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We have derived patterns that describe the statistical interplanetary magnetic field (IMF) dependencies of ionospheric convection in the high-latitude region of the northern hemisphere. The observations of plasma motion were made with the HF coherent backscatter radar located at Goose Bay, Labrador, over the period September 1987 to June 1993. The area covered by the measurements extended poleward of 65°Λ to a working limit of about 85°Λ. Distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics. The patterns are the first derived from direct ground-based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies [Rich and Hairston, 1994; Weimer, 1995]. We show the dependence of the convection on IMF angle in the GSM y–z plane for three intervals of IMF magnitude in this plane. Except for predominantly northward IMF, the convection is primarily two-cell. The dusk cell is larger in terms of both spatial extent and potential variation/The effect of IMF By is apparent in the global shaping of the cells and the orientation of the overall pattern in MLT; for By + (By−) the dusk (dawn) cell is more round (crescent-shaped) and the pattern more rotated toward earlier MLTs. The By effect on the nightside convection is pronounced and is hemispherically antisymmetric, like the well-known day side By effect. For IMF increasingly northward, the convection trajectories on the dayside become increasingly distorted, evolving through a three-cell to a four-cell circulation. The additional cells appear on either side of the noon meridian and result in sunward flow. The overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with Weimer [1995]. There are significant differences with the statistical patterns derived from magnetometer measurements, which tend to show domination by the dawn rather than the dusk cell.

A combination of simultaneous measurements from high, low and ground altitude instruments has been used to infer the rapid evolution of the coupled nightside magnetosphere-ionosphere during substorms. Reversals from an eastward zonal convection to westward zonal convection become apparent inside the volume of the substorm bulge a few minutes after the intensification of the westward electrojet. These reversals persist for periods of 10–20 min and appear to have a one-to-one correspondence with the occurrence of dipolarizations. The changes in convection are accompanied by changes in precipitation. Flux depletion regions (FDR) are measured by the DMSP satellites inside the surge, near the equatorward portion of the westward electrojet intensification. The poleward boundary of every FDR is collocated with a convection reversal and an arc intensification that marks a poleward transition into a region initially dominated by intense discrete electron precipitation and velocity dispersed ion structures (VDIS). Convection in the FDR constitutes an eastward electrojet channel that may produce a transient recovery signature as observed by ground magnetometers. The observations of FDR's and the fast westward flows that accompany them are consistent with the scenario of a rarefaction and/or a neutral line in the near-earth tail that produces fast earthward flows after the breakup. The arc intensification at the poleward boundary of the depletion region and the collocated transient convection reversal favor an enhancement of the magnetosphere-ionosphere coupling and thus the continuation of the substorm expansion in a multiple cell convection system. Arguments are presented to explain how a series of pseudobreakups modify the near Earth magnetic field into an increasingly dipolar geometry until a breakup is possible.

We have studied a hydromagnetic wave event in the early noon sector of the cusp region, using HF radar data collected in the southern hemisphere and ground-based magnetic data from both hemispheres. There were three distinct pulsations, which appear to have been driven by changes in the solar wind, beginning with the passage of a powerful interplanetary shock. This shock triggered the first pulsation, a strong ssc, at 1346 UT. It was followed at 1402 UT by a second pulsation, which had a stable dominant frequency of 3.3 mHz and lasted about half an hour. Although this frequency was close to that of the ssc, the new pulsation was clearly differentiated from the ssc by an abrupt 180° change of phase. A further such phase discontinuity at 1434 UT (near noon MLT) marked the start of a third distinct ULF pulsation with a clearly lower frequency of 2.8 mHz, which continued for another half hour, until 1505 UT, and was correlated with the magnitude of the IMF. These latter two pulsations yielded a successful calibration of the radar observations with ground-based magnetometer measurements; the pulsation current systems in the northern and southern hemispheres were found to be highly correlated and conjugate. The 3.3-mHz pulsation was observed over a range of ∼10° in invariant latitude, and its amplitude showed a strong, broad maximum close to the latitude of the cusp as inferred from an independent radar-satellite analysis of cusp signatures. The zonal E × B motion associated with the pulsation showed a linear latitudinal decrease in phase (25° per degree of latitude) while the meridional motion had nearly constant phase. The longitudinal phase variation corresponded to an m value of ∼10 with a source at later MLT. We discuss the possibility that the dominant wave activity near 3 mHz in the latter two pulsations was due to a hydromagnetic surface wave at the magnetopause stimulated by the ssc and modified by the forcing action of the solar wind.

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

Strong interplanetary magnetic field (IMF) and seasonal effects in the convection of nightside ionospheric plasma are described. The findings are based on a statistical analysis of observations made with the Johns Hopkins University/ Applied Physics Lab (JHU/APL) HF radar located at Goose Bay, Labrador. For positive sign of the IMF dusk-dawn component, By greater than 0 the dawn cell is more crescent shaped and the dusk cell more round while for BY less than 0 these pairings of size and shape are reversed. The more extreme crescent /round cell dichotomy is obtained for BY greater than 0. The return flows associated with the crescent-shaped cell dominate at midnight MLT (magnetic local time); the reversal in the zonal velocity in the 67 deg-69 deg lambda (magnetic latitude) interval occurs 2.5 hr earlier in summer than in winter. The maximum effects are obtained on the nightside for the pairings By greater than 0, summer and BY less than 0, winter; the first produces the more structured cell in the morning, the second in the evening, and this cell dominates the return flow at midnight. The difference in the zonal flow reversals for these pairings exceeds 4 hr in MLT.

During geomagnetically disturbed conditions the midlatitude ionosphere is subject to intense poleward directed electric fields in the dusk-midnight sector. These electric fields lead to the generation of a latitudinally narrow westward directed flow channel in the subauroral region called a subauroral polarization stream (SAPS). If the magnetic field lines are treated as equipotentials, electrodynamic events such as SAPS are expected to occur simultaneously at magnetically conjugate locations with similar features. In this paper we present simultaneous observations of a SAPS event in both hemispheres made by midlatitude SuperDARN radars with conjugate fields-of-view. We analyze the relation between the geomagnetic conditions and the characteristics of the channels such as latitudinal location, electric field, total potential variations across the channels, and Pedersen current. The results suggest a strong correlation between the strength of the ring current and the latitudinal location of the channel. An inter-hemispheric comparison of the characteristics of the channel indicates that the potential variations across the channels are similar while the electric fields, Pedersen currents and latitudinal widths of the channel exhibit differences that are consistent with equal potential variations. We attribute these differences to seasonal differences in ionospheric conductivity between the hemispheres and magnetic distortion effects in the inner magnetosphere.

[1] We apply a statistical technique called Principal Component Analysis (PCA) for examining underlying patterns of polar cap convection and illustrate potential applications of the PCA-based dimension reduction. Two principal components are identified: the first mode (PC1) is related to “uniform variation” of the flow speed at all MLTs, and is primarily governed by IMF Bz. The second mode (PC2) is related to “dawn-dusk asymmetry”, and is predominantly driven by IMF By. PCA gives the relative variance contribution of the two modes: PC1 giving ∼42% of the total variance and PC2 ∼17% of the total variance, which is about 40% of that from PC1. Due to the orthogonality of the principal components, the degree of dawn-dusk asymmetry can be represented byP2, where P2 is a component value when the observed data are projected along PC2. We identified P2 as proportional to IMF By, which leads to stronger dawn flows for By > 0 and stronger dusk flows for By

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

[1] The Super Dual Auroral Radar Network (SuperDARN) is a chain of HF radars that monitor plasma dynamics in the ionosphere. In recent years, SuperDARN has expanded to midlatitudes in order to provide enhanced coverage during geomagnetically active periods. A new type of backscatter from F region plasma irregularities with low Doppler velocity has been frequently observed on the nightside during quiescent conditions. Using three years of data from the Blackstone, VA radar, we have implemented a method for extracting this new type of backscatter from routine observations. We have statistically characterized the occurrence properties of the Sub Auroral Ionospheric Scatter (SAIS) events, including the latitudinal relationships to the equatorward edge of the auroral oval and the ionospheric projection of the plasmapause. We find that the backscatter is confined to local night, occurs on ≈70% of nights, is fixed in geomagnetic latitude, and is equatorward of both the auroral region and the plasmapause boundary. We conclude that SAIS irregularities are observed within a range of latitudes that is conjugate to the inner magnetosphere (plasmasphere).

We report on the response of high-latitude ionospheric convection during the magnetic storm of March 20-21 1990. IMP-8 measurements of solar wind plasma and interplanetary magnetic field (IMF), ionospheric convection flow measurements from the Wick and Goose Bay coherent radars, EISCAT, Millstone Hill and Sondrestrom incoherent radars and three digisondes at Millstone Hill, Goose Bay and Qaanaaq are presented. Two intervals of particular interest have been identified. The first starts with a storm sudden commencement at 2243 UT on March 20 and includes the ionospheric activity in the following 7 h. The response time of the ionospheric convection to the southward turning of the IMF in the dusk to midnight local times is found to be approximately half that measured in a similar study at comparable local times during more normal solar wind conditions. Furthermore, this response time is the same as those previously measured on the dayside. An investigation of the expansion of the polar cap during a substorm growth phase based on Faraday's law suggests that the expansion of the polar cap was nonuniform. A subsequent reconfiguration of the nightside convection pattern was also observed, although it was not possible to distinguish between effects due to possible changes in By and effects due to substorm activity. The second interval, 1200-2100 UT 21 March 1990, included a southward turning of the IMF which resulted in the Bz component becoming -10 nT. The response time on the dayside to this change in the IMF at the magnetopause was approximately 15 min to 30 min which is a factor of ~2 greater than those previously measured at higher latitudes. A movement of the nightside flow reversal, possibly driven by current systems associated with the substorm expansion phases, was observed, implying that the nightside convection pattern can be dominated by substorm activity.

The azimuthal structure of field-aligned current (FAC) systems is one of the target subjects of the Freja satellite's magnetometer observation. In the event of May 28, 1993, the satellite observed large-amplitude fluctuations of the latitudinal, as well as the longitudinal, magnetic field components along a postmidnight-to-morning orbit. By combining radar and ground magnetometer data, we inferred that the associated FAC system has a periodic structure consisting of multiple FAC sheets inclined significantly from the azimuthal (east-west) direction and moving westward and possibly poleward. Such a structure may be interpreted in terms of the meandering of an auroral electrojet, and perhaps as the ionospheric projection of boundary waves.

The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.

Meridional radar scans of electron density from the Sondrestrom incoherent scatter radar (Greenland, 66.99°N, 50.95°W) have been used to identify latitudinally narrow, field-aligned depletions of the auroral F region ionosphere. Observations of these so-called auroral cavities have been reported in earlier case studies in close proximity to E layer arcs at the poleward edge of the nightside oval (Doe et al. 1993). These radar data indicated that the cavities and arcs remained as collocated pairs for periods as long as an hour, while coordinated imaging and satellite measurements indicated that the pairs were extended in magnetic local time

[1] Recent analysis of a short period of observations has led to the hypothesis that enhanced meso-scale flows from well within the region of open polar cap field lines may cross the nightside polar cap boundary into the closed field line region and contribute to the triggering of equatorward (earthward) meso-scale flows across the ionospheric (equatorial) portion of plasma sheet fields lines and lead to PBIs and streamers. This includes the streamers that have been postulated to bring new plasma equatorward (earthward) and lead to substorm onset. Meso-scale structure of flow within the polar cap, often studied near the dayside polar cap boundary, has not previously been generally recognized as significant within the nightside polar cap. Here we have taken advantage of new capabilities to measure polar cap convection by the Resolute Bay incoherent scatter radar and the Rankin Inlet PolarDARN radar, coordinated with THEMIS all-sky imager observations, to study flow measurements from well within the polar cap to near the polar cap boundary. We present evidence that flow structures moving from the polar cap toward the nightside polar cap boundary may be important for triggering the flows that lead to substorm onset streamers. The new observations also have given evidence that the flow structures come from deep within the polar cap, and have given unexpected evidence that a continuation of flow structures moving from the polar cap toward the nightside polar cap boundary after substorm onset may be important in controlling the poleward expansion and duration of post-onset auroral activity.

[1] Over the past few years, the prominent role of solar wind dynamic pressure in enhancing dayside and nightside reconnection and driving-enhanced ionospheric convection has been documented by both ground and spaceborne instruments. For a previous case study of an abrupt increase in solar wind dynamic pressure, Super Dual Auroral Radar Network (SuperDARN) measurements of plasma convection within the dayside polar ionosphere revealed an immediate enhancement of plasma convection. The convection enhancement variation closely follows the variation in solar wind pressure. The dayside enhancement was followed by a nightside convection increase about 40 min later, which has similar variation characteristics as seen on the dayside. We now use SuperDARN flow measurements during a large number of solar wind pressure enhancements to conduct a superposed epoch analysis of the effects of solar wind pressure fronts on the dayside and nightside ionospheric convection. The results for the dayside show an increase of convection for nearly all interplanetary magnetic field (IMF) Bz values. The response is more pronounced and immediate (within minutes) for southward IMF, with a duration of 20–30 min. The response time scales increase to 5–10 min for northward IMF, and the enhanced flows last for 30–50 min. We also find a significant enhancement of nightside convection, particularly for small values of IMF By, that follows about 10–15 min after the dayside response and can last for 40–50 min.

[1] We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

[1] Under geomagnetically quiet conditions, the daytime midlatitude ionosphere is mainly influenced by solar radiation: typically, electron densities in the ionosphere peak around solar noon. Previous observations from the Millstone Hill incoherent scatter radar (ISR) have evidenced the presence of evening electron densities higher than daytime densities during the summer. The recent development of midlatitude Super Dual Auroral Radar Network (SuperDARN) radars over North America and Japan has revealed an evening enhancement in ground backscatter during the summer. SuperDARN observations are compared to data from the Millstone Hill ISR, confirming a direct relation between the observed evening enhancements in electron densities and ground backscatter. Statistics over a year of data from the Blackstone radar show that the enhancement occurs during sunset for a few hours from April to September. The evening enhancement observed by both SuperDARN and the Millstone Hill ISR is shown to be related to recent satellite observations reporting an enhancement in electron densities over a wide range of longitudes in the Northern Hemisphere midlatitude sector during summer time. Finally, global results from the International Reference Ionosphere (IRI) and the horizontal wind model (HWM07) are presented in relation with previously published experimental results and proposed mechanisms of the evening enhancement, namely, thermospheric horizontal winds and geomagnetic field configuration. It is shown that the IRI captures the features of the evening enhancement as observed by SuperDARN radars and satellites.

[1] We identified 17 geomagnetic storms between January 2007 and December 2008. Using particle measurements, each storm is classified as a high-flux event or a low-flux event. High-flux events are those storms associated with enhanced flux of relativistic electrons at geosynchronous orbit, and low-flux events show no such enhancement. We study the characteristics of ultralow frequency waves between 0 and 80 mHz during the recovery phase of the high-flux storms using magnetic field data. By determining the wave propagation direction and magnetic perturbation direction, we are able to calculate how the power is distributed between toroidal, poloidal, and compressional wave modes. We find that on the nightside most wave power is compressional at all frequencies below 80 mHz. On the dayside compressional wave power dominates the lowest frequencies below 20 mHz; toroidal waves carry most energy between 20 and 50 mHz, and poloidal waves are associated with higher power levels between 50 and 80 mHz. This suggests that on the nightside electrons are predominantly transported and energized by mechanisms involving the compressional wave mode; on the dayside, electron energization is achieved by acceleration mechanisms involving toroidal waves and mechanisms dependent on poloidal and compressional waves.

[1] The Super Dual Auroral Radar Network (SuperDARN) is a network of HF radars that are traditionally used for monitoring phenomena in the Earth's ionosphere at high latitudes. The radar backscatter is due primarily to reflections from plasma irregularities in the ionosphere, known as ionospheric scatter, and to signal reflected from the ground, known as ground scatter. In recent years, SuperDARN has expanded to midlatitudes to provide improved coverage of the auroral region during times of enhanced geomagnetic activity. In addition to high-speed auroral flows, the radars commonly see a variety of low-velocity plasma drift associated with the quiet time midlatitude ionosphere. The traditional method of distinguishing between scatter types in SuperDARN data was developed for high latitudes and depends solely on the Doppler velocity and Doppler spectral width of each data point. This method has proven inadequate for identifying quiet time midlatitude ionospheric scatter. In this paper, we present a new technique for the classification of SuperDARN data, which operates on a distributed range time basis and involves procedures similar to “depth first search.” Using the new method for classification of ground and ionospheric scatter, we show a dramatic improvement in the determination of ionospheric scatter within extended (>1 h) events. Compared to the traditional method, the number of ionospheric measurements resolved increases by more than 50%. The new classification algorithm identifies discrete events of ionospheric scatter and can be applied to statistical analysis of event occurrence and characteristics.

[1] Recent expansion of the SuperDARN network to mid-latitudes and the addition of a new high-time resolution mode provides new opportunities to observe mid-latitude ultra-low frequency waves and other ionospheric sub-auroral features at high temporal resolution. On 22 February 2008, the Blackstone SuperDARN radar and THEMIS ground magnetometers simultaneously observed substorm Pi2 pulsations. Similarities in measurements from the Blackstone radar and a magnetometer at Remus suggest a common generating mechanism. Cross-phase analysis of magnetometer data places these measurements at the ionospheric projection of the plasmapause, while fine spatial and temporal details of the radar data show evidence of field line compressions. About 1 min prior to ground Pi2 observation, 2 Earthward-moving Bursty Bulk Flows (BBFs) were observed by THEMIS probes D and E in the near-Earth plasma sheet. We conclude that the first 2 pulses of the Pi2s observed at Blackstone and Remus result from compressional energy generated by BBFs braking against the magnetospheric dipolar region.

Meteor wind data from the first year of operation of the Falkland Islands SuperDARN radar (52 degrees S, 59 degrees W) are used to characterize the atmospheric tides and background winds in the upper mesosphere above the South Atlantic. Strong (>40 ms(-1)) semidiurnal tides are observed in the winter time and large amplitude (>60 ms(-1)) bursts of quasi two-day wave activity are seen in January 2011. Data are in good agreement with those presented from the SAAMER meteor radar (54 degrees S, 68 degrees W). Comparison with SuperDARN meteor wind data from a geographically similar Northern Hemisphere site at Goose Bay (53 degrees N 60 degrees W) reveal clear interhemispheric differences especially in the semidiurnal and terdiurnal components of the tides. The winter time amplitudes of the tides are much stronger in the Southern Hemisphere than in the north. Background winds are observed to be significantly more polewards and westwards throughout the year than those predicted by the empirical horizontal wind model HWM07. UK Natural Environment Research Council NE/G018707/1, NE/G019665/1 Goose Bay radar operations SuperDARN Upper Atmosphere Facility under US National Science Foundation AGS-0849031, AGS-0946900

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Observations are presented of the ionospheric convection in cross sections of the polar cap and auroral zone as part of the study of the interaction of the Earth's magnetosphere with the magnetic cloud of January 13-15, 1988. For strongly northward IMF, the convection in the Southern Hemisphere is characterized by a two-cell convection pattern comfined to high latitudes with sunward flow over the pole. The strength of the flows is comparable to that later seen under southward IMF. Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. When the cloud's magnetic field turns southward, the convection is characterized by a two-cell pattern extending to lower latitude with antisunward flow over the pole. There is no evident interhemispheric difference in the structure and strength of the convection. Superimposed dawn-dusk asymmetries in the flow patterns are observed which are only in part attributable to the east-west component of the magnetic field.

Field aligned resonances at ULF pulsation frequencies are important mechanisms for transporting energy from the solar wind to the ionosphere. Observations by the Goose Bay HF radar in the post midnight sector have established that a stable discrete spectrum of resonant frequencies in the mHz range exists. This can be ascribed to the existence of magnetospheric cavity resonances which drive the resonances. The cavity between the magnetopause and the turning point is usually assumed to be the cause of such phenomena. In this paper an alternative cavity in the magnetotail is studied and its eigenmodes and eigenfrequencies are computed. While this cavity produces frequencies which fit the observations, the excitation of such modes by the solar wind would be difficult. It is concluded that a cavity between the magnetopause and the turning point is the most probable source of the pulsations.

[1] Using line‐of‐sight measurements of horizontal plasma drift from the Super Dual Auroral Radar Network (SuperDARN) located in the Northern and Southern hemispheres over a period extending from 1998 to 2002, statistical models of the high‐latitude convection electric field are derived for various ranges of interplanetary magnetic field (IMF) magnitude and orientation and for several ranges of dipole tilt angle. Direct comparison of the corresponding convection patterns in each hemisphere shows that under neutral tilt conditions (dipole tilt angle magnitude

[1] A spherical cap harmonic analysis (SCHA) technique is introduced for mapping the 2-D high-latitude ionospheric convection pattern based on Super Dual Auroral Radar Network (SuperDARN) velocity measurements. The current method for generating such maps is the FIT technique which generates global-scale maps over the entire convection region. This is accomplished by combining observations with a statistical model to prevent unphysical solutions in areas away from the observation points and by forcing the plasma flow to zero at the low-latitude boundary of the convection zone. Both constraints distort the mapped convection and require a preconception of where the plasma flow lines should close. By focusing on mapping the convection over a region well covered by velocity observations, the SCHA technique is freed of these constraints and more accurately reproduces local convection. We generate large-scale convection maps from SuperDARN data for various interplanetary magnetic field (IMF) conditions during periods of widespread radar coverage to show the patterns are consistent with expectations for various IMF configurations. We validate the SCHA maps by comparing them with the 2-D ion drifts measured by the DMSP satellites and with the 2-D convection vectors obtained by merging SuperDARN measurements at beam crossings. The SCHA technique is shown to perform comparably to the FIT technique over regions of good data coverage. For limited data coverage and over regions of highly variable flow, particularly near the equatorward edge of the mapping region, the SCHA technique provides a better solution for mapping ionospheric convection based on SuperDARN radar observations.

The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F-region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere. These resonances are seen in the interval from local midnight to morning, and have discrete, latitude-dependent frequencies at approximately 1.3, 1.9, 2.6–2.7, and 3.2–3.4 mHz. We show that these frequencies are compatible with MHD waveguide modes, with antisunward propagation and reflection at the magnetopause and at turning points on dipolar field lines.

[1] We present a technique to measure the magnetic field-aligned vorticity of mesoscale plasma flows in the F region ionosphere using line-of-sight velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN). Vorticity is often used as a proxy for magnetic field-aligned current (FAC) intensity in the ionosphere but also provides information about turbulent processes in the ionosphere and magnetosphere. Using 6 years (2000–2005 inclusive) of vorticity measurements made by six SuperDARN radars in the Northern Hemisphere, we have compiled, for the first time, maps of average vorticity across the northern polar ionosphere. These maps have been subdivided according to different seasonal and interplanetary magnetic field (IMF) conditions. The variations in the morphology of the vorticity maps with IMF direction match very closely those seen in maps of average FAC intensity (determined using different methods and instrumentation), suggesting that vorticity is a good proxy for FAC in an averaged sense. The variations in the morphology of the vorticity maps with season show differences from those seen in the FAC maps, illustrating that ionospheric conductance plays a major role in determining the differences between measurements of vorticity and FAC.

The technique of F region sounding by HF coherent radar is applied to the study of field line resonances equatorward of a region of shear flow in the early morning sector. The motions were predominantly in the geomagnetic east-west direction, indicating north-south electric fields. These oscillations had pronounced peaks in their latitudinal power distribution. For the pulsation at 1.95 mHz, a latitudinal phase shift of 180 deg was observed across the peaks in all the look directions of the radar, and a longitudinal wavelength corresponding to an m value of 3 was obtained. For the 2.6-mHz pulsation, the phase shifts across the peaks had a variation with a look direction that indicated a significant longitudinal as well as latitudinal variation; for this activity, an m value of about 16 is estimated. These features are interpreted in terms of the field line resonance theory, and the possible sources of the pulsation energy are discussed.

A critical examination of the accuracy of ionospheric vector velocity determinations, using realistic modeled flow patterns that are time-stationary but not spatially uniform, is presented. Under certain circumstances the actual and inferred flow fields are found to exhibit considerable discrepancy, sometimes not even agreeing in the sense of flow direction. It is shown that the natural curvature present in ionospheric convection on varying spatial scales can introduce significant error in the velocity estimate, particularly when the radius of curvature of the flow structure is less than or equal to the radar range to the scattering volume. It is argued that the ionospheric convection should be measured by bidirectional or multidirectional observations of a common ionospheric volume and that a synthesis of coherent and incoherent radar observations from different sites is preferable to multidirectional single-station observations using either radar alone.

Data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used to analyze the magnetic fields and auroral structures which were associated with a field line resonance which occurred near local midnight and in the early morning sector. The electric fields of this resonance, which have a frequency of about 1.95 mHz, were observed by the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and had a maximum at about 70.7° invariant latitude. Effects of the field line resonance were seen in the 5577 A, meridian scanning photometer data and the magnetometer data from the CANOPUS array. The field line resonance was accompanied by precipitating energetic electrons (energies greater than 3 keV) in a narrow auroral arc which was 3°–4° equatorward of the resonance structure seen in the radar data and approximately 1° equatorward of field lines threading the inner boundary of the proton plasma sheet. The electron precipitation in this arc was modulated at the frequency of the field line resonance. The effects of the resonance are also seen as pulsations in the magnetometer data, and the horizontal polarization of the pulsations showed a change in sense of rotation across the arc. The oscillations in the precipitating electrons might have been caused by modulation in the ELF/VLF growth rates due to the presence of the magnetohydrodynamic waves associated with the resonance, by mode conversion to kinetic Alfven waves, or by the formation of electrostatic ion cyclotron waves due to field-aligned currents associated with a second field line resonance collocated with the arc. The evidence presented here suggests that the modulation of ELF/VLF by magnetohydrodynamic waves on field lines near the plasmapause was the most likely cause of the auroral oscillations.

Night and early morning data from the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and from the magnetometers in the Canadian CANOPUS array often show structured spectra with distinct spectral peaks at 1.3, 1.9, and 2.6 mHz. These frequencies are very stable and vary by less than ±5% for the 6 days of data we have analysed. The radar measurements of the F-region drift velocities indicate that these spectral peaks are often associated with field line resonances of the shear Alfvén wave, and that the resonances are seen at lower latitudes as the frequency increases. The magnetometer data indicate that the magnetohydrodynamic (MHD) waves have westward phase velocities. We shall show that these observations are compatible with the formation of MHD cavity modes in the early morning and nightside magnetosphere, between the magnetopause, at approximately 14.5 RE, and turning points in the dipolar magnetosphere, outside the plasmasphere. Likely sources of energy are compressional pulses from the solar wind or Kelvin–Helmholtz instabilities in the low-latitude boundary layer.

The high temporal and spatial resolution of the Polar Anglo-American Conjugate Experiment radar data have permitted the evolution and formation of a transient plasma velocity signature in the cusp ionosphere to be studied. The signature satisfies a number of the predictions of theoretical models that require the formation of vortices. A much weaker enhancement in velocities is shown in the Northern Hemisphere, which is consistent with the vector superposition arguments presented by Crooker (1979) for the antiparallel merging model.

[1] Over the past few years the prominent role of solar wind dynamic pressure in enhancing dayside and nightside reconnection and driving enhanced ionospheric convection has been documented by both ground and spaceborn instruments. Super Dual Auroral Radar Network (SuperDARN) observations show that solar wind pressure fronts induce significantly enhanced ionospheric convection in the dayside ionosphere. In parallel, Defense Meteorological Satellite Program (DMSP) precipitating particle measurements and POLAR Ultra-Violet Imager (UVI) images have demonstrated that sudden solar wind pressure increases also significantly reduce the size of the polar cap, especially on the nightside, suggesting an enhancement of magnetotail reconnection. MHD models of the interaction of the magnetosphere with solar wind pressure fronts have reproduced the enhancement of dayside reconnection but have failed so far to account for the observed closing of the polar cap on the nightside and the suggested magnetotail reconnection increase. We use SuperDARN measurements of ionospheric convection within the nightside polar ionosphere, including near the magnetic separatrix, to evaluate the strength of the observed nightside reconnection enhancement after an abrupt increase in solar wind dynamic pressure on 6 November 2000 and compare it with similar observations on the dayside. We show that an enhancement of nightside convection occurs after a sudden increase in solar wind pressure, delayed by about 40 min compared with the observed dayside convection enhancement. The nightside enhanced flows are observed crossing the open-closed boundary determined by POLAR UVI data, indicating an enhancement of tail reconnection that is possibly due to the pressure increase and is, in addition to the tail reconnection, associated with the more immediate closing of the polar cap.

[1] ACE SWEPAM measurements of solar wind field-aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross-polar-cap gradients in the intensity of otherwise-steady polar rain. The generally good agreement of the distribution functions, f, from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field-aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low-energy electrons and maintain charge quasi-neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field-aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.

The Geospace Environment Modeling (GEM) Program is directed toward modeling the coupled solar wind/magnetosphere/ionosphere system. The inter-calibration of ground-based observations of the ionosphere and satellite observations has been identified as an essential step in tying together the data to produce a global picture of geospace. On October 10, 1988 the DMSP-F9 satellite passed through the Southern Hemisphere cusp while a coheret scatter HF-radar was observing 10-m scale irregularities present in the ionosphere. The combined data indicate that these irregularities were being generated in the cusp, and that the cusp was a region of greater than normal electric field turbulence. The radar data indicate that the cusp was colocated with the region where the ionospheric convection rotated from sunward to anti-sunward with increasing latitude. These observations provide an unambiguous case where simultaneous satellite and ground-based observations of the cusp can be compared.

[1] We report the first sub-auroral Pi2 pulsations observations by a SuperDARN type HF radar. The Wallops radar LOS measurements obtained at ionospheric altitudes, ∼56° magnetic latitude, 23 hour magnetic local time, are shown to be highly correlated with ground magnetic field perturbations obtained at Ottawa. The period of the Pi2 pulsations is 118 s and the m-number is ∼2.3. The availability of both ionospheric LOS measurements and ground based magnetic field perturbations enable us to constrain the properties of the wave. A predominantly shear Alfven mode wave is able to explain both the amplitude and phase relations of our observations. Although the solar wind dynamic pressure is fairly stable the IMF Bz is highly variable thereby preventing an unambiguous identification of a solar wind trigger. Rather, magnetotail observations of a clear dipolarization and an intensification of the auroral westward electrojet producing a modest ground magnetic bay indicate that the wave is associated with the onset of a weak substorm.

[1] During geomagnetic storms the ability of the Super Dual Auroral Radar Network (SuperDARN) to measure ionospheric convection becomes limited when the radars suffer from absorption and the auroral disturbance expands equatorward of the radar sites. To overcome these shortcomings, it was decided to construct a SuperDARN radar at middle latitudes on the grounds of the NASA Wallops Flight Facility. This paper presents the first comprehensive analysis of Doppler measurements from the Wallops radar, which commenced operations in May 2005. Wallops measurements are compared with the Goose Bay radar during the onset of a geomagnetic storm on 31 August 2005: Goose Bay measured the onset of geomagnetic activity at high latitude while Wallops monitored the expansion of convection to middle latitudes. Average convection patterns binned by the Kp geomagnetic index are also presented. During weak-moderate geomagnetic activity (Kp ≤ 3) the Wallops radar observes ionospheric irregularities between 50° and 60° magnetic latitude drifting westward across much of the nightside. When these measurements are incorporated into the calculation of an average SuperDARN convection pattern, the streamlines of polar cap outflow on the nightside become kinked in a manner reminiscent of the Harang discontinuity. This morphology arises quite naturally when the two-cell convection at high latitudes merges with the prevailing westward convection at middle latitudes. During increased geomagnetic activity (Kp ≥ 3), Wallops is able to measure the expansion of auroral electric fields to middle latitudes and the average SuperDARN cross-polar cap potential is increased by 25%.

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.

[1] In this letter we present the first two-dimensional observations of sub-auroral ion drift (SAID) variability within the sub-auroral polarization stream (SAPS) using a new mid-latitude SuperDARN radar located at Wallops Island, VA. The radar data are complemented with observations from the DMSP F15, TIMED, and NOAA-18 spacecraft to confirm that a backscatter feature observed at the equatorward edge of the auroral oval is a manifestation of SAPS/SAID. During a several hour long period on August 6, 2005, the velocity data indicate that significant changes in the SAPS flow occurred on time scales of a few minutes. The Wallops HF radar observations demonstrate that the SAPS phenomenon is a source of small-scale irregularities extending over many hours of MLT and that the electric fields associated with SAID in particular are highly variable.

[1] This letter reports observations of large-scale ionospheric plasma flows from the SuperDARN radar network and global auroral images from the Polar ultraviolet imager during two large and rare substorm events. The substorm events were characterized by predominantly westward expansion for one and predominantly eastward expansion (or lack of westward expansion) for the other. The westward expansion case was associated with a positive y-component of interplanetary magnetic field (IMF), whereas the eastward expansion case was associated with a negative y-component of IMF. The concurrent plasma flow measurements from the SuperDARN radar network suggest that the westward (eastward) expansion is embedded in the large, round dusk (dawn) convection cell. A simple explanation is offered for the observed preponderance of westward expansions, namely, that the pre-midnight onset location favors the westward plasma flow that is associated with the dusk cell. Eastward expansion occurs only when a large IMF By negative condition causes the midnight sector to be dominated by eastward plasma flows.

[1] In this paper we present observations of the F region cusp ionosphere from the Super Dual Auroral Radar Network (SuperDARN) Finland and European Incoherent Scatter (EISCAT) Svalbard radars in a new joint experiment. On 16 December 2002 the EISCAT Svalbard radar was operated in a fast elevation scan along beam 9 of the SuperDARN Finland radar, giving altitude/latitude profiles of the SuperDARN Finland backscatter volume every 2 min. Combining the two independent data sets, we monitor an isolated polar cap patch of high electron density (1011.5 m−3) that slowly formed in the dark cusp ionosphere during an interval of northward interplanetary magnetic field. The patch formed on a stirred lobe cell, excluding the possibility for solar EUV ionized plasma to drift in from the south. This data set represents an unparalleled example of patch formation within the polar cap, locally by particle impact ionization. After the interplanetary magnetic field turned southward and By changed polarity, the patch started to move poleward into the polar cap. Enhanced velocity gave rise to enhanced HF backscatter power and enhanced spectral widths, consistent with gradient drift instability as the formation mechanism of HF backscatter targets.

[1] The database of the nine radars of the Super Dual Auroral Radar Network (SuperDARN) in the northern hemisphere has been analyzed for information on factors that influence the convection of plasma in the high-latitude ionosphere. The velocity measurements were collected over the period 1998–2002. The data were first used to derive a new statistical model of convection that improves upon the earlier one-radar model of Ruohoniemi and Greenwald (1996) in its specification of the dependence of the convection pattern on the magnitude and direction of the IMF in the GSM Y-Z plane. We then derived average patterns for secondary sortings by season, year, and radar. Such dependencies as emerged were most clearly seen by contrasting the results for By+ and By−. The seasonal effect in the convection pattern is found to have similarities to that of the sign of By. In particular, the combination of By+/summer (By−/winter) reinforces the tendency of the By sign factor to sculpt the dusk and dawn cells into more round/crescent (crescent/round) shapes and to shift the crescent cell across the midnight MLT meridian. However, these combinations are associated with lower estimates of the total cross polar cap potential drop, ΦPC, while the nonreinforcing combinations produce elevated ΦPC, especially By−/summer. There is an overall tendency for ΦPC to increase from winter to summer, although the pure seasonal effect on the potential drop is weaker than that of the By−sign/season factor. We did not find pronounced differences among the patterns derived for the 5 individual years, which spanned the most recent interval of solar cycle maximum. Sorting by radar, we found few differences among the patterns for By+, but for By−, variations emerged that are consistent with a possible dependence on universal time (UT). The impacts of season and UT on convection in the high-latitude ionosphere thus depends on the IMF, especially the sign of By. We speculate that variability in the ionospheric conductivity has a greater effect on magnetosphere-ionosphere coupling under By− conditions.

A case study of medium-scale travelling ionospheric disturbances (TIDs) that are correlated with solar wind Alfvén waves is presented. The HF radar ground-scatter signatures of TIDs caused by atmospheric gravity waves with periods of 20-40min are traced to a source at high latitudes, namely pulsed ionospheric flows (PIFs) due to bursts in the convection electric field and/or the associated ionospheric current fluctuations inferred from ground magnetic field perturbations. The significance of PIFs and TIDs in the context of solar-terrestrial interaction is that Alfvénic fluctuations of the interplanetary magnetic field (IMF) observed in the solar wind plasma streaming from a coronal hole correlate with PIFs and TIDs. The link between the solar wind Alfvén waves and TIDs is corroborated by the ground magnetic field signatures of ionospheric current fluctuations that are associated with the IMF-By oscillations and TIDs. The observed PIFs and the associated negative-to-positive deflections of the ground magnetic field X component are interpreted as ionospheric signatures of magnetic reconnection pulsed by solar wind Alfvén waves at the dayside magnetopause. Although the clarity of the radar line-of-sight velocity data may have been affected by anomalous HF propagation due to intervening TIDs, the application of a pure state filtering technique to analyze the radar data time series reveals a one-to-one correspondence between PIFs, TIDs and solar wind Alfvén waves. The spectra of solar wind and ground magnetic field perturbations are similar to those of PIFs and TIDs. The ground-scatter signatures indicate TID wavelengths, phase velocities and travel times that are consistent with ray tracing, which shows a subset of possible gravity wave group paths that reach the F region from a source in the E region after the wave energy first travel downward to the upper mesosphere where the waves are reflected upward. The observed one-to-one correspondence between the convection electric field bursts and TIDs is consistent with the modeling results for large-scale TIDs by Millward et al.(1993a,b). The correlation with solar wind Alfvén waves points to very direct coupling of energy in the solar wind into the subauroral atmosphere.

[1] Auroral boundary algorithms for SuperDARN ionospheric convection observations are introduced. Both the equatorward boundary of scatter (EBS) and the convection reversal boundary (CRB) are found and compared with boundaries based on DMSP particle precipitation observations. These comparisons provide an intercalibration between the two schemes and insight into the relationship between disparate phenomena. The EBS is reliably poleward of the equatorward boundary of precipitation (EBP) at most local times but centers well on the equatorwardmost precipitation between 1600 and 2200 MLT when within 8° latitude of the radar station, indicating a correspondence with the SLERPS (slow long-lived E region plasma structures) of Jayachandran. Near dawn the EBS coincides better with the structured–unstructured boundary (SUB) than the EBP, as one might expect if the ionospheric irregularities responsible for this scatter were produced by the gradient-drift instability. The CRB correlates well with the open-closed boundary (OCB) as seen by DMSP. There is, however, an equatorward offset of the CRB relative to the OCB that varies from zero near noon to ∼1° near dawn and dusk and is largest near midnight. This offset is consistent with a small viscous-like interaction between the magnetosheath and the low-latitude boundary layer, resulting in a degree of antisunward flow on closed field lines. The offset decreases under the influence of southward interplanetary magnetic field, as one might expect if increased dayside merging interfered with this effect.

[1] This paper reports results from a detailed analysis of theta auroras that occurred during the passage of a magnetic cloud on 8 November 2000. The interplanetary magnetic field (IMF) was generally northward for more than 12 hours, while the y-component of the IMF changed signs several times. Auroral images of the Northern Hemisphere acquired from the Ultraviolet Imager (UVI) on board the Polar satellite showed clear development of theta auroras during the prolonged northward IMF period. It is found that the theta aurora can be enhanced by compression of the magnetosphere associated solar wind pressure pulses. We also examine large-scale ionospheric plasma convection flow data from the SuperDARN radar network to study the ionospheric plasma flow pattern associated with the theta aurora. The radar data, when available, showed that two different plasma flows can coexist within the day-night part of the theta aurora. The nightside extension of the theta aurora is always on a region of antisunward convecting flows, whereas there is no consistent flow pattern found in the dayside portion of the theta aurora studied. In general, plasma flow outside the theta aurora cannot be determined because of lack of scattered radar signals, but from a few instances it suggested that the nightside theta aurora is flanked by antisunward flows. In contrast to the prevailing view that theta auroras reside in regions of closed magnetic flux that convect sunward, this study suggests one more aspect of the theta aurora and provides constraints for models and theories.

[1] We present maps of ionospheric precipitation regions, based on 11 years of DMSP particle data, binned by the interplanetary magnetic field (IMF), with superposed SuperDARN convection streamlines gathered under similar conditions. The convection patterns are transformed into an inertial coordinate system. The maps, which include both the nightside and dayside, are created in a fully automated fashion, with, for example, the cusp centered at its centroid latitude for each half hour bin of MLT, with a latitudinal width equal to the statistical difference between the poleward and equatorward edges. The mantle asymmetry about noon does not fit the pattern expected from simple theoretical considerations (namely, that the mantle should be thicker postnoon for positive By in the Northern Hemisphere). The mantle is appreciably thicker prenoon than postnoon, especially for positive By but also even for negative B y This asymmetry matches the SuperDARN convection flows, in which, irrespective of the sign of B y , most of the conversion of closed field lines to open occurs prenoon. Quantitatively expressed, for southward IMF, the potential encompassed by flux crossing the open-closed boundary prenoon (0600-1200 MLT) exceeds that for postnoon (1200-1800 MLT) by 30 kV to 15 kV for By > 3 nT and by 30 kV to 20 kV for By 3 nT, 31 kV reconnects from 1800 to 2400 MLT, but only 14 kV reconnects from 0000 to 06000 MLT. The convection reversal boundary (CRB) consistently coincides with the nightside open-closed particle boundary (OCB). On the dayside, the CRB lies equatorward (poleward) of the OCB in the postnoon (prenoon) sector for B y 0). This shift is consistent with the effects of an interhemispheric current produced by the partial penetration of the IMF By into the frontside magnetosphere.

[1] Much of our current understanding of magnetospheric electrodynamics is based on the assumption that magnetic field lines often behave as electrostatic equipotentials. This assumption has allowed hemispheric patterns of ionospheric convection to be interpreted in terms of the large-scale circulation of plasma in the magnetosphere. However, the extent to which the equipotential field-line assumption is justified for different regions of the magnetosphere has not been adequately explored, largely because of the sparseness of magnetospheric measurements. In this paper, a mathematical formalism is developed that allows conjugate magnetospheric and ionospheric measurements to be compared with each other using a model magnetic field. The technique is demonstrated during an event interval in which Super Dual Auroral Radar Network measurements of ionospheric plasma drift are mapped to the magnetosphere using the Tsyganenko T01 magnetic field model and compared with conjugate measurements from the Cluster spacecraft electron drift instrument. The degree of consistency between the conjugate measurements is discussed in terms of (1) the accuracy of the magnetic field model, (2) the validity of the assumption of equipotential magnetic field lines, and (3) the presence of inductive electric fields in the magnetosphere. It is also shown how conjugate plasma drift measurements can be used to identify small inaccuracies in the location of ionospheric foot points specified by the magnetic model.