On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

doi:10.1007/S00585-001-1523-2

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On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

Journal Article•DOI•

On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

Michael Pinnock¹, Alan S. Rodger¹•Institutions (1)

British Antarctic Survey¹

31 Dec 2000-Annales Geophysicae (Copernicus GmbH)-Vol. 18, Iss: 12, pp 1523-1530

TL;DR: In this paper, the authors examined the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern using four SuperDARN radars and found that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site.

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Abstract: Previous work has shown that ionospheric HF radar backscatter in the noon sector can be used to locate the footprint of the magnetospheric cusp particle precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plasma across it to derive the reconnection electric field in the ionosphere. This work used only single radar data sets with a field of view limited to ∼2 h of local time. In this case study using four of the SuperDARN radars, we examine the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern. The variation with longitude of the latitude of the radar scatter with cusp characteristics shows a bay-like feature. It is shown that this feature is shaped by the variation with longitude of the poleward flow component of the ionospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cusp ions and find that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF radar data is described.

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Journal Article•DOI•

The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field

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Michael Pinnock¹, Gareth Chisham¹, I. J. Coleman¹, Mervyn P. Freeman¹, Marc R. Hairston², Jean-Paul Villain³ - Show less +2 more•Institutions (3)

Natural Environment Research Council¹, University of Texas at Dallas², Centre national de la recherche scientifique³

31 Jul 2003-Annales Geophysicae

TL;DR: In this paper, the authors used ionospheric data from the SuperDARN radar network and a DMSP satellite to obtain a comprehensive description of the spatial and temporal pattern of day-side reconnection.

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Abstract: . Using ionospheric data from the SuperDARN radar network and a DMSP satellite we obtain a comprehensive description of the spatial and temporal pattern of day-side reconnection. During a period of southward interplanetary magnetic field (IMF), the data are used to determine the location of the ionospheric projection of the dayside magnetopause reconnection X-line. From the flow of plasma across the projected X-line, we derive the reconnection rate across 7 h of longitude and estimate it for the total length of the X-line footprint, which was found to be 10 h of longitude. Using the Tsyganenko 96 magnetic field model, the ionospheric data are mapped to the magnetopause, in order to provide an estimate of the extent of the reconnection X-line. This is found to be ~ 38 R E in extent, spanning the whole dayside magnetopause from dawn to dusk flank. Our results are compared with previously reported encounters by the Equator-S and Geotail spacecraft with a reconnecting magnetopause, near the dawn flank, for the same period. The SuperDARN observations allow the satellite data to be set in the context of the whole magnetopause reconnection X-line. The total potential associated with dayside reconnection was ~ 150 kV. The reconnection signatures detected by the Equator-S satellite mapped to a region in the ionosphere showing continuous flow across the polar cap boundary, but the reconnection rate was variable and showed a clear spatial variation, with a distinct minimum at 14:00 magnetic local time which was present throughout the 30-min study period. Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionoshere interactions) – Space plasma physics (magnetic reconnection)

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46 citations

Cites background from "On determining the noon polar cap b..."

...…auroral emissions or radar Doppler spectral width boundary) related to the cusp precipitation will also be offset from the true location of the PCB. Pinnock and Rodger (2001) illustrated the self-consistency of this approach, deriving typical cusp ion travel times from SWB and convection data…...
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...The location of the magnetopause was then mapped into the ionosphere by field-line tracing, to give an estimate of the location of the PCB....
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...…merging hypothesis varied with dipole tile angle; it ranged between 3 h (summer) and 8 h (winter) of MLT. Previous experimental data have suggested values between 3 h (Pinnock and Rodger, 2001) and greater than 8 h of MLT (Crooker et al., 1991) for the dayside merging line in the ionosphere....
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...Higher order components were found, but the magnitude of these perturbations on the dayside gives a maximum tilt of the PCB of the order of 10 degrees with respect to the zero order circle (see Pinnock et al., 1999, p. 447 for discussion of this), which results in an error of 20% in determining the reconnection electric field for a flow vector of any magnitude within± 45◦ of the normal to the assumed circular PCB. Significant perturbations of the auroral oval shape are found for IMFBz northward (Hones et al., 1989), significant IMFBy component, and after substorm onset (Frank and Craven, 1988)....
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...The assumption that the arc of a circle can approximate the PCB across the local time sector spanned by the radar and satellite data (06:00–17:00 MLT) and over the 30-min interval of interest is justified by reference to previous observations of the poleward boundary of the auroral oval that is a proxy for the PCB. Holzworth and Meng (1975), in deriving a mathematical expression for the Feldstein (1963) statistical oval, found that the poleward boundary of the auroral oval is approximated to zero order by a circle offset from the magnetic pole, for all geomagnetic activity levels....
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Journal Article•DOI•

An investigation of latitudinal transitions in the SuperDARN Doppler spectral width parameter at different magnetic local times

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Gareth Chisham¹, Mervyn P. Freeman¹•Institutions (1)

Natural Environment Research Council¹

02 Apr 2004-Annales Geophysicae

TL;DR: In this paper, the authors performed a statistical study of the nature of similar spectral width transitions at other magnetic local times (MLTs) and showed that the probability of a transition occurring at any latitude, identified independently in each MLT sector, is continuous with MLT from the cusp, through the morning sector, to the nightside.

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Abstract: . Latitudinal transitions from low to high Doppler spectral width in backscatter measured by the Super Dual Auroral Radar Network (SuperDARN) are now routinely used as proxies for the polar cap boundary (PCB) in the cusp-region ionosphere. In this paper we perform a statistical study of the nature of similar spectral width transitions at other magnetic local times (MLTs). This analysis illustrates that these latitudinal spectral width transitions exist at all magnetic local times, and that the latitude, gradient, and amplitude of the transitions vary systematically with MLT. In particular, the probability of a transition occurring at any latitude, identified independently in each MLT sector, is continuous with MLT from the cusp, through the morning sector, to the nightside. This suggests that the transition represents the PCB, as this is known to be what it represents in the cusp region. However, the picture in the afternoon sector (12:00-18:00 MLT) is more complex with no clearly preferred transition latitudes. Key words. Ionosphere (ionosphere-magnetosphere interactions; instruments and techniques). Magnetospheric physics (magnetopause, cusp, and boundary layers.)

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33 citations

Cites background or methods from "On determining the noon polar cap b..."

..., 1999; Milan and Lester, 2001), and the Doppler spectral width boundary (SWB) from the Super Dual Auroral Radar Network (SuperDARN) (Baker et al., 1995, 1997; Pinnock and Rodger, 2001; Chisham et al., 2001, 2002)....
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...…et al., 1996, 2001), the equatorward edge of HF radar backscatter (Milan et al., 1999; Milan and Lester, 2001), and the Doppler spectral width boundary (SWB) from the Super Dual Auroral Radar Network (SuperDARN) (Baker et al., 1995, 1997; Pinnock and Rodger, 2001; Chisham et al., 2001, 2002)....
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Journal Article•DOI•

Space- and ground-based investigations of solar wind–magnetosphere–ionosphere coupling

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Steve Milan¹, J. A. Wild¹, Adrian Grocott¹, N. C. Draper¹•Institutions (1)

University of Leicester¹

01 Jan 2006-Advances in Space Research

TL;DR: In this paper, a brief review of the understanding of the coupled solar wind-magnetosphere-ionosphere system that can be attained from observations of the size of the ionospheric polar caps from space and the ground is provided.

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22 citations

Journal Article•DOI•

What controls the local time extent of flux transfer events

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Stephen E. Milan¹, Stephen E. Milan², Suzanne M. Imber¹, J. A. Carter¹, M.-T. Walach¹, Benoît Hubert³ - Show less +2 more•Institutions (3)

University of Leicester¹, University of Bergen², University of Liège³

01 Feb 2016-Journal of Geophysical Research

TL;DR: In this paper, the authors compare two sequences of the ionospheric signatures of flux transfer events observed in global auroral imagery and coherent ionosphere radar measurements, and discuss the ramifications of these differences for solar wind-magnetosphere coupling.

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Abstract: Flux transfer events (FTEs) are the manifestation of bursty and/or patchy magnetic reconnection at the magnetopause. We compare two sequences of the ionospheric signatures of flux transfer events observed in global auroral imagery and coherent ionospheric radar measurements. Both sequences were observed during very similar seasonal and interplanetary magnetic field (IMF) conditions, though with differing solar wind speed. A key observation is that the signatures differed considerably in their local time extent. The two periods are 26 August 1998, when the IMF had components BZ≈−10 nT and BY≈9 nT and the solar wind speed was VX≈650 km s−1, and 31 August 2005, IMF BZ≈−7 nT, BY≈17 nT, and VX≈380 km s−1. In the first case, the reconnection rate was estimated to be near 160 kV, and the FTE signatures extended across at least 7 h of magnetic local time (MLT) of the dayside polar cap boundary. In the second, a reconnection rate close to 80 kV was estimated, and the FTEs had a MLT extent of roughly 2 h. We discuss the ramifications of these differences for solar wind-magnetosphere coupling.

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21 citations

Journal Article•DOI•

An unusual geometry of the ionospheric signature of the cusp: implications for magnetopause merging sites

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Gareth Chisham¹, Michael Pinnock¹, I. J. Coleman¹, Marc R. Hairston², Anthony Walker³ - Show less +1 more•Institutions (3)

Natural Environment Research Council¹, University of Texas at Dallas², University of Natal³

31 Jan 2002-Annales Geophysicae

TL;DR: The spectral width boundary (SWB) in the cusp represents a very good proxy for the equatorward edge of cusp ion precipitation in the dayside ionosphere.

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Abstract: . The HF radar Doppler spectral width boundary (SWB) in the cusp represents a very good proxy for the equatorward edge of cusp ion precipitation in the dayside ionosphere. For intervals where the Interplanetary Magnetic Field (IMF) has a southward component (Bz Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (magnetic reconnection)

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20 citations

Cites background or methods from "On determining the noon polar cap b..."

...We suggested earlier that poleward excursions of the SWB, such as observed here, can be used as a diagnostic of the extent of the merging gap (Rodger, 2000; Pinnock and Rodger, 2001)....
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...…a growing understanding of the factors which result in the high spectral width values observed in the cusp region (André et al., 1999, 2000) and also those which influence the offset of the SWB from the actual PCB (Rodger and Pinnock, 1997; Lockwood, 1997; Rodger, 2000; Pinnock and Rodger, 2001)....
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...5b. 4.4 The morning sector reconnection potential We suggested earlier that poleward excursions of the SWB, such as observed here, can be used as a diagnostic of the extent of the merging gap (Rodger, 2000; Pinnock and Rodger, 2001)....
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..., 1999, 2000) and also those which influence the offset of the SWB from the actual PCB (Rodger and Pinnock, 1997; Lockwood, 1997; Rodger, 2000; Pinnock and Rodger, 2001)....
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...Pinnock and Rodger (2001) have observed this characteristic proxy variation in the combined SWB variation of four SuperDARN HF radars covering∼ 6 h of MLT....
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References

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Journal Article•DOI•

Modeling the Earth's magnetospheric magnetic field confined within a realistic magnetopause

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Nikolai A. Tsyganenko¹•Institutions (1)

Goddard Space Flight Center¹

01 Apr 1995-Journal of Geophysical Research

TL;DR: In this paper, the authors describe a new approach to the problem of fitting the magnetosphereic magnetic field to spacecraft data, based on fits to a large number of observed crossing (allowing a parametrization by the solar wind pressure).

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Abstract: Empirical data-based models of the magnetosphereic magnetic field have been widely used during recent years. However, the existing models (Tsyganenko, 1987, 1989a) have three serious deficiencies: (1) an unstable de facto magnetopause, (2) a crude parametrization by the K(sub p) index, and (3) inaccuracies in the equatorial magnetotail B(sub z) values. This paper describes a new approach to the problem; the essential new features are (1) a realistic shape and size of the magnetopause, based on fits to a large number of observed crossing (allowing a parametrization by the solar wind pressure), (2) fully controlled shielding of the magnetic field produced by all magnetospheric current systems, (3) new flexible representations for the tail and ring currents, and (4) a new directional criterion for fitting the model field to spacecraft data, providing improved accuracy for field line mapping. Results are presented from initial efforts to create models assembled from these modules and calibrated against spacecraft data sets.

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950 citations

"On determining the noon polar cap b..." refers methods in this paper

...…the IMF) and that they transit a ®eld-aligned distance of 13 Re to reach the ionosphere (obtained from the Tsyganenko 96 magnetospheric ®eld model (Tsyganenko, 1995)) then the cusp ion transit times derived above correspond to ions with energies ranging between 3.6 keV (100 s) and 900 eV (200…...
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...…the IMF) and that they transit a ®eld-aligned distance of 13 Re to reach the ionosphere (obtained from the Tsyganenko 96 magnetospheric ®eld model (Tsyganenko, 1995)) then the cusp ion transit times derived above correspond to ions with energies ranging between 3.6 keV (100 s) and 900 eV (200 s)....
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Journal Article•

Excitation and decay of solar-wind driven flows in the magnetosphere-ionosphere system

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Stanley W. H. Cowley, Mike Lockwood

01 Feb 1992-Annales Geophysicae

TL;DR: In this paper, the authors considered the high-latitude ionospheric flows and their excitation and decay and proposed a flow-free equilibrium configuration for a magnetosphere which contains a given (arbitrary) amount of open flux.

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Abstract: Basic concepts of the form of high-latitude ionospheric flows and their excitation and decay are discussed in the light of recent high time-resolution measurements made by ground-based radars. It is first pointed out that it is in principle impossible to adequately parameterize these flows by any single quantity derived from concurrent interplanetary conditions. Rather, even at its simplest, the flow must be considered to consist of two basic time-dependent components. The first is the flow driven by magnetopause coupling processes alone, principally by dayside reconnection. These flows may indeed be reasonably parameterized in terms of concurrent near-Earth interplanetary conditions, principally by the interplanetary magnetic field (IMF) vector. The second is the flow driven by tail reconnection alone. As a first approximation these flows may also be parameterized in terms of interplanetary conditions, principally the north-south component of the IMF, but with a delay in the flow response of around 30-60 min relative to the IMF. A delay in the tail response of this order must be present due to the finite speed of information propagation in the system, and we show how "growth" and "decay" of the field and flow configuration then follow as natural consequences. To discuss the excitation and decay of the two reconnection-driven components of the flow we introduce that concept of a flow-free equilibrium configuration for a magnetosphere which contains a given (arbitrary) amount of open flux. Reconnection events act either to create or destroy open flux, thus causing departures of the system from the equilibrium configuration. Flow is then excited which moves the system back towards equilibrium with the changed amount of open flux. We estimate that the overall time scale associated with the excitation and decay of the flow is about 15 min. The response of the system to both impulsive (flux transfer event) and continuous reconnection is discussed in these terms.

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621 citations

"On determining the noon polar cap b..." refers background in this paper

...The three arrows mark the times of ¯ow transients in the ionosphere (discussed in the text) account of the dynamics of the OCB associated with the reconnection process, such as have been suggested by Cowley and Lockwood (1992)....
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Journal Article•DOI•

The cusp and the cleft/boundary layer: Low-altitude identification and statistical local time variation

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Patrick T. Newell, Ching-I. Meng

01 Dec 1988-Journal of Geophysical Research

TL;DR: In this paper, the authors investigated the distinction between the low-altitude cusp and the cleft (with the latter identified as the ionospheric signature of low-latitude boundary layer (LLBL)) on both a statistical and a case study basis.

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Abstract: Particles of roughly magnetosheath energies precipitate at low altitudes throughout the dayside, in a band referred to as the cusp or cleft. Recently it has been suggested that the cusp proper is a more limited region of the cleft localized near noon, although the criteria for distinguishing between the two regions have been unclear. An investigation into the distinction between the low-altitude cusp and the cleft (with the latter herein identified as the ionospheric signature of the low-latitude boundary layer (LLBL)) was performed on both a statistical and a case study basis. One year of DMSP F7 electron and ion data, comprising in all 5609 individual dayside passes, was employed. It was found that the average energy of precipitating particles allows for a clear morphological distinction between the cusp proper and the cleft/LLBL. Often both regions are observed on a given pass at the same MLT, each with its own characteristic properties. The probability of observing the cusp was found to be sharply peaked at 1200 MLT, while the probability of observing the cleft/LLBL was near unity away from noon and had a minimum at noon. The cusp was found to be 0.8°–1.1° magnetic latitude (MLAT) thick essentially independent of MLT, whereas the cleft was thinnest at noon and widened rapidly at local times away from noon. The ion number flux in the cusp was statistically 3.6 times higher than in the cleft. The peak flux within the cusp was located on average closer to the equatorward than to the poleward boundary. Yearly average composite spectrograms of precipitation in the two regions as a function of local time show that the properties of the cusp change comparatively little with local time, but that the peak ion energy flux in the cleft increases smoothly from roughly magnetosheath values close to noon to about plasma sheet boundary layer values near 0600 MLT.

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401 citations

"On determining the noon polar cap b..." refers methods in this paper

...The ion energies derived compare favourably with those observed in the low-altitude cusp by DMSP satellite (see e.g. Newell and Meng, 1988), the higher energies being close to those often observed at the equatorward edge of the cusp particle precipitation (Newell and Meng, 1991, see their Fig....
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Journal Article•DOI•

HF radar signatures of the cusp and low-latitude boundary layer

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K. B. Baker¹, J. R. Dudeney², R. A. Greenwald, Michael Pinnock², Patrick T. Newell, Alan S. Rodger², N. Mattin², C.-I. Meng - Show less +4 more•Institutions (2)

Johns Hopkins University¹, British Antarctic Survey²

01 May 1995-Journal of Geophysical Research

TL;DR: In this article, an intercalibration study made using the Polar Anglo-American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites is used to provide clear identifications of the ionospheric cusp and the low-latitude boundary layer (LLBL).

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Abstract: Continuous ground-based observations of ionospheric and magnetospheric regions are critical to the Geospace Environment Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground-based instruments and satellites in order to clearly place the ground-based observations in context with the corresponding in situ satellite measurements. HF-radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo-American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low-latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibration indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed in the cusp.

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231 citations

"On determining the noon polar cap b..." refers background or methods in this paper

...The results rearm the intimate link between radar scatter showing the characteristics de®ned in Baker et al. (1995) and the cusp particle precipitation and must be taken in to account when the radar scatter is used as a proxy for the open/ closed ®eld line boundary (e.g. Baker et al., 1997;…...
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...Rather the technique has been employed to provide further evidence for the link between HF radar backscatter characteristics and cusp particle precipitation (Baker et al., 1995) and to understand the morphology of the high spectral width boundary around noon....
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...The location of HF radar backscatter showing characteristics of the cusp (Baker et al., 1995) shows a poleward bay around the noon region....
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...This is consistent with the equatorward edge of the high spectral width region mapping to the equatorward edge of the cusp particle precipitation (Baker et al., 1990, 1995) and the sharp increase in Pc1 wave activity associated with the equatorward edge of the cusp particle precipitation (e.g.…...
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...…cusp particle precipitation into the ionosphere and HF radar backscatter from the ionospheric F region has been reported on by a number of workers (Baker et al., 1990, 1995; Rodger et al., 1995; Yeoman et al., 1997; Milan et al., 1999) and has been used as an alternative means of determining the…...
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Journal Article•DOI•

Observations of an enhanced convection channel in the cusp ionosphere

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Michael Pinnock¹, Alan S. Rodger, J. R. Dudeney², K. B. Baker, Patrick T. Newell, R. A. Greenwald³, M. E. Greenspan⁴ - Show less +3 more•Institutions (4)

NASA Headquarters¹, British Antarctic Survey², Johns Hopkins University³, Boston University⁴

01 Mar 1993-Journal of Geophysical Research

TL;DR: In this paper, the authors describe combined observations by the PACE HF backscatter radar and the DMSP F9 polar-orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp.

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Abstract: Transient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high-latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar-orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line-of-sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.

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174 citations

"On determining the noon polar cap b..." refers background in this paper

...The two arrows marked``FCE'' are at the times that ¯ow channel events, possible ¯ux transfer event (FTE) signatures (Pinnock et al., 1993 ), are observed whilst the third arrow is at the time that the IMF changed, returning back to a By dominated regime....
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