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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 authors evaluate how such wave activity modifies the radar's autocorrelation function and conclude that it explains the spectra seen in the cusp of the open/closed field line boundary.
Abstract: The SuperDARN radars are proving to be a very powerful experimental tool for exploring solar wind-magnetosphere-ionosphere interactions. They measure the autocorrelation function (ACF) of the transmitted signal backscattered from the ionospheric irregularities, and derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behaviour inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Although no studies have explained these characteristics, they are routinely used to detect the cusp in the radar data and estimate the open/closed field line boundary. Both satellite and magnetometer data show in the same region broadband wave activity in the Pc1, Pc2 frequency band. In this study, we evaluate how such wave activity modifies the radar's ACF, and conclude that it explains the spectra seen in the cusp.

51 citations


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

  • ...Andre et al. (1999) have identi®ed the cause of the high Doppler spectral widths....

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  • ...…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. Erlandson and Anderson, 1996) being the cause of the large Doppler spectral widths (Andre et al., 1999)....

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Book ChapterDOI
01 Jan 1998
TL;DR: In this paper, the authors look at the topological reconnection classes that can take place, both at the magnetopause and in the cross-tail current sheet and discuss the implications for identifying the open-closed boundary when reconnection is giving velocity filter dispersion of signatures.
Abstract: Of all the various definitions of the polar cap boundary that have been used in the past, the most physically meaningful and significant is the boundary between open and closed field lines. Locating this boundary is very important as it defines which regions and phenomena are on open field lines and which are on closed. This usually has fundamental implications for the mechanisms invoked. Unfortunately, the open-closed boundary is usually very difficult to identify, particularly where it maps to an active reconnection site. This paper looks at the topological reconnection classes that can take place, both at the magnetopause and in the cross-tail current sheet and discusses the implications for identifying the open-closed boundary when reconnection is giving velocity filter dispersion of signatures. On the dayside, it is shown that the dayside boundary plasma sheet and low-latitude boundary layer precipitations are well explained as being on open field lines, energetic ions being present because of reflection of central plasma sheet ions off the two Alfven waves launched by the reconnection site (the outer one of which is the magnetopause). This also explains otherwise anomalous features of the dayside convection pattern in the cusp region. On the nightside, similar considerations place the open-closed boundary somewhat poleward of the velocity-dispersed ion structures which are a signature of the plasma sheet boundary layer ion flows in the tail.

36 citations


Additional excerpts

  • ...This has been illustrated and discussed in Lockwood (1998) for both magnetopause and magnetotail reconnection....

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Journal ArticleDOI
TL;DR: In this paper, the authors examined the phenomenology of the penetration of the interplanetary magnetic field (IMF) B(sub y) component into the magnetosphere and investigated the implications for the magnetopause merging site.
Abstract: Magnetosheath plasma peertated into the magnetospere creating the particle cusp, and similarly the interplanetary magnetic field (IMF) B(sub y) component penetrates the magnetopause. We reexamine the phenomenology of such penetration to investigate implications for the magnetopause merging site. Three models are popular: (1) the 'antiparallel' model, in which merging occurs where the local magnetic shear is largest (usually high magnetic latitude); (2) a tilted merging line passing through the subsolar point but extending to very high latitudes; or (3) a tilted merging line passing through the subsolar point in which most merging occurs within a few Earth radii of the equatorial plane and local noon (subsolar merging). It is difficult to distinguish between the first two models, but the third implies some very different predictions. We show that properties of the particle cusp imply that plasma injection into the magnetosphere occurs most often at high magnetic latitudes. In particular, we note the following: (1) The altitude of the merging site inferred from midaltitude cusp ion pitch angle dispersion is typically 8-12 R(sub E). (2) The highest ion energy observable when moving poleward through the cusp drops long before the bulk of the cusp plasma is reached, implying that ions are swimming upstream against the sheath flow shortly after merging. (3) Low-energy ions are less able to enter the winter cusp than the summer cusp. (4) The local time behavior of the cusp as a function of B(sub y) and B(sub z) corroborates predictions of the high-latitude merging models. We also reconsider the penetration of the IMF B(sub y) component onto closed dayside field lines. Our approach, in which closed field lines ove to fill in flux voids created by asymmetric magnetopause flux erosion, shows that strich subsolar merging cannot account for the observations.

34 citations


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

  • ...If the ®eld-aligned distance is less, for example 10 Re as suggested by Newell et al. (1995), then the corresponding energy range would be 2.1 keV (100 s) to 530 eV(200 s)....

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Journal ArticleDOI
TL;DR: In this paper, an analysis of a cusp ion step between two poleward moving events of enhanced ionospheric electron temperature, observed by the European Incoherent Scatter (EISCAT) radar, is presented.
Abstract: We present an analysis of a cusp ion step, observed by the Defense Meteorological Satellite Program (DMSP) F10 spacecraft, between two poleward moving events of enhanced ionospheric electron temperature, observed by the European Incoherent Scatter (EISCAT) radar. From the ions detected by the satellite, the variation of the reconnection rate is computed for assumed distances along the open-closed field line separatrix from the satellite to the X line, do. Comparison with the onset times of the associated ionospheric events allows this distance to be estimated, but with an uncertainty due to the determination of the low-energy cutoff of the ion velocity distribution function, ƒ(ν). Nevertheless, the reconnection site is shown to be on the dayside magnetopause, consistent with the reconnection model of the cusp during southward interplanetary magnetic field (IMF). Analysis of the time series of distribution function at constant energies, ƒ(ts), shows that the best estimate of the distance do is 14.5±2 RE. This is consistent with various magnetopause observations of the signatures of reconnection for southward IMF. The ion precipitation is used to reconstruct the field-parallel part of the Cowley D ion distribution function injected into the open low-latitude boundary layer in the vicinity of the X line. From this reconstruction, the field-aligned component of the magnetosheath flow is found to be only −55±65 km s−1 near the X line, which means either that the reconnection X line is near the stagnation region at the nose of the magnetosphere, or that it is closely aligned with the magnetosheath flow streamline which is orthogonal to the magnetosheath field, or both. In addition, the sheath Alfven speed at the X line is found to be 220±45 km s−1, and the speed with which newly opened field lines are ejected from the X line is 165±30 km s−1. We show that the inferred magnetic field, plasma density, and temperature of the sheath near the X line are consistent with a near-subsolar reconnection site and confirm that the magnetosheath field makes a large angle (>58°) with the X line.

27 citations


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

  • ...It should be stressed that we are not proposing that the technique for deriving the time-of-¯ight of cusp ions be used to infer properties of the reconnection X-line on the magnetopause, which can be done with greater accuracy using satellite particle data (e.g. Lockwood et al., 1995)....

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Journal ArticleDOI
TL;DR: In this paper, a long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF, punctuated by a number of IMF directional discontinuities during which the IMF north-south component, Bz changed polarity abruptly.
Abstract: We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large-scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north-south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Alesund (75° magnetic latitude) by meridian scanning photometers and all-sky cameras, were as follows: The interval of negative IMF Bz was characterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at ∼0.5° MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4–5 min. The net effect over the first hour of IMF Bz < 0 conditions was to move the equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (initial brightenings) are followed by a switch-off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equivalently, flux erosion events.

26 citations