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  • 1
    Digitale Medien
    Digitale Medien
    Radar observations of auroral zone flows during a multiple-onset substorm (1995)
    Springer
    Annales geophysicae 13 (1995), S. 1144-1163 
    Details
    ISSN: 0992-7689
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every \sim12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the preexisting extended current system driven by the large-scale flow. These filaments are \sim1 h MLT wide (\sim600 km), and initially expand poleward to a width of \sim300 km before contracting equatorward and coalescing with the preexisting current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s−1 or less during the first few minutes, before surging equatorward at 0.5-1.0 km s−1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed \sim100 mho, before decaying over a few minutes. This value compares with Hall conductivities of \sim20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-995-1144-2
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  • 2
    Digitale Medien
    Digitale Medien
    Substorm associated radar auroral surges: a statistical study and possible generation model (1998)
    Springer
    Annales geophysicae 16 (1998), S. 441-449 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Substorms ; Auroral surges ; Plasma convection ; Sub-auroral ion drifts
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Substorm-associated radar auroral surges (SARAS) are a short lived (15–90 minutes) and spatially localised (∼5° of latitude) perturbation of the plasma convection pattern observed within the auroral E-region. The understanding of such phenomena has important ramifications for the investigation of the larger scale plasma convection and ultimately the coupling of the solar wind, magnetosphere and ionosphere system. A statistical investigation is undertaken of SARAS, observed by the Sweden And Britain Radar Experiment (SABRE), in order to provide a more extensive examination of the local time occurrence and propagation characteristics of the events. The statistical analysis has determined a local time occurrence of observations between 1420 MLT and 2200 MLT with a maximum occurrence centred around 1700 MLT. The propagation velocity of the SARAS feature through the SABRE field of view was found to be predominately L-shell aligned with a velocity centred around 1750 ms−1 and within the range 500 m s−1 and 3500 m s−1. This comprehensive examination of the SARAS provides the opportunity to discuss, qualitatively, a possible generation mechanism for SARAS based on a proposed model for the production of a similar phenomenon referred to as sub-auroral ion drifts (SAIDs). The results of the comparison suggests that SARAS may result from a similar geophysical mechanism to that which produces SAID events, but probably occurs at a different time in the evolution of the event.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-998-0441-y
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  • 3
    Digitale Medien
    Digitale Medien
    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
    Schlagwort(e): Ionosphere (Auroral ionosphere) ; Magnetospheric physics (Magnetopause, cusp, and boundary layers; Magnetosphere-ionosphere interaction)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-998-0549-0
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  • 4
    Digitale Medien
    Digitale Medien
    Simultaneous ionospheric and magnetospheric observations of azimuthally propagating transient features during substorms (1998)
    Springer
    Annales geophysicae 16 (1998), S. 754-763 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Ionosphere (Auroral, ionosphere) ; Magnetospheric Physics (Magnetotail; Storms and substorms)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract During the 6th August 1995, the CUTLASS Finland HF radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s. Data from such scans, during the substorm expansion phase, revealed pulses of equatorward flow exceeding ∼600 m s−1 with a duration of ∼5 min and a repetition period of ∼8 min. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. These transient features, which propagate eastwards away from local midnight, have been interpreted as ionospheric current vortices associated with fieldaligned current pairs. The present study reveals that these ionospheric convection features appear to have an accompanying signature in the magnetosphere, comprising a dawnward perturbation and dipolarisation of the magnetic field and dawnward plasma flow, measured in the geomagnetic tail by the Geotail spacecraft, located at L = 10 and some four hours to the east, in the postmidnight sector. These signatures are suggested to be the consequence of the observation of the same field aligned currents in the magnetosphere. Their possible relationship with bursty Earthward plasma flow and magnetotail reconnection is discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-998-0754-x
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  • 5
    Digitale Medien
    Digitale Medien
    The polarization of Pc5 ULF waves around dawn: A possible ionospheric conductivity gradient effect (1995)
    Springer
    Annales geophysicae 13 (1995), S. 159-167 
    Details
    ISSN: 0992-7689
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-995-0159-z
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  • 6
    Digitale Medien
    Digitale Medien
    The response of ionospheric convection in the polar cap to substorm activity (1995)
    Springer
    Annales geophysicae 13 (1995), S. 147-158 
    Details
    ISSN: 0992-7689
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°〈Lambda〉-78°〈Lambda〉. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°〈Lambda〉 by the time of the expansion phase onset. A westward electrojet, centred at 65.4°〈Lambda〉, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°〈Lambda〉 at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the “distant” neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-995-0147-3
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  • 7
    Digitale Medien
    Digitale Medien
    CUTLASS observations of a high-m ULF wave and its consequences for the DOPE HF Doppler sounder (1999)
    Springer
    Annales geophysicae 17 (1999), S. 1493-1497 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Ionosphere (ionosphere-magnetosphere interactions) ; Magnetospheric physics (magnetosphere-ionosphere interactions; MHD waves and instabilities)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract The CUTLASS (Co-operative UK Twin Located Auroral Sounding System) Finland HF radar, whilst operating in a high spatial and temporal resolution mode, has measured the ionospheric signature of a naturally occurring ULF wave in scatter artificially generated by the Tromsø Heater. The wave had a period of 100 s and exhibited curved phase fronts across the heated volume (about 180 km along a single radar beam). Spatial information provided by CUTLASS has enabled an m-number for the wave of about 38 to be determined. This high-m wave was not detected by the IMAGE (International Monitor for Auroral Geomagnetic Effects) network of ground magnetometers, as expected for a wave of a small spatial scale size. These observations offer the first independent confirmation of the existence of the ground uncorrelated ULF wave signatures previously reported in measurements recorded from an HF Doppler sounder located in the vicinity of Tromsø. These results both demonstrate a new capability for geophysical exploration from the combined CUTLASS-EISCAT ionospheric Heater experiment, and provide a verification of the HF Doppler technique for the investigation of small scale ULF waves.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-999-1493-3
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  • 8
    Digitale Medien
    Digitale Medien
    A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar (1999)
    Springer
    Annales geophysicae 17 (1999), S. 707-711 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Interplanetary physics (ionosphere-magnetosphere interaction) Magnetospheric physics (magnetopause, cusp, and boundary layers; solar wind-magnetosphere interactions)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corresponding transient plasma flows were seen in the near-conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ≈226 RE upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ≈77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78° and 83° magnetic latitude, near the ionospheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magnetopause reconnection.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-999-0707-z
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  • 9
    Digitale Medien
    Digitale Medien
    ULF wave occurrence statistics in a high-latitude HF Doppler sounder (1999)
    Springer
    Annales geophysicae 17 (1999), S. 749-758 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Ionosphere (auroral ionosphere; ionosphere -magnetosphere interactions) ; Magnetospheric physics (MHD waves and instabilities)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-999-0749-2
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  • 10
    Digitale Medien
    Digitale Medien
    Statistical observations of the MLT, latitude and size of pulsed ionospheric flows with the CUTLASS Finland radar (1999)
    Springer
    Annales geophysicae 17 (1999), S. 855-867 
    Details
    ISSN: 0992-7689
    Schlagwort(e): Magnetospheric physics (magnetosphere-ionosphere interactions; plasma convection; solar wind-magnetosphere interactions)
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Physik
    Notizen: Abstract A study has been performed on the occurrence of pulsed ionospheric flows as detected by the CUTLASS Finland HF radar. These flows have been suggested as being created at the ionospheric footprint of newly-reconnected field lines, during episodes of magnetic flux transfer into the terrestrial magnetosphere (flux transfer events or FTEs). Two years of both high-time resolution and normal scan data from the CUTLASS Finland radar have been analysed in order to perform a statistical study of the extent and location of the pulsed ionospheric flows. We note a great similarity between the statistical pattern of the coherent radar observations of pulsed ionospheric flows and the traditional low-altitude satellite identification of the particle signature associated with the cusp/cleft region. However, the coherent scatter radar observations suggest that the merging gap is far wider than that proposed by the Newell and Meng model. The new model for cusp low-altitude particle signatures, proposed by Lockwood and Onsager and Lockwood provides a unified framework to explain the dayside precipitation regimes observed both by the low-altitude satellites and by coherent scatter radar detection.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s00585-999-0855-1
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