Observations Of Echoes Research Articles

Estimation of Humidity Profiles with the L-Band Boundary Layer Radar-RASS Measurements

A radar remote-sensing technique of humidity profiles, which was originally developed for the middle and upper atmosphere radar (the MU radar), is applied to the lower troposphere radar (LTR) with Radio Acoustic Sounding System (RASS) operated on L-band (1357.5 MHz) in order to expand the height range into the atmospheric boundary layer. Radar volume reflectivity and turbulence energy dissipation rate are derived from LTR observations of turbulence echoes. Virtual temperature profiles are simultaneously monitored by using the RASS technique. The mixing ratio of water vapor (q) is then calculated by solving a first-order differential equation of q versus height. An observational campaign of the LTR with RASS was conducted on May 28-29, 2002 at a meteorological station in Feixi, China as a part of the Coordinated Enhanced Observing Period (CEOP) project. From the observed clear air echo intensity we successfully determined a humidity profile at the height of 0.2-2.2 km. Precipitable water vapor (PWV) derived from the propagation delay of GPS radio signals was incorporated in the estimation algorithm as a constraint of the q profiles.A kytoon was moored at about 160 m during the radar operation, and monitored temperature, pressure and humidity, though not continuously, which are used for defining the lower boundary values in retrieving q profiles. By using the RASS temperature, and kytoon data, we could successfully estimate continuous q profiles. The radar-derived q profiles generally agreed with radiosonde data. This study successfully demonstrated a potential of the humidity estimation with the LTR in the atmospheric boundary layer. Future long-term observation is expected to investigate the precision under various meteorological conditions.

THE METEOR FLUX: IT DEPENDS HOW YOU LOOK

In this paper, we use radar observations from a 50 MHz radar stationed near Salinas, Puerto Rico, to study the variability of specular as well as non-specular meteor trails in the E-region ionosphere. The observations were made from 18:00 to 08:00 h AST over various days in 1998 and 1999 during the Coqui II Campaign [Urbina et al., 2000, Geophys. Rev. Lett. 27, 2853–2856]. The radar system had two sub-arrays, both produced beams pointed to the north in the magnetic meridian plane, perpendicular to the magnetic field, at an elevation angle of approximately 41 degrees. The Coqui II radar is sensitive to at least two types of echoes from meteor trails: (1) Specular reflections from trails oriented perpendicular to the radar beam, and (2) scattering, or, non-specular reflections, from trails deposited with arbitrary orientations. We examine and compare the diurnal and seasonal variability of echoes from specular and non-specular returns observed with the Coqui II radar. We also compare these results with meteor head echo observations made with the Arecibo 430 MHz radar. We use common region observations of these three types of meteor echoes to show that the diurnal and seasonal variability of specular trails, nonspecular trails, and head echoes are not equivalent. The implications of these results on global meteor mass flux estimates obtained from specular meteor observations remains to be examined.

Observation of coherent echoes with narrow spectra near 150 km altitude during daytime away from the dip equator

We present the rare observation of backscatter echoes perpendicular to the geomagnetic field from the 150 km altitude vicinity over a region situated away from the dip equator, namely, at Gadanki (13.5°N, 79.2°E, 6.4° dip latitude). A layer of weak echoes, having Signal‐to‐Noise Ratio (SNR) of the order of −12 to −3 dB, was seen drifting downward from 157 km at 0930 LT to about 148 km at 1028 LT on July 12, 1997. The Doppler velocity as well as Doppler Width was in the range 10–20 ms−1. While there was no quasi‐periodicity as such in terms of echo occurrence, the echo properties were in other respects very similar to 150 km echoes seen in equatorial regions. This included the descending phase with time during the morning hours. These constitute the first observations of daytime 150 km echoes away from the dip‐equator and only add to the mystery regarding the origin of these structures.

Common volume coherent and incoherent scatter radar observations of mid-latitude sporadic E-layers and QP echoes

Abstract. Common-volume observations of sporadic E-layers made on 14-15 June 2002 with the Arecibo incoherent scatter radar and a 30MHz coherent scatter radar imager located on St. Croix are described. Operating in dual-beam mode, the Arecibo radar detected a slowly descending sporadic E-layer accompanied by a series of dense E-region plasma clouds at a time when the coherent scatter radar was detecting quasi-periodic (QP) echoes. Using coherent radar imaging, we collocate the sources of the coherent scatter with the plasma clouds observed by Arecibo. In addition to patchy, polarized scattering regions drifting through the radar illuminated volume, which have been observed in previous imaging experiments, the 30MHz radar also detected large-scale electrostatic waves in the E-region over Puerto Rico, with a wavelength of about 30km and a period of about 10min, propagating to the southwest. Both the intensity and the Doppler shifts of the coherent echoes were modulated by the wave.

First polar mesosphere summer echoes observed at Davis, Antarctica (68.6°S)

We report the first observations of polar mesosphere summer echoes (PMSE) above the high‐latitude Southern Hemisphere (SH) station Davis, Antarctica (68.6°S, 78.0°E geographic; 74.6°S magnetic). Observations were obtained using a 55 MHz atmospheric radar, the first stage of which was commissioned late in the austral summer of 2002–2003. The radar commenced mesosphere observations with approximately 20 kW of transmitted power in October 2003. PMSE were recorded from 19 November to 3 December 2003 and, after a break in radar operation, from 27 January to 21 February 2004. We present the initial seasonal and diurnal occurrence morphology from 180 hours of Davis PMSE observations. Our initial findings reveal that SH PMSE show similar backscatter echo characteristics and occurrence properties to those reported for the Northern Hemisphere (NH).

Observations of meteor-head echoes using the Jicamarca 50MHz radar in interferometer mode

Abstract. 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.

Investigation of the Mesospheric PMSE Conditions by Use of the New Overshoot Effect

We present a model for the recently discovered PMSE overshoot effect. The overshoot effect is created during radar observations of the mesospheric Polar Mesospheric Summer Echoes (PMSE), when the PMSE strength is modified by artificial electron heating over a short period which in our cases were 20 s. Thereafter the heater is switched off for a long period (160 s) so the plasma and dust charges are allowed to relax back to their undisturbed values. This heater cycling is then repeated throughout the observing run. This results in a series of what we have called overshoot characteristic curves (OCC) which contain information on the state of the PMSE dusty plasma. The form of the OCC strongly support a model where the radar scattering is controlled by many small dust structures within the PMSE region, each of which will produce an electron gradient, with a strength which will depend mainly on the density and size of the dust, the electron density and temperature. It is the electron gradients which cause the PMSE radar scattering. With this model, and assuming an average dust structure to represent the PMSE dust structures, we follow the changes of the dust charging and the electron density through one cycle where the heater is switched on and off, and show how the resulting relative scattering efficiency reproduce observed OCC. We show that we can obtain values for the increase of electron temperature as the heater is switched on, the dust content, and from the relaxation time of the OCC the photodetachment rate or an upper limit to it. Other information may possibly also be obtained from the OCC, such as the electron density from electron heating calculations when the temperature is known, and information on to what degree absorption of plasma during the electron heating phase is important.

Accuracy of Wind Fields Observed by a Bistatic Doppler Radar Network

The accuracy of wind vectors derived from bistatic dual-Doppler synthesis was investigated. The investigation was based on mathematical examinations and observation data analyses in both convective and stratiform cases. Accurate wind vectors were calculated within a range of 40 8, b , 1508, where b was the bistatic scatter angle. The accuracy is mainly governed by b as the first principle, while sidelobe contamination and the sensitivity of the low-gain bistatic antenna are practically the main obstacles in retrieving accurate wind fields in convective and stratiform echoes, respectively. The sidelobe contamination around strong convective echoes with large gradients of reflectivity can be eliminated by comparing the measured bistatic reflectivity and a clean bistatic radar reflectivity, which is derived from the measured transmitting radar reflectivity and a bistatic radar equation. The radar equation includes the bistatic resolution volume and a bistatic antenna pattern estimated by the longtime observation of stratiform echoes. The equation also leads to the distribution of minimum detectable reflectivity of the bistatic receiver. The multiple bistatic radar observation in stratiform echoes produced three kinds of horizontal wind vectors, which were almost coincident with each other, in the overdetermined region. The composite wind fields are calculated by a simple variational method.

Observation of echoes in reorientation processes of nematic liquid crystals

Fast electrically driven reorientation processes of thermotropic nematic liquid crystals were investigated by means of 1H NMR. The reorientation time is determined by the electric field and can be varied between several hundred milliseconds and less than 50 μs. Although the reorientation of the nematic director is limited to an angular range of 90°, echoes occur for reorientation times below 4 ms. It turned out that the separation of the sidebands is up to two orders of magnitude higher than it would be expected in MAS experiments. This behaviour can be understood in analogy to MAS by means of a time-dependent dipolar Hamiltonian; an exact description of the echo position is given for the heteronuclear dipolar interaction. The homogeneous sample rotation of the MAS experiment must be replaced by a more complex term which describes the director reorientation. Due to the non-linearity of this term, the separation of the sidebands is no longer proportional to the frequency of the sample rotation but can reach up to 20 times the maximum director frequency. The experimental results are presented together with a theoretical interpretation.

Finland HF and Esrange MST radar observations of polar mesosphere summer echoes

Abstract. Peculiar near range echoes observed in summer with the SuperDARN HF radar in Finland are presented. The echoes were detected at four frequencies of 9, 11, 13 and 15 MHz at slant ranges of 105–250 km for about 100 min. Interferometer measurements indicate that the echoes are returned from 80–100 km altitudes with elevation angles of 20°–60°. Echo power (< 16 dB), Doppler velocity (between –30 and + 30 ms-1) and spectral width (< 60 ms-1) fluctuate with periods of several to 20 min, perhaps due to short–period atmospheric gravity waves. When the HF radar detected the echoes, a vertical incidence MST radar, located at Esrange in Sweden (650 km north of the HF radar site), observed polar mesosphere summer echoes (PMSE) at altitudes of 80–90 km. This fact suggests that the near range HF echoes are PMSE at HF band, although both radars did not probe a common volume. With increasing radar frequency, HF echo ranges are closer to the radar site and echo power becomes weaker. Possible mechanisms to explain these features are discussed.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides; instruments and techniques)

Seasonal and long‐term variations of PMSE from VHF radar observations at Andenes, Norway

The observation of polar mesosphere summer echoes (PMSE) with VHF radars is an important possibility for the investigation of the polar mesospheric region during summer. This region is characterized by the lowest temperatures in the Earth's atmosphere and the existence of mesospheric ice clouds, visually or by lidars observed as noctilucent clouds (NLC). Using measurements at 53.5 MHz in Andenes, Norway, with the ALOMAR SOUSY radar between 1994–1997 and with the ALWIN radar between 1999–2001 mean seasonal, solar cycle and long‐term variations of PMSE have been derived. The seasonal variation of PMSE is characterized by a strong increase during end of May/beginning of June, a rather high level in June and July, and a more gradual decrease during August. The occurrence rate of PMSE is strongly positive correlated with the mesospheric ionization level mainly caused by solar cycle variations of the solar Lyman α radiation and also by the flux of precipitating high energetic particles. Long‐term trends of PMSE are only very small and not significant due to the limited data series.

Scattering characteristics of high‐resolution meteor head echoes detected at multiple frequencies

Meteor data collected at the Kwajalein Missile Range (KMR) during the peak of the 1998 Leonid storm comprise the only simultaneous observations of meteor head echoes and trails using seven frequencies (very high frequency (VHF), ultrahigh frequency (UHF), L‐, S‐, C‐, Ka‐, and W‐band spanning 160 MHz to 95 GHz). The primary sensor was the ARPA Long‐Range Tracking and Instrumentation Radar (ALTAIR) radar operating at 160 MHz with 30 m range resolution and 422 MHz with 7.5 m range resolution, including both interferometric and polarization capabilities. This paper presents an analysis of this high‐resolution data set with the following results: First, these observations support the theory that head echo scattering arises from an ionized region with a density sufficiently high that its plasma frequency exceeds the radar frequency (overdense reflection). Second, radar cross section (RCS) decreases rapidly with decreasing wavelength because higher frequencies must penetrate further into the increasing density of the plasma surrounding the meteoroid to reach its reflection point. Third, head echo angle measurements indicate that most of the observed meteors are sporadics not originating from the Leonid radiant. Fourth, polarization ratios showed that head echo reflections result from plasmas with a circular cross section. Fifth, the highest RCS values are detected near 105 km altitude, where the meteoroid gives up the most kinetic energy during its decent. This paper presents the first analyses of a three‐frequency head echo as well as the polarization ratios and RCS characteristics from numerous two‐frequency head echoes, which will allow us to develop a better understanding of meteor physics.

Polar mesosphere summer echo observations at HF frequencies using the HAARP Gakona Ionospheric Observatory

We present initial polar mesosphere summer echo (PMSE) observations over Alaska in the HF band. Echoes were detected near 85 km over 50% of the observation time even though it was late in the PMSE season. The signal‐to‐noise ratio sometimes exceeded 25 dB. We also present the effects of different transmitted powers and antenna pointing directions on PMSE strength and occurrence. There is some evidence that using 4.5 MHz electron heating occurred, even at the low duty cycle used.

Noctilucent clouds, PMSE and 5‐day planetary waves: A case study

Analysis of global meteorological assimilations between mid‐July and late‐August 2000 shows strong 5‐day planetary waves in the middle atmosphere. Observations of temperature, zonal wind, noctilucent clouds and polar mesosphere summer echoes (PMSE) near Kiruna, Sweden, all at heights between 80 and 95 km, show variations correlating with the passage of the 5‐day waves. Temperature variations correlated with the 5‐day wave reach 15 K peak‐to‐peak and correspond to modulation of PMSE occurrence by up to 50%. These observations appear to be the first experimental evidence of amplification of 5‐day waves at the summer mesosphere which was predicted theoretically in 1976. A close linear relation is found between mean daily temperature and mean daily occurrence of PMSE. This can be explained if temperature is the primary factor controlling PMSE occurrence and time and height variations within each day between 80 and 90 km altitude reach 30 – 50 K.

Horizontal structure of wind velocity field around the mesopause region derived from meteor radar observations

The horizontal structure of the horizontal wind velocity is derived by the meteor echo observation with the MU radar. By dividing the meteor echoes into four azimuth sectors, horizontal wind velocities at four locations with about 100 km horizontal separation are determined with a time and height resolutions of 1 h and 2 km . The typical wind difference was about 20 m/ s for 100 km separation with a significant temporal variation. Examples of the discrimination of tides and gravity waves with a same wave period was demonstrated using horizontal wind structure analysis. A simultaneous observation with the OH airglow imaging showed that the uniformity/non-uniformity of the wind field well corresponds to the uniformity/non-uniformity in the gravity wave in the airglow layer.

Cubane shaped clusters, precursors for aluminophosphate frameworks: a solid state multinuclear NMR study, in time and frequency domains

A detailed solid state NMR study of molecular aluminophosphates is presented. The crystallographic structures of [Al4(HPO4)4(C2H5OH)12]Cl4·4C2H5OH (compound 1) and [Al4(HPO4)4(C2H5OH)12]Br4·4C2H5OH (compound 2) were obtained at low temperature. 13C CP MAS NMR dynamics were carefully studied: non-exponential growth of magnetisation was observed for 13C CP even for moderately coupled 13CH3 groups. This approach was extended for the first time to 31P CP MAS NMR. Variable contact time as well as inversion recovery cross polarisation (IRCP) experiments showed unambiguously that the 1H → 31P magnetisation transfer was also not exponential. The 31P IRCP MAS experiment proved to be a powerful tool for the study of complex amorphous materials, allowing editing of the spectra. Variable contact time experiments under fast MAS led to the observation of strong dipolar oscillations. The subsequent analysis of the CP data in the frequency domain led to the direct accurate determination of 31P–1H distances. This approach can be used as an alternative method for the measurement of distances in solid state NMR. 27Al NMR quadrupolar parameters for 1 and 2 were derived by using several static and MAS techniques, in both time and frequency domains. The time domain response to a two-pulse sequence led to the very unusual observation of multiple quadrupolar Solomon echoes. The spectroscopic data related to 1 and 2 are a good starting point and pertinent tools for the study of more complex derivatives.

Plasma density distribution along the magnetospheric field: RPI observations from IMAGE

A new technique is introduced that remotely measures the plasma density profile in the plasmasphere. Radio plasma imager (RPI) echo observations provide echo delay time as function of frequency, from which the plasma density as function of position along the magnetic field line can be calculated. An example from the nightside plasmasphere (L=3) shows the density having its minimum value near the equator and rapidly increasing densities along the field line above 40° magnetic latitude. The density increases at a faster rate toward the ionosphere than the field strength. The index of the power law of the density as a function of field strength increases from a few tenths near the equator to close to unity near 40° and greater than 2 near the ionosphere.

Spatial domain interferometric VHF radar observations of spread meteor echoes

This paper is concerned with short lived (<2 s) meteor trails which produced VHF radar echoes, spread in range >10 km . The Doppler spectrum of these echoes in each range bin is very broad and asymmetric with many discrete peaks. Spatial domain Interferometer observations of Spread Meteor Echoes were made at the Indian National MST Radar Facility to locate the echoing regions on the trail producing the discrete peaks in the broad Doppler spectra of each range bin. We report that the mean direction of arrival was found to be similar for all Doppler frequencies in each range bin and varies for successive bins by much less than one degree. The echoing regions on the trail are located nearly parallel to the radar beam direction. Further, the directions of arrival were found to be not necessarily normal to the geomagnetic field. The observations are discussed in terms of ionization structures developed within the meteor trail.

Modelling of particle charging in the polar summer mesosphere: Part 2—Application to measurements

A model describing the charging of aerosol particles in the vicinity of the polar summer mesosphere is applied to in situ measurements of electron and ion number densities. More general results from this model are presented in a companion paper (Rapp and Lübken, 2001). Here we apply this model to various in situ measurements. Charging of the aerosols causes disturbances in the electron and ion number density profiles which depend on various parameters, such as the electron/ion production rate Q, the electron/ion recombination coefficient α, the aerosol size, r A, and the aerosol number density N A. We have analyzed measurements from rocket flights where at least two of these parameters are known (in most cases Q and α). Under most circumstances the disturbance in the electron density profile Δ n e (‘biteout’) depends on r A and N A in a different way than the disturbance in the positive ion density profile Δ n i. In many cases this allows to unambiguously determine r A and N A from measurements of Δ n e and Δ n i. From a total of 11 flights found in the literature we have analyzed in detail four cases which are characterized by simultaneous observations of noctilucent clouds (NLC) and/or polar mesosphere summer echoes (PMSE) by ground based or in situ techniques. In all cases our model successfully explains the observed plasma disturbances in terms of aerosol charging. During NLC conditions we arrive at particle radii of 32– 60 nm and number densities of 110– 850 cm −3 . Assuming that the particles consist of water ice the amount of ice in the particles corresponds to gas phase water vapor concentrations of 4– 14 ppm v which is rather large compared to ‘standard’ model values. These large values support the idea that water vapor is collected by the aerosols in a larger horizontal area and/or extended altitude range when they sediment from the mesopause region (88 km) to typical NLC altitudes (82 km) . In one flight we arrive at much smaller (1 nm) and much more abundant (several 100,000 cm −3 ) aerosol particles.

CUTLASS HF radar observations of high-velocity E-region echoes

Abstract. A short event of high-velocity E-region echo observations by the Pykkvibaer HF radar is analysed to study echo parameters and the echo relation to the Farley-Buneman plasma instability. The echoes were detected in several beams aligned closely to the magnetic L-shell direction. Two echo groups were identified: one group corresponded to the classical type 1 echoes with velocities close to the nominal ion-acoustic speed of 400 ms–1 , while the other group had significantly larger velocities, of the order of 700 ms–1 . The mutual relationship between the echo power, Doppler velocity, spectral width and elevation angles for these two groups was studied. Plotting of echo parameters versus slant range showed that all ~700 ms–1 echoes originated from larger heights and distances of 500–700 km, while all ~400 ms–1 echoes came from lower heights and from farther distances; 700–1000 km. We argue that both observed groups of echoes occurred due to the Farley-Buneman plasma instability excited by strong ( ~70 mVm–1 ) and uniformly distributed electric fields. We show that the echo velocities for the two groups were different because the echoes were received from different heights. Such a separation of echo heights occurred due to the differing amounts of ionospheric refraction at short and large ranges. Thus, the ionospheric refraction and related altitude modulation of ionospheric parameters are the most important factors to consider, when various characteristics of E-region decametre irregularities are derived from HF radar measurements.Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; polar ionosphere)