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Non-Gaussian statistics of large-scale fields are routinely observed in data from atmospheric and oceanic campaigns and global models. Recent direct numerical simulations (DNSs) showed that large-scale intermittency in stably stratified flows is due to the emergence of sporadic, extreme events in the form of bursts in the vertical velocity and the temperature. This phenomenon results from the interplay between waves and turbulent motions, affecting mixing. We provide evidence of the enhancement of the classical small-scale (or internal) intermittency due to the emergence of large-scale drafts, connecting large- and small-scale bursts. To this aim we analyze a large set of DNSs of the stably stratified Boussinesq equations over a wide range of values of the Froude number ($Fr\approx 0.01-1$). The variation of the buoyancy field kurtosis with $Fr$ is similar to (though with smaller values than) the kurtosis of the vertical velocity, both showing a non-monotonic trend. We present a mechanism for the generation of extreme vertical drafts and vorticity enhancements which follows from the exact equations for field gradients., Comment: 7 pages, 4 figures

Northwest to southeast phase fronts with southwestward moving features are commonly observed in the nighttime midlatitude ionosphere during the solstice months at low solar activity. These features are identified as nighttime MSTIDs (medium scale traveling ionospheric disturbances). Initially, they were considered to be a manifestation of neutral atmospheric gravity waves. Later on, investigations showed that the nighttime MSTIDs are electrified in nature and mostly confined to the mid and low latitude ionosphere. Although the overall characteristics of the nighttime MSTIDs are mostly well understood, the causative mechanisms are not well known. Perkins instability mechanism was believed to be the cause of nighttime MSTIDs, however, the growth rate of the instability is too small to explain the perturbations observed. Recently, model simulations and observational studies suggest that coupling between sporadic-E layers and other type of E-region instabilities, and the F region may be relevant to explain the generation of the MSTIDs.In the present study simultaneous observation from OI 630 nm all-sky airglow imager, GPS-TEC, ionosonde and Meteor radars, are used to investigate the role of E and F region coupling on the generation of MSTIDs .Nighttime MSTIDs observed on three nights (14 March 2020, 23 March 2020 and 28 May 2020) in the OI 630 nm airglow images over Kuehlungsborn (54°07'N; 11°46'E, 53.79N mag latitude), Germany, are presented. Simultaneous detrended GPS-TEC measurements also shows presence of MSTIDs on these nights. In addition, simultaneous ionosonde observations over Juliusruh (54°37.7'N 13°22.5'E) show spread-F in the ionograms as well as sporadic-E layer occurrence. Furthermore, we also investigate the MLT region wind variations during these nights. The role of Es-layers and the interplay between the winds and Es-layers role on the generation of the MSTIDs will be discussed in detail in this presentation.

Atmospheric structures due to gravity waves, turbulence, Kelvin Helmholtz instabilities, etc. in the mesosphere are being studied with a varying of ground-based and satellite-based instruments. At scales less than 100 km, they are mainly studied with airglow imagers, lidars, and radars. Typical radar observations have not been able to resolve spatial and temporal ambiguities due to the strength of radar echoes, the size of the system, and/or the nature of the atmospheric irregularities. In this work we observed spatially and temporally resolved structures of PMSE with unprecedented horizontal resolution, using the improved radar imaging accuracy of the Middle Atmosphere Alomar Radar System (MAARSY) with the aid of a multiple-input multiple output (MIMO) technique. The studies are performed in both the brightness of the mesospheric echoes and their Doppler velocities. The resolutions achieved are less than 1 km in the horizontal direction, less than 300m in altitude, and less than 1 minute in time, in an area of ~15km x 15km around 85km of altitude. We present a couple of wavelike monochromatic events, one drifting with the background neutral wind, and one propagating against the neutral wind. Horizontal wavelengths, periods, and vertical and temporal coverage of the events are described and discussed. A theory of stratified turbulence is employed in the present study. In particular, it is shown that the structure that propagates with the background wind is a large-scale turbulent KHI event. Some important turbulence characteristics, such as a turbulent dissipation rate, buoyancy Reynolds number, and Froude number, support our conclusion.

The Ionospheric Continuous-wave E-region Bistatic Experimental Auroral Radar (ICEBEAR) is a VHF coherent scatter radar that operates with a field-of-view centered on 58°N, 106°W and measures characteristics of ionospheric E-region plasma density irregularities. The initial operations of ICEBEAR utilized a wavelength-spaced linear receiving array to determine the angle of arrival of the ionospheric scatter at the receiver site. Initially only the shortest baselines were used to determine the angle of arrival of the scatter. This publication uses this linear antenna array configuration and expands on the initial angle of arrival determination by including all the cross-spectra available from the antenna array to determine both the azimuthal angle of arrival and the azimuthal extent of the incoming ionospheric scatter. This is accomplished by fitting Gaussian distributions to the complex coherence of the signal between different antennas and deriving the azimuthal angle and extent based on the best fit. Fourteen hours of data during an active ionospheric period (March 10, 2018, 0–14 UT) were analyzed to investigate the Gaussian fitting procedure and determine its feasibility for implementation with ICEBEAR. A comparison between mapped scatter, both neglecting azimuthal extent and including azimuthal extent is presented. It demonstrates that the azimuthal extent of the ionospheric E-region scatter is very important for accurately portraying and analyzing the ICEBEAR measurements.

We describe in this study the analysis of small and large horizontal-scale gravity waves from datasets composed of images from multiple mesospheric airglow emissions as well as multistatic specular meteor radar (MSMR) winds collected in early November 2018, during the SIMONe–2018 (Spread-spectrum Interferometric Multi-static meteor radar Observing Network) campaign. These ground-based measurements are supported by temperature and neutral density profiles from TIMED/SABER (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry) satellite in orbits near Kühlungsborn, northern Germany (54.1∘ N, 11.8∘ E). The scientific goals here include the characterization of gravity waves and their interaction with the mean flow in the mesosphere and lower thermosphere and their relationship to dynamical conditions in the lower and upper atmosphere. We have obtained intrinsic parameters of small- and large-scale gravity waves and characterized their impact in the mesosphere via momentum flux (FM) and momentum flux divergence (FD) estimations. We have verified that a small percentage of the detected wave events is responsible for most of FM measured during the campaign from oscillations seen in the airglow brightness and MSMR winds taken over 45 h during four nights of clear-sky observations. From the analysis of small-scale gravity waves (λh < 725 km) seen in airglow images, we have found FM ranging from 0.04–24.74 m2 s−2 (1.62 ± 2.70 m2 s−2 on average). However, small-scale waves with FM > 3 m2 s−2 (11 % of the events) transport 50 % of the total measured FM. Likewise, wave events of FM > 10 m2 s−2 (2 % of the events) transport 20 % of the total. The examination of large-scale waves (λh > 725 km) seen simultaneously in airglow keograms and MSMR winds revealed amplitudes > 35 %, which translates into FM = 21.2–29.6 m2 s−2. In terms of gravity-wave–mean-flow interactions, these large FM waves could cause decelerations of FD = 22–41 m s−1 d−1 (small-scale waves) and FD = 38–43 m s−1 d−1 (large-scale waves) if breaking or dissipating within short distances in the mesosphere and lower thermosphere region.

A new incoherent scatter radar called EISCAT 3D is being constructed in northern Scandinavia. It will have the capability to produce volumetric images of ionospheric plasma parameters using aperture synthesis radar imaging. This study uses the current design of EISCAT 3D to explore the theoretical radar imaging performance when imaging electron density in the E region and compares numerical techniques that could be used in practice. Of all imaging algorithms surveyed, the singular value decomposition with regularization gave the best results and was also found to be the most computationally efficient. The estimated imaging performance indicates that the radar will be capable of detecting features down to approximately 90×90 m at a height of 100 km, which corresponds to a ~0.05° angular resolution. The temporal resolution is dependent on the signal-to-noise ratio and range resolution. The signal-to-noise ratio calculations indicate that high-resolution imaging of auroral precipitation is feasible. For example, with a range resolution of 1500 m, a time resolution of 10 s, and an electron density of 2·10¹¹ m⁻³, the correlation function estimates for radar scatter from the E region can be measured with an uncertainty of 5 %. At a time resolution of 10 s and an image resolution of 90×90 m, the relative estimation error standard deviation of the image intensity is 10 %. Dividing the transmitting array into multiple independent transmitters to obtain a multiple-input–multiple-output (MIMO) interferometer system is also studied, and this technique is found to increase imaging performance through improved visibility coverage. Although this reduces the signal-to-noise ratio, MIMO has successfully been applied to image strong radar echoes as meteors and polar mesospheric summer echoes. Use of the MIMO technique for incoherent scatter radars (ISRs) should be investigated further. En ny inkoherent spredningsradar, kalt EISCAT 3D, med lokasjoner i Nord-Norge, -Sverige og -Finland er under bygging. Radaren vil kunne produsere volumetriske bilder av plasmaparametre gjennom apertursynteseradar-avbildning. Denne studien bruker den nåværende planen for EISCAT 3D til å utforske den teoretiske avbildningsytelsen for avbildningaer av elektrontetthet i E-laget og sammenligner numeriske teknikker som vil kunne brukes i praksis. Blant de undersøkte avbildningsalgoritmene ga singulærverdidekomponering med regularisering best resultater og den var også blant de mest effektive algoritmene. Radaren vil kunne oppdage variasjoner med størrelser ned mot 90x90 ved 100 km distanse. Dette tilsvarer ei vinkeloppløsning på 0,05°. Tidsoppløsninga er avhengig av signal-til-støyraten, og distanseoppløsninga gjennom pulskodinga. Signal- og støyeffektberegninger tyder på at avbildning av nordlysnedbør er mulig med denne oppløsninga. Et eksempel med 1500 m distanseoppløsning, 10 s tidsoppløsning og ei elektrontetthet på 2·10¹¹ m⁻³ viser at korrelasjonsfunksjonen for spredning fra E-laget kan måles med ei usikkerhet på 5 %. Når avbildningsoppløsninga er 90x90 m, er usikkerheta på 10 %. Ved å dele opp senderantennefeltet i flere uavhengige sendere får man et multiple-input–multiple-output (MIMO)-system som viser seg å gjennomføre avbildninga bedre. Sjøl om signalet da blir svakere, har MIMO blitt brukt for å avbilde sterke radarekko slik som meteorer og polare mesosfæriske sommerekko. Bruken av MIMO-teknikken for inkoherent spredningsradarer burde undersøkes nærmere.

During September of 2019, a state-of-the-art multistatic meteor radar system called SIMONe (Spread Spectrum Interferometric Multistatic meteor radar Observing Network) was installed in southern Patagonia, Argentina. Its main goal being the study of mesospheric waves in one of the (theoretically predicted) most dynamically active regions of the world. SIMONe Patagonia consists of 5 linearly polarized Yagi antennas in a pentagon configuration on transmission, and 5 dual-polarization single Yagi antennas on reception, situated between 30 and 270 km from the transmitters, which locate at 49.6° S. Combining measurements from the 5 links allows for more accurate estimations of mean winds and horizontal momentum flux for altitudes between 75 and 105 km. Furthermore, given the significantly higher amount of meteor detections, one can determine wind fields within the limits of the illuminated volume every 1 hour and 1 km in time and height, respectively. Preliminary results indicate a dominant semidiurnal oscillation in both the zonal and meridional wind components, as well as an enhanced and sustained (in time) gravity wave activity, especially above 90 km of altitude. In addition, momentum flux analysis reveals that the gravity wave activity is stronger than in other parts of the Southern Hemisphere, confirming that Patagonia is indeed a very active region at mesospheric heights.

The Ionospheric Continuous-wave E-region Bistatic Experimental Auroral Radar (ICEBEAR) is located in Canada and has a field of view centered at (58°N, 106°W) overlooking the terrestrial auroral zone. This 49.5 MHz coherent scatter radar measures plasma density irregularities in the E-region ionosphere using a pseudo random noise phase modulated continuous-wave (CW) signal. ICEBEAR uses this coded CW signal to obtain simultaneous high temporal (1 s) and spatial (1.5 km) resolutions of E-region plasma density turbulence over a 600 km x 600 km field of view, providing insights into the Farley-Buneman plasma density instability and wave-like structures evident in the coherent scatter. The initial results from ICEBEAR were obtained with a 1D receiving array, providing azimuthal angle of arrival details of the incoming scattered signal. This azimuthal determination, along with the range determined using the coded signal, allowed the scatter to be mapped in 2D. A recent reconfiguration of the receiving array has allowed the elevation angle of the received signal to be calculated, providing 3D determination of the location of the plasma density irregularities. This presentation will demonstrate the capabilities of ICEBEAR, displaying measurements of highly dynamic plasma density irregularities with wave-like behaviour on 1 second time scales.

Investigation of turbulence in the polar mesopause is essential for a better understanding of dynamical or mixing processes in the region. Polar Mesospheric Summer Echoes (PMSEs), occurring at mesopause altitudes during the summer season, are known to be a result of turbulence-induced fluctuations in the refractive index. The presence of ice particles controls and reduce the free-electron diffusivity in D region plasma, which in turn leads to complex, strong radar echoes at very high frequencies.Often, Doppler spectral width of radar measurements are associated with the strength of turbulence in the target area and traditionally used to estimate turbulent kinetic energy dissipation rates, a fundamental parameter of the turbulence processes. Besides the cooling of summer mesopause region induced by GW drag, the turbulence produced by GW breaking contributes to the total energy budget due to release of turbulent kinetic energy to heat. We use PMSE spectral width measurements observed by Middle Atmosphere Alomar Radar System (MAARSY) during summer of 2016 to study their summer temporal mean profiles as well as temporal evolution and connection to the atmospheric turbulence at PMSE altitudes - 80 and 90 km. The current theoretical models suggest that the radar reflectivity should correlate to the strength of the turbulence; however, such a relation is mainly observed for the weaker PMSEs. The mean summer behaviour of estimated turbulent kinetic energy dissipation rates shows an increase from lower altitudes up to 90 km. It should be noticed that spectral width measurements contain additional broadening rather than turbulence, so derived energy dissipation rates are “upper values” than expected from pure turbulence. The results are still slightly lower than those known from climatology obtained from rocket soundings, mostly at altitudes close to the maximum occurrence of PMSE, 86-87 km.We discuss a possible consequence of spectral width measurements under strong PMSEs. In such conditions, the strength of the echo does not correlate with the turbulence intensity, and the observed spectral width is weaker. However, the uniform distribution of spectral width values throughout the echo power is expected from the present theoretical understandings. Based on previous studies, strong PMSEs can also be observed during fossil turbulence. The interpretation of connection the spectral with measurements under fossil turbulence with the turbulence energy dissipation rates and the possibility of using PMSEs for the turbulence studies will be discussed.

The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69 $$^\circ $$ N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.

This talk presents a solution to the problem of specular meteor radar interferometry cast on the complex plane. This technique is applicable to all meteor radars at the output of the receiver's pulsed Doppler signal processing chain. Specular meteor trail radar echoes are modeled as exponentially decaying sinusoids impinging on a sparse interferometric antenna array. A set of 10 underdense meteor trail radar parameters are estimated using nonlinear least squares optimization of the physical model parameters. Meteor trail spatial locations are determined by estimating all parameters through global optimization of the complex-valued interferometric angle-of-arrival equations. Statistical precision of the estimated meteor radar parameters of Doppler, diffusion coefficient, angle-of-arrival, and height are subsequently characterized by calculation of the full covariance matrix. The determination of statistical uncertainties in the measured parameters is a key innovation in the field, as it lays the groundwork for calculating the statistical precision of the subsequently derived wind field.

Sensitive long-wavelength radar observations of absolute velocity never previously published from Jicamarca are brought to bear on the long-standing problem of radar detection of slow-moving meteors. Attention is devoted to evaluating the ionization coefficient β(V) in the critically important velocity range of 11-20 km/s in recent laboratory measurements of Thomas et al. (2016). Theoretical predictions for β(V) based on the laboratory data, on Jones (1997), on Janches et al. (2014) and on Verniani and Hawkins (1964) are used to correct the incoming meteor velocities measured with the sensitive Jicamarca HPLA radar operating at 6 m wavelength. All corrected distributions are consistent with the predictions of the Nesvorný model in showing pronounced monotonic increases down to the escape velocity (11km/s). Such distributions may be essential to explaining the pronounced ledge in nighttime electron density and the rapid disappearance of electrons in meteor trails in the altitude range of 80-85 km.

A recent study has hypothesized that polar mesospheric summer echoes (PMSEs) might consist mainly of localized isotropic scattering. These results have been inferred from indirect measurements. Using radar imaging with the Middle Atmosphere Alomar Radar System (MAARSY), we observed horizontal structures that support our previous findings. We observe that small-scale irregularities, causing isotropic scattering, are organized in patches. We find that patches of PMSEs, as observed by the radar, are usually smaller than 1 km. These patches occur throughout the illuminated volume, supporting that PMSEs are caused by localized isotropic or inhomogeneous scattering. Furthermore, we show that imaging can be used to identify side lobe detections, which have a significant influence even for narrow beam observations. Improved spectra estimations are obtained by selecting the desired volume to study parameters such as spectral width and to estimate the derived energy dissipation rates. In addition, a combined wide beam experiment and radar imaging is used to resolve the radial velocity and spectral width at different volumes within the illuminated volume.

Publisher's version available at https://doi.org/10.1029/2018RS006688 We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements.

The Andenes specular meteor radar shows meteor-trail diffusion rates increasing on average by ~ 20 % at times and locations where a lidar observes noctilucent clouds (NLCs). This high-latitude effect has been attributed to the presence of charged NLC but this study shows that such behaviors result predominantly from thermal tides. To make this claim, the current study evaluates data from three stations, at high-, mid-, and low-latitudes, for the years 2012 to 2016, comparing diffusion to show that thermal tides correlate strongly with the presence of NLCs. This data also shows that the connection between meteor-trail diffusion and thermal tide occurs at all altitudes in the mesosphere, while the NLC influence exists only at high-latitudes and at around peak of NLC layer. This paper discusses a number of possible explanations for changes in the regions with NLCs and leans towards the hypothesis that relative abundance of background electron density plays the leading role. A more accurate model of the meteor trail diffusion around NLC particles would help researchers determine mesospheric temperature and neutral density profiles from meteor radars.

This talk presents a solution to the problem of specular meteor radar interferometry cast on the complex plane. This technique is applicable to all meteor radars at the output of the receiver's pulsed Doppler signal processing chain. Specular meteor trail radar echoes are modeled as exponentially decaying sinusoids impinging on a sparse interferometric antenna array. A set of 10 underdense meteor trail radar parameters are estimated using nonlinear least squares optimization of the physical model parameters. Meteor trail spatial locations are determined by estimating all parameters through global optimization of the complex-valued interferometric angle-of-arrival equations. Statistical precision of the estimated meteor radar parameters of Doppler, diffusion coefficient, angle-of-arrival, and height are subsequently characterized by calculation of the full covariance matrix. The determination of statistical uncertainties in the measured parameters is a key innovation in the field, as it lays the groundwork for calculating the statistical precision of the subsequently derived wind field.

Plasma irregularities in the postsunset equatorial valley region ionosphere are investigated experimentally and through numerical simulation. Coherent radar backscatter observed at the Jicamarca Radio Observatory shows two classes of irregularities in different altitude bands – one mainly below about 125 km and the other mainly above. Irregularities in both bands are organized into wavefronts with wavelengths of a few km. However, only the irregularities in the high-altitude band exhibit consistent propagation speeds and directions. Some previous observations of irregularities in the nighttime electrojet suggest that gravity waves may sometimes influence their morphology. The possibility that the valley-region irregularities are also related to gravity waves (GWs) is therefore investigated numerically. A model of a GW packet propagating through a tidal wind field is used to drive another model which predicts the resulting ionospheric electrodynamics. The combined simulation shows that GWs can induce field-aligned currrents and excite resistive drift waves which could be responsible for the valley-region irregularities in the high-altitude band. The GWs also induce irregularities in the upper E region directly through simple dynamo action which subsequently deform under the influence of shear flow. This may explain the irregularities in the low-altitude band.

The initial results of a two year simultaneous optical-radar meteor campaign are described. Analysis of 105 double-station optical meteors having plane of sky intersection angles greater than 5 degrees and trail lengths in excess of 2 km also detected by the Middle Atmosphere Alomar Radar System (MAARSY) as head echoes was performed. These events show a median deviation in radiants between radar and optical determinations of 1.5 degrees, with 1/3 of events having radiant agreement to less than one degree. MAARSY tends to record average speeds roughly 0.5 km/s and 1.3 km higher than optical records, in part due to the higher sensitivity of MAARSY as compared to the optical instruments. More than 98% of all head echoes are not detected with the optical system. Using this non-detection ratio and the known limiting sensitivity of the cameras, we estimate that the limiting meteoroid detection mass of MAARSY is in the 10-9 kg to 10-10 kg (astronomical limiting meteor magnitudes of +11 to +12) appropriate to speeds from 30-60 km/s. There is a clear trend of higher peak RCS for brighter meteors between 35 and -30 dBsm. For meteors with similar magnitudes, the MAARSY head echo radar cross-section is larger at higher speeds. Brighter meteors at fixed heights and similar speeds have consistently, on average, larger RCS values, in accordance with established scattering theory. However, our data show RCS ~ v/2, much weaker than the normally assumed RCS ~ v^3, a consequence of our requiring head echoes to also be detectable optically. Most events show a smooth variation of RCS with height broadly following the light production behavior. A significant minority of meteors show large variations in RCS relative to the optical light curve over common height intervals, reflecting fragmentation or possibly differential ablation., Comment: 44 pages, 13 figures, accepted April 19, 2017 to Planetary and Space Science

We present observations of polar mesospheric summer echoes (PMSE) using the Middle Atmosphere Alomar Radar System in Northern Norway (69.30° N, 16.04° E). The radar is able to resolve PMSE at high spatial and temporal resolution and to perform pulse-to-pulse beam steering. In this experiment, 81 oblique beam directions were used with off-zenith angles up to 25°. For each beam pointing direction and range gate, coherent radar imaging was applied to determine the mean backscatter location. The location of the mean scatterer in the beam volume was calculated by the deviation from the nominal off-zenith angle of the brightest pixel. It shows that in tilted beams with an off-zenith angle greater than 5°, structures appear at the altitudinal edges of the PMSE layer. Our results indicate that the mean influence of the location of the maximum depends on the tilt of the beam and on the observed area of the PMSE layer. At the upper/lower edge of the PMSE layer, the mean backscatter has a greater/smaller off-zenith angle than the nominal off-zenith angle. This effect intensifies with greater off-zenith beam pointing direction, so the beam filling factor plays an important role in the observation of PMSE layers for oblique beams.

In this paper we study the effects of absorption and Faraday rotation on measurements of polar mesosphere summer echoes (PMSE). We found that such effects can produce significant reduction of signal-to-noise ratio (SNR) when the D region electron densities (Ne) are enhanced, and VHF radar systems with linearly polarized antennas are used. In particular we study the expected effects during the strong solar proton event (SPE) of July 2000, also known as the Bastille day flare event. During this event, a strong anti-correlation between the PMSE SNR and the D-region Ne was found over three VHF radar sites at high latitudes: Andoya, Kiruna, and Svalbard. This anti-correlation has been explained (a) in terms of transport effects due to strong electric fields associated to the SPE and (b) due to a limited amount of aerosol particles as compared to the amount of D-region electrons. Our calculations using the Ne profiles used by previous researchers explain most, if not all, of the observed SNR reduction in both time (around the SPE peak) and altitude. This systematic effect, particularly the Faraday rotation, should be recognized and tested, and possibly avoided (e.g., using circular polarization), in future observations during the incoming solar maximum period, to contribute to the understanding of PMSE during enhanced D region Ne.

Specular meteor radars have provided essential information about the mesospheric/lower thermosphere (MLT) dynamics. The Canadian Meteor Orbit Radar has been conducting continuous meteor echo observations in a fixed, stable configuration since 2002. Here we present estimates of the neutral air density variations derived from observations of the meteor peak flux altitude. Using a simple model assuming a linear trend and a sinusoidal solar cycle we derived a trend of a decreasing neutral density of 5.8 ± 1.1% per decade at approximately 91 km altitude and an amplitude across the most recent solar cycle, the solar cycle of 2.4 ± 0.7% for solar cycle 23/24. The long-term trend of decreasing neutral air density in the MLT is in good agreement with the model results from Akmaev et al. (2006).

The Middle Atmosphere Alomar Radar System (MAARSY) on the Norwegian island of Andoya is a 53.5 MHz monostatic radar with an active phased array antenna. The total array consists of 433 3-element linearly polarized Yagi antennas and can be configured to receive with multiple antenna sections (currently up to 16 complex receiving channels). In order to exploit its multiple-receiver capability for improving the space-time ambiguities of atmospheric/ionospheric targets, the phase difference between receiving channels has to be measured with good precision. Such phases are intrinsic to the system and are due to different cable lengths, pointing positions, filters, attenuators, amplifiers, antenna impedances, etc. In this work, we have operated MAARSY in a radio passive mode to observe the strong radio signals of Cassiopeia A and Cygnus A sources and calibrate the receiving system. By using the so-called fringe-stopping method, we have been able to calibrate the 16 complex channels, including the smaller antenna module that can be used, i.e., an Hexagon consisting of 7 Yagi antennas. The measured phases have been obtained with a mean standard deviation of ∼ 5 °. We have tested the validity of such phases using meteor-head echoes with different configurations and pointing directions. Given that the procedure is easy to implement, it should be used in a routine manner either to corroborate the stability of the system or to measure new phases after upgrades or repairs.

[1] For the first time, equatorial plasma depletions (EPDs) have been imaged in the longitude-altitude plane using radiotomography. High-resolution (~10 km) reconstructions of electron density were derived from total electron content (TEC) measurements provided by a receiver array in Peru. TEC data were obtained from VHF/UHF signals transmitted by the Coherent Electromagnetic Radio Tomography (CERTO) beacon on the C/NOFS satellite. EPDs generated premidnight were observed near dawn. On one night, the bubble densities were highly reduced, 100–1000 km wide, and embedded within a layerlike ionosphere. Three nights later, the EPDs exhibited similar features but were embedded in a locally uplifted ionosphere. The C/NOFS in situ instruments detected a dawn depletion where the reconstruction showed lifted EPDs, implying that the postmidnight electric fields raised sections of ionosphere to altitudes where embedded/reactivated fossil EPDs were detected as dawn depletions. Satellites flying under domelike distortions of the ionosphere may observe these distortions as broad plasma decreases (BPDs).

A multi-beam incoherent scatter radar technique has been developed at the Jicamarca Radio Observatory in order to maximize the number of ionospheric parameters that can be estimated simultaneously. The technique interleaves radar observations with antenna beams pointing perpendicular and oblique to the Earth's magnetic field. For the estimation of the ionospheric parameters, we have modeled the magnetic aspect angle variations of the signal power and cross-correlation data measured in multiple directions. The data model, formulated in terms of soft-target radar equations, was built based on the theories of incoherent scattering and magneto-ionic propagation. Applying a nonlinear least-squares inversion algorithm, we have succeeded in measuring simultaneously ionospheric electron densities Ne, electron-to-ion temperature ratios T e / T i as well as vertical and zonal plasma drifts. In the past, radar users had to choose between either perpendicular-to- B or oblique modes, thus, the application of this technique extends the current capabilities of the Jicamarca radar.

[1] We show here that VHF signals scattered from the 150 km region above Jicamaca exhibit two distinct types of features. In one type (type A), the Doppler spectral width increases with the echo strength and the corresponding signal-to-noise ratio (SNR). A second type (type B) of higher SNR echoes exhibits SNR-independent Doppler spectral widths that are much narrower than those observed in the first type. The type A echo population is by far the dominant population. Comparisons with earlier data sets collected at Jicamarca and elsewhere suggest that the type A and type B are likely to be associated with a naturally enhanced incoherent scattering (NEIS) process and the unstable growth of field-aligned irregularities (FAIs), respectively. We conjecture that small radar systems operated near the geomagnetic equator that have reported 150 km echo observations detected FAI echoes and that the NEIS echoes can only be seen by high sensitivity systems.

[1] We report the role that the ambiguity function (AF) plays on the determination of signal to noise ratio (SNR) collected from meteor head echoes. Theoretical analysis shows that any measured meteor SNR exhibits temporal ripples whose shape is related to both the transmitted pulse envelope and the filter impulse response of the receiver. These theoretical findings are corroborated with (1) experimental meteor data recorded with Jicamarca 50 MHz radar (11.95° S, 76.87° W) and (2) simulated meteor data obtained by replicating the acquisition system of Jicamarca. A statistical analysis of the experimental meteor data reveals that at least 14% of the population collected each day at Jicamarca exhibits these ripples. On the remaining 86% of meteor events, the ripples cannot be distinguished due to noise, contamination from other sources of scattering (i.e., nonspecular echoes), and ensemble average applied to the data. In general, these ripples demonstrate the importance of obtaining an accurate model of a radar system to avoid misinterpretation of SNR.

Radar cross sections (RCS) of mesospheric layers at 50 MHz observed at Jicamarca, Peru, range from 10−18 to 10−16 m−1, three orders of magnitudes smaller than cross sections reported for polar mesospheric winter echoes during solar proton events and six orders of magnitude smaller than polar mesospheric summer echoes. Large RCS are found in thick layers around 70 km that also show wide radar spectra, which is interpreted as turbulent broadening. For typical atmospheric and ionospheric conditions, volume scattering RCS for stationary, homogeneous, isotropic turbulence at 3 m are also in the range 10−18 to 10−16 m−1, in reasonable agreement with measurements. Moreover, theory predicts maximum cross sections around 70 km, also in agreement with observations. Theoretical values are still a matter of order-of-magnitude estimation, since the Bragg scale of 3 m is near or inside the viscous subrange, where the form of the turbulence spectrum is not well known. In addition, steep electron density gradients can increase cross-sections significantly. For thin layers with large RCS and narrow spectra, isotropic turbulence theory fails and scattering or reflection from anisotropic irregularities may gain relevance.

In this work we present a novel use of the Poker Flat Incoherent Scatter Radar (PFISR) to study meteor-head echoes with wide (W) beams. Until now, most of the meteor-head echo studies have been performed with High-Power Large-Aperture Radars (HPLARs) using very narrow (N) beams. At PFISR we have implemented an antenna compression approach using a defocusing scheme, similar to Chirp (linear frequency modulation) in pulse compression. The resulting effective beam is ∼ 3 times wider than the narrowest PFISR beam. Using the signal-to-noise ratio (SNR) as a proxy measurement of cross-section, from the combined W and N beam experiments, our main results are: (1) observed meteors in the W beam are approximately half the number of meteors observed in the N beam, (2) we detected 10 times more large cross-section (strong) meteors ( > 15 dB if they were measured by the N mainlobe) than using only the N beam, and (3) more than 15% of the total N meteors were observed in the N sidelobes, therefore being at least 20 dB stronger if they were observed in the N mainlobe. Our results are summarized in a corrected distribution of relative meteor cross-sections as if all of them were observed with the N mainlobe, namely correcting their SNR values depending on where in the beam they were detected (sidelobes or mainlobe). In addition, we show a qualitative meteor cross-section distribution that one can obtain combining W and N beams. The resulting distribution is incomplete, since the W beam is not sensitive enough to detect the very small (weak) meteors, but could provide new information about the large cross-section events.

We present the first clear observations of meteor shower activity from meteor-head echoes detected by a high-power large-aperture radar (HPLAR). Such observations have been performed at the Jicamarca VHF radar using its interferometric capabilities allowing the discrimination of meteor shower echoes from the much more frequent sporadic meteors. Until now, HPLARs were unable to distinguish meteor shower from the much more common sporadic meteor ones. In this work we have been able to detect and characterize the η -Aquariids (ETA) as well as the Perseids (PER) showers. The shower activity is more conspicuous for the ETA than for the PER shower due to the more favorable geometry. Namely, PER meteors come from low elevation angles, experiencing more filtering due to the combined Earth–atmosphere–radar instrument. In both cases, there is an excellent agreement between the measured mean velocity of the shower echoes and their expected velocity, within a fraction of 1 km s −1 . Besides the good agreement with expected visual results, HPLARs observe meteors with a variety of particles sizes and masses, not observed by any other technique. Taking into account the different viewing volumes, compare to optical observations Jicamarca observes more than 1000 times more ETA meteors. Our results indicate that Jicamarca and other HPLARs are able to detect the echoes from meteor showers, but without interferometric capabilities such populations are difficult to identify just from their velocity distributions, particularly if their velocity distributions are expected to be similar to the more dominant distributions of sporadic meteors.

We present experimental evidence and modeling results which indicate that eastward thermospheric wind is the primary controlling factor of equatorial spread-F initiation in the post-sunset ionosphere. Eastward wind-driven Pedersen currents are able to polarize F-region density perturbations with westward tilting wavefronts into rapidly growing modes to trigger the formation of spread-F bubbles. The described process is so rapid that seeding requirements of spread-F initiation by external factors such as gravity waves are effectively eliminated.

Electric fields have been determined at three longitudes corresponding to Peru, India, and the Philippines. We compare these fields to applying a frequency-dependent linear transfer function (TF) to the dawn-to-dusk component of the interplanetary electric field (IEF). The TF is based on four years of simultaneous observations of the IEF and equatorial data. The model gives good results for the prompt penetrating electric field (PPE) in the case of an oscillatory IEF with a period in the 1–2 h range, when the interplanetary magnetic field remains southward for a long period and, to a lesser extent, when the IEF can be described as a square wave. There is evidence that a disturbance dynamo (DD) effect contributes on the dayside, where it leads to suppression of the normal quiet time pattern. A very strong counter-electrojet was seen at two locations during a time of persistent B z south and was not predicted by the model or a linear scaling of the IEF. This suggests that suppression (and even reversal) of the E-region dynamo can occur in a large storm. Both the data and the model yielded a long-lived response to a sustained southward interplanetary magnetic field. Previously suggested B y effects on equatorial electric fields are confirmed by a sequence of three distinct spikes in the B y component of the IMF, one of which had no associated B z change and yet was reproduced by two independent ground magnetometer-based electric field determinations. The sometimes remarkable agreement of a linear relationship between the equatorial and interplanetary electric fields shown here and elsewhere remains somewhat mysterious for such an apparently complex system.

Penetration of the interplanetary electric field (IEF) to the middle- and low-latitude ionosphere has been investigated for nearly four decades. Most previous studies focused on the correlation between the interplanetary and ionospheric electric field perturbations. Very little attention has been paid to a quantitative relationship except for a recent case analysis by Kelley et al. [2003. Penetration of the solar wind electric field into the magnetosphere/ionosphere system. Geophysical Research Letters 30(4), 1158. doi: 10.1029/2002GL016321 ]. In this paper, we present a statistical result of the efficiency of IEF penetration to the dayside equatorial ionosphere; the efficiency is defined as the ratio of the change of the equatorial ionospheric electric field to the change of the IEF. The Jicamarca incoherent scatter radar has made continuous operation with a coherent scatter mode since 2001, and the radar data of equatorial ionospheric electric fields are used in our statistics. On the basis of data statistics, we derive an empirical value of 9.6% for the efficiency of penetration. We apply this empirical formula to the observations and numerical simulations of storm-time penetration electric fields over a prolonged interval of southward interplanetary magnetic field. The prediction of the formula is in good agreement with case studies and with results from first-principle simulations of the coupled magnetosphere–ionosphere–thermosphere system. We conclude that the IEF can continuously penetrate to the low-latitude ionosphere without significant attenuation for many hours during the main phase of magnetic storms.

Four years of magnetometer data from two locations in Peru, one at the equator and one off the equator, have been converted to electric fields and their frequency characteristics (fluctuation spectra) examined. In the frequency range from 0.1 to 30 cycles per hour, the average spectrum monotonically decreases. However, it deviates from a power law in the range 0.3–3 cycles per hour especially for high levels of activity. The integrated power above 0.15 cycles per hour is a strong function of K p indicating that much of the fluctuations in the ionospheric equatorial field are of solar wind or magnetospheric origin. This result is in agreement with a previous power spectral study of low, middle, and high latitude fields using radars. The observed field strengths are lower than the ones observed in a previous study using balloon data at middle and high latitudes when the fields are projected to the equatorial plane. Simultaneous interplanetary electric field (IEF) data are compared to the equatorial field to determine how strong a relationship exists and to determine the amplitude and phase of their ratio as a function of frequency—an estimate of the average transfer function of the system. This function displays a bandpass-like form with a peak near 0.5 cycles per hour. This peak and evidence for a ringing of the time domain response suggests a weakly resonant system indicating some capacitance in addition to the inductance of the ring current and the resistance of the ionosphere. Case studies show that application of this function to IEF data yields good results and supports the notion that the response of the equatorial field to long-duration IEF polarities can last for many hours. Application of the function to test inputs such as pulses and triangle waves support this result. At high frequencies, we suggest that mapping of small-scale MHD turbulence is less effective than high frequency related transitions in the IEF.

The 50-MHz MST radar at Jicamarca Radar Observatory (JRO) can detect atmospheric turbulence on the Bragg scale of 3 m in the daytime mesosphere (∼60–85 km). Since 2002, the radar was operated for a certain number of days each year collecting 1-min Doppler spectra in four off-vertical (2.5°) beam directions and 150 m resolution. The spectral widths have been used to compute the kinetic energy dissipation rate e due to atmospheric turbulence. A small beam broadening effect has been removed from the observed spectral widths. The daily median energy dissipation rates e increase from 5 to 30 mW/kg between 67 and 80 km, and the eddy diffusivities increase from 3 to 20 m 2 /s, consistent with similar studies conducted by two other large 50-MHz radars in Japan and India. The energy dissipation rates are about the same magnitude as the e estimates for low-latitudes from a global model and are larger than the averages from rocket observations at high-latitudes, confirming previous comparisons.

Sounding rocket experiments were conducted during the NASA EQUIS II campaign on Kwajalein Atoll designed to elucidate the electrodynamics and layer structure of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each comprised of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets deployed trails of trimethyl aluminum (TMA) which yielded wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. Electron density profiles were also measured with rocket beacons and an ionosonde. Strong plasma shear flow was evident in both experiments. Bottom-type scattering layers were observed mainly in the valley region, below the shear nodes, in westward-drifting plasma strata. The layers were likely produced by wind-driven interchange instabilities as proposed by Kudeki and Bhattacharyya (1999). In both experiments, the layers were patchy and distributed periodically in space. Their horizontal structure was similar to that of the large-scale plasma depletions that formed later at higher altitude during ESF conditions. We argue that the bottom-type layers were modulated by the same large-scale waves that seeded the ESF. A scenario where the large-scale waves were themselves produced by collisional shear instabilities is described.

We present here the first Jicamarca observations of incoherent scatter radar (ISR) spectra detected from E- and D-region altitudes. In the past such observations have not been possible at Jicamarca due a combined effect of strong equatorial electrojet (EEJ) clutter and hardware limitations in the receiving system. The observations presented here were made during weak EEJ conditions (i.e., almost zero zonal electric field) using an improved digital receiving system with a wide dynamic range and a high data throughput. The observed ISR spectra from E- and D-region altitudes are, as expected, narrow and get even narrower with decreasing altitude due to increasing ion-neutral collision frequencies. Therefore, it was possible to obtain accurate spectral measurements using a pulse-to-pulse data analysis. At lower altitudes in the D-region where signal correlation times are relatively long we used coherent integration to improve the signal-to-noise ratio of the collected data samples. The spectral estimates were fitted using a standard incoherent scatter (IS) spectral model between 87 and 120 km, and a Lorentzian function below 110 km. Our preliminary estimates of temperature and ion-neutral collisions frequencies above 87 km are in good agreement with the MSISE-90 model. Below 87 km, the measured spectral widths are larger than expected, causing an overestimation of the temperatures, most likely due to spectral distortions caused by atmospheric turbulence.

Vertical shear in the zonal plasma drift speed is apparent in incoherent and coherent scatter radar observations of the bottomside F region ionosphere made at Jicamarca from about 1600–2200 LT. The relative importance of the factors controlling the shear, which include competition between the E and F region dynamos as well as vertical currents driven in the E and F regions at the dip equator, is presently unknown. Bottom-type scattering layers arise in strata where the neutral and plasma drifts differ widely, and periodic structuring of irregularities within the layers is telltale of intermediate-scale waves in the bottomside. These precursor waves appear to be able to seed ionospheric interchange instabilities and initiate full-blown equatorial spread F. The seed or precursor waves may be generated by a collisional shear instability. However, assessing the viability of shear instability requires measurements of the same parameters needed to understand shear flow quantitatively - thermospheric neutral wind and off-equatorial conductivity profiles. Keywords. Ionosphere (Equatorial ionosphere; ionospheric irregularities) – Space plasma physics (Waves and instabilities)

It is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather.

We present preliminary radar results obtained with a new method of spectrum estimation of moderately overspread equatorial spread F (ESF) echoes over Jicamarca. This new method is similar to the classical pulse-to-pulse methods but with nonuniform pulse spacings, i.e., using aperiodic transmitter pulses, allowing us to avoid range and frequency aliasing, at the cost of extra clutter at some ranges. We also present different ways of dealing with the extra clutter, depending on the spectral characteristics of the clutter. We have identified two kinds of clutter: ground clutter (due to mountains and transmitter pulses) and ionospheric clutter (due to ESF and equatorial electrojet echoes). Preliminary radar results are very encouraging, and ways of improving them are proposed based on Monte Carlo simulations. Particularly, we propose alternating the phases of the uncoded aperiodic pulses using random binary phase codes to flatten the spectrum of the ionospheric clutter. The theoretical implications of the newly estimated spectra, particularly from topside echoes, are presented and discussed in a companion paper.

A theory predicting the shape of coherent scatter radar Doppler spectra from field aligned plasma irregularities is presented. The theory pertains to electrostatic turbulence in guiding center plasmas and utilizes a test particle approach. Numerous simplifying assumptions are made which limit the applicability of the theory. We find that the theory may hold for the case of topside equatorial spread F irregularities in the so-called inertial regime. Here, the Doppler spectra are predicted to be essentially Gaussian in shape with a width equal to the RMS velocity of the plasma in the scattering volume. The theory is tested against the output of a direct numerical simulation of the inertial interchange instability and against new data from Jicamarca taken with an aperiodic pulsing technique. An experimental means of inferring the wave number spectrum of ionospheric irregularities using ground-based radar is outlined.

We present results of recent observations of meteor-head echoes obtained with the high-power large-aperture Jicamarca 50MHz radar (11.95°S, 76.87°W) in an interferometric mode. The large power-aperture of the system allows us to record more than 3000 meteors per hour in the small volume subtended by the 1° antenna beam, albeit when the cluttering equatorial electrojet (EEJ) echoes are not present or are very weak. The interferometry arrangement allows the determination of the radiant (trajectory) and speed of each meteor. It is found that the radiant distribution of all detected meteors is concentrated in relative small angles centered around the Earth's Apex as it transits over the Jicamarca sky, i.e. around the corresponding Earth heading for the particular observational day and time, for all seasons observed so far. The dispersion around the Apex is ~18° in a direction transverse to the Ecliptic plane and only 8.5° in heliocentric longitude in the Ecliptic plane both in the Earth inertial frame of reference. No appreciable interannual variability has been observed. Moreover, no population related to the optical (larger meteors) Leonid showers of 1998-2002 is found, in agreement with other large power-aperture radar observations. A novel cross-correlation detection technique (adaptive match-filtering) is used in combination with a 13 baud Barker phase-code. The technique allows us to get good range resolution (0.75km) without any sensitivity deterioration for the same average power, compared to the non-coded long pulse scheme used at other radars. The matching Doppler shift provides an estimation of the velocity within a pulse with the same accuracy as if a non-coded pulse of the same length had been used. The velocity distribution of the meteors is relatively narrow and centered around 60kms-1. Therefore most of the meteors have an almost circular retrograde orbit around the Sun. Less than 8% of the velocities correspond to interstellar orbits, i.e. with velocities larger than the solar escape velocity (72kms-1). Other statistical distributions of interest are also presented.

The 150-km altitude region of the equatorial ionosphere is known to exhibit meter-scale electron density irregularities during daytime hours that scatter the VHF radar signals propagating perpendicular to the geomagnetic field in the region. We show here that VHF signals scattered from the 150-km region above the Jicamarca Radio Observatory located near Lima, Peru, exhibit two distinct types of features. In one type the Doppler spectral width increases with the echo strength and the corresponding signal-to-noise ratio (SNR). A second type of higher SNR echoes exhibits SNR-independent Doppler spectral widths narrower that those observed in the first type. This classification of the equatorial 150-km radar returns was discovered within a data set consisting of 9 days of Jicamarca observations conducted in January 2009. The low SNR echo population with SNR-dependent spectral widths is by far the dominant population — more than 90% of the observations was of this type. While the underlying physics governing the generation of the 150-km density irregularities corresponding to these two types of echoes remains uncertain, comparisons with earlier data sets collected at Jicamarca and elsewhere is suggesting that the population of weaker SNR echoes is likely to be associated with a naturally enhanced incoherent scattering (NEIS) process while the second population with the unstable growth of field-aligned irregularities (FAI) in the electron density of the region (see Figure 1). We conjecture that small radar systems operated near the geomagnetic equator that have reported 150-km echo observations are mainly detecting the second type of FAI-related radar returns. Echoes produced by NEIS can only be seen by high sensitivity systems such as Jicamarca and possibly the Indian MST radar at Gadanki. In addition, we show and discuss the strong dependence of 150-km echoes (altitude and intensity) on solar radiation, a parameter that has been overlooked in most theories trying to explain these echoes. Figure 2 shows an example of how 150-km echoes are modified during a solar flare. It is hoped that the categorization of the 150-km returns being proposed here and the dependence on solar radiation, may lead to a better understanding of the 150-km scattering phenomena and contribute toward the resolution of the long standing mystery surrounding the generation of the 150-km irregularities in the low latitude ionosphere.

We present wind and spectral width measurements in a multiple beam experiment using polar mesospheric summer echoes (PMSE) as backscatterers. For each beam pointing direction, coherent radar imaging was applied using Capon's method to weight the time series of several spatially separated receivers. Thereby we estimate the radial velocity and spectral width for several points inside the beam volume. These estimates are used to calculate a 3D map of turbulence in PMSE. The radial velocities are used in a gravity wave analysis with an increased number of virtual beam directions while keeping the original Nyquist frequency and increasing the spatial resolution.

En: Modeling the Ionosphere–Thermosphere System (Geophysical Monograph Series),1st Edition by J. D. Huba (Editor), Robert W. Schunk (Editor), G. V. Khazanov (Editor).

A new technique for measuring E region plasma density profiles in the equatorial electrojet using a bistatic coherent scatter radar is described. The technique utilizes the Faraday rotation of the obliquely and coherently scattered signal. Plasma density versus altitude is inferred from the rate of Faraday rotation as a function of range and elevation angle. A narrow beam width is required to minimize returns from unwanted azimuths, but this can be achieved in a bistatic experiment using relatively small antenna arrays with widely spaced elements. We give a sample time sequence of daytime electron density profiles that were measured with the new technique at altitudes between 95 and 110 km. Scatter from pure two-stream waves makes it possible to measure both the bottomside and topside density profiles during the day. The importance of this new technique becomes evident when one realizes that only a few rocket flights have provided density profiles through these altitudes at the magnetic equator; the region has been inaccessible to any remote sensing technique until now.

We report new observations made on April 11 2000 over Jicamarca of daytime spread-F-like irregularities. These irregularities are extremely rare and were first reported by Woodman et al. [1985]. The main features of these new observations, which are similar to the previous ones, are that they were: (1) two orders of magnitude stronger than the incoherent scatter echoes at the F peak, (2) observed between 1400 and 1600 LT; (3) characterized by small Doppler shifts (|υd| < 1 m s−1); and (4) characterized by long correlation times, i.e., narrow spectral widths. The latter is interpreted as evidence that these irregularities are field aligned (very aspect sensitive in the north-south direction). On this occasion the irregularities occurred at higher altitudes than before, between 950 and 1450 km compared to 600–1000 km. The echoes in these recent observations were quite localized in the east-west direction (they were much stronger in the west beam than in the east beam). Furthermore, using interferometry we determined that the irregularities were in the form of blobs, aspect sensitive in the east-west (as well as north-south) direction with aspect angle widths as small as 0.2 degrees, particularly when the echoes were strong.

On the basis of scattering theory, we present four different contributions to angle-of-arrival (AOA) measurements using spaced antenna (SA) radar systems. We show that the measurement of the scattering parameters is needed to estimate most AOA contributions (e.g., tilt angle of layers). We analyze a general SA system (i.e., receiving antennas not necessarily symmetrically placed about the transmitter) and show, theoretically, that such systems can be used for vertical velocity corrections without explicitly knowing the scattering parameters (e.g., correlation lengths). The theoretical AOA expressions are compared with lower atmospheric data (3–21 km) obtained with the Jicamarca VHF radar system in Perú. We compare the AOAs obtained by a time and a frequency domain method, giving both methods essentially the same information. While the results are comparable, frequency domain AOAs tend to exhibit more outliers. From examination of time series of these AOA measurements, we find some short, but predominantly long-period oscillations (12 hours) with amplitudes of -1. Por pares