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  • 1
    Electronic Resource
    Electronic Resource
    Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study (1999)
    Springer
    Annales geophysicae 17 (1999), S. 159-172 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (auroral ionosphere; plasma convection) ; Magnetospheric physics (magnetopause, cusp, and boundary layers)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The dynamics of the cusp region and post-noon sector for an interval of predominantly IMF By, Bz 〈 0 nT are studied with the CUTLASS Finland coherent HF radar, a meridian-scanning photometer located at Ny Ålesund, Svalbard, and a meridional network of magnetometers. The scanning mode of the radar is such that one beam is sampled every 14 s, and a 30° azimuthal sweep is completed every 2 minutes, all at 15 km range resolution. Both the radar backscatter and red line (630 nm) optical observations are closely co-located, especially at their equatorward boundary. The optical and radar aurora reveal three different behaviours which can interchange on the scale of minutes, and which are believed to be related to the dynamic nature of energy and momentum transfer from the solar wind to the magnetosphere through transient dayside reconnection. Two interpretations of the observations are presented, based upon the assumed location of the open/closed field line boundary (OCFLB). In the first, the OCFLB is co-located with equatorward boundary of the optical and radar aurora, placing most of the observations on open field lines. In the second, the observed aurora are interpreted as the ionospheric footprint of the region 1 current system, and the OCFLB is placed near the poleward edge of the radar backscatter and visible aurora; in this interpretation, most of the observations are placed on closed field lines, though transient brightenings of the optical aurora occur on open field lines. The observations reveal several transient features, including poleward and equatorward steps in the observed boundaries, “braiding” of the backscatter power, and 2 minute quasi-periodic enhancements of the plasma drift and optical intensity, predominantly on closed field lines.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0159-5
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  • 2
    Electronic Resource
    Electronic Resource
    CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1411-1422 
    Details
    ISSN: 0992-7689
    Keywords: Magnetospheric physics (auroral phenomena; magnetopause, cusp and boundary layers; magnetosphere - ionosphere interaction)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The CUTLASS Finland radar has been run in a two-beam special scan mode, which offered excellent temporal and spatial information on the flows in the high-latitude ionosphere. A detailed study of one day of this data revealed a convection reversal boundary (CRB) in the CUTLASS field of view (f.o.v) on the dayside, the direction of plasma flow either side of the boundary being typical of a dawn-cell convection pattern. Poleward of the CRB a number of pulsed transients are observed, seemingly moving away from the radar. These transients are identified here as the ionospheric signature of flux transfer events (FTEs). Equatorward of the CRB continuous backscatter was observed, believed to be due to the return flow on closed field lines. The two-beam scan offered a new and innovative opportunity to determine the size and velocity of the ionospheric signatures associated with flux transfer events and the related plasma flow pattern. The transient signature was found to have an azimuthal extent of 1900 ± 900 km and an poleward extent of ∼250 km. The motion of the transient features was in a predominantly westward azimuthal direction, at a velocity of 7.5 ± 3 km.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1411-0
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  • 3
    Electronic Resource
    Electronic Resource
    Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern (1999)
    Springer
    Annales geophysicae 17 (1999), S. 877-891 
    Details
    ISSN: 0992-7689
    Keywords: Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers; MHD waves and instabilities)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1–2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0877-8
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  • 4
    Electronic Resource
    Electronic Resource
    A comparison of veloCity measurements from the CUTLASS Finland radar and the EISCAT UHF system (1999)
    Springer
    Annales geophysicae 17 (1999), S. 892-902 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionospheric irregularities; plasma convection)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The CUTLASS Finland radar, which comprises an integral part of the SuperDARN system of HF coherent radars, provides near continuous observations of high-latitude plasma irregularities within a field-of-view which extends over some four million square kilometres. Within the Finland radar field-of-view lie both the EISCAT mainland and EISCAT Svalbard incoherent scatter radar facilities. Since the CUTLASS Finland radar commenced operation, in February 1995, the mainland EISCAT UHF radar has been run in common programme 1 and 2 modes for a total duration exceeding 1000 h. Simultaneous and spatially coincident returns from these two radars over this period provide the basis for a comparison of irregularity drift veloCity and F-region ion veloCity. Initial comparison is limited to velocities from four intervals of simultaneous radar returns; intervals are selected such that they exhibit a variety of veloCity signatures including that characteristic of the convection reversal and a rapidly fluctuating veloCity feature. Subsequent comparison is on a statistical basis. The velocities measured by the two systems demonstrate reasonable correspondence over the veloCity regime encountered during the simultaneous occurrence of coherent and incoherent scatter; differences between the EISCAT UHF measurements of F-region ion drift and the irregularity drift velocities from the Finland radar are explained in terms of a number of contributing factors including contamination of the latter by E-region echoes, a factor which is investigated further, and the potentially deleterious effect of discrepant volume and time sampling intervals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0892-9
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  • 5
    Electronic Resource
    Electronic Resource
    Letter to the editor A comparison of F-region ion velocity observations from the EISCAT Svalbard and VHF radars with irregularity drift velocity measurements from the CUTLASS Finland HF radar (2000)
    Springer
    Annales geophysicae 18 (2000), S. 589-594 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionospheric irregularities; plasma convection; instruments and techniques)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract During August 1998, the UK EISCAT special programme SP-UK-CSUB, which combines operation of both the mainland VHF and Svalbard UHF incoherent scatter radars, was run for several hours around magnetic midnight on four consecutive days. The CUTLASS Finland HF coherent scatter radar was, at these times, operating in a discretionary mode, sounding on all 16 beams, one at high-time resolution. This study presents a comparison of the velocities measured by coherent and incoherent techniques during the SP-UK-CSUB experiments. Agreement, particularly between the ion velocities measured by the EISCAT Svalbard radar and irregularity drift measurements by the Finland radar, is remarkable, thereby validating the scientific integrity of both data sets. This work highlights the substantive contribution to our understanding of the solar-terrestrial environment which can be made by use in concert of incoherent and HF coherent scatter radars.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0589-6
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  • 6
    Electronic Resource
    Electronic Resource
    Investigation of the relationship between optical auroral forms and HF radar E region backscatter (2000)
    Springer
    Annales geophysicae 18 (2000), S. 608-617 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (particle precipitation; plasma waves and instabilities)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The SuperDARN HF radars have been employed in the past to investigate the spectral characteristics of coherent backscatter from L-shell aligned features in the auroral E region. The present study employs all-sky camera observations of the aurora from Husafell, Iceland, and the two SuperDARN radars located on Iceland, Þykkvibær and Stokkseyri, to determine the optical signature of such backscatter features. It is shown that, especially during quiet geomagnetic conditions, the backscatter region is closely associated with east-west aligned diffuse auroral features, and that the two move in tandem with each other. This association between optical and radar aurora has repercussions for the instability mechanisms responsible for generating the E region irregularities from which radars scatter. This is discussed and compared with previous studies investigating the relationship between optical and VHF radar aurora. In addition, although it is known that E region backscatter is commonly observed by SuperDARN radars, the present study demonstrates for the first time that multiple radars can observe the same feature to extend over at least 3 h of magnetic local time, allowing precipitation features to be mapped over large portions of the auroral zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0608-7
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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Simultaneous observations at different altitudes of ionospheric backscatter in the eastward electrojet (1997)
    Springer
    Annales geophysicae 16 (1997), S. 55-68 
    Details
    ISSN: 0992-7689
    Keywords: Auroral ionosphere ; Ionospheric irregularities ; Plasma convection
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A common feature of evening near-range ionospheric backscatter in the CUTLASS Iceland radar field of view is two parallel, approximately L-shell-aligned regions of westward flow which are attributed to irregularities in the auroral eastward electrojet region of the ionosphere. These backscatter channels are separated by approximately 100–200 km in range. The orientation of the CUTLASS Iceland radar beams and the zonally aligned nature of the flow allows an approximate determination of flow angle to be made without the necessity of bistatic measurements. The two flow channels have different azimuthal variations in flow velocity and spectral width. The nearer of the two regions has two distinct spectral signatures. The eastern beams detect spectra with velocities which saturate at or near the ion-acoustic speed, and have low spectral widths (less than 100ms−1), while the western beams detect lower velocities and higher spectral widths (above 200ms−1). The more distant of the two channels has only one spectral signature with velocities above the ionacoustic speed and high spectral widths. The spectral characteristics of the backscatter are consistent with E-region scatter in the nearer channel and upper-E-region or F-region scatter in the further channel. Temporal variations in the characteristics of both channels support current theories of E-region turbulent heating and previous observations of velocity-dependent backscatter cross-section. In future, observations of this nature will provide a powerful tool for the investigation of simultaneous E- and F-region irregularity generation under similar (nearly co-located or magnetically conjugate) electric field conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-0055-9
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    Substorm correlated absorption on a 3200 km trans-auroral HF propagation path (1996)
    Springer
    Annales geophysicae 14 (1996), S. 182-190 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A high-frequency transmitter located at Clyde River, NWT, Canada, and a receiver located near Boston, USA, provide a 3200 km trans-auroral, near-meridional propagation path over which the propagation characteristics have been measured. Out of the fourteen frequencies in the HF band sampled every hour for the duration of the experimental campaign (16 January-8 February 1989), the signal level measurements of 6.800 MHz transmissions were selected in order to determine the extent and occurrence of auroral absorption. The median level of auroral absorption along the path is found to increase with geomagnetic activity, quantified by the index Kp, with the increase being greater in the post-midnight sector than in the pre-midnight sector. This asymmetric behaviour is attributed to the precipitation of high energy electrons into the midnight and morning sector auroral D region. The measured diurnal variation in the median level of absorption is consistent with previous models describing the extent and magnitude of auroral absorption and electron precipitation. Individual substorms, identified from geosynchronous satellite data, are found to cause short-lived absorption events in the HF signal level of \sim30 dB at 6.800 MHz. The occurrence of substorm correlated auroral absorption events is confined to the midnight and morning sectors, consistent with the location of the electron precipitation. The magnitude of absorption is related to the magnetotail stress during the substorm growth phase and the magnetotail relaxation during the substorm expansion phase onset. The absorption magnitude and the occurrence of substorms during the period of the campaign increase at times of high Kp, leading to an increase in median auroral absorption during disturbed periods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-0182-8
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