On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics
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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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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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
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
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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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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01 Sep 1997
TL;DR: In this paper, a favorable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously.
Abstract: A favourable conjunction of HF coherent radar backscatter, meridian scanning photometer data and an overflight of the DMSP F13 spacecraft has enabled the study of the ionospheric signature of the cusp with these three important techniques simultaneously. Strong HF backscatter power, poleward-moving red line auroral forms and latitude-dispersed ion precipitation features are all observed to be collocated. The precipitation of ions in the 0.1–2 keV energy range is found to be very closely associated with the production of the F region irregularities detected by the HF radar.
83 citations
"On determining the noon polar cap b..." refers methods in this paper
...…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 OCB at noon (Baker et al., 1997; Pinnock et…...
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TL;DR: In this paper, optical all-sky imager and photometer data from South Pole station and the PACE HF radar at Halley, Antarctica from two case studies are used to show that their respective ionospheric signatures of the magnetospheric cusp are collocated to better than about 1° latitude.
Abstract: Simultaneous optical all-sky imager and photometer data from South Pole station and the PACE HF radar at Halley, Antarctica from two case studies are used to show that their respective ionospheric signatures of the magnetospheric cusp are collocated to better than about 1° latitude. The plasma convection reversal as identified in the PACE data is usually observed within the region showing cusp precipitation, as expected from contemporary models of this region of geospace.
74 citations
"On determining the noon polar cap b..." refers methods in this paper
...…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 OCB at noon (Baker et al.,…...
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TL;DR: Goose Bay HF-radar data have been used to determine the dayside reconnection electric field which transports energy from the solar wind into the Earth's magnetosphere and ionosphere.
Abstract: Goose Bay HF-radar data have been used to determine the dayside reconnection electric field which transports energy from the solar wind into the Earth's magnetosphere and ionosphere. The speed of the ionospheric plasma flow perpendicular to the open/closed boundary is determined in the rest frame of the boundary along each of the 16 beam directions of the HF radar. The observations were made during one of the Geospace Environment Modeling program's boundary layer campaigns. The period from 1200 to 1600 UT on March 29, 1992, was one of generally southward interplanetary magnetic field (IMF). The y component of the IMF was negative for most of the time. Despite the generally steady IMF conditions, the merging rate observed by the radar shows a great deal of temporal structure. The radar observations have been compared with the results from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. Initially, the merging inferred from the radar observations accounts for a significant portion of the total polar cap potential drop, suggesting that a majority of the potential drop was generated within the radar field of view and must therefore be due to magnetic merging at the magnetopause. At the end of the period, however, the potential drop derived from the radar measurements is distinctly less than that derived from the AMIE procedure. At that time, however, satellite and ground magnetometer data show that a substorm was in progress, and there is substantial evidence for a strong nightside contribution to the polar cap potential drop. An additional feature that appears in this data set is that the orientation of the open/closed magnetic field separatrix with respect to magnetic latitude is well correlated to the y component of the IMF.
68 citations
"On determining the noon polar cap b..." refers background or methods or result 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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...…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; Pinnock et al., 1999 )....
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...In HF radar studies (Baker et al., 1997; Pinnock et al., 1999) which measured the reconnection electric ®eld (E rec ), it was assumed that the critical measurement was plasma ¯ow perpendicular to the equatorward edge of the high spectral width region (which was assumed to be the OCB)....
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...Baker et al. (1997) performed a similar analysis and found comparable tilts in the orientation of the cusp Fig....
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...…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 OCB at noon (Baker et al., 1997; Pinnock et al., 1999)....
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TL;DR: In this article, a statistical study of Pc 1 waves was performed using electric field data recorded in the ionosphere by the Dynamics Explorer 2 satellite, which was performed by applying an automated wave detection algorithm to over 900 hours of data recorded at invariant latitudes (INV) greater than 40°.
Abstract: A statistical study of Pc 1 waves has been performed using electric field data recorded in the ionosphere by the Dynamics Explorer 2 satellite. The study was performed by applying an automated wave detection algorithm to over 900 hours of data recorded at invariant latitudes (INV) greater than 40° from December 6, 1981, to February 16, 1983. A total of 390 Pc 1 waves in the electric field were identified using a selection technique based on spectral peak detection in the frequency range from 0.2 to 6.0 Hz. Most events were observed at frequencies between 0.4 and 2.0 Hz and in the dawn (0400–0600 MLT) and noon (1000–1500 MLT) sectors from 50° to 62° INV. Detection of events at high latitudes (INV > 65°) was limited by noise associated with auroral zone electric fields. Significant differences in the properties of ionospheric Pc 1 waves were observed in the dawn and noon sectors. First, noon sector waves were clearly ordered by the equatorial gyrofrequencies (ƒ 10 (mV/m)2/Hz, in the noon sector compared with just 23% in the dawn sector. Third, for events with a magnetic field component, the median value of ΔE/ΔB was 410 km/s in the dawn and 330 km/s in the noon sector, although the median value of the ratio between (ΔE/ΔB) and the Alfven velocity was similar in both sectors. Fourth, more short-duration events (2–8 s) were observed in the noon than in the dawn sector. We conclude that the source of noon sector ionospheric Pc 1 waves is electromagnetic ion cyclotron waves generated in the equatorial magnetosphere. The source of the dawn sector waves is less certain, as many waves were observed only in the electric field.
62 citations
"On determining the noon polar cap b..." refers background in this paper
...…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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01 Oct 1998
TL;DR: In this article, the CUTLASS Finland coherent HF radar was employed to demonstrate that changes in propagation mode from 1 2F to 1 1/2F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed.
Abstract: Observations from the CUTLASS Finland coherent HF radar on 23 February 1996 are employed to demonstrate that changes in propagation mode from 1/2F to 1 1/2F and back again, determined from elevation angle measurements, do not significantly alter the ranges over which ionospheric backscatter is observed. This indicates that the latitudinal extent of backscatter in the dayside auroral oval and cusp region correspond to the boundaries of geophysical processes, as opposed to limits in the illumination of the F region ionosphere by the radar. Hence, the HF radar technique is confirmed as an excellent diagnostic of the cusp and other dayside regions.
57 citations
"On determining the noon polar cap b..." refers background in this paper
...Milan et al. (1998) have shown that the cusp particle precipitation region forms a ``hard'' target for the radars, in the sense that the radar backscatter from it marks a true geophysical boundary and its location is relatively insensitive to HF propagation conditions....
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