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1

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Meridian-scanning photometer, coherent HF radar, and magnetometer observations of the cusp: a case study (1999)

Milan, S. E. ; Lester, M. ; Cowley, S. W. H. ; [et al.]

Springer

Annales geophysicae 17 (1999), S. 159-172

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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

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Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern (1999)

Milan, S. E. ; Yeoman, T. K. ; Lester, M. ; [et al.]

Springer

Annales geophysicae 17 (1999), S. 877-891

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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

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Multi-instrument ground-based observations of a travelling convection vortices event (1996)

Lühr, H. ; Lockwood, M. ; Sandholt, P. E. ; [et al.]

Springer

Annales geophysicae 14 (1996), S. 162-181

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ISSN: 0992-7689

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract A coordinated ground-based observational campaign using the IMAGE magnetometer network, EISCAT radars and optical instruments on Svalbard has made possible detailed studies of a travelling convection vortices (TCV) event on 6 January 1992. Combining the data from these facilities allows us to draw a very detailed picture of the features and dynamics of this TCV event. On the way from the noon to the drawn meridian, the vortices went through a remarkable development. The propagation velocity in the ionosphere increased from 2.5 to 7.4 km s−1, and the orientation of the major axes of the vortices rotated from being almost parallel to the magnetic meridian near noon to essentially perpendicular at dawn. By combining electric fields obtained by EISCAT and ionospheric currents deduced from magnetic field recordings, conductivities associated with the vortices could be estimated. Contrary to expectations we found higher conductivities below the downward field aligned current (FAC) filament than below the upward directed. Unexpected results also emerged from the optical observations. For most of the time there were no discrete aurora at 557.7 nm associated with the TCVs. Only once did a discrete form appear at the foot of the upward FAC. This aurora subsequently expanded eastward and westward leaving its centre at the same longitude while the TCV continued to travel westward. Also we try to identify the source regions of TCVs in the magnetosphere and discuss possible generation mechanisms.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s00585-996-0162-z

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4

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Simultaneous optical and radar signatures of poleward-moving auroral forms (2000)

Thorolfsson, A. ; Cerisier, J.-C. ; Lockwood, M. ; [et al.]

Springer

Annales geophysicae 18 (2000), S. 1054-1066

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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 Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s00585-000-1054-2

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5

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The dynamic cusp aurora on 30 November 1997: response to southward turning of the IMF (1999)

Sandholt, P. E. ; Farrugia, C. J. ; Lybekk, B.

Springer

Annales geophysicae 17 (1999), S. 1155-1165

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ISSN: 0992-7689

Keywords: Mangetospheric physics (magnetopause, cusp and boundary layers; solar wind-magnetosphere interactions; auroral phenomena)

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract We document the detailed dynamics of the dayside aurora in the ≈1200–1600 MLT sector in response to a sharp southward turning of the interplanetary magnetic field (IMF) under negative IMF By conditions. Features not documented in previous work are elucidated by using two meridan scanning photometers (separated by 2 h) and an all-sky auroral imager in Ny Ålesund, Svalbard (75.5^MLAT) in combination with magnetograms from stations on Svalbard, covering the latitude range 71^–75^MLAT. The initial auroral response may be divided into three phases consisting of: (1) intensification of both the red (630.0 nm) and green (557.7 nm) line emissions in the cusp aurora near 1200 MLT and ≈100 km equatorward shift of its equatorward boundary, at ≈75^MLAT, (2) eastward and poleward expansions of the cusp aurora, reaching the 1430 MLT meridian after 5–6 min, and (3) east-west expansion of the higher-latitude aurora (at ≈77^–78^MLAT) in the postnoon sector. The associated magnetic disturbance is characterized by an initial positive deflection of the X-component at stations located 100–400 km south of the aurora, corresponding to enhanced Sunward return flow associated with the merging convection cell in the post-noon sector. The sequence of partly overlapping poleward moving auroral forms (PMAFs) during the first 15 min, accompanied by corresponding pulsations in the convection current, was followed by a strong westward contraction of the cusp aurora when the ground magnetograms indicated a temporary return to the pre-onset level. These observations are discussed in relation to the Cowley-Lockwood model of ionospheric response to pulsed magnetopause reconnection.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s00585-999-1155-5

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6

Electronic Resource

Characteristics of the Polar Cusp Region (1985)

EGELAND, A. ; SANDHOLT, P. E.

Oxford, UK : Blackwell Publishing Ltd

Annals of the New York Academy of Sciences 452 (1985), S. 0

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ISSN: 1749-6632

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Natural Sciences in General

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1749-6632.1985.tb30003.x

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7

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Auroral and magnetic variations in the polar cusp and cleft — Signatures of magnetopause boundary-layer dynamics (1988)

Sandholt, P. E. ; Egeland, A.

Springer

Astrophysics and space science 144 (1988), S. 171-199

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ISSN: 1572-946X

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract By combining continuous ground-based observations of polar cleft/cusp auroras and local magnetic variations with electromagnetic parameters obtained from satellites in polar orbit (low-altitude cleft/cusp) and in the magnetosheath/interplanetary space, different electrodynamic processes in the polar cleft/cusp have been investigated. One of the more controversial questions in this field is related to the observed shifts in latitude of cleft/cusp auroras and the relationship with the interplanetary magnetic field (IMF) orientation, local magnetic disturbances (DP2 and DPY modes) and magnetospheric substorms. A new approach which may contribute to clarifying these complicated relationships — simultaneous ground-based observations of the midday and evening-midnight sectors of the auroral oval—is illustrated. A related topic is the spatial relationship between the cleft/cusp auroras and the ionospheric convection currents. A characteristic feature of the polar cusp and cleft regions during negative IMFB Z is repeated occurrence of certain short-lived auroral structures which seem to move in accordance with the local convection pattern. Satellite measurements of particle precipitation, magnetic field and ion drift components permit detailed investigations of the electrodynamics of these cusp/cleft structures. Information on electric field components, Birkeland currents, Poynting flux, height-integrated Pedersen conductivity, and Joule heat dissipation rate has been derived. These observations are discussed in relation to existing models of temporal plasma injections from the magnetosheath.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00793180

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