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Journal ArticleDOI

A technique for accurately determining the cusp-region polar cap boundary using SuperDARN HF radar measurements

30 Apr 2003-Annales Geophysicae (Copernicus GmbH)-Vol. 21, Iss: 4, pp 983-996
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)
Citations
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Journal ArticleDOI
TL;DR: The Super Dual Auroral Radar Network (SuperDARN) as discussed by the authors has been operating as an international co-operative organization for over 10 years and has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions.
Abstract: The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.

690 citations


Cites methods from "A technique for accurately determin..."

  • ...A better understanding of these spectral width distributions led to the development of a reliable and objective method for determining the SWB known as the ‘C-F threshold technique’ (Chisham and Freeman 2003, 2004)....

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Journal ArticleDOI
22 Nov 2006
TL;DR: The first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures was presented in this article, where two bursts of sunward plasma flow across the noon portion of the open/closed field line boundary (OCB) indicating magnetic flux closure at the dayside were observed in SuperDARN radar data during a period of strongly northward IMF.
Abstract: . We present the first substantial evidence for the occurrence of dual lobe reconnection from ionospheric flows and auroral signatures. The process of dual lobe reconnection refers to an interplanetary magnetic field line reconnecting with lobe field lines in both the northern and southern hemispheres. Two bursts of sunward plasma flow across the noon portion of the open/closed field line boundary (OCB), indicating magnetic flux closure at the dayside, were observed in SuperDARN radar data during a period of strongly northward IMF. The OCB is identified from spacecraft, radar backscatter, and auroral observations. In order for dual lobe reconnection to take place, we estimate that the interplanetary magnetic field clock angle must be within ±10° of zero (North). The total flux crossing the OCB during each burst is small (1.8% and 0.6% of the flux contained within the polar cap for the two flows). A brightening of the noon portion of the northern auroral oval was observed as the clock angle passed through zero, and is thought to be due to enhanced precipitating particle fluxes due to the occurrence of reconnection at two locations along the field line. The number of solar wind protons captured by the flux closure process was estimated to be ~2.5×1030 (4 tonnes by mass), sufficient to populate the cold, dense plasma sheet observed following this interval.

74 citations


Additional excerpts

  • ...…spectral width of a radar backscatter echo in comparison with closed field lines (e.g. Baker et al., 1995; Milan et al., 1999, 2000; Moen et al., 2001, 2002; Chisham and Freeman, 2003, 2005; Oksavik et al., 2004) and therefore the spectral width of radar backscatter can be used to identify the OCB....

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Journal ArticleDOI
TL;DR: The spectral width boundary (SWB) measured by the Super Dual Auroral Radar Network (SuperDARN) has been shown to be a reliable ionospheric proxy for the open-closed magnetic field line boundary (OCB) at certain magnetic local times (MLTs) as mentioned in this paper.
Abstract: The open-closed magnetic field line boundary (OCB) is best measured at the foot points of the boundary in the Earth's ionosphere where continuous and extensive spatiotemporal measurements can be made. The ability to make routine observations of this type is crucial if accurate global measurements of energy transfer processes occurring at the boundary, such as magnetic reconnection, are to become a reality. The spectral width boundary (SWB) measured by the Super Dual Auroral Radar Network (SuperDARN) has been shown to be a reliable ionospheric proxy for the OCB at certain magnetic local times (MLTs). However, the reliability of the SWB proxy in the afternoon sector ionosphere (12:00-18:00 MLT) has been questionable. In this paper we undertake a statistical comparison of the latitudinal locations of SWBs measured by SuperDARN and particle precipitation boundaries (PPBs) measured by the Defense Meteorological Satellite Program (DMSP) spacecraft, concentrating on the PPB which best approximates the location of the OCB. The latitudes of SWBs and PPBs were identified using automated algorithms applied to 5 years (1997-2001) of data measured in the 12:00-18:00 MLT range. A latitudinal difference was measured between each PPB and the nearest SWB within a ±10 min universal time (UT) window and within a ±1 h MLT window. The results show that when the SWB is identified at higher geomagnetic latitudes (poleward of ~74), it is a good proxy for the OCB, with 76% of SWBs lying within 3 of the OCB. At lower geomagnetic latitudes (equatorward of ~74), the correlation is poor and the results suggest that most of the SWBs being identified represent ionospheric variations unassociated with the OCB, with only 32% of SWBs lying within 3 of the OCB. We propose that the low level of precipitating electron energy flux, typical of latitudes well equatorward of the OCB in the afternoon sector, may be a factor in enhancing spectral width values at these lower latitudes. A consequence of this would be low latitude SWBs unrelated to the OCB.

69 citations


Cites methods from "A technique for accurately determin..."

  • ...Any modification to the technique must also address the problem identified byChisham and Freeman (2004) of occasional boundary misidentifications when there is no low spectral width region equatorward of an isolated high spectral width region....

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  • ...Hence, we would argue that the uncertainty of the OCB location determined from SWB measurements is similar to that of the measurement of the SWB location itself, which is ∼1◦ latitude (Chisham and Freeman, 2003)....

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  • ...By determining the probability distribution of the SWB with latitude and MLT,Chisham and Freeman (2004) showed how the preferred latitudinal location of SWBs in data measured by the Halley SuperDARN radar became less clearly defined in the afternoon sector ionosphere (see their Fig. 12). As we are using data from the Goose Bay and Finland SuperDARN radars in this study, we first present the equivalent latitudeMLT probability distributions for the SWB locations identified by the Goose Bay and Finland radars for the 5-year SWB database. Full details regarding the determination of these distributions are presented in Chisham and Freeman (2004) (see their Sec....

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  • ...We employ the “C-F threshold technique” to identify the SWB (Chisham and Freeman, 2003, 2004)....

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  • ...This preprocessing increases the accuracy of the estimated SWB location by effectively thinning the distributions of spectral width found poleward and equatorward of the SWB, making it easier to distinguish between them (Chisham and Freeman, 2003)....

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Journal ArticleDOI
TL;DR: In this paper, the authors compared the FUV OCB estimates from three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) over all magnetic local times.
Abstract: A statistical comparison of the latitude of the open/closed magnetic field line boundary (OCB) as estimated from the three far ultraviolet (FUV) detectors onboard the IMAGE spacecraft (the Wideband Imaging camera, WIC, and the Spectrographic Imagers, SI-12 and SI-13) has been carried out over all magnetic local times. A total of over 400 000 OCB estimations were compared from December 2000 and January and December of 2001–2002. The modal latitude difference between the FUV OCB proxies from the three detectors is small, <1°, except in the predawn and evening sectors, where the SI-12 OCB proxy is found to be displaced from both the SI-13 and WIC OCB proxies by up to 2° poleward in the predawn sector and by up to 2° equatorward in the evening sector. Comparing the IMAGE FUV OCB proxies with that determined from particle precipitation measurements by the Defense Meteorological Satellites Program (DMSP) also shows systematic differences. The SI-12 OCB proxy is found to be at higher latitude in the predawn sector, in better agreement with the DMSP OCB proxy. The WIC and SI-13 OCB proxies are found to be in better agreement with the DMSP OCB proxy at most other magnetic local times. These systematic offsets may be used to correct FUV OCB proxies to give a more accurate estimate of the OCB latitude.

62 citations


Cites background from "A technique for accurately determin..."

  • ..., 1991, 1996; Sotirelis and Newell, 2000) and measurements from incoherent and coherent scatter radars (Baker et al., 1995; Blanchard et al., 1996, 2001; Milan et al., 1999; Chisham et al., 2001, 2002; Lester et al., 2001; Chisham and Freeman, 2003), or a combination of all of these (Milan et al....

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  • ...…1996; Sotirelis and Newell, 2000) and measurements from incoherent and coherent scatter radars (Baker et al., 1995; Blanchard et al., 1996, 2001; Milan et al., 1999; Chisham et al., 2001, 2002; Lester et al., 2001; Chisham and Freeman, 2003), or a combination of all of these (Milan et al., 2003)....

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Journal ArticleDOI
TL;DR: In this paper, a detailed spatiotemporal measurements of the reconnection electric field in the Northern Hemisphere ionosphere during an extended interval of northward interplanetary magnetic field were presented.
Abstract: . This study presents, for the first time, detailed spatiotemporal measurements of the reconnection electric field in the Northern Hemisphere ionosphere during an extended interval of northward interplanetary magnetic field. Global convection mapping using the SuperDARN HF radar network provides global estimates of the convection electric field in the northern polar ionosphere. These are combined with measurements of the ionospheric footprint of the reconnection X-line to determine the spatiotemporal variation of the reconnection electric field along the whole X-line. The shape of the spatial variation is stable throughout the interval, although its magnitude does change with time. Consequently, the total reconnection potential along the X-line is temporally variable but its typical magnitude is consistent with the cross-polar cap potential measured by low-altitude satellite overpasses. The reconnection measurements are mapped out from the ionosphere along Tsyganenko model magnetic field lines to determine the most likely reconnection location on the lobe magnetopause. The X-line length on the lobe magnetopause is estimated to be ~6–11 RE in extent, depending on the assumptions made when determining the length of the ionospheric X-line. The reconnection electric field on the lobe magnetopause is estimated to be ~0.2mV/m in the peak reconnection region. Key words. Space plasma physics (Magnetic reconnection) – Magnetospheric physics (Magnetopause, cusp and boundary layers) – Ionosphere (Plasma convection)

57 citations


Cites methods from "A technique for accurately determin..."

  • ...…width backscatter in the cusp correlates well with the equatorward edge of cusp particle precipitation and so is often used as a proxy for the ionospheric projection of the merging line (which equates to the polar cap boundary under those conditions) (Baker et al., 1995; Chisham and Freeman, 2003)....

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References
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Journal ArticleDOI
TL;DR: In this article, it was found that a model with a southward interplanetary magnetic field leads to a natural explanation of the SD currents and speculative aspects of the problem as they appear at this time are discussed.
Abstract: : It was found that a model with a southward interplanetary magnetic field leads to a natural explanation of the SD currents. Speculative aspects of the problem as they appear at this time are discussed. It should be remembered that this problem is amenable to revolutionary progress by observations from rockets or satellites which go out more than a few earth's radii.

3,818 citations


"A technique for accurately determin..." refers background in this paper

  • ...The polar cap boundary (PCB) in the cusp ionosphere represents the ionospheric projection of the separatrix between newly-opened and closed geomagnetic field lines (Dungey, 1961) across which mass, momentum, energy and magnetic flux are transferred during dayside reconnection....

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Journal ArticleDOI
TL;DR: The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E and F-regions of the high-latitude ionosphere as mentioned in this paper.
Abstract: The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E and F-regions of the high-latitude ionosphere. Currently, the network consists of the STARE VHF radar system in northern Scandinavia, a northern-hemisphere, longitudinal chain of HF radars that is funded to extend from Saskatoon, Canada to central Finland, and a southern-hemisphere chain that is funded to include Halley Station, SANAE and Syowa Station in Antarctica. When all of the HF radars have been completed they will operate in pairs with common viewing areas so that the Doppler information contained in the backscattered signals may be combined to yield maps of high-latitude plasma convection and the convection electric field. In this paper, the evolution of DARN and particularly the development of its SuperDARN HF radar element is discussed. The DARN/SupperDARN network is particularly suited to studies of large-scale dynamical processes in the magnetosphere-ionosphere system, such as the evolution of the global configuration of the convection electric field under changing IMF conditions and the development and global extent of large-scale MHD waves in the magnetosphere-ionosphere cavity. A description of the HF radars within SuperDARN is given along with an overview of their existing and intended locations, intended start of operations, Principal Investigators, and sponsoring agencies. Finally, the operation of the DARN experiment within ISTP/GGS, the availability of data, and the form and availability of the Key Parameter files is discussed.

1,051 citations


"A technique for accurately determin..." refers background in this paper

  • ...SuperDARN (The Super Dual Auroral Radar Network) is a network of coherent scatter HF radars (Greenwald et al., 1995) which measure backscatter from magnetic field-aligned decametre-scale ionospheric iregularities....

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  • ...SuperDARN is a network of HF radars designed to determine the large-scale convection electric field in the polar ionospheres (Greenwald et al., 1995)....

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Journal ArticleDOI
TL;DR: In this article, the authors synthesize a descriptive model of plasma structures in the high-latitude F layer that unifies most of the diverse and independent observations, including the formation of 1000 km-scale "patches" in the polar cap from solar-produced plasma that is transported poleward from lower latitudes; the reconfiguration of patches as they convect into the auroral region and become the latitudinally confined, but longitudinally extended, plasma density enhancements near the equatorward auroral boundary; and the production of localized enhancements and depletions
Abstract: The most intense, F region irregularities in the high-latitude ionosphere appear to be produced by convective plasma processes and in particular, by the fluid (gradient drift) interchange instability. Such irregularities are produced by convectively mixing plasma across a mean plasma density gradient with the transport of higher-density plasma into regions of lower-density plasma (and vice versa) leading to the development of an irregularity spectrum that extends in scale from about 10 km down to the ion gyroradius. The mean plasma density gradient that must be present to allow irregularity production by this interchange process appears to be associated with larger-scale (>10 km) plasma structure produced by other means. Because much of the recent progress on this research topic stems from this recognition, a significant portion of this review is dedicated to a description of the characteristics and processes of 10-km plasma structure and their relationships to those of smaller-scale irregularities. From this review, we synthesize a descriptive model of plasma structures in the high-latitude F layer that unifies most of the diverse and independent observations. For the large-scale plasma processes, the model includes (1) the formation of 1000-km-scale “patches” in the polar cap from solar-produced plasma that is transported poleward from lower latitudes; (2) the reconfiguration of patches as they convect into the auroral region and become the latitudinally confined, but longitudinally extended, plasma density enhancements near the equatorward auroral boundary; and (3) the production of localized enhancements and depletions along the poleward auroral boundary by soft-particle precipitation and large but localized electric fields. In the model, the most intense, smaller-scale irregularities are in spatial proximity to these large-scale plasma features, the implication being that the presence of the latter allows formation of the former. The irregularity characteristics are consistent with production by the instability and a morphology controlled by (1) a “slip” velocity (i.e., plasma drift relative to the neutral gas) that is moderately small except in regions of nonuniform plasma convection or under time-varying conditions (e.g., substorms, pulsation events) and (2) a highly conducting auroral E layer that damps irregularity growth and enhances decay. The final irregularity spectrum appears to be produced by (1) global convective processes acting on solar-produced plasma at the largest scales (>50 km), (2) particle precipitation at scales greater than 10 km, (3) perhaps some form of wave activity around 10 km, and (4) the instability at the smaller scales (<10 km).

452 citations

Journal ArticleDOI
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.
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.

401 citations

Journal ArticleDOI
TL;DR: Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed in this paper, including wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.
Abstract: Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.

299 citations


"A technique for accurately determin..." refers background or methods in this paper

  • ...…magnetometer observations of ULF wave characteristics have also been used to identify the location of the cusp (Menk et al., 1992) but are unlikely to resolve the PCB to better than 2◦ due to spatial integration effects (Southwood and Hughes, 1983) and typical magnetometer separation distances....

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  • ...to resolve the PCB to better than 2 ◦ due to spatial integration effects (Southwood and Hughes, 1983) and typical magnetometer separation distances....

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