VHF Radar Research Articles

Strong correlation between quasiperiodic echoes and plasma drift in the E region

AbstractSimultaneous observations of quasiperiodic (QP) echoes and plasma drift in the ionospheric E region were conducted in Fuke (19.5°N, 109.1°E), Hainan province, China, to investigate the QP striation tilts under varying plasma drift conditions. The E region field‐aligned irregularities (FAIs), observed using the Hainan VHF radar, and the drift velocities of the plasma blobs in the Es layer, recorded by the Hainan Digisonde operating in drift mode, are reported. The QP echoes and drift data recorded during the entire year of 2013 were analyzed and compared. A surprising consistency between the striation tilt of the QP echoes and the drift direction of the plasma blobs was discovered. A negative echo striation of the QP FAIs was recorded when the measured drift direction of the plasma blobs was southward, whereas a positive echo striation was observed during the northward drift. Furthermore, the echo trace was continuous, whereas the QP striation changed from negative to positive, and vice versa. Thus, it can be concluded that the morphology of the QP echoes may be controlled by the background wind fields in the E region. The northward/southward‐drifting striated FAIs in the observation region of a coherent scatter radar might induce the positive/negative QP echo striation in the range‐time‐intensity plots.

Extended observations of polar mesosphere winter echoes over Andøya (69°N) using MAARSY

AbstractContinuous observations of Polar Mesosphere Winter Echoes (PMWE) have been conducted at the Norwegian island Andøya (69.30°N, 16.04°E) since autumn 2004 using the ALWIN VHF radar (until 2008) and the Middle Atmosphere Alomar Radar System (MAARSY) (since 2011). Using the more sensitive MAARSY compared to the ALWIN radar results in more detections characterized by smaller volume reflectivity values down to 4·10−18m−1 and a greater altitudinal coverage, 50–88 km compared to previous observations between 65 and 75 km. The results obtained by MAARSY show that the PMWE season starts clearly at the beginning of September with a mean seasonal occurrence rate of about 16%, but a strong seasonal variability with maxima up to 70% in the seasonal variation of the individual years. The end of the winter season is hard to determine since mesospheric echoes have also been observed below altitudes of 80km during nonwinter months, particularly around the end of May, i.e., the beginning of the polar mesospheric summer echo season, indicating that the physical mechanism for creating the lower mesospheric echoes is present during the early summer months as well.

The science case for the EISCAT_3D radar

The EISCAT (European Incoherent SCATer) Scientific Association has provided versatile incoherent scatter (IS) radar facilities on the mainland of northern Scandinavia (the EISCAT UHF and VHF radar systems) and on Svalbard (the electronically scanning radar ESR (EISCAT Svalbard Radar) for studies of the high-latitude ionised upper atmosphere (the ionosphere). The mainland radars were constructed about 30 years ago, based on technological solutions of that time. The science drivers of today, however, require a more flexible instrument, which allows measurements to be made from the troposphere to the topside ionosphere and gives the measured parameters in three dimensions, not just along a single radar beam. The possibility for continuous operation is also an essential feature. To facilitatefuture science work with a world-leading IS radar facility, planning of a new radar system started first with an EU-funded Design Study (2005–2009) and has continued with a follow-up EU FP7 EISCAT_3D Preparatory Phase project (2010–2014). The radar facility will be realised by using phased arrays, and a key aspect is the use of advanced software and data processing techniques. This type of software radar will act as a pathfinder for other facilities worldwide. The new radar facility will enable the EISCAT_3D science community to address new, significant science questions as well as to serve society, which is increasingly dependent on space-based technology and issues related to space weather. The location of the radar within the auroral oval and at the edge of the stratospheric polar vortex is also ideal for studies of the long-term variability in the atmosphere and global change. This paper is a summary of the EISCAT_3D science case, which was prepared as part of the EU-funded Preparatory Phase project for the new facility. Three science working groups, drawn from the EISCAT user community, participated in preparing this document. In addition to these working group members, who are listed as authors, thanks are due to many others in the EISCAT scientific community for useful contributions, discussions, and support.

Ionospheric variation during pulsating aurora

AbstractWe have statistically analyzed data from the European Incoherent Scatter (EISCAT) UHF/VHF radars in Tromsø (69.60°N, 19.20°E), Norway, to reveal how the occurrence of pulsating auroras (PsAs) modifies the electron density profile in the ionosphere. By checking five winter seasons' (2007–2012) observations of all‐sky aurora cameras of the National Institute of Polar Research in Tromsø, we have extracted 21 cases of PsA. During these PsA events, either the UHF or VHF radar of EISCAT was operative and the electron density profiles were obtained along the field‐aligned or vertical direction near the zenith. From these electron density measurements, we calculated hmE (E region peak height) and NmE (E region peak density), which are proxies for the energy and flux of the precipitating PsA electrons, respectively. Then, we examined how these two parameters changed during the evolution of 21 PsA events in a statistical fashion. The results can be summarized as follows: (1) hmE is lower (the energy of precipitation electrons is higher) during the periods of PsA than that in the surrounding interval; (2) when NmE is higher (flux of PsA electrons is larger), hmE tends to be lower (precipitation is harder); (3) hmE is lower and NmE is larger in the later magnetic local time; and (4) when the AE index during the preceding substorm is larger, hmE is lower and NmE is larger. These tendencies are discussed in terms of the characteristics of particles and plasma waves in the source of PsA in the magnetosphere. In addition to the statistics of the EISCAT data, we carried out several detailed case studies, in which the altitude profiles of the electron density were derived by separating the On and Off phases of PsA. This allows us to estimate the true altitude profiles of the PsA ionization, which can be used for estimating the characteristic energy of the PsA electrons and better understanding the wave‐particle interaction process in the magnetosphere.

Energetic electron precipitation associated with pulsating aurora: EISCAT and Van Allen Probe observations

AbstractPulsating auroras show quasi‐periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also subrelativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler mode wave‐particle interactions. The height‐resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Tromsø VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from ~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe‐A satellite was very close to Tromsø and the satellite observed rising tone emissions of the lower band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave‐particle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of subrelativistic electrons and the pulsating aurora.

Characteristics of PMSE associated with the geomagnetic disturbance driven by corotating interaction region and high‐speed solar wind streams in the declining solar cycle 23

AbstractWe report interannual variations of the correlation between the reflectivity of polar mesospheric summer echoes (PMSEs) and solar wind parameters (speed and dynamic pressure), and AE index as a proxy of geomagnetic disturbances, and cosmic noise absorption (CNA) in the declining phase (2001–2008) of solar cycle 23. PMSEs are observed by 52 MHz VHF radar measurements at Esrange (67.8°N, 20.4°E), Sweden. In approaching the solar minimum years, high‐speed solar wind streams emanate from frequently emerging coronal holes, leading to 7, 9, and 13.5 day periodicities in their arrival at Earth. Periodicities of 7 and/or 9 days are found in PMSE reflectivity in 2005–2006 and 2008. Periodicity‐resolved correlations at 7 and 9 days of both D region ionization observed by cosmic noise absorption (CNA) and PMSE with solar wind speed and AE index vary from year to year but generally increase as solar minimum is approached. PMSEs have a higher periodicity‐resolved correlation with AE index than the solar wind speed. In addition, cross correlation of PMSE reflectivity with AE index is mostly higher than with CNA in solar minimum years (2005–2008). This can signify that high‐speed solar wind stream‐induced high‐energy particles possibly have strong influence on CNA, but not as much as on PMSE, especially for the years of significant periodicities occurring.

Direct measurement of lower thermospheric neutral density using multifrequency incoherent scattering

AbstractIncoherent scatter (IS) is sensitive to collisional properties of the ion gas when the mean free path is close to the radar wave number. However, it has been traditionally difficult to infer the rate of collisions from IS measurements because of ambiguities in the theory for measurements at a single wave number (k). We demonstrate that multifrequency measurements to achieve diversity in k can allow for direct inference of the composition‐weighted ion‐neutral collision frequency in the upper mesosphere and lower thermosphere. By direct, we mean that no significant constraints are imposed on the interpretation of the IS spectra and that interpretation relies only on the IS formalism (rather than a steady state ion‐momentum equation, for example). The technique is demonstrated using measurements from the European Incoherent Scatter VHF and UHF radar systems. This technique can be used to investigate neutral atmosphere variations as well as the validity of collision models commonly used in the IS formalism.

Plasma parameter estimation from multistatic, multibeam incoherent scatter data

AbstractMultistatic incoherent scatter radars are superior to monostatic facilities in the sense that multistatic systems can measure plasma parameters from multiple directions in volumes limited by beam dimensions and measurement range resolution. We propose a new incoherent scatter analysis technique that uses data from all receiver beams of a multistatic, multibeam radar system and produces, in addition to the plasma parameters typically measured with monostatic radars, estimates of ion velocity vectors and ion temperature anisotropies. Because the total scattered energy collected with remote receivers of a modern multistatic, multibeam radar system may even exceed the energy collected with the core transmit‐and‐receive site, the remote data improve the accuracy of all plasma parameter estimates, including those that could be measured with the core site alone. We apply the new multistatic analysis method for data measured by the tristatic European Incoherent Scatter VHF radar and the Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) multibeam receiver and show that a significant improvement in accuracy is obtained by adding KAIRA data in the multistatic analysis. We also demonstrate the development of a pronounced ion temperature anisotropy during high‐speed ionospheric plasma flows in substorm conditions.

Meteor heights during the recent solar minimum

Abstract. Average meteor heights have been continuously observed using a SKiYMET VHF radar at Collm (51.3° N, 13.0° E) since late summer of 2004. Initially, the daily mean meteor height was about 89.4 km. Since that time, average meteor heights have decreased. This is consistent with earlier results on middle atmosphere temperature change from the literature and from earlier results of low-frequency reflection height changes measured at Kühlungsborn and Collm. During the recent solar minimum 2008/2009 the meteor heights further decreased. Linear fitting of a trend and a solar cycle to the heights reveals a linear decrease of about −56 m year−1 and a solar cycle effect of +450 m per 100 sfu. Assuming that meteor heights, on a long-term average, approximately refer to a level of constant pressure, this decrease can be converted to a mean middle atmosphere linear temperature decrease of −0.23 K year−1 and a solar cycle effect of +1.8 K per 100 sfu during the last decade, which is in the range of observed trends reported in the literature.

Interferometry observations of low-latitude E-region irregularity patches using the Sanya VHF radar

Interferometry plays an important role in revealing fine-scale structures of ionospheric irregularity. By placing an additional receiving array of four 5-element Yagi antennas in the north of the main East-West array of the Sanya VHF radar, multiple interferometry baselines with components parallel and perpendicular to the magnetic meridian are formed. These baselines allow us to study the three-dimensional (3D) behavior of low-latitude ionospheric field-aligned irregularity (FAI) over Sanya. Using multiple non-collinear receiving baselines, an experiment for which the Sanya VHF radar operated as an interferometer was performed on 10 July 2013. Ionospheric E-region FAI echoes with periods of several minutes were observed during 0745-0915 UT; mean Doppler velocity was around −30 m/s with spectral widths of about 50 m/s. The interferometry results show fine-scale structures of E-region FAI with a zonal scale size of 15 km or less. In addition, we found that the periodic variations of echo intensity shown in radar range-time-intensity (RTI) maps were produced by spatially separated E-region irregularity patches. The patches drifted westward with a velocity of about 40–60 m/s and could be associated with propagating gravity waves. These results provide a clearer picture of the characteristics of FAI and thus help to investigate the generation and dynamics of low-latitude ionospheric irregularities in the Chinese sector.

Short vertical-wavelength inertia-gravity waves generated by a jet–front system at Arctic latitudes – VHF radar, radiosondes and numerical modelling

Abstract. Inertia-gravity waves with very short vertical wavelength (λz≤1000 m) are a very common feature of the lowermost stratosphere as observed by the 52 MHz radar ESRAD (Esrange MST radar) in northern Scandinavia (67.88° N, 21.10° E). The waves are seen most clearly in radar-derived profiles of buoyancy frequency (N). Here, we present a case study of typical waves from 21 February to 22 February 2007. Good agreement between N2 derived from radiosondes and by radar shows the validity of the radar determination of N2. Large-amplitude wave signatures in N2 are clearly observed by the radar and the radiosondes in the lowermost stratosphere, from 9 km to 14–16 km height. Vertical profiles of horizontal wind components and potential temperature from the radiosondes show the same waves. Mesoscale simulations with the Weather Research and Forecasting (WRF) model are carried out to complement the analysis of the waves. Good agreement between the radar and radiosonde measurements and the model (except for the wave amplitude) shows that the model gives realistic results and that the waves are closely associated to the upper-level front in an upper-troposphere jet–front system. Hodographs of the wind fluctuations from the radiosondes and model data show that the waves propagate upward in the lower stratosphere confirming that the origin of the waves is in the troposphere. The observations and modelling all indicate vertical wavelengths of 700 ± 200 m. The radiosonde hodograms indicate horizontal wavelengths between 40 and 110 km and intrinsic periods between 6 and 9 h. The wave amplitudes indicated by the model are however an order of magnitude less than in the observations. Finally, we show that the profiles of N2 measured by the radar can be used to estimate wave amplitudes, horizontal wavelengths, intrinsic periods and momentum fluxes which are consistent with the estimates from the radiosondes.

Polar summer mesospheric extreme horizontal drift speeds during interplanetary corotating interaction regions (CIRs) and high‐speed solar wind streams: Coupling between the solar wind and the mesosphere

AbstractWe report the observation of echo extreme horizontal drift speed (EEHS, ≥ 300 m s−1) during polar mesospheric (80–90 km) summer echoes (PMSEs) by the VHF (52 MHz) radar at Esrange, Sweden, in years of 2006 and 2008. The EEHS occur in PMSEs as correlated with high‐speed solar wind streams (HSSs), observed at least once in 12–17% of all hours of observation for the two summers. The EEHS rate peaks occur either during high solar wind speed in the early part of the PMSE season or during the arrival of interplanetary corotating interaction regions (CIRs) followed by peaks in PMSE occurrence rate after 1–4 days, in the latter part of the 2006 summer. The cause of EEHS rate peaks is likely under the competition between the interval of the CIR and HSS passage over the magnetosphere. A candidate process in producing EEHS is suggested to be localized strong electric field, which is caused by solar wind energy transfer from the interaction of CIR and HSS with the magnetosphere in a sequential manner. We suggest that EEHS are created by strong electric field, estimated as > 10–30 V m−1 at 85 km altitude, exceeding the mesospheric breakdown threshold field.

Short Study on Performance of Air Surveillance Augmented by Twin Radars

Detection of targets at low altitudes with a low Radar Cross Section (RCS) such as drones and stealth could challenge Air Surveillance systems and this situation must be addressed in a cost–effective way. This paper summarizes the newly emerged technical challenges of the Control and Reporting Centre (CRC) and gives solutions on how to overcome recognized shortcomings. In addition it highlights problems and requirements related to drone maneuvering capabilities and their very low RCS. This article summarizes challenges and gives a synthesized solution to the network centric approach based on the short distance Gaussian bi–static radar concept called twin radar coherent signal fusion. The idea is to explore advantages of the twin radar concept such as doubled detection range, increased, superior angular resolution of the system and increased plot update frequency. Examples for the twin radar performance demonstration are based on VHF radar, which has unique capabilities such as exploitation of multipath and resonance frequency for drones. A few peculiarities of the new signal processing are introduced with the extended air traffic control and monitoring network structure.

Characteristics of the E- and F-region field-aligned irregularities in middle latitudes: Initial results obtained from the Daejeon 40.8 MHz VHF radar in South Korea

We present preliminary observations of the field-aligned-irregularities (FAIs) in the E and F regions during the solar minimum (2009 - 2010) using the 40.8 MHz coherent backscatter radar at Daejeon (<TEX>$36.18^{\circ}N$</TEX>, <TEX>$127.14^{\circ}E$</TEX>, <TEX>$26.7^{\circ}N$</TEX> dip latitude) in South Korea. The radar, which consists of 24 Yagi antennas, observes the FAIs using a single beam with a peak power of 24 kW. The radar has been continuously operated since December 2009. Depending on the manner of occurrence of the backscatter echoes, the E-region echoes are largely divided into two types: quasi-periodic (QP) and continuous echoes. Our observations show that the QP echoes occur frequently above an altitude of 105 km in the post-sunset period and continuous echoes occur preferentially around an altitude of 105 km in the post-sunrise period. QP echoes appear as striated discrete echoes for a period of about 10 - 20 min. The QP-type echoes occur more frequently than the continuous-type echoes do and the echo intensity of the QP type is stronger than that of the continuous type. In the F region, the FAIs occur at night at an altitude interval of 250 - 450 km. As time proceeds, the occurrence height of the FAIs gradually increases until early in the morning and then decreases. The duration of the F-region FAIs is typically a few hours at night, although, in rare cases, FAIs persist throughout the night or appear even after sunrise. We discuss the similarities and differences of the FAIs observed by the Daejeon radar in comparison with other radar observations.

Echo Modeling and Detection Performance of Wideband VHF Radar

Echo Modeling and Detection Performance of Wideband VHF Radar

Ambiguity Resolution Based on Matrix Completion in Direction of Arrival Estimation for an Interferometric Array VHF Radar

Ambiguity Resolution Based on Matrix Completion in Direction of Arrival Estimation for an Interferometric Array VHF Radar

Design and Realization of Anti-Broadband Electromagnetic Interference Wired Phone

First analyzing electromagnetic compatibility system of the wired circuit, and then electromagnetic shielding, PCB board electromagnetic compatibility and filtering are used to develop an anti-broadband electromagnetic interference wired phone. It can resolve practical problems of SLT dialing error in strong electromagnetic interference, voice unclear and intermittent communication. After 30 days test, experiment shows that in 50m radiating field of high-power VHF radar or shortwave radio, speech intelligibility of phone designed was higher than that of general phone and had no whistling or ringing.

Design of Anti-Broadband Electromagnetic Interference Wired Phone Based on Speech Enhancement

In order to develop an anti-broadband electromagnetic interference wired phone, speech enhancement, electromagnetic shielding, PCB board electromagnetic compatibility and filtering are the usual measures. Realization of speech enhancement can resolve practical problems of SLT dialing error in strong electromagnetic interference, voice unclear and intermittent communication. The experiment shows that in 50m radiating field of high-power VHF radar or shortwave radio, speech intelligibility of phone designed was higher than that of general phone and had no whistling or ringing.

우주환경 지상관측기 자료통합시스템 개발

Korea Astronomy and Space Science Institute, Daejeon 305-348, KoreaE-mail: jhbaek@kasi.re.kr(Received September 06, 2013; Accepted September 27, 2013)ABSTRACT We have developed a data integration system for ground-based space weather facilities in Korea Astronomy and Space Science Institute (KASI). The data integration system is necessary to analyze and use ground-based space weather data efficiently, and consists of a server system and data monitoring systems. The server system consists of servers such as data acquisition server or web server, and storage. The data monitoring systems include data collecting and processing applications and data display monitors. With the data integration system we operate the Space Weather Monitoring Lab (SWML) where real-time space weather data are displayed and our ground-based observing facilities are monitored. We expect that this data integration system will be used for the highly efficient processing and analysis of the current and future space weather data at KASI.Key words: space weather; data integration system; ground-based observational system: solar telescope, magnetometer, VHF radar

Statistical analysis of radar observed F region irregularities from three longitudinal sectors

Abstract. Equatorial Spread F (ESF) is a manifestation of ionospheric interchange instabilities in the nighttime equatorial F region. These instabilities generate plasma density irregularities with scale sizes ranging from centimetres to thousands of kilometres. The irregularities can be detected from a variety of instruments such as digisonde, coherent and incoherent scatter radars, in situ space probes, and airglow photometers. In the present study, occurrence statistics of the ESF, based on various parameters are presented using data obtained from the VHF radars located at three longitudinally separated equatorial stations: Christmas Island (2° N, 202.6° E, 2.9° N dip latitude), São Luís (2.59° S, 315.8° E, 0.5° S dip latitude) and Jicamarca (12° S, 283.1° E, 0.6° N dip latitude). The ESF parameters presented here are the onset altitude, onset time (onset refers to first appearance of signal in the radar field of view) of the bottom-type and plume, and the peak altitude of the plume. Recent studies have used these parameters to classify the spread F occurrence characteristics. The present study reveals novel features namely, the dependence of ESF parameters on the seasonal, solar flux, declination angle and longitudinal dependence from the three radar sites. In addition, we also present an empirical model to determine the nature of these ESF parameters as a function of the solar flux which may enable us to forecast (with 30 min to 1 h tolerance) the plume occurrence at any longitude located in between São Luís and Christmas Island.