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 extents of magnetic reconnection at the Earth's magnetopause were investigated using coordinated observations of multiple spacecraft and radars for three conjunction events, and the results indicated that both spatially patchy and spatially continuous and extended reconnections are possible forms of active reconnection.
Abstract: . Magnetic reconnection can vary considerably in spatial
extent. At the Earth's magnetopause, the extent generally corresponds to the
extent in local time. The extent has been probed by multiple spacecraft
crossing the magnetopause, but the estimates have large uncertainties because
of the assumption of spatially continuous reconnection activity between
spacecraft and the lack of information beyond areas of spacecraft coverage.
The limitations can be overcome by using radars examining ionospheric flows
moving anti-sunward across the open–closed field line boundary. We therefore
infer the extents of reconnection using coordinated observations of multiple
spacecraft and radars for three conjunction events. We find that when
reconnection jets occur at only one spacecraft, only the ionosphere conjugate
to this spacecraft shows a channel of fast anti-sunward flow. When
reconnection jets occur at two spacecraft and the spacecraft are separated by
Re, the ionosphere conjugate to both spacecraft shows a
channel of fast anti-sunward flow. The consistency allows us to determine the
reconnection jet extent by measuring the ionospheric flows. The
full-width-at-half-maximum flow extent is 200, 432, and 1320 km,
corresponding to a reconnection jet extent of 2, 4, and 11 Re. Considering
that reconnection jets emanate from reconnections with a high reconnection
rate, the result indicates that both spatially patchy (a few Re) and
spatially continuous and extended reconnections ( > 10 Re) are
possible forms of active reconnection at the magnetopause. Interestingly, the
extended reconnection develops from a localized patch via spreading across
local time. Potential effects of IMF Bx and By on the reconnection
extent are discussed.
13 citations
Cites background from "On determining the noon polar cap b..."
...…by soft (∼ 100 eV) electron precipitation (Ponomarenko et al., 2007), and evidence has shown that the longitudinal extent of large spectral widths correlates with the extent of PMAFs (Moen et al., 2000) and of poleward flows across the open–closed field line boundary (Pinnock and Rodger, 2000)....
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TL;DR: In this article, the authors demonstrate that high-resolution 557.7-nm all-sky images are useful tools for investigating the spatial and temporal evolution of merging on the dayside magnetopause.
Abstract: We demonstrate that high-resolution 557.7-nm all-sky images are useful tools for investigating the spatial and temporal evolution of merging on the dayside magnetopause. Analysis of ground and satellite measurements leads us to conclude that high-latitude merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 30 s to 3 min. Variations of 557.7 nm emissions were observed at a 10 s cadence at Ny-Alesund on 19 December 2001, while significant changes in the IMF clock angle were reaching the magnetopause. The optical patterns are consistent with a scenario in which merging occurs around the rim of the high-latitude cusp at positions dictated by the IMF clock angle. Electrons energized at merging sites represent plausible sources for 557.7 nm emissions in the cusp. Polar observations at the magnetopause have directly linked enhanced fluxes of > or = 0.5 keV electrons with merging. Spectra of electrons responsible for some of the emissions, measured during a DMSP F15 overflight, exhibit "inverted-V" features, indicating further acceleration above the ionosphere. SuperDARN spectral width boundaries, characteristic of open-closed field line transitions, are located at the equatorward edge of the 557.7nm emissions. Optical data suggest that with IMF B(sub Y) > 0, the Northern Hemisphere cusp divides into three source regions. When the IMF clock angle was approx. 150 deg structured 557.7-nm emissions came from east of the 13:00 MLT meridian. At larger clock angles the emissions appeared between 12:00 and 13:00 MLT. No significant 557.7-nm emissions were detected in the prenoon MLT sector. MHD simulations corroborate our scenario, showing that with the observed large dipole-tilt and IMF clock angles, merging sites develop near the front and eastern portions of the high-altitude cusp rim in the Northern Hemisphere and near the western part of the cusp rim in the Southern Hemisphere.
7 citations
Cites background from "On determining the noon polar cap b..."
...Due to time-of-flight effects the spectral-width boundary may be displaced poleward of the open-closed boundary (Pinnock and Rodger, 2001)....
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Dissertation•
20 Dec 2002
Abstract: SuperDARN est un reseau de radars HF coherents dedie a l'etude de la convection du plasma ionospherique a haute latitude qui trouve ses principales applications dans l'etude des relations Soleil/Terre. On s'interesse ici aux effets des interactions entre l'onde radar emise et les gradients d'ionisation de differentes echelles et a leur impact sur la mesure. Des etudes sont menees pour detecter la signature du mouvement cyclotron des ions, superpose aux mouvements turbulents, dans les spectres mesures. Ensuite, l'effet des moyennes echelles (100 m a 10 km) est mis en evidence sur la mesure des largeurs spectrales. Des etudes statistiques montrent que la determination des parametres est affectee par le rapport entre la frequence emise et la frequence plasma et par la distance de l'echo. Nous proposons une interpretation en terme de decorrelation du front d'onde au cours de la propagation, validee par la mise en place de simulations s'appuyant sur des parametres realistes de l'ionosphere.
5 citations
TL;DR: A statistical analysis of the equatorward and poleward auroral boundary movement during substorm onsets, the related solar wind activity, GOES 8 and 10 magnetic field, and the westward electrojet ( AL ) index is undertaken, during the years 2000-2002.
4 citations
TL;DR: In this paper , the authors simulate five solar storm periods with the coupled model consisting of the Open GGCM coupled with the Coupled Thermosphere IonosphereModel (CTIM) and the Rice Convection Model (RCM), to estimate the location and dynamics of the OCB.
Abstract: Abstract. It is well known that the polar cap, delineated by the open–closed field line boundary (OCB),
responds to changes in the interplanetary magnetic field (IMF).
In general, the boundary moves equatorward when the IMF turns southward and contracts
poleward when the IMF turns northward. However,
observations of the OCB are spotty and limited in local time,
making more detailed studies of its IMF dependence difficult.
Here, we simulate five solar storm periods with the coupled model consisting of the Open
Geospace General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere
Model (CTIM) and the Rice Convection Model (RCM),
i.e., the OpenGGCM-CTIM-RCM, to estimate the location and dynamics of the OCB.
For these events, polar cap boundary location observations are also obtained from Defense Meteorological
Satellite Program (DMSP) precipitation spectrograms and compared with the model output.
There is a large scatter in the DMSP observations and in the model output.
Although the model does not predict the OCB with high fidelity for every observation,
it does reproduce the general trend as a function of IMF clock angle.
On average, the model overestimates the latitude of the open–closed field line boundary
by 1.61∘. Additional analysis of the simulated polar cap boundary dynamics across
all local times shows that the MLT of the largest polar cap expansion closely correlates
with the IMF clock angle, that the strongest correlation occurs when the IMF is southward, that
during strong southward IMF the polar cap shifts sunward, and that the polar cap rapidly
contracts at all local times when the IMF turns northward.
2 citations
References
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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).
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.
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•
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.
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.
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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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
"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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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).
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.
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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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.
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.
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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