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Journal ArticleDOI

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

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.
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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Citations
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Journal ArticleDOI
TL;DR: In this article, the authors employed observations from several sources to determine the location of the polar cap bound-ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified.
Abstract: This study employs observations from several sources to determine the location of the polar cap bound- ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral im- ages from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the mag- netosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, al- lowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and de- creases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to de- crease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap bound- ary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times sup- ports this finding.

211 citations


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

  • ...Pinnock and Rodger (2001) had four radars at their disposal, and consequently, a greater proportion of the merging gap was imaged....

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  • ...On the dayside, Baker et al. (1997) and Pinnock and Rodger (2001) used SuperDARN observations to estimate the dayside reconnection voltage, employing the spectral width boundary as an estimate of the location of the OCB....

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Journal ArticleDOI
01 Jan 2007
TL;DR: In this paper, changes in the open flux content of the ionospheric polar cap, estimated from auroral, radar, and low-Earth orbit particle measurements, are used to determine dayside and nightside reconnection rates during 73 hours of observation spread over nine intervals.
Abstract: [1] Changes in the open flux content of the ionospheric polar cap, estimated from auroral, radar, and low-Earth orbit particle measurements, are used to determine dayside and nightside reconnection rates during 73 hours of observation spread over nine intervals. We identify 25 episodes of nightside reconnection and examine statistically the rates and durations of reconnection, as well as possible triggers for the onset of reconnection, such as changes in solar wind ram pressure or orientation of the interplanetary magnetic field. Approximately half of the events can possibly be identified with a trigger, the other half appearing spontaneous. On average 0.3 GWb of open flux are closed in each event, with average durations and reconnection rates being 70 min and 85 kV. We find no evidence for a low background rate of nightside reconnection between these events and conclude that substorms and other large reconnection bursts provide the major or only source of flux closure on the nightside.

173 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the role of the cusp auroral processes in the production of irregularities, and found that the occurrence rate of the GPS phase scintillation is highest inside the auroral cusp, regardless of the scintillillation strength and the interplanetary magnetic field (IMF).
Abstract: The climatology map of the GPS phase scintillation identifies two regions of high scintillation occurrences: around magnetic noon and around magnetic midnight. The scintillation occurrence rate is higher around noon, while the scintillation level is stronger around magnetic midnight. This paper focuses on the dayside scintillation region. In order to resolve the role of the cusp auroral processes in the production of irregularities, we put the GPS phase scintillation in the context of the observed auroral morphology. Results show that the occurrence rate of the GPS phase scintillation is highest inside the auroral cusp, regardless of the scintillation strength and the interplanetary magnetic field (IMF). On average, the scintillation occurrence rate in the cusp region is about 5 times as high as in the region immediately poleward of it. The scintillation occurrence rate is higher when the IMF Bz is negative. When partitioning the scintillation data by the IMF By, the distribution of the scintillation occurrence rate around magnetic noon is similar to that of the poleward moving auroral form (PMAF): there is a higher occurrence rate at earlier (later) magnetic local time when the IMF By is positive (negative). This indicates that the irregularities which give rise to scintillations follow the IMF By-controlled east-west motion of the aurora and plasma. Furthermore, the scintillation occurrence rate is higher when IMF By is positive when the cusp is shifted toward the post noon sector where it may get easier access to the higher density plasma. This suggests that the combined auroral activities (e.g., PMAF) and the density of the intake solar EUV ionized plasma are crucial for the production of scintillations.

70 citations

Journal ArticleDOI
TL;DR: In this paper, the first statistical study on auroral oval boundaries derived from small and medium-scale field-aligned currents (FACs) was presented, and the results were used for the first time.
Abstract: . In this paper we present the first statistical study on auroral oval boundaries derived from small- and medium-scale field-aligned currents (FACs,

66 citations


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

  • ...The boundaries of the auroral oval can be determined locally by ground-based observations such as radars (Pinnock and Rodger, 2000; Moen et al., 2004; Aikio et al., 2006) or low-altitude satellite observations (Newell et al., 1996; Wang et al., 2005), while optical imagers from high-altitude…...

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Journal ArticleDOI
TL;DR: In this paper, a series of threshold algorithms were applied to a simulated cusp-region spectral width data set, to assess the accuracy of different algorithms and showed that simple threshold algorithms correctly identified the boundary location in, at most, 50% of the cases and that the average boundary error is at least ~ 1−2 range gates (~ 1° latitude).
Abstract: . Accurately measuring the location and motion of the polar cap boundary (PCB) in the high-latitude ionosphere can be crucial for studies concerned with the dynamics of the polar cap, e.g. the measurement of reconnection rates. The Doppler spectral width characteristics of backscatter received by the SuperDARN HF radars have been previously used for locating and tracking the PCB in the cusp region. The boundary is generally observed in meridional beams of the SuperDARN radars and appears as a distinct change between low spectral width values observed equatorward of the cusp region, and high, but variable spectral width values observed within the cusp region. To identify the spectral width boundary (SWB) between these two regions, a simple algorithm employing a spectral width threshold has often been applied to the data. However, there is not, as yet, a standard algorithm, or spectral width threshold, which is universally applied. Nor has there been any rigorous assessment of the accuracy of this method of boundary determination. This study applies a series of threshold algorithms to a simulated cusp-region spectral width data set, to assess the accuracy of different algorithms. This shows that simple threshold algorithms correctly identify the boundary location in, at the most, 50% of the cases and that the average boundary error is at least ~ 1–2 range gates (~ 1° latitude). It transpires that spatial and temporal smoothing of the spectral width data (e.g. by median filtering), before application of a threshold algorithm can increase the boundary determination accuracy to over 95% and the average boundary error to much less than a range gate. However, this is sometimes at the cost of temporal resolution in the motion of the boundary location. The algorithms are also applied to a year’s worth of spectral width data from the cusp ionosphere, measured by the Halley SuperDARN radar in Antarctica. This analysis highlights the increased accuracy of the enhanced boundary determination algorithm in the cusp region. Away from the cusp, the resulting SWB locations are often dependent on the choice of threshold. This suggests that there is not a sharp latitudinal SWB in regions of the dayside ionosphere away from the cusp, but that there is a shallower latitudinal gradient in spectral width near the boundary location. Key words. Ionosphere (instruments and techniques) – Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

53 citations


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

  • ...This is the general approach used by Baker et al. (1997), Pinnock et al. (1999), Pinnock and Rodger (2001), and Chisham et al. (2001)....

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  • ...Pinnock and Rodger (2001) excluded all backscatter with spectral widths less than 150 m/s from a composite of scans from 4 SuperDARN radars and then smoothed the remaining equatorward backscatter boundary to obtain their estimate of the boundary....

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References
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Journal ArticleDOI
TL;DR: In this paper, the future direction of ground-based imaging of magnetospheric boundaries is discussed and the key issues are reliable identification of further magnetosphere boundaries such as near-Earth neutral x-line, improvements to the groundbased experiment network to improve the spatial and temporal resolution, and the coverage, development of data assimilation and visualisation techniques.

26 citations


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

  • ...Taking account of the typical tilt angles reported by Baker et al. (1997) and Pinnock et al. (1999) for the Northern Hemisphere, we estimate that the maximum error in their E rec values would be 15%....

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  • ...Our ®ndings support the suggestion of Rodger (2000) in that a bay-like feature is found to exist in the cusp radar scatter....

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  • ...These angles are very similar to those reported in the previous paper for the Northern Hemisphere radar but larger than the Southern Hemisphere results....

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  • ...Rodger (2000) suggested that the longitudinal variation of the poleward ¯ow component in the throat ¯ow region would result in a characteristic bay-like feature, with the cusp radar backscatter displaced the greatest distance poleward where the poleward ¯ow component maximises....

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  • ...The upper and lower patterns are for By < 0 and By > 0 in the Northern Hemisphere (Rodger, 2000) observed at Wind at 14:36:38 UT and at IMP-8 at 15:09:18 UT, and the ionospheric response is seen at 15:20:06....

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