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  • 1
    Electronic Resource
    Electronic Resource
    Validation of the CUTLASS HF radar gravity wave observing capability using EISCAT CP-1 data (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1392-1399 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionosphere - atmosphere interactions) ; Meteorology and atmospheric dynamics (thermospheric dynamics) ; Radio science (ionospheric propagations)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Quasi-periodic fluctuations in the returned ground-scatter power from the SuperDARN HF radars have been linked to the passage of medium-scale gravity waves. We have applied a technique that extracts the first radar range returns from the F-region to study the spatial extent and characteristics of these waves in the CUTLASS field-of-view. Some ray tracing was carried out to test the applicability of this method. The EISCAT radar facility at Tromsø is well within the CUTLASS field-of-view for these waves and provides a unique opportunity to assess independently the ability of the HF radars to derive gravity wave information. Results from 1st March, 1995, where the EISCAT UHF radar was operating in its CP-1 mode, demonstrate that the radars were in good agreement, especially if one selects the electron density variations measured by EISCAT at around 235 km. CUTLASS and EISCAT gravity wave observations complement each other; the former extends the spatial field of view considerably, whilst the latter provides detailed vertical information about a range of ionospheric parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1392-z
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  • 2
    Electronic Resource
    Electronic Resource
    ULF wave occurrence statistics in a high-latitude HF Doppler sounder (1999)
    Springer
    Annales geophysicae 17 (1999), S. 749-758 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (auroral ionosphere; ionosphere -magnetosphere interactions) ; Magnetospheric physics (MHD waves and instabilities)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Ultra low frequency (ULF) wave activity in the high-latitude ionosphere has been observed by a high frequency (HF) Doppler sounder located at Tromsø, Norway (69.71°N, 19.2°E geographic coordinates). A statistical study of the occurrence of these waves has been undertaken from data collected between 1979 and 1984. The diurnal, seasonal, solar cycle and geomagnetic activity variations in occurrence have been investigated. The findings demonstrate that the ability of the sounder to detect ULF wave signatures maximises at the equinoxes and that there is a peak in occurrence in the morning sector. The occurrence rate is fairly insensitive to changes associated with the solar cycle but increases with the level of geomagnetic activity. As a result, it has been possible to characterise the way in which prevailing ionospheric and magnetospheric conditions affect such observations of ULF waves.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0749-2
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  • 3
    Electronic Resource
    Electronic Resource
    The synthesis of travelling ionospheric disturbance (TID) signatures in HF radar observations using ray tracing (1999)
    Springer
    Annales geophysicae 18 (1999), S. 56-64 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (auroral ionosphere; ionosphere atmosphere interactions; ionospheric disturbances)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Characteristic signatures are often observed in HF radar range-time-intensity plots when travelling ionospheric disturbances (TIDs) are present. These signatures, in particular the variation of the F-region skip distance, have been synthesised using a ray tracing model. The magnitude of the skip variation is found to be a function of the peak electron density perturbation associated with the TID and radar frequency. Examination of experimental observations leads to an estimate of the peak electron density perturbation amplitude of around 25% for those TIDs observed by the CUTLASS radar system. The advantage of using the skip variation over the radar return amplitude as an indicator of density perturbation is also discussed. An example of a dual radar frequency experiment has been given. The investigation of the effect of radar frequency on the observations will aid the optimisation of future experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0056-4
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  • 4
    Electronic Resource
    Electronic Resource
    SABRE observations of structured ionospheric flows during substorm expansion phase onset (1994)
    Springer
    Annales geophysicae 12 (1994), S. 1027-1038 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The irregularity velocity patterns observed by the SABRE coherent radar at substorm expansion phase onset, which is identified by magnetometer observations of Pi2 pulsations, are occasionally highly structured. In all the examples of structured velocity patterns examined, the SABRE viewing area is located at longitudes within the inferred substorm current wedge. Three types of structured velocity regime are apparent depending on the level of magnetic activity and the position of the radar viewing area relative to the substorm enhanced currents and the Pi2 pulsation generation region. Firstly, vortex-like velocity patterns are observed and these may be caused by the field-aligned currents associated with the substorm current wedge. Secondly, regions of equatorward velocity are also observed at times of substorm expansion phase onset moving longitudinally across the SABRE viewing area. The longitudinal movement is usually westward although an example of eastward motion has been observed. The phase velocity of these regions of equatorward flow is typically 1–3 km s−1. The observed equatorward velocities occur at the poleward edge or poleward of the background convection velocities observed by SABRE. These equatorward velocities may be related to the westward travelling surge and to the expansion (eastwards as well as westwards) of the brightening arc region at substorm onset. Thirdly, the flow rotates equatorward within the field of view but does not then appear to move longitudinally. These equatorward velocities may relate to the earthward surge of plasma from the magnetotail at substorm onset.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-994-1027-y
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  • 5
    Electronic Resource
    Electronic Resource
    Spatial observations by the CUTLASS coherent scatter radar of ionospheric modification by high power radio waves (1997)
    Springer
    Annales geophysicae 15 (1997), S. 1412-1421 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Results are presented from an experimental campaign in April 1996, in which the new CUTLASS (Co-operative UK twin-located Auroral Sounding System) coherent scatter radar was employed to observe artificial field aligned irregularities (FAI) generated by the EISCAT (European Incoherent SCATter) heating facility at Tromso, Norway. The distribution of back-scatter intensity from within the heated region has been investigated both in azimuth and range with the Finland component of CUTLASS, and the first observations of artificial irregularities by the Iceland radar are also presented. The heated region has been measured to extend over a horizontal distance of 170 ± 50 km, which by comparison with a model of the heater beam pattern corresponds to a threshold electric field for FAI of between 0.1 and O.OlV/m. Differences between field-aligned and vertical propagation heating are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-1412-4
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  • 6
    Electronic Resource
    Electronic Resource
    First complementary observations by ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1519-1522 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere ; Auroral ionosphere ; Polar ionosphere ; Radio science (ionospheric physics)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Experimental results are presented from ionospheric tomography, the EISCAT Svalbard radar and the CUTLASS HF radar. Tomographic measurements on 10 October 1996, showing a narrow, field-aligned enhancement in electron density in the post-noon sector of the dayside auroral zone, are related to a temporal increase in the plasma concentration observed by the incoherent scatter radar in the region where the HF radar indicated a low velocity sunwards convection. The results demonstrate the complementary nature of these three instruments for polar-cap ionospheric studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1519-2
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  • 7
    Electronic Resource
    Electronic Resource
    New techniques for auroral irregularity studies with COSCAT (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1241-1250 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionospheric irregularities) ; Radio science (Instruments and techniques)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The COSCAT system enables the detection of E-region auroral backscatter with the EISCAT remote receivers at magnetic aspect angles close to 90°. This is achieved by utilising a low-power transmitter stationed in Oulu, Finland. Many important observations of E-region irregularities have been achieved with this simple experiment. Recent studies have attempted to push the COSCAT system to its experimental limits. Firstly, the CW signal has been phase-modulated with 13-bit Barker codes with baud lengths of 40, 70 and 100 μs. Interpretation of the received power allows the spatial distribution of the auroral scatterers to be determined. The second advance is in the use of a sophisticated correlator program which allows data to be buffered within the correlator at very high time resolution. This enables the coherent backscatter power to be sampled every 12.5 ms and the full auto-correlation function to be measured every 100 ms. These measurements allow the COSCAT system to be employed for the first time in an investigation of the growth and decay of the auroral irregularities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1241-0
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  • 8
    Electronic Resource
    Electronic Resource
    Initial backscatter occurrence statistics from the CUTLASS HF radars (1997)
    Springer
    Annales geophysicae 15 (1997), S. 703-718 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A statistical study of the occurrence of ground and ionospheric backscatter within the fields-of-view of the CUTLASS HF radars, at an operating frequency of 10 MHz, during the first 20 months of operation has been undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be highly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. In general, ionospheric backscatter is observed at far ranges during the local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more near-range E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on geomagnetic activity and radar operating frequency are also investigated. The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as well as geophysical processes. A brief assessment of the possible impact of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the ‘instrumental’ aspect of backscatter occurrence, is essential for a full interpretation of HF coherent radar observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-0703-0
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  • 9
    Electronic Resource
    Electronic Resource
    SuperDARN studies of the ionospheric convection response to a northward turning of the interplanetary magnetic field (1998)
    Springer
    Annales geophysicae 16 (1998), S. 549-565 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (Auroral ionosphere) ; Magnetospheric physics (Magnetopause, cusp, and boundary layers; Magnetosphere-ionosphere interaction)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative Bz and positive By, to positive Bz and small By, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (∼2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within ∼2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of ∼7 and ∼14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further ∼6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-0549-0
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  • 10
    Electronic Resource
    Electronic Resource
    Interferometric evidence for the observation of ground backscatter originating behind the CUTLASS coherent HF radars (1997)
    Springer
    Annales geophysicae 15 (1997), S. 29-39 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Interferometric techniques allow the Super-DARN coherent HF radars to determine the elevation angles of returned backscatter, giving information on the altitude of the scatter volume, in the case of ionospheric backscatter, or the reflection altitude, in the case of ground backscatter. Assumptions have to be made in the determination of elevation angles, including the direction of arrival, or azimuth, of the returned signals, usually taken to be the forward look-direction (north) of the radars, specified by the phasing of the antenna arrays. It is shown that this assumption is not always valid in the case of ground backscatter, and that significant returns can be detected from the backward look-direction of the radars. The response of the interferometer to backscatter from behind the radar is modelled and compared with observations. It is found that ground backscatter from a field-of-view that is the mirror image of the forward-looking field-of-view is a common feature of the observations, and this interpretation successfully explains several anomalies in the received backscatter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-0029-y
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