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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
    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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  • 3
    Electronic Resource
    Electronic Resource
    Multi-instrument probing of the polar ionosphere under steady northward IMF (1999)
    Springer
    Annales geophysicae 18 (1999), S. 90-98 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (polar ionosphere) ; Magnetospheric physics (magnetosphere-ionosphere interactions; polar cap phenomena)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Observations are presented of the polar ionosphere under steady, northward IMF. The measurements, made by six complementary experimental techniques, including radio tomography, all-sky and meridian scanning photometer optical imaging, incoherent and coherent scatter radars and satellite particle detection, reveal plasma parameters consistent with ionospheric signatures of lobe reconnection. The optical green-line footprint of the reconnection site is seen to lie in the sunward plasma convection of the lobe cells. Downstream in the region of softer precipitation the reverse energy dispersion of the incoming ions can be identified. A steep latitudinal density gradient at the equatorward edge of the precipitation identifies the general location of an adiaroic boundary, separating the open field lines of polar lobe cells from the closed field of viscous-driven cells. Enhancements in plasma density to the south of the gradient are interpreted as ionisation being reconfigured as it is thrust against the boundary by the antisunward flow of the viscous cells near noon. Each of the instruments individually provides valuable information on certain aspects of the ionosphere, but the paper demonstrates that taken together the different experiments complement each other to give a consistent and comprehensive picture of the dayside polar ionosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0090-2
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  • 4
    Electronic Resource
    Electronic Resource
    Reconfiguration and closure of lobe flux by reconnection during northward IMF: possible evidence for signatures in cusp/cleft auroral emissions (1999)
    Springer
    Annales geophysicae 17 (1999), S. 996-1011 
    Details
    ISSN: 0992-7689
    Keywords: Magnetospheric physics (magnetopause, cusp and boundary layers; solar-wind-magnetosphere interactions) ; Space plasma physics (magnetic reconnection)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Observations are presented of the response of the dayside cusp/cleft aurora to changes in both the clock and elevation angles of the interplanetary magnetic field (IMF) vector, as monitored by the WIND spacecraft. The auroral observations are made in 630 nm light at the winter solstice near magnetic noon, using an all-sky camera and a meridian-scanning photometer on the island of Spitsbergen. The dominant change was the response to a northward turning of the IMF which caused a poleward retreat of the dayside aurora. A second, higher-latitude band of aurora was seen to form following the northward turning, which is interpreted as the effect of lobe reconnection which reconfigures open flux. We suggest that this was made possible in the winter hemisphere, despite the effect of the Earth’s dipole tilt, by a relatively large negative X component of the IMF. A series of five events then formed in the poleward band and these propagated in a southwestward direction and faded at the equatorward edge of the lower-latitude band as it migrated poleward. It is shown that the auroral observations are consistent with overdraped lobe flux being generated by lobe reconnection in the winter hemisphere and subsequently being re-closed by lobe reconnection in the summer hemisphere. We propose that the balance between the reconnection rates at these two sites is modulated by the IMF elevation angle, such that when the IMF points more directly northward, the summer lobe reconnection site dominates, re-closing all overdraped lobe flux and eventually becoming disconnected from the Northern Hemisphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-0996-2
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  • 5
    Electronic Resource
    Electronic Resource
    Probing discrete auroral arcs by ionospheric tomography (1998)
    Springer
    Annales geophysicae 16 (1998), S. 574-582 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (Auroral ionosphere) ; Magnetospheric physics (Auroral phenomena) ; Radio science (Ionospheric physics)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Optical observations of 557.7 nm and 630.0 nm emissions from discrete auroral arcs in the post-noon sector have been related to localised field-aligned enhancements in the spatial distribution of E- and F-layer electron density respectively seen in images reconstructed by ionospheric tomography. Results from two case studies are presented in which meridian scanning photometer and all-sky camera observations on Svalbard have been compared to electron-density structures found by tomographic inversion of measurements made by reception of radio signals at a chain of four stations at high latitude. The F-layer features are long-lived and show exact correspondence to the red-line emissions. Transient arcs in green-line intensity result in E-region structures that are resolved in one case, but not in another where the dynamic auroral forms are separated by less than one degree of latitude. The signature of an inverted-V precipitation event is clearly evident in one example.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-0574-z
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  • 6
    Electronic Resource
    Electronic Resource
    On the possible role of cusp/cleft precipitation in the formation of polar-cap patches (1999)
    Springer
    Annales geophysicae 17 (1999), S. 1298-1305 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionospheric irregularities; particle precipitation; polar ionosphere)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection flow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward flow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward flow across the polar cap.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-999-1298-4
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  • 7
    Electronic Resource
    Electronic Resource
    A comparison of optical and coherent HF radar backscatter observations of a post-midnight aurora (1997)
    Springer
    Annales geophysicae 15 (1997), S. 1388-1398 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A poleward-progressing 630 nm optical feature is observed between approximately 0100 UT and 0230 UT (0400 MLT to 0530 MLT) by a meridian-scanning photometer (MSP) located at Ny Alesund, Svalbard. Simultaneous coherent HF radar measurements indicate a region of poleward-expanding back-scatter with rapid sunward plasma flow velocity along the MSP meridian. Spatial maps of the backscatter indicate a stationary backscatter feature aligned obliquely with respect to the MSP meridian, which produces an impression of poleward-expansion as the MSP progresses to later MLT. Two interpretations of the observations are possible, depending on whether the arc system is considered to move (time-dependent) or to be stationary in time and apparent motion is produced as the MSP meridian rotates underneath it (time-independent). The first interpretation is as a poleward motion of an east-west aligned auroral arc. In this case the appearance of the region of backscatter is not associated with the optical feature, though the velocities within it are enhanced when the two are co-located. The second interpretation is as a polar arc or theta aurora, common features of the polar cap under the prevailing IMF northwards conditions. In this case the backscatter appears as an approximately 150 km wide region adjacent to the optical arc. In both interpretations the luminosity of the optical feature appears related to the magnitude of the plasma flow velocity. The optical features presented here do not generate appreciable HF coherent backscatter, and are only identifiable in the backscatter data as a modification of the flow by the arc electrodynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-1388-0
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  • 8
    Electronic Resource
    Electronic Resource
    ESR and EISCAT observations of the response of the cusp and cleft to IMF orientation changes (2000)
    Springer
    Annales geophysicae 18 (2000), S. 1009-1026 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (ionosphere – magnetosphere interactions; auroral ionosphere; plasma temperature and density)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Troms⊘ and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Ålesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened field lines for the observed negative IMF By. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating, northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward Bz, and explain their morphology in the context of previous theoretical work.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-1009-7
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  • 9
    Electronic Resource
    Electronic Resource
    Ionospheric signatures of magnetospheric boundaries in the post-noon sector (1999)
    Springer
    Annales geophysicae 18 (1999), S. 74-80 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; polar ionosphere)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Spatial structures in ionospheric electron density revealed in a tomographic image have been identified with auroral forms and related to their sources in precipitating particles observed by DMSP satellites. The observations of plasma enhancements relate to discrete auroral arcs seen in the post-noon sector, identified by both red- and green-line emissions measured by a meridional scanning photometer. The features lie within a very narrow latitudinal band on L-shells where the satellite detectors observed electron precipitation classified as from the boundary plasma sheet (BPS). The harder particles are identified with an E-region structure, while further north the precipitation is softer, resulting in a localised F-layer blob and 630.0 nm emissions. A steep gradient in plasma density represent a signature in the ionosphere of the central plasma sheet (CPS)/BPS boundary. A transition to a less-structured F-layer is found on crossing the convection reversal boundary.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-0074-2
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  • 10
    Electronic Resource
    Electronic Resource
    Plasma structure within poleward-moving cusp/cleft auroral transients: EISCAT Svalbard radar observations and an explanation in terms of large local time extent of events (2000)
    Springer
    Annales geophysicae 18 (2000), S. 1027-1042 
    Details
    ISSN: 0992-7689
    Keywords: Ionosphere (polar ionosphere) ; Magnetospheric physics (magnetopause; cusp and boundary layers; solar wind-magnetosphere interactions)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We report high-resolution observations of the southward-IMF cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) Svalbard radar (ESR), and compare them with observations of dayside auroral luminosity, as seen at a wavelength of 630 nm by a meridian scanning photometer at Ny Ålesund, and of plasma flows, as seen by the CUTLASS (co-operative UK twin location auroral sounding system) Finland HF radar. The optical data reveal a series of poleward-moving transient red-line (630 nm) enhancements, events that have been associated with bursts in the rate of magnetopause reconnection generating new open flux. The combined observations at this time have strong similarities to predictions of the effects of soft electron precipitation modulated by pulsed reconnection, as made by Davis and Lockwood (1996); however, the effects of rapid zonal flow in the ionosphere, caused by the magnetic curvature force on the newly opened field lines, are found to be a significant additional factor. In particular, it is shown how enhanced plasma loss rates induced by the rapid convection can explain two outstanding anomalies of the 630 nm transients, namely how minima in luminosity form between the poleward-moving events and how events can re-brighten as they move poleward. The observations show how cusp/cleft aurora and transient poleward-moving auroral forms appear in the ESR data and the conditions which cause enhanced 630 nm emission in the transients: they are an important first step in enabling the ESR to identify these features away from the winter solstice when supporting auroral observations are not available.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-000-1027-5
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