Observations of an enhanced convection channel in the cusp ionosphere
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.
Citations
More filters
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
Natural Environment Research Council1, University of Leicester2, University of Saskatchewan3, Johns Hopkins University Applied Physics Laboratory4, La Trobe University5, Nagoya University6, University of KwaZulu-Natal7, National Institute of Polar Research8, Centre national de la recherche scientifique9
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 background or methods from "Observations of an enhanced convect..."
...…in the two hemispheres in response to changes in the direction of the y-component of the interplanetary magnetic field (IMF) (Greenwald et al. 1990), and identifying the first spatial signatures of the footprints of flux transfer events (FTE) in the ionospheric cusp (Pinnock et al. 1991, 1993)....
[...]
...Pinnock et al. (1993) presented the first HF radar observations of transient convection enhancements thought to be associated with FTEs, using line-of-sight velocity measurements from the PACE Halley radar and DMSP ion drift velocity measurements....
[...]
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
TL;DR: Recently, new ground-based observations, combined with in situ satellite measurements, have led the way in reinterpreting cusp signatures, which have stimulated new interest in the solar wind-magnetosphere-ionosphere coupling chain this paper.
Abstract: Earth's cusps are magnetic field features in the magnetosphere associated with regions through which plasma from the Sun can have direct access to the upper atmosphere. Recently, new ground-based observations, combined with in situ satellite measurements, have led the way in reinterpreting cusp signatures. These observations, combined with theoretical advances, have stimulated new interest in the solar wind-magnetosphere-ionosphere coupling chain. This coupling process is important because it causes both momentum and energy from the solar wind to enter into the near-Earth region. Here we describe the current ideas concerning the cusps and the supporting observational evidence which have evolved over the past 30 years. We include discussion on the plasma entry process, particle motion between the magnetopause and ionosphere, ground optical and radar measurements, and transient events. We also review the important questions that remain to be answered.
194 citations
01 Jul 2000
TL;DR: In this article, the first spacecraft-borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause were presented, during an interval of interplanetary magnetic field Bz ≈ −10 nT and By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw vw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in
Abstract: We present the first spacecraft-borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause. During an interval of interplanetary magnetic field Bz ≈ −10 nT, By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in the 14 to 16 magnetic local time (MLT) sector which subsequently progresses antisunward (eastward) at 2 km s−1 toward the 19 MLT sector. The poleward portion of the bifurcation is interpreted as a poleward-moving auroral form (PMAF) as has previously been observed by ground-based optical instrumentation and identified as the auroral signature of flux transfer events. Ground-based measurements of the associated plasma drift, made with the Cooperative U.K. Twin Located Auroral Sounding System (CUTLASS) Finland HF radar, show poleward (1 km s−1) and westward (1 km s−1) convection flow, consistent with the By tension force, as well as poleward-moving regions of backscatter. International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers within the radar field of view observe poleward-progressing, 10 min period, X component magnetic deflections, which are consistent with the effect of Hall currents associated with the plasma flow. The combined radar and optical observations suggest that the PMAFs can be 3500 km or 7 hours of MLT in length. The antisunward motion of the bifurcation of the auroral oval is interpreted as an expansion of the reconnection X line along the flank of the magnetopause.
179 citations
References
More filters
TL;DR: In this paper, large-scale revisions of the OGO 6 dawn-dusk measurement models are made, showing that the deformations of the two-cell patterns lead to sunward convection in dayside polar regions, while maintaining the integrity of the night-side convection pattern.
Abstract: The present analysis of electric field measurements from the Dynamics Explorer 2 satellite, which extends previous empirical models, emcompasses much data from polar crossings entering and exiting the high latitudes in all magnetic local time zones. The goal is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns. Significant large-scale revisions of the OGO 6 dawn-dusk measurement models are made. The deformations of the two-cell patterns lead to sunward convection in dayside polar regions, while maintaining the integrity of the nightside convection pattern.
1,060 citations
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
TL;DR: In this paper, the signatures of localized ionospheric traveling convection vortices were analyzed using 20-second resolution magnetometer data from an array of temporary stations operated around Sondre Stromfjord, Greenland during the summer of 1986.
Abstract: Analysis of 20-second resolution magnetometer data from an array of temporary stations operated around Sondre Stromfjord, Greenland, during the summer of 1986 shows the signatures of localized ionospheric traveling convection vortices. An example of an isolated event of this kind observed near 08 local time is presented in detail. This event consists of a twin vortex pattern of convection consistent with the presence of two field-aligned current filaments separated by about 600 km in the east-west direction. This system of currents is observed to move westward (tailward) past the array of stations at about 4 km/sec. The event is associated with relative quiet time ionospheric convection and occurs during an interval of northward IMF. It is, however, associated with a large fluctuation in both the Z and Y components of the IMF and with a large sudden decrease in the solar wind number density. The propagation of the system is inconsistent with existing models of FTE current systems, but nevertheless appears to be related to a readjustment of the magnetopause boundary to a sudden change in the solar wind dynamic pressure and/or to a change in reconnection brought about by a sudden reorientation of the IMF.
329 citations
TL;DR: In this paper, the authors argue that surges in the reconnection rate on the magnetopause give rise to bubble-like regions of plasma containing a twisted field with energetic streaming particles in the outer layers.
301 citations
TL;DR: In this paper, a model of flux transfer events at the dayside magnetopause is proposed, which is based on non-stationary reconnection along a single X line over a large longitudinal segment.
Abstract: A model of flux transfer events (FTEs) at the dayside magnetopause is proposed, which is based on non-stationary reconnection along a single X line over a large longitudinal segment. An individual field line coming from the magnetospheric side constitutes after reconnection a loop-like structure in the magnetopause current layer and leaves into the magnetosheath. In the presence of a By component of the magnetic field in the transition region of the magnetopause each loop is twisted in the y direction. Whereas traditionally FTEs are considered to be localized flux tubes with a helical field inside, in the present model field lines come from the magnetospheric side over a large longitudinal segment, have a loop-like structure in the magnetopause resulting in the FTE signatures, and leave over the same segment in longitude into interplanetary space.
296 citations