wind is eastward. The occurrence of MJO is observed during the summer and winter months. These results are the first of their kind in two aspects: first, they show the PWs with enhanced altitude coverage covering up to 110 km, and second, they show the PWs not contaminated due to equatorial electro jet influence.

Simulation of Mesosphere Wind Measurement with Multiple Emission Lines of the O2(0-1) Band Using Space-Based Doppler Asymmetric Spatial Heterodyne

For space-based atmospheric wind measurements, full-link simulation is critical for the optimization of the instrument indicators and the evaluation of the measurements’ performance. This paper presents observation simulations and error verification of the mesosphere wind measurement with four emission lines of the O2(0-1) band by using the space-based Doppler Asymmetric Spatial Heterodyne (DASH), named the Mesosphere Wind Image Interferometer (MWII). The passive wind measurement principle and the DASH concept are first described. The full-link simulation consists of radiation simulation, the instrument forward model, and the wind retrieval model. The four emission lines at about 866.5 nm of the O2(0-1) band were selected as the observation targets. The radiation characteristics of the target lines were studied and calculated, as well as the background radiation. Based on the LOS radiation integral model, a numerical simulation of the raw observation data was carried out using the instrument model. The interference fringe priority strategy and joint wind decision method were proposed to achieve multiple-emission-line wind retrieval with higher precision. In the simulation, multiple-line retrieval could improve the precision by more than 30% compared to single-line retrieval under the same conditions. The error simulation indicated that the wind profile precision was 3–9 m/s in the altitude range of 50–110 km, with an average accuracy of about 1 m/s, proving that the scheme of MWII has good altitude coverage of the whole mesosphere and a part of the lower thermosphere.

Continuous temperature soundings at the stratosphere and lower mesosphere with a ground-based radiometer considering the Zeeman effect

Abstract. Continuous temperature observations at the stratosphere and lower mesosphere are rare. Radiometry opens the possibility of observing microwave emissions from two oxygen lines to retrieve temperature profiles at all altitudes. In this study, we present observations performed with a temperature radiometer (TEMPERA) at the MeteoSwiss station at Payerne for the period from 2014 to 2017. We reanalyzed these observations with a recently developed and improved retrieval algorithm accounting for the Zeeman line splitting in the line center of both oxygen emission lines at 52.5424 and 53.0669 GHz. The new temperature retrievals were validated against MERRA2 reanalysis and the meteorological analysis NAVGEM-HA. The comparison confirmed that the new algorithm yields an increased measurement response up to an altitude of 53–55 km, which extends the altitude coverage by 8–10 km compared to previous retrievals without the Zeeman effect. Furthermore, we found correlation coefficients comparing the TEMPERA temperatures with MERRA2 and NAVGEM-HA for monthly mean profiles to be in the range of 0.8–0.96. In addition, mean temperature biases of 1 and −2 K were found between TEMPERA and both models (MERRA2 and NAVGEM-HA), respectively. We also identified systematic altitude-dependent cold and warm biases compared to both model data sets.

Studies of relative contributions of internal gravity waves and 2‐D turbulence to tropospheric and lower‐stratospheric temporal wind spectra measured by a network of VHF windprofiler radars using a decade‐long data set in Canada

AbstractTropospheric and lower‐stratospheric motions at mesoscales and larger are a mixture of waves and two‐dimensional (2‐D) turbulence. Determining their relative importance is necessary, since waves are capable of coordinated systematic momentum transport accompanying the wave propagation, and associated wind forcing, in ways that 2‐D turbulence is not. This can impact weather forecasting. Using a network of ten windprofiler radars in eastern Ontario and western Quebec in Canada, plus an additional one in the Arctic, the relative roles of internal gravity (buoyancy) waves and two‐dimensional turbulence are examined at temporal scales from about 3–4 hrs to several tens of hours (horizontal spatial scales of typically one or two hundred kilometres to a few thousand kilometres), with the purpose of investigating the respective roles of these two distinct characteristic fluid motions as functions of location, season and year. The emphasis is on studies of spectral slope variability, rather than absolute spectral magnitudes, giving a perspective not previously substantially presented. In particular, we have found a frequency band in which gravity‐wave Doppler shifting produces distinctly different spectral slopes than those predicted for 2‐D turbulence, and these differences are employed to distinguish the flow fields. The network used (excluding the Arctic site) covers an area of ∼106 km2 and includes a variety of different terrains. Radial velocities have been recorded on time scales of minutes for data lengths covering durations of up to 12 years. Altitude coverage is from 1 km to typically 14 km, at 500 m resolution. Results suggest a region from ∼2 to ∼5 km altitude (deeper for some radars) where waves are weaker and 2‐D turbulence appears to be generally more significant, but where occasional bursts of gravity‐wave activity can occur, while above typically 6–8 km, gravity waves increase in significance. There are distinct site‐to‐site variations.

Effect of OH emission on the temperature and wind measurements derived from limb-viewing observations of the 1.27 µm O2 dayglow

Abstract. The O2(a1Δg) emission near 1.27 µm is well-suited for remote sensing of global wind and temperature in near-space by limb-viewing observations to its bright signal and extended altitude coverage. However, vibrational–rotational emission lines of the OH dayglow produced by the hydrogen–ozone reaction (H+O3→OH•+O2) overlap the infrared atmospheric band emission (a1Δg→X3Σg) of O2. The main goal of this paper is to discuss the effect of OH emission on the wind and temperature measurements derived from the 1.27 µm O2 dayglow limb-viewing observations. The O2 dayglow and OH dayglow spectrum over the spectral region and altitude range of interest is calculated by using the line-by-line radiative transfer model and the most recent photochemical model. The method of four-point sampling of the interferogram and sample results of measurement simulations are provided for both O2 dayglow and OH dayglow. It is apparent from the simulations that the presence of OH dayglow as an interfering species decreases the wind and temperature accuracy at all altitudes, but this effect can be reduced considerably by improving OH dayglow knowledge.

The Near-Space Wind and Temperature Sensing Interferometer: Forward Model and Measurement Simulation

Wind and temperature observation in near space has been playing an increasingly important role in atmospheric physics and space science. This paper reports on the near-space wind and temperature sensing interferometer (NWTSI), which employs a wide-angle Michelson interferometer to observe O2(a1Δg) dayglow near 1.27 μm from a limb-viewing satellite, and presents the instrument modeling and observation simulations from the stratosphere to the mesosphere and lower thermosphere. The characteristics of atmospheric limb-radiance spectra and line selection rules are described. The observational strategy of using two sets of three emission lines with a line-strength difference of one order of magnitude is proved to be suitable for extending altitude coverage. The forward modeling and measurement simulation of the expected NWTSI observations are provided, and the measurement uncertainty of the wind and temperature is discussed. The signal-to-noise ratio (SNR) and the limb-view weight work together to affect the precision of the wind and temperature measurements. The simulated results indicate a wind measurement precision of 1 to 3 m/s and a temperature precision of 1 to 3 K over an altitude range from 40 to 80 km, which meets the observing requirement in measurement precision for near-space detection.

Simulation and application of the emission line O19P18 of O2(a1Δg) dayglow near 1.27 μm for wind observations from limb-viewing satellites.

The O2(a1Δg) emission near 1.27 μm has relatively bright signal and extended altitude coverage and provides an important means to remotely sense the compositional structures and dynamical features of the upper atmosphere globally. In this paper, we report the simulation and application of O2(a1Δg) dayglow near 1.27 μm for wind observations from limb-viewing satellites. A line by line radiative transfer model of the O2(aΔ1g,υ'=0)→O2(XΣ3g,υ″=0) band is developed by taking both multiple scattering radiative transfer and nonlocal thermal equilibrium (non-LTE) models into account. The emission line O19P18 (7772.030 cm-1) with weak self-absorption, bright radiation intensity, and large spectral separation range is proved to be suitable for limb-viewing wind detection, due to its advantages of significantly lower cost, risk, and platform requirements. In order to ascertain the wind precision of O19P18, observations by a DASH-type (the Doppler asymmetric spatial heterodyne) instrument are simulated. The simulated results indicate a wind measurement precision of 1-2 m/s over an altitude range of 40 to 70 km in general, and possibly to 2-4 m/s due to a strong dependence on the spectral interference of the scattered sunlight background.

Validation of INSAT-3D sounder data with in situ measurements and other similar satellite observations over India

Abstract. To date, several satellites measurements are available which can provide profiles of temperature and water vapour with reasonable accuracies. However, the temporal resolution has remained poor, particularly over the tropics, as most of them are polar orbiting. At this juncture, the launch of INSAT-3D (Indian National Satellite System) by the Indian Space Research Organization (ISRO) on 26 July 2013 carrying a multi-spectral imager covering visible to long-wave infrared made it possible to obtain profiles of temperature and water vapour over India with higher temporal and vertical resolutions and altitude coverage, besides other parameters. The initial validation of INSAT-3D data is made with the high temporal (3 h) resolution radiosonde observations launched over Gadanki (13.5° N, 79.2° E) during a special campaign and routine evening soundings obtained at 12:00 UTC (17:30 LT). We also compared INSAT-3D data with the radiosonde observations obtained from 34 India Meteorological Department stations. Comparisons were also made over India with data from other satellites like AIRS, MLS and SAPHIR and from ERA-Interim and NCEP reanalysis data sets. INSAT-3D is able to show better coverage over India with high spatial and temporal resolutions as expected. Good correlation in temperature between INSAT-3D and in situ measurements is noticed except in the upper tropospheric and lower stratospheric regions (positive bias of 2–3 K). There is a mean dry bias of 20–30 % in the water vapour mixing ratio. Similar biases are noticed when compared to other satellites and reanalysis data sets. INSAT-3D shows a large positive bias in temperature above 25° N in the lower troposphere. Thus, caution is advised when using these data for tropospheric studies. Finally it is concluded that temperature data from INSAT-3D are of high quality and can be directly assimilated for better forecasts over India.

Simulation study for measurement of horizontal wind profiles in the polar stratosphere and mesosphere using ground-based observations of ozone and carbon monoxide lines in the 230–250 GHz region

Abstract. Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure stratospheric–mesospheric winds at altitudes between 20 and 80 km to verify model datasets. Here we demonstrate the feasibility of horizontal wind profile measurements using ground-based passive millimetre-wave spectroradiometric observations of ozone lines centred at 231.28, 249.79, and 249.96 GHz. Vertical profiles of horizontal winds are retrieved from forward and inverse modelling simulations of the line-of-sight Doppler-shifted atmospheric emission lines above Halley station (75°37′ S, 26°14′ W), Antarctica. For a radiometer with a system temperature of 1400 K and 30 kHz spectral resolution observing the ozone 231.28 GHz line we estimate that 12 h zonal and meridional wind profiles could be determined over the altitude range 25–74 km in winter, and 28–66 km in summer. Height-dependent measurement uncertainties are in the range 3–8 m s−1 and vertical resolution ∼ 8–16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5 h for measuring either zonal or meridional winds is possible, reducing to 0.5 h for a radiometer with a 700 K system temperature. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85 ± 12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined.

Ionospheric response to tidal waves measured by dynasonde techniques

AbstractAtmospheric tides are known to have a dramatic influence on thermospheric and ionospheric structure and variability. Considerable effort goes into understanding characteristics of tidal modes, their interactions with planetary and gravity waves and other tidal modes, as well as their influence on the background state of the thermosphere‐ionosphere system. For the altitude interval between roughly 120 and 400 km, this effort is somewhat hindered by the lack of global observations. We propose a new method of determining tidal variability by making use of dynasonde measurements. The NeXtYZ inversion procedure produces altitude profiles of the ionospheric parameters with a vertical resolution typically better than 1 km. This, together with the typical 2 min cadence of the instrument, results in extensive data sets with wide temporal and altitude coverage. At any given altitude we have nonuniform sampling due to the natural ionospheric variability. A Lomb‐Scargle implementation is used to obtain equivalent results at all altitudes and locations. We report height profiles of the first three tidal harmonics derived from dynasonde measurements. The data analyzed include the vertical electron density profiles, the ionospheric X (east‐west) “tilt” measurement, and the derived zonal plasma density gradient. Both the tilt and the gradient are shown to be sensitive tracers of atmospheric waves. We use data from Wallops Island and San Juan, for two time intervals: 6 May‐ 6 June and 9 October‐ 8 November 2013, thus capturing seasonal, latitudinal, and altitude variations of tidal amplitude and phase. This proves the potential of using dynasonde‐capable instruments as a data source for tidal studies in the thermosphere.

Iron Requirement in Hypoxic Conditions

While highlanders in the Andes and Himalayas are well adapted to live at high altitude, adequate acclimatization time to slowly adjust to hypoxic conditions is among the most important aspects for ascending mountaineers. Accordingly, most reports emphasize on mechanisms that cope with reduced oxygenation. Notably, during a journey to high altitude coverage of the elevated iron demand is equally critical. Thus, it's crucial to focus on the crosstalk between oxygen and iron homeostasis. I will discuss the role of iron in the oxygen sensing process and erythropoietin (Epo) synthesis as well as in gene expression control mediated by the hypoxia-inducible factor-2 (HIF-2). The blood hormone Epo that is abundantly expressed by the kidney under hypoxic conditions stimulates erythropoiesis in the bone marrow, a process that requires high quantities of iron. To ensure that sufficient iron is provided, hypoxia-induced soluble factors - such as the novel Epo-controlled erythroferrone that is expressed in erythroid precursor cells - reach the liver where they reduce expression of the iron hormone hepcidin. In turn, suppression of hepcidin allows both, elevated iron release from storage organs and enhanced absorption of dietary iron by enterocytes. As recently observed in sojourners at high altitude, however, iron uptake may be hampered by reduced appetite and gastrointestinal bleeding. Overall, adequate systemic iron availability is an important prerequisite to adjust not only to high-altitude induced hypoxia but to treat disease-related hypoxic conditions. Disclosures No relevant conflicts of interest to declare.

Evaluation of wind profiles from the NERC MST radar, Aberystwyth, UK

Abstract. This study quantifies the uncertainties in winds measured by the Aberystwyth Mesosphere–Stratosphere–Troposphere (MST) radar (52.4° N, 4.0° W), before and after its renovation in March 2011. A total of 127 radiosondes provide an independent measure of winds. Differences between radiosonde and radar-measured horizontal winds are correlated with long-term averages of vertical velocities, suggesting an influence from local mountain waves. These local influences are an important consideration when using radar winds as a measure of regional conditions, particularly for numerical weather prediction. For those applications, local effects represent a source of sampling error additional to the inherent uncertainties in the measurements themselves. The radar renovation improved the signal-to-noise ratio (SNR) of measurements, with a corresponding improvement in altitude coverage. It also corrected an underestimate of horizontal wind speeds attributed to beam formation problems, due to pre-renovation component failure. The root mean square error (RMSE) in radar-measured horizontal wind components, averaged over half an hour, increases with wind speed and altitude, and is 0.8–2.5 m s−1 (6–12% of wind speed) for post-renovation winds. Pre-renovation values are typically 0.1 m s−1 larger. The RMSE in radial velocities is <0.04 m s−1. Eight weeks of special radar operation are used to investigate the effects of echo power aspect sensitivity. Corrections for echo power aspect sensitivity remove an underestimate of horizontal wind speeds; however aspect sensitivity is azimuthally anisotropic at the scale of routine observations (≈1 h). This anisotropy introduces random error into wind profiles. For winds averaged over half an hour, the RMSE is around 3.5% above 8 km, but as large as 4.5% in the mid-troposphere.

Limb Viewing Hyper Spectral Imager (LiVHySI) for airglow measurements onboard YOUTHSAT-1

The Limb Viewing Hyper Spectral Imager (LiVHySI) is one of the Indian payloads onboard YOUTHSAT (inclination 98.73°, apogee 817km) launched in April, 2011. The Hyper-spectral imager has been operated in Earth’s limb viewing mode to measure airglow emissions in the spectral range 550–900nm, from terrestrial upper atmosphere (i.e. 80km altitude and above) with a line-of-sight range of about 3200km. The altitude coverage is about 500km with command selectable lowest altitude. This imaging spectrometer employs a Linearly Variable Filter (LVF) to generate the spectrum and an Active Pixel Sensor (APS) area array of 256×512 pixels, placed in close proximity of the LVF as detector. The spectral sampling is done at 1.06nm interval. The optics used is an eight element f/2 telecentric lens system with 80mm effective focal length. The detector is aligned with respect to the LVF such that its 512 pixel dimension covers the spectral range. The radiometric sensitivity of the imager is about 20 Rayleigh at noise floor through the signal integration for 10s at wavelength 630nm. The imager is being operated during the eclipsed portion of satellite orbits. The integration in the time/spatial domain could be chosen depending upon the season, solar and geomagnetic activity and/or specific target area. This paper primarily aims at describing LiVHySI, its in-orbit operations, quality, potential of the data and its first observations. The images reveal the thermospheric airglow at 630nm to be the most prominent. These first LiVHySI observations carried out on the night of 21st April, 2011 are presented here, while the variability exhibited by the thermospheric nightglow at O(1D) 630nm has been described in detail.

基于植被覆盖度的藏羚羊栖息地时空变化研究

The distribution and dynamics of vegetation can reflect the distribution and changes of wildlife habitat to some extent. Tibetan Antelope,well-known as‘the numen of plateau',is an endemic animal on the Qinghai-Tibetan Plateau.However,the survival and distribution of Tibetan Antelope have been threatened by environmental deterioration and illegal poaching. This animal has been listed as an Class I species by the Convention on International Trade in Endangered Species( CITES) and as Category I by the Key Protected Wildlife List of China. This situation has attracted a great deal of attentions from government and conservation professionals. The population distribution of Tibetan Antelope remains unkown in Altun National Nature Reserve. The Altun National Nature Reserve,the largest inland nature reserve of China established in May of 1983,is one of the four "no man' s-lands"in China. Due to its unique natural conditions,this reserve is preserving the rich natural resources,rare wild animals and plants.In this study,we analyzed the dynamics of vegetation in Altun National Nature Reserve based on remote sensing image data and GIS tool. The Normalized Difference Vegetation Index( NDVI) data were collected in 2000,2005 and 2010. We merged the regions,which were covered by vegetation in the study years and the suitable habitats were identified through interpreting vegetation types,which can be used by Tibetan Antelopes as grazing pastures. Based on NDVI datasetscollected in different years,we calculated the vegetation coverage and analyzed the spatial and temporal variations of vegetation coverage. Moreover,the changes of habitat were analyzed on the basis of vegetation coverage variations. The results showed that the vegetation coverage of the reserve was at a low level,and grids with low vegetation coverage took up about 50% of the regions that were covered by vegetation. The grids with high vegetation coverage were mainly distributed in the southwest of Kardun inspection station. The vegetation coverage in the regions where Tibetan Antelopes mainly lived was relatively higher,and that in the regions where the animal lambed was lower. The upward trends of vegetation coverage over time reflected the increase of the reserve's carrying capacity for animals,and it may also imply that the area of suitable habitat of this animal increased. The vegetation coverage of alpine steppe dominated by feather grass experienced a significant upward trend,also suggesting that carrying capacity of the reserve for Tibetan Antelope increased. The change of spatial distribution of feather grass steppe coverage indicated that habitat of Tibetan Antelopes might spread from east to southwest. The relationship between the altitude and vegetation coverage was not significant correlated. The study of habitat variations based on vegetation coverage is meaningful and can be used for better protecting this wildlife. Furthermore,we suggested that more factors could be integrated with vegetation coverage to precisely identify suitable habitat for animals,and more field surveys and monitoring should be conducted in the future.

On Rayleigh lidar capability enhancement for the measurement of short-period waves at upper mesospheric altitudes

We used a combination of simultaneous measurements made with Rayleigh lidar and O2 airglow monitoring to improve lidar investigation capability to cover a higher altitude range. We fed in instantaneous O2 airglow temperatures rather than model values at the peak altitude for a subsequent integration method of temperature retrieval using Rayleigh lidar backscattered signals. Using this method, errors in the lidar temperature estimates converge at higher altitudes indicating better altitude coverage compared with regular methods where model temperatures are used rather than real-time measurements. This improvement enables the measurement of short-term waves at upper mesospheric altitudes (∼90 km). Using two case studies, we show that above 60 km the amplitude of a few short-term waves drastically increases while some of the short-term waves show either damping or saturation. We claim that by using such combined measurements, significant and cost-effective progress can be made in the understanding of short-term wave processes that are important for coupling across different atmospheric regions.

The O2(b1Σ) dayglow emissions: application to middle and upper atmosphere remote sensing1This article is part of a Special issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career.

The O2atmospheric band transition is observed in the altitude region between ∼40 and 200 km in the dayglow and between ∼80 and 100 km in the nightglow. Wallace and Hunten (J. Geophys. Res. 73, 4813 (1968)) presented the first detailed analysis of the sources and sinks of this O2airglow emitting state. Because of its extended altitude coverage, bright signal, and spectral and photometric properties, this emission provides an important means to remotely sense the thermal, dynamical, and compositional structures of the upper atmosphere. In this paper we present a photochemical and emission–absorption model that calculates the spectral brightnesses of the four brightest vibrational manifolds of this band system that, for the first time, extends from the mesosphere to the thermosphere. This model incorporates the latest rate constants, cross sections, and spectral parameters relevant to this emission, some of which were not considered in previous remote sensing retrieval applications. The model results are compared with our previous rocket experiments to assess the utility of this emission for upper atmospheric remote sensing and to identify key future measurement challenges. This model, together with improved instrument capabilities, permits us to study the atmosphere from 40 up to 200 km, a region where the strongest coupling between the lower atmosphere and upper atmosphere occurs.

Observations of nighttime equatorial-upper-E-valley region irregular structures from São Luís radar and their occurrence statistics: A manifestation of vertical coupling between E and F regions

Radar observations of nighttime irregular structures in the equatorial upper-E-valley region are presented. These structures are observed strictly during occurrences of an ascending irregular bottomtype or bottomside F layer, during or prior to the appearance of a high-rising F region plume and a momentarily intensification of E-region irregularities. The slope and the altitude coverage of these structures are proportional to the ascending rate of the irregular bottomtype/bottomside F layer and the altitude coverage of the plume respectively. These structures are found to consist of substructures separated in times suggesting the presence of quasi-periodic striations within these irregular structures. On the basis of occurrence statistics during October 2001–December 2008, the occurrence rate of these structures is found to be 3.5%, indicating the nature of rare occurrence of these structures over São Luís. Moreover, their occurrence remains confined to the maximum solar-flux period (2001–2003) and to the summer months (October–January). The presence of these structures is a manifestation of the vertical coupling of E and F regions owing to the ambient electric fields of the ionosphere and the fringe-field associated with the F region plume.

Multifrequency Decoding of a Phased Array Doppler Sodar

Abstract Doppler sodar is being used for studying the lower part of atmospheric boundary layer (ABL) and wind profiling. To obtain maximum altitude coverage multifrequency transmission has been used along with more transmitted power. In this article, the implementation of multifrequency transmission of a Doppler sodar and its decoding to extract the atmospheric parameters are presented. This article also shows the advantage of profiling using multifrequency sodar operation. The range of frequency used for transmission is between 1700 and 2100 Hz. The decoded Doppler spectra have shown significant improvement in signal-to-noise ratio (SNR) as well as higher altitude coverage compared to single-frequency transmission and reception. Wind profiles obtained from sodar have been compared with data obtained from high-resolution GPS sonde balloons, which were launched from a place close to the sodar system. The authors observed that 30% more wind data height coverage in when transmission is in multifrequency mode; the consistency in wind estimate is also improved compared to the single-frequency transmission.

HF radio wave acceleration of ionospheric electrons: Analysis of HF‐induced optical enhancements

The shape of the HF‐pump modified electron energy distribution has long been a central question in the field of ionospheric high‐frequency radio wave modification experiments. Here we present estimates of the enhanced differential electron flux, from 1.9 to 100 eV based on optical multiwavelength (6300, 5577, 8446, and 4278 Å) data and 930 MHz incoherent scatter radar measurements of ion temperature, electron temperature and concentration. According to our estimate, the electron energy distribution is depleted in the energy range of 2 to approximately 4 eV, probably caused by electron excitation of vibrational states in N2. At the higher energies the electron energy distribution has a nonthermal tail. Further we show that the altitude variations of the four optical emissions should differ both in altitude coverage and center altitude.