UHF Radar Research Articles

Localized mesosphere-stratosphere-troposphere radar echoes from theEregion at 69°N: Properties and physical mechanisms

[1] We present the first observations, to our knowledge, of a new class of high-latitude mesosphere-stratosphere-troposphere radar echoes from the E region as observed with the Arctic Lidar Observatory for Middle Atmosphere Research wind radar during the period 2004–2008. These echoes occur primarily during the summer months and in the altitude range from 93 to 114 km, with a pronounced peak of maximum occurrence at about 100 km. The echoes are rather short with typical durations of ∼20 min, with some examples lasting as long as 3 h. The echoes typically cover only a few hundred meters in the vertical and show both small Doppler velocities (±1–2 m/s) as well as very narrow spectral widths (just a few meters per second when converted to Doppler velocities). The echoes are highly aspect sensitive indicative of a specular-scattering mechanism and reveal a distinct diurnal variation with maxima of occurrence around noon and midnight. The latter is related to the semidiurnal tidal components of the zonal and meridional wind where times of occurrence correspond to large values of corresponding vertical wind shears. Considering possible physical mechanisms, turbulence with large Schmidt number scatter is likely ruled out as is auroral backscatter. Finally, a strong case for a close correspondence of the echoes to sporadic E layers is presented on the basis of comparisons to ionosonde data, occurrence patterns of sporadic layers, simultaneous and common volume lidar measurements of a sporadic Fe layer, as well as simultaneous measurements of sporadic E layers with the European Incoherent Scatter UHF radar at a horizontal distance of 130 km. Applying the theory of partial reflections to the observed electron density gradients, we are able to demonstrate that the observed echo strengths can likely be explained on the basis of this scattering mechanism.

Variance of the vertical ion speed measured with the European Incoherent Scatter (EISCAT) UHF radar in the polar lower ionosphere at Tromsø, Norway

[1] The vertical component of the ion velocity measured with the European Incoherent Scatter (EISCAT) Tromso UHF radar (69.6°N, 19.2°E) in the lower ionosphere (from 95 to 130 km) was found to be characterized by notably large variances at oscillation periods of 2–8 min. Of particular interest was the geomagnetic activity dependence above ∼106 km, which showed larger variances (100–500 m2 s−2) during periods of geomagnetic disturbance than during quiet periods. Below 106 km, the variance was less sensitive to the geomagnetic activity. Height profiles of the variance during the disturbed period showed steep increases with heights above 106 km, then reaching a peak at 120 km, where the ion gyrofrequency is equal to the modeled ion-neutral collision frequency. A theoretical prediction well reproduced the height profile for the geomagnetically disturbed period by assuming oscillations in the meridional component of the electric field. The theoretical study suggested that the electric field oscillation is a possible cause of large variances of the vertical ion speed in the polar lower ionosphere. However, below 106 km, other mechanisms were necessary.

Generation of ionospheric ducts by the HAARP HF heater

We report an investigation of ionospheric ducts having the shape of large plasma sheets, generated by vertically transmitted HAARP HF heater waves in several experiments conducted in Gakona, Alaska. Theory predicts that O-mode heater wave-created ionospheric ducts form parallel-plate waveguides within the meridional plane, and those generated by the X-mode heater waves are orthogonal to the meridional plane. Our theoretical prediction is supported by measurements of ionosonde data (namely ionograms), range–time–intensity (RTI) plots of UHF and HF backscatter radars, as well as magnetometer data analyses. When these plasma sheets experienced E×B drifts, they were intercepted by the HAARP UHF radar and seen as slanted stripes in the RTI plots. This striking feature was also observed in our earlier experiments using the Arecibo UHF radar.

Majority of PMSE spectral widths at UHF and VHF are compatible with a single scattering mechanism

Motivated by a recent controversy in the literature about the agreement or disagreement of spectral widths (after conversion to Doppler-velocities) of PMSE observed simultaneously with the EISCAT VHF and UHF radars in Tromsø, Norway, this issue was reconsidered taking into account a much larger data set than in any of the previous studies. While these previous studies only considered case studies of a few minutes in length, we considered a total of 380min of simultaneous and common volume observations with these two radars at a time resolution of 30s, i.e., a total of 760 samples. This study is important to discern whether PMSE at these two frequencies originate from the same coherent scattering mechanism or if different physical mechanisms are at work. We considered both shape and width of the considered spectra and found that – within the available spectral resolution – the VHF spectra are on average well described by a Gauss shape (with spectral parameter n=2—which is derived from a generalized formulation of the autocorrelation function), whereas the UHF-spectra show a small deviation from this shape with an average n of 1.6. Spectral widths do largely agree but show a small systematic difference, i.e., the UHF spectra are on average 0.1m/s narrower than the VHF spectra at an average spectral width of 3.5m/s. This small systematic effect is largely explained considering the overall effect of beam-, shear-, and wave-broadening. This means that the slightly different beam widths of the VHF and UHF-radars and the hence slightly different observing volumes account for the small systematic difference in mean spectral width. Finally, we demonstrated that the small deviation of UHF-spectra from a perfect Gauss shape can be either due to relatively small signal-to-noise ratios of the UHF-data and/or a superposition of incoherent and coherent scatter in cases where both contributions add equally to the total observed power. Since VHF PMSE are much more stronger than the incoherent scatter background this effect cannot be recognized in VHF observations. This superposition effect may occasionally also lead to larger deviations between VHF and UHF spectra in that it can potentially narrow the UHF-spectra significantly. However, excluding these rather rare and exotic cases, we conclude that the majority of our observations are compatible with a single coherent scattering at both VHF and UHF.

Lower-thermospheric wind fluctuations measured with an FPI during pulsating aurora at Tromsø, Norway

Abstract. Simultaneous observations were conducted with a Fabry-Perot Interferometer (FPI) at a wavelength of 557.7 nm, an all-sky camera at a wavelength of 557.7 nm, and the European Incoherent Scatter (EISCAT) UHF radar during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA-2) campaign in January 2009. This paper concentrated on two events during periods of pulsating aurora. The lower-thermospheric wind velocity measured with the FPI showed obvious fluctuations in both vertical and horizontal components. Of particular interest is that the location of the fluctuations was found in a darker area that appeared within the pulsating aurora. During the same time period, the EISCAT radar observed sporadic enhancements in the F-region backscatter echo power, which suggests the presence of low-energy electron (1 keV or lower) precipitation coinciding with increase in amplitude of the electromagnetic wave (at the order of 10 Hz or higher). While we have not yet identified the dominant mechanism causing the fluctuations in FPI-derived wind velocity during the pulsating aurora, the frictional heating energy dissipated by the electric-field perturbations may be responsible for the increase in ionospheric thermal energy thus modifying the local wind dynamics in the lower thermosphere.

An olden but golden EISCAT observation of a quiet‐time ionospheric trough

Incoherent scatter measurements were carried out on 9 November 1987, showing the presence of an ionospheric trough in the F region. The experiment was made using the EISCAT UHF radar, and it consisted of an azimuthal scan with constant beam elevation and a meridional scan. Since the radar rotates with the Earth, beams with different directions from subsequent scans meet in the same MLT‐ CGMLat pixel in nonrotating frame. If the ionosphere is not too variable, these can be combined to give an average value of electron density and ion/electron temperature in each pixel. Furthermore, since different beams passing through the same pixel give different ion velocity components, it is also possible to obtain the velocity vector. The geomagnetic conditions during the observations were quiet enough for assuming a quasi‐stationary ionosphere. It was found that both ion and electron temperatures have minima within the trough region and increase at the poleward wall. Ion velocity measurements, together with a convection model, suggest that the density depletion within the trough is due to recombination of F region plasma convecting for a long time in the dusk convection cell beyond the terminator. The northern edge of the trough is associated with soft particle precipitation. The southern edge is steeper than the northern edge, and is built by sunlit plasma brought to the trough region by corotation. The trough is thus a result of a combination of transport and precipitation processes rather than stagnation.

Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005

Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromsø (69.6°N, 19.2°E) over ∼23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day‐to‐day variations (∼5–30 ms−1) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17–22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be ∼58 km and ∼76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than ∼24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms−1) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5–6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere.

Solar activity dependence of ion upflow in the polar ionosphere observed with the European Incoherent Scatter (EISCAT) Tromsø UHF radar

The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromsø UHF radar between 1984 and 2008. In agreement with other work we find that the upward ion flux is generally higher when solar activity is high than when it is low. Ion upflow events and also the upward velocity behave the opposite: they are more frequently seen and higher, respectively, at times of low solar activity. In any year about 30–40% ion upflow is accompanied by ∼500 K higher electron temperature than the background temperature at 400 km altitude. Electron and ion heating in connection with upflow is nearly twice as prevalent during high solar activity as it is at low activity. The acceleration of ions by pressure gradients and ambipolar electric field becomes larger when solar activity is low than when it is high. This variation of the average acceleration is caused by the different shapes of electron density profiles for low and high solar activities. Ions start to flow up at above 450 km altitude when solar activity was high, and lower, at 300–500 km altitude, at low solar activity. It is suggested that the solar activity influences long‐term variations of the ion upflow occurrence because it modulates the density of neutral particles, the formation of the F2 density peak, and ion‐neutral collision frequencies in the thermosphere and ionosphere.

Links between a stratospheric sudden warming and thermal structures and dynamics in the high‐latitude mesosphere, lower thermosphere, and ionosphere

We analyzed neutral winds, ambipolar diffusion coefficients, and neutral temperatures observed by the Nippon/Norway Tromsø Meteor Radar (NTMR) and ion temperatures observed by the European Incoherent Scatter (EISCAT) UHF radar at Tromsø (69.6°N, 19.2°E), during a major stratospheric sudden warming (SSW) that occurred in January 2009. The zonal winds at 80–100 km height reversed approximately 10 days earlier than the zonal wind reversal in the stratosphere and the neutral temperature at 90 km decreased simultaneously with the zonal wind reversal at the same altitude. We found different variations between geomagnetically quiet nighttime ion temperatures at 101–110 km and 120–142 km for about 10 days around the SSW. Our results from the ground‐based observations agree well with the satellite observations shown in an accompanying paper. Thus, this study indicates that a SSW is strongly linked to thermal structure and dynamics in the high‐latitude mesosphere, lower thermosphere, and ionosphere.

PMSE-observations with the EISCAT VHF and UHF-radars: Statistical properties

Observations of polar mesosphere summer echoes (PMSE) have been carried out with the EISCAT VHF radar during five summer seasons between 2003 and 2007 as well as with the EISCAT UHF radar in July 2004 and 2005. From a total of 523 h of observations with the VHF radar, PMSE have been detected for more than 300 h overall. In July 2004 and 2005 when PMSE were detected by both radars, the occurrence rate of the VHF PMSE is as large as 85.0% whereas the UHF PMSE are much rarer with an occurrence rate of 5.3%. Comparison of the year-to-year variation between the occurrence rate of PMSE at 224 MHz and noctilucent clouds (NLC) observed with a nearby lidar shows similar properties. The year-to-year variation of the occurrence rate of PMSE at 53.5 MHz, however, is quite different. This might imply that PMSE at 224 MHz require the presence of larger ice particles than at 53.5 MHz, and hence show similar behavior as NLC. In contrast, the correlations of the occurrence rate of PMSE at 224 MHz with the solar and geomagnetic activities both show positive correlations which are, however less significant compared with the PMSE at 53.5 MHz. In addition, the occurrence rate and volume reflectivities of PMSE derived from the observations at 224 MHz both show remarkable characteristics of seasonal variation and diurnal variation. As expected from previous PMSE observations at 53.5 MHz, the seasonal variation is determined by the condition of supersaturation with respect to ice. The diurnal variations, however, are much more complicated, as they can be significantly influenced by the temperature changes due to tidal activity as well as the ionization level due to the solar radiation and high-energy particle precipitation. Taking into account the ‘turbulence with large Schmidt number’-theory of PMSE, a comparison between the volume reflectivities of simultaneous VHF PMSE and UHF PMSE is carried out and shows 99% of the observations with reflectivity ratios larger than a minimum value predicted by theory. Last but not least, we considered electron densities derived from UHF incoherent scatter observations at altitudes above the PMSE-layers (i.e., 88 and 90 km) during cases with VHF PMSE with and without UHF PMSE as well as a comparison of spectral widths from observations with both radars. This reveals that the occurrence of UHF-PMSE most likely requires larger electron densities and larger spectral widths (corresponding to enhanced neutral turbulence) at altitudes above 84 km.

Interaction of a high-power UHF beam with the ionosphere

[1] In 1992 a rocket was launched into a high-power radio beam to study its interaction with the ionosphere. The frequency of the beam, 5.1 MHz, was optimized for such interactions since the frequency used was below the maximum plasma frequency and hence was reflected in a manner that optimized such an interaction. Fortuitously, the rocket passed quite close to the diagnostic Arecibo UHF (430 MHz) radar beam and remarkably, as reported here, we find that there was an interaction as well with the UHF beam. In retrospect, however, we see that the energy density of the UHF radar was more than an order of magnitude higher than the plasma energy density. And, although the quiver velocity of the electrons in the beam is only 4% of their thermal speed, the ponderomotive force is quite a bit larger than the other forces in the medium. This force creates a drift velocity perpendicular to the magnetic field at the beam edge which, we show, is unstable to the drift wave instability and is likely to create the heated ions and fluctuating electric field and plasma density we observe. These results suggest that any transmission of high-power radio waves from a solar power satellite to the ground will interact with the ionosphere in a manner that must be studied before such an expensive project is initiated.

Multi‐scale analysis of the 25–27 July 2006 convective period over Niamey: Comparison between Doppler radar observations and simulations

AbstractThe present study investigates the multi‐scale processes associated with a sequence of convective events that occurred over Niamey during the period 25–26 July 2006. This period corresponds to the active stage of the first intense monsoon surge over Sahel for 2006. During this two‐day period, two successive sequences of mesoscale convective systems (MCSs) were located ahead of and in phase with the trough of an African Easterly Wave (AEW). They were followed by suppressed or isolated convection behind the trough and in the vicinity of the ridge. The large AMMA‐SOP dataset, in particular the UHF radar and the MIT Doppler radar in Niamey, are used in combination with a low‐resolution (5 km) cloud‐resolving model to understand the convection organization and its interaction with the environment. Several initial and boundary conditions have been tested, but only the simulation starting with the ECMWF AMMA reanalysis succeeds in reproducing the observed features; this emphasizes the importance of the initial state. From the simulated MCSs, the along‐line component of the apparent source of momentum due to the convection is found to be up to 1 m s−1h−1. It seems that MCSs globally reduce the monsoon flow and generate southerlies at mid levels which can reinforce the rotation of the wind at the passage of the trough. During the afternoon of 26 July, the local convection over Niamey resulted from some favourable factors (humidity, CAPE, CIN) that triggered convection, while inhibiting factors (mid‐level dry layer, weaker low‐level wind shear pointing to the north, anticyclonic curvature of the streamlines at 700 hPa) prevented it organizing itself and propagating. In particular, the low‐level wind shear seems of critical importance. Copyright © 2010 Royal Meteorological Society

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

Studies of polar mesosphere summer echoes with the EISCAT VHF and UHF radars: Information contained in the spectral shape

The nature of PMSE in the VHF and UHF frequency range is considered taking into account the shape of corresponding Doppler spectra. Assuming a turbulence-based model of PMSE it is argued that for cases where a VHF radar detects strong PMSE, the UHF radar could either detect enhanced coherent scattering caused by the same physical process as in the VHF (i.e., turbulence with large charged ice particles), there could be incoherent scattering modified by the charged ice particles, or there could be a mixture of both. In order to distinguish these cases a simple but robust method is introduced to characterize the shape of the Doppler spectra derived from observations at both frequencies. Spectral shapes are quantified with one simple fitting parameter of a generalized fit to the autocorrelation function (=Fourier transform of the Doppler spectrum). This parameter takes a value of 1 for a Lorentzian spectrum indicative of pure incoherent scatter from the D-region, a value of 2 for coherent scatter owing to turbulence, and a value of less than 1 for incoherent scatter modified by the presence of charged aerosol particles. This method is applicable to observations at altitudes between ∼70 and ∼90 km. Simultaneous observations with the EISCAT VHF and UHF radar are presented in which all three cases mentioned above are identified. For the case of incoherent scatter modified by the presence of charged aerosol particles we quantify the radius of the involved ice particles to exceed ∼5 nm. Most importantly, however, for the case where the UHF-signal exceeded the incoherent scatter signal significantly, the spectrum revealed a clear Gaussian shape indicative of a coherent scattering process with identical spectral width as for the VHF-observations. This finding gives strong support that both echoes are created by the same turbulence-based mechanism and not by different mechanisms as speculated by several previous authors.

71 W (19.7 W/mm) SiC BJTs for long-pulse UHF radar applications

In this paper, we present 71 W SiC bipolar junction transistors (BJTs) using state of the art technology. The devices were fabricated on a commercial n-type 4H–SiC substrate using a double-mesa etch and interdigitated emitter–base finger design. When operating under common-emitter configuration and long pulse RF conditions of 15 ms pulse width and 25% duty cycle, the packaged devices without internal matching exhibited 8.5 dB power gain and 71 W output power with a 50.7% power added efficiency (PAE) at 500 MHz. The power density is 19.7 W/mm normalized to total emitter finger length. The normally-off characteristic and superior long pulse RF performance makes these SiC transistors promising for use in compact power amplifiers in long-pulse UHF radar systems.

Radar and rocket comparison of UHF radar scattering from auroral electrojet irregularities: Implications for a nanosatellite radar

This article presents the estimates for UHF radar volume scattering coefficient of auroral electrojet irregularities based on previous measurements of absolute radar scattering coefficients and in situ rocket spectra. Both estimates indicate that a low‐earth–orbiting nanosatellite with a 0‐dB gain UHF antenna will be sensitive to bistatic scatter into space from the irregularities illuminated by the ground‐based Poker Flat Incoherent Scatter Radar (PFISR). It is also shown that the change of scattering geometry due to the satellite motion is sufficiently slow to “lock on” the target. That is, the satellite will acquire a statistically large number of samples in a sufficiently small time window to resolve the scattering signal power as a function of the Bragg scattering angle with respect to the magnetic field lines.

Measurements and simulation of ionospheric scattering on VHF and UHF radar signals: Channel scattering function

The design and operation of transionospheric VHF and UHF radars requires knowledge of amplitude and phase scintillation due to ionospheric scattering. Phase coherence is of particular importance where long coherent integration periods and large bandwidths are required. A thin phase screen, parabolic equation based, Trans‐Ionospheric Radio Propagation Simulator (TIRPS) is described. Modeled channel scattering functions (CSFs) are compared to experimental VHF and UHF data derived from the Advanced Research Projects Agency Long‐range Tracking and Instrumentation Radar on Kwajalein Island (9.4°N, 166.8°E). TIRPS quantitatively reproduces the experimental results, including the quasi‐parabolic profile observed in the measured CSFs under strong turbulence conditions. Variations in the simulated CSF with ionospheric phase screen parameters are also presented. Under conditions of high integrated strength of turbulence (CkL), a low phase spectral index (p = 1), indicating relatively dense small‐scale irregularities, produces pronounced range spreading. Conversely, when the spectral index is high (p = 4), indicative of strong focusing/defocusing by large‐scale irregularities, there is increased Doppler spreading and, when the outer scale of irregularities is large, a greater likelihood of asymmetry of the CSF about the zero Doppler axis.

Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency

Abstract. Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-static HF radio scatter on the London-Tromsø-St. Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was near and then above the critical frequency of the F2 layer by up to 0.4 MHz. An increase of the electron density up to 25% accompanied by strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation of striations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization.

First estimates of volume distribution of HF-pump enhanced emissions at 6300 and 5577 Å: a comparison between observations and theory

Abstract. We present bi-static observations of radio-wave induced optical emissions at 6300 and 5577 Å from a night-time radio-induced optical emission ionospheric pumping experiment at the HIPAS (Fairbanks) facility in Alaska. The optical observations were made at HIPAS and from HAARP located 285 km south-east. From these observations the altitude distribution of the emissions is estimated with tomography-like methods. These estimates are compared with theoretical models. Other diagnostics used to support the theoretical calculations include the new Poker Flat AMISR UHF radar near HIPAS. We find that the altitude distribution of the emissions agree with electron transport modeling with a source of accelerated electrons located 20 km below the upper-hybrid altitude.

Urban-breeze circulation during the CAPITOUL experiment: observational data analysis approach

An experimental study focused on the urban-breeze circulation observed in Toulouse, South-West of France, during the Intensive Observation Period number 5 (IOP5, 3rd and 4th July 2004) of the CAPITOUL experiment (Feb. 2004 to Feb. 2005) is presented. The influence of the urban breeze circulation on the structure and the dynamics of the Atmospheric Boundary Layer is analysed by using ground stations, wind profilers (UHF radar), radio soundings and aircraft data. The IOP5 is a summertime anticyclonic situation with low wind, strong insulation and a deep boundary layer with a sheared wind profile.