MHz Radar Research Articles

Radar measurements of electric fields in the topside of the equatorial electrojet: First results

We show that the vertical polarization electric field (Ep) in the topside of the equatorial electrojet can be determined from the Doppler spectra of type‐2 echoes obtained with a radar that is appropriately displaced in latitude from the magnetic dip equator. Using a 49.92 MHz radar on Christmas Island (2.9° magnetic dip latitude), we found unexpectedly large Ep at altitudes at least as high as 120 km over the dip equator. Another new and related finding is the transient appearance of type‐1 echoes at 99 km over Christmas Island, which likely was produced by an Ep of at least 11 mV/m that must have appeared at 114 km altitude over the dip equator. Elevated electron temperatures inferred from the type‐1 Doppler spectra are consistent with the presence of anomalous energetics.

50 MHz radar observations of mid‐latitude E‐region irregularities at Camp Santiago, Puerto Rico

A 50 MHz radar interferometer was used near Salinas, Puerto Rico, to probe the meter‐scale E‐region plasma density irregularities during two campaigns conducted in 1998. During the February–April period E‐region echoing layers were primarily observed between 90 and 100 km heights. The layers were typically thin (∼1 km) and unstructured, although in several cases short period (∼90 s) layer oscillations were observed. During the June–July period E‐region echoes showed more varied characteristics. In addition to low altitude layers, quasi‐periodic structures with descending echoing layers were observed at altitudes above 100 km. Zonal motions detected during descending layer events were at times variable and oscillated between westward and eastward directions while the layer descent rates remained fixed.

A stratigraphic link across 1100 km of the Antarctic Ice sheet between the Vostok ice‐core site and Titan Dome (near South Pole)

Isochronous internal ice‐sheet layering, measured from airborne 60 MHz radar, was traced between Lake Vostok and the Titan Ice Dome (100 km from South Pole Station), Antarctica. Three layers were selected between Ridge B and Titan Dome, and between Ridge B and Lake Vostok. This layering can be used to correlate the existing Vostok ice core across 1100 km of the ice sheet interior. Our correlation is also matched to the new EPICA ice‐core site, by using an existing radar link between Vostok and Dome C stations. Thus, three East Antarctic ice domes are linked stratigraphically for the first time through internal ice‐sheet radar layering. Our results indicate that the basal layers of ice at Titan Dome are around 165,000 years old suggesting that this location and, by inference, the South Pole Station, are prime sites for a high‐resolution ice core from the last glacial‐interglacial cycle.

Midlatitude type 1 echoes observed with the Chung-Li 52 MHz radar

This paper presents observational characteristics of midlatitude ionospheric E region type 1 echoes measured with a 52 MHz Doppler radar located at the National Central University, Chung-Li, Taiwan. Two events, on August 13 and October 15, 1997, are considered which show clear evidence of type 1 echo occurrence with peak Doppler velocities at about −300 m/s and +270 m/s, respectively. On the other hand, interferometry measurements indicate that the drift motion of the scattering region is eastward in both events, at about 80–100 m/s.

Micrometeor Observations Using the Arecibo 430 MHz Radar: I. Determination of the Ballistic Parameter from Measured Doppler Velocity and Deceleration Results

We present a sample of radar meteors detected during the November 1997 Leonids shower period using the narrow-beam, high-power Arecibo Observatory 430-MHz radar. During this period ∼7700 events were detected over 73 h of observations that included six mornings. Near apex-crossing, 6–10 events per minute were observed in the ∼300-m diameter beam. From these events a total of 390 meteors are characterized by a clear linear deceleration as derived from the radial Doppler speed determined from the meteor-echo leading-edge (head-echo). We interpret our results in terms of the meteor ballistic parameter—the ratio of the meteoroid mass to cross-sectional area—yielding a physical characterization of these particles prior to any assumptions regarding meteoroid shape and mass density. In addition, we compare these measurements with the results of a numerical solution of the meteor deceleration equation and find them in good agreement. The size and dynamical mass of the meteoroids are estimated considering these particles to be spheres with densities of 3 g/cm 3. We also discuss atmospheric energy-loss mechanisms of these meteroids. We believe these are the first radar meteor decelerations detected since those ones reported by J. V. Evans (1966, J. Geophys. Res. 71, 171–188) and F. Verniani (1966, J. Geophys. Res. 71, 2749–2761; 1973, J. Geophys. Res. 78, 8429–8462) and the first ones for meteors of this size.

Doppler Studies of Near-Antapex UHF Radar Micrometeors

A “radar micrometeor” is the radar-scattering signature from the free electrons in the plasma generated by entry of a dust-sized meteoroid into the atmosphere. We report the first direct Doppler measurements, made using the Arecibo Observatory 430-MHz radar, of the so-called meteor head echo. Our observations demonstrate that this region is moving with the speed of the meteoroid as determined from the meteor head-echo altitude–time trajectory and that this radar return is distinct spatially and in velocity from the much more commonly observed trail echo. We also report the first observations of near-antapex micrometeors which are characterized by the very slow atmospheric speeds expected from low-ecliptic-inclination objects entering the atmosphere from behind Earth's orbital path. Of the 32 meteors observed during four early evening hours of observations on 10 January 1997, velocities were determined for 18 of the meteors of which 7 were at or just below Earth escape velocity (11.2 km/s). We give heliocentric orbits for the 11 meteor events with speeds greater than the escape velocity and present a detailed analysis of these orbital parameters and their possible origins. One particle was determined to be interstellar: a preliminary analysis indicates that the ecliptic coordinates of the radiant relative to the local standard of rest (LSR) (with the solar motion relative to the nearby stars removed) are λ=43.02°, β=−43.28°, V=−25.11 km/s or, in system II galactic coordinates l II=219.8°, b II=−52.4°, V=−25.1 km/s.

A low-cost glacier-mapping system

AbstractAn old portable 60 MHz radar has been upgraded with a new digital data-processing and -acquisition system and a new antenna construction enabling a fast and low-cost installation on a Twin Otter aircraft. Augmented by a laser altimeter and kinematic global positioning system (GPS), the system has the capability of acquiring accurate data on location and ice-surface elevation, and adequate-quality data on ice thickness. The system has been applied successfully in mapping the Nioghalvfjerdsfjorden glacier, northeast Greenland, in spite of the difficult conditions with melting water on the glacier surface. The measurements from the floating part of the glacier have been evaluated by comparison of radar data with laser-altimeter and in situ measurements.

Measurement of ocean wave height and direction by means of HF radar: An empirical approach

High-frequency (HF) radar has been used for 20 years for remote sensing of ocean surface currents and ocean waves. Backscattered Doppler spectra contain two discrete lines, whose frequencies (Bragg frequency) determine the current speed, and four continuous side bands enabling inversion techniques to be used for retrieving ocean wave spectra. Recently, a new HF radar has been developed at the University of Hamburg (Germany). Data of a 34-day experiment reveal a high correlation between the standard deviation of the Bragg frequencies and the significant wave height weighted by an azimuthal function. Applying empirical regression curves it is possible to determine the significant wave height and the mean wave direction from intersecting beams of two radar stations. Compared to inversion techniques, the new method is applicable to data with a lower signal-to-noise ratio, i.e. it allows larger ranges. Two radar sites are required for current measurements. The optimum distance between two 30 MHz radars is about 20 km and, with the new method, needs not be reduced for the purpose of simultaneous wave measurements.

First passive radar observations of auroral E‐region irregularities

On August 27, 1998 we observed auroral E‐region irregularities using the Manastash Ridge Radar system. These are the first radio science results from passive radar observations of commercial FM broadcasts near 100 MHz. These are also the first reported observations of 1.5 m irregularities in 15 years. The data show scatter most consistent with type‐1 and type‐3 ion‐acoustic turbulence as observed by 50 MHz and 140 MHz radars. The instrument is extraordinarily low in cost and provides data free of range and doppler velocity aliasing problems. The range and velocity resolution achieved by this radar is substantially superior to that of other comparable coherent scatter radars, and the time resolution is similar.

A 4-yr Climatology of Pressure Disturbances Using a Barometer Network in Central Illinois

This paper presents a climatology of coherent disturbances detected during 1991‐95 by a network of barometers with a diameter of about 50 km located in a very flat terrain centered on the Flatland Atmospheric Observatory in east-central Illinois. An automatic, wavelet-based adaptive filter is used to extract the waveforms of all disturbance events with amplitudes larger than a frequency-dependent threshold. The extracted events cover characteristic temporal scales from about 30 min to 6 h, that is, the range that includes mesoscale disturbances that affect the weather and the forecasts. The analysis resulted in two classes of events. One class, called coherent events, or CEs, consists of disturbances that propagated coherently through the barograph network and for which the phase propagation velocity, dominant period, and horizontal wavelength could be estimated with good accuracy. The propagation directions of 97% of the CEs were between 08 and 1808 (i.e., had an eastward component) and the speeds of 96% were between 10 and 50 ms 21 with a mode at 25‐30 m s21. The other class, called incoherent events, or IEs, consists of disturbances that had significant amplitudes but that did not propagate coherently across the network, so that the propagation velocity could not be estimated. This class consists of localized disturbances and wave packets with short periods and/or wavelengths, or with pressure signatures that were too different at the network stations. The extracted events are attributed to gravity waves, wave packets, gravity currents, pressure jumps, solitary waves, bores, etc. The rate of occurrence of events had a strong seasonal dependence, with a maximum in fall and winter and a minimum in summer. The CEs occurred about 20%‐21% of the total time in fall and winter and 12% in summer, while all events occurred 34% in both fall and winter and 23% in summer. The seasonal dependence of events confirms the strong relation of these disturbances to the baroclinicity of the atmosphere. Concurrent vertical velocity fluctuations observed by the 50-MHz radar at the Flatland Atmospheric Observatory showed that many of the large-amplitude events extended up to at least 7 km, the highest altitude reliably observed by the radar.

Turbulence-induced fluctuations in ionization and application to PMSE

The temporal evolution of a turbulent layer is calculated in detail by solving the hydrodynamic equations. The turbulence is initiated by a Kelvin-Helmholtz instability. The field of potential-temperature fluctuations serves as a tracer for modeling entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere. This entrainment modeling provides the input to a turbulence advection model capable of calculating the spectra and cospectra of ions and electrons. The turbulence advection model is used as a subgrid-scale model and is required because, given present or foreseeable computer capabilities, numerical solutions cannot span the enormous range of spatial scales from the depth of the shear layer to the smallest scales on which the most massive ions diffuse. The power spectrum of electron number-density fluctuations obtained from the turbulence advection model is compared with that measured by a rocket during the STATE (Structure and Atmospheric Turbulence Environment) experiment; agreement is found for a case of massive ions. The radar cross section for Bragg scattering is calculated from the electron number-density power spectrum and is used to calculate the signal-to-noise ratio (S/N) for the Poker Flat 50 MHz radar. The resultant S/N is then compared with the radar measurements obtained during the STATE experiment. These comparisons support the hypothesis that massive ions can cause polar mesosphere summer echoes from turbulent layers. Large-scale morphology of the turbulent layer obtained from rocket and radar measurements is reproduced by the hydrodynamic solution.

Investigations of the possible relationship between PMSE and tides using a VHF MST radar

The possible relationship between Polar Mesospheric Summer Echoes (PMSE), tidal winds and vertical wind shear associated with the tidal winds is examined. PMSE were observed by the Esrange VHF MST Radar (ESRAD ) 52 MHz radar at 67°56′N, 21°04′E during May–August 1997. Three data series in the beginning, middle and the end of the PMSE season are considered. The seasonal variation of PMSE shows a steep increase in occurrence at the end of May and fade‐out in mid‐August. Daily variation of echo power in the middle of the season has a minimum around 20–22 UT. Wind measurements reveal terdiurnal, semidiurnal and diurnal tides. Between 82–87 km we found a strong wind shear which tends to increase through the season, but does not show significant correlation with SNR (signal‐to‐noise ratio). It seems likely that PMSE appearance is conditioned by small scale dynamics and tidal effects on aerosols, and not by large scale wind shear.

Detection of a meteor contrail and meteoric dust in the Earth's upper mesosphere

In 1983 a series of small rockets were launched from the Poker Flat Rocket Range near Fairbanks, Alaska to study what has come to be called Polar Mesospheric Summer Echoes (PMSE). We report here on a fortuitous simultaneous 50-MHz radar and rocket detection of what seems to be a meteor contrail produced over the Poker Flat Rocket Range. The two data sets are mutually consistent and taken together suggest some very interesting properties for the trails of large meteors. Most notable is the first evidence that the ablated material can coagulate into particles the order of 50 nm in radius. This estimate is based primarily on the fall speed deduced from both the Doppler shift of the VHG radar signal and the time rate of change of the target as it fell through the beam. In addition the very existence of the radar target, the extremely sharp edges of the trail, and the existence of electron density structures inside the trail more than an order of magnitude smaller than the Kolmogorov microscale, all require large charged aerosols and a very high Schmidt number. Curiously the environment leading to PMSE (the study of which was our primary mission), is very similar to the properties of a large meteor trail some minutes after it is formed. In the PMSE case ice particles grow and become charged by the plasma and, when more than half the charge is tied up on the ice, the plasma diffusion coefficient becomes so small that structure can be supported at VHF scattering scales. In the late-time meteor case large aerosols coagulate and tie up both natural charge in the plasma and the original meteor trail electrons, Following the work of Rosinski and Snow (1961) and Hunten et al. (1980) we conclude that the incident meteor was the order of 100 g and would have had a visual magnitude of about - 5. This dust production process may resolve some open questions concerning long-lived meteor radar echoes. For example, in the event studied the electron density was well into the underdense condition and yet was detected for over 6 min. Classical meteor scatter theory has no explanation for such a long duration underdense event. c 1998 Elsevier Science Ltd. All rights reserved

Incoherent scatter from space shuttle and rocket engine plumes in the ionosphere

Enhanced echoes from the 430 MHz radar at Arecibo were observed during burns of the space shuttle orbital maneuver subsystem (OMS) engines near 317 km altitude. Similar radar signatures of enhanced backscatter were also obtained by the Millstone Hill radar observing the plume of a Centaur engine burning in the ionosphere. A theoretical model of incoherent scatter is presented to explain the radar backscatter observations. The theory considers molecular ion beams generated in the exhaust plume as a result of charge exchange between the ambient O+ ions and the high‐speed exhaust molecules (primarily H2O). The field‐aligned gyromotion of the pickup ions affects the radio wave scattering from the random thermal fluctuations of electron density. Numerical calculations are carried out for plasmas modified by the space shuttle or Centaur engines, and reasonable agreement with observations is found for the total scattered power. Incoherent backscatter spectra respond to characteristics of the exhaust plume such as vector flow velocity, temperature, and composition. The nonequilibrium velocity distributions for the ions in the pickup ion plume are similar to the distributions found in strongly convecting auroral region ionospheres. The incoherent scatter from the plume ions can be used to validate techniques used to study naturally disturbed plasmas. The predictions of our radar scatter calculations will be tested in future experiments using the space shuttle OMS engines over incoherent scatter radars located at equatorial latitudes and midlatitudes.

Excitation of oscillating two stream instability by upper hybrid pump waves in ionospheric heating experiments at Tromso

It is shown that upper hybrid waves generated by the HF heater wave in the Tromso' heating experiments can excite oscillating two stream instability near the upper hybrid resonance layer of the heater wave. The sidebands of the investigated parametric instability are obliquely propagating Langmuir waves. These Langmuir waves can be excited in milliseconds and have a broad angular distribution. Thus, those waves propagating obliquely at small angles with respect to the geomagnetic field can contribute to the zero off‐set frequency component of the plasma line spectrum detected by a backscatter radar. The analyses also show that the wavelengths of the Langmuir waves have an upper bound and consequently, explain why the zero off‐set frequency plasma line has been detected by the EISCAT 933 MHz (UHF) radar, but not by the EISCAT 244 MHz (VHF) radar as reported in Stubbe et. al. [1992].

The terdiurnal tide in the mesosphere and lower thermosphere over London, Canada (43°N, 81°W)

Radars have been used successfully for many years to measure atmospheric diurnal and semidiurnal tides, but studies of the atmospheric terdiurnal tide have only rarely been reported to date in the literature. In this paper we investigate the amplitude, phase, and vertical wavelength for the terdiurnal tide using the University of Western Ontario (UWO) 2 MHz radar near London, Canada. Emphasis is placed on short‐term, monthly, seasonal, and annual variations of the terdiurnal tide. The results of the present study bring out certain new aspects of the terdiurnal tide at midlatitudes in the northern hemisphere. We show that terdiurnal tides have significant amplitudes in the mésosphère and lower thermosphère; indeed, the terdiurnal amplitudes are often comparable to the diurnal ones. We also draw attention to the annual variability and especially highlight the occurrence of the terdiurnal tide in non winter months. For example, the occurrence of the terdiurnal tide in spring months is not uncommon. Thus it seems that the 8‐hour component observed over London, Canada, cannot be explained by the terdiurnal tide being due to solar heating alone. Particular cases of the terdiurnal tide are examined, and the observed amplitude and phase profiles of the terdiurnal tide at certain times of the year are discussed in terms of nonlinear interaction between the diurnal and semidiurnal tides. We suspect that this effect may also explain the fact that diurnal and semidiurnal tides at our station often seem suppressed relative to other sites [Thayaparan et al., 1995a]. On the basis of our previous and present observations, it is also possible that large terdiurnal amplitudes observed at certain times of the year may be partly explained in terms of tidal/gravity wave interactions. However, this possibility needs to be further investigated in the future.

Type-1 echoes from the mid-latitude E-Region ionosphere

Abstract. This paper presents more data on the properties of type-1 irregularities in the nighttime mid-latitude E-region ionosphere. The measurements were made with a 50-MHz Doppler radar system operating in Crete, Greece. The type-1 echoes last from several seconds to a few minutes and are characterized by narrow Doppler spectra with peaks corresponding to wave phase velocities of 250–350 m/s. The average velocity of 285 m/s is about 20% lower than nominal E-region ion-acoustic speeds, probably because of the presence of heavy metallic ions in the sporadic-E-layers that appear to be associated with the mid-latitude plasma instabilities. Sometimes the type-1 echoes are combined with a broad spectrum of type-2 echoes; at other times they dominate the spectrum or may appear in the absence of any type-2 spectral component. We believe these echoes are due to the modified two-stream plasma instability driven by a polarization electric field that must be larger than 10 mV/m. This field is similar in nature to the equatorial electrojet polarization field and can arise when patchy nighttime sporadic-E-layers have the right geometry.

Amplitude, phase, and period variations of the quasi 2‐day wave in the mesosphere and lower thermosphere over London, Canada (43°N, 81°W), during 1993 and 1994

Because there is as yet no conclusive evidence pointing to the source of the quasi 2‐day wave, further observations are required in order to resolve questions about the generating mechanism as well as to better understand the role of the wave in the middle atmosphere. In this paper we investigate the period, amplitude, phase, and vertical wavelength for the quasi 2‐day wave in the mesosphere and lower thermosphere, using the University of Western Ontario 2 MHz radar near London, Canada, and in addition, we look at possible coupling with atmospheric tides. The results of the present study bring out certain new aspects of the quasi 2‐day wave at midlatitude in the northern hemisphere that have not been previously reported. In particular, we show that the period of the quasi 2‐day wave determined from our study is smaller (46–47 hours) than the 51‐ to 52‐h period generally suggested by other northern hemisphere results, and our periods also showed variability as a function of time. We also draw attention to the annual variability and especially highlight the occurrence of the wave in nonsummer months. For example, a brief occurrence in April is not uncommon. Our study also contributes additional midlatitude geographical data that should aid in developing a better picture of the quasi 2‐day wave.

Remote sensing of water salinity at decameter wavelengths

The possibility of remote sensing of surface water salinity in the decametric wave range is studied. The theoretical consideration of sensitivities to salinity variations proves the decametric range to be best for measuring small water salinities [0.3-5 parts per thousand (/sup 0///sub 00/) at 30 MHz]. The conclusion is confirmed by 30 MHz radar experiments, which show a good agreement with the in situ data in the range 0.3-30 /sup 0///sub 00/. Radar contrasts of about 0.35 db and 1.6 db are observed when salinity changes from 0.3 /sup 0///sub 00/ to 1.2 /sup 0///sub 00/ and to 30 /sup 0///sub 00/, correspondingly.

Harmonics of the ion acoustic frequency in the heater induced ion spectrum

Odd harmonics of the ion acoustic frequency in the 224MHz radar ion line spectrum have been found during HF heating experiments at Tromsø. This new feature appears in a narrow height range near the matching height where the ion line is characterized by enhanced ion acoustic shoulders and cascade or line spectra are observed in the plasma line channel. An explanation and a first estimate are given in terms of the parametric decay theory. A numerical calculation using an extended Zakharov model also demonstrates the existence of such ion line harmonics.