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.

Using two conjunction events of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) imagers and spacecraft as well as the Super Dual Auroral Radar Network (SuperDARN) and Poker Flat Incoherent Scatter Radars, we show that longitudinally narrow flow bursts in the nightside polar cap can precede poleward boundary intensifications (PBIs) that are followed by equatorward moving north‐south (N‐S) arcs, including those leading to substorm onset instability within the near‐Earth plasma sheet. The association between the ionospheric flows and PBIs indicates that enhanced flows on open field lines may contribute to parallel potential drop increase, triggering of magnetotail reconnection, and to the earthward flows leading to N‐S arcs and to substorm onset. We also investigated differences between N‐S arc sequences that do and do not lead to substorm expansion onset. We found that the two types of N‐S arcs have similar characteristics, indicating that their corresponding plasma sheet flow properties could also be similar. There is, however, one difference between the sequences of N‐S arc evolution. Each N‐S arc leads to small intensification of the growth phase arc, and when the onset‐related N‐S arc reaches the equatorward portion of the auroral oval, the preexisting growth phase arc is much brighter than at the times of non‐onset‐related N‐S arcs. Assuming that the growth phase arc is related to pressure gradients at the inner edge of the plasma sheet, this difference indicates that the near‐Earth plasma pressure distribution at the time of plasma sheet fast flows is crucial in substorm triggering. These observations suggest that substorm onset instability is possible only when the preexisting inner plasma sheet pressure is sufficiently large.

The THEMIS All‐Sky Imager at Fort Smith, Canada observed a sudden appearance and subsequent evolution of auroral streamers on April 15, 2006. The event took place in an oval that was optically dark, and evolved into a 20‐minute period of intense equatorward streaming of red aurora. We characterize the incipient event as isolated streamers, a phenomenon previously linked with bursty bulk flows in the plasma sheet. Thanks to the high time and spatial resolution of THEMIS ASI, the observed streamer reveals some detailed features hitherto not reported. Aside from their exceptionally high speed and fine transient structures, the streamers are found to exhibit an unusual convergent motion (equatorward from high latitudes and poleward from low latitudes) to form a complete flow channel. Our analysis shows that this observation is best explained with a new theory on the origin of auroral streamers.

The overarching objective of the ILWS Geospace program is to facilitate system level science. This demands synoptic observations such as global auroral imaging. At present, there is no funded mission during ILWS that incorporates a global auroral imager. The imaging community needs to move now to address this important gap. While doing so, it is interesting to take stock of global auroral observations that have not been achieved, or that have been achieved only to a limited extent. These include simultaneous imaging across all relevant scales, spectral resolution of sufficient quality to allow for global maps of characteristic energy and energy flux of precipitating electrons, continuous global auroral imaging for time periods spanning long-duration geomagnetic events, systematic interhemispheric conjugate observations, auroral observations magnetically conjugate to in situ measurements, and automatic classification of auroral images. These observations can be achieved within the next decade. If they are, then they will facilitate exciting new science.

RESUME With the world’s largest accessible land mass under the auroral oval, Canada has exploited its unique position by deploying wide-spread instrument arrays dedicated to the study of high-latitude geospace. The Canadian GeoSpace Monitoring (CGSM) array is currently being upgraded, and during the Swarm mission will cover a large portion of Canada with magnetic, optical, and radio (ionosonde, riometer, and Canadian SuperDARN) measurements. The THEMIS ground-based camera and magnetometer array will augment this coverage further at 20 additional sites throughout Canada, and at higher time resolution. Together with these arrays and others throughout the world, Swarm’s precision multi-point measurements of magnetic fields, electric fields and plasma parameters will bring a powerful and comprehensive new dataset to studies of auroral electrodynamics and auroral arc formation, global ionospheric current and convection systems, low-frequency plasma instabilities, magnetosphere-ionosphere-thermosphere coupling, and ion heating and outflow. 1. SCIENTIFIC THEMES Swarm will be able to address a broad spectrum of scientific questions in regions ranging from the Earth’s core to the magnetosphere. For this reason, it will operate more as a facility with a wide range of users than as a targeted scientific mission. The scientific themes reviewed below explore aspects of the scientific potential of Swarm in the area of I-T-M science. The original Swarm proposal to ESA describes the basic scientific motivations of the mission; this document emphasizes scientific questions of particular relevance to the Canadian space research program, through the scientific interest of its participants and collaborators, and through the ground and space-based instrumentation that Canada can bring to bear on Swarm-related science. 1.1. Auroral electrodynamics and field-line

In this study the nightside ionospheric plasma convection pattern during two substorm events is investigated from global SuperDARN observations. We find that, a postmidn ight anticlockwise convection vortex (PoACV) usually emerges at latitudes higher than the auroral brightening region after the substorm expansion phase onset. Meanwhile, an east-to-west flow reversal region wrapping around the int ensified auroras extends into the postmidnight sector. A premidnight clockwise vortex is gradually attenuated or even absent, during the substorm expansion, The substorm current system inferred from the relative positions of the PoACV and the auroral brightening region is in general northeast- southwest aligned, implying a mixture of a meridional current system (MCS) and a zonal system associated with the substorm current wedge (SCW). the dynamic change of ionospheric convection pattern around the midnight sector associated during the substorm EP. In par- ticular, we investigate the different evolution of convect ion vor- tices in the pre- and post-midnight sectors after the substo rm onset. Possible generation mechanisms of the convection vor- tices and the associated current system will be discussed.

Radar measurements of the global convection pattern and satellite observations of nightside low-energy ion populations are presented for magnetically disturbed conditions. The results form a consistent picture, namely that t he ions are secondary ions (mostly O+) produced in the auroral zone ionospheric F-region during bombardment by primary auroral electrons, and that these ions subsequently undergo TAI (transverse acceleration of ions) which raises their e nergy from the ionospheric value of about 0.1 eV to suprathermal and sub-auroral energies from a few tens of eV to a few hundred eV. The ions subsequently undergo a combination of bounce motion and ExB drift along the streamlines of the convection pattern, which has recently been shown to include some new features. We can classify three populations of these low-energy ions, namely DAPS (Dusk Auroral Positive Stream) ions, SAPS (Sub-Auroral Polarization (or Positive) Stream) ions and MAPS (Morning Auroral Positive Stream) ions. When conditions are not too disturbed, these ions show some characteristic features such as energy bands with integer energy ratios and a dispersive signature (decreasing energy with decreasing latitude). It is proposed that these low-en ergy ions, particularly the MAPS ions on the dawnside, may alter the shielding conditions in the inner magnetosphere.

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

This study employs observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap boundary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times supports this finding.Key words. Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)

Ps 6 disturbances and associated omega bands are often considered to be part of the phenomenology of the recovery phase of substorms. We note cases of the initiation of Ps 6 activity at or very near the time of onset, either of a substorm expansive phase, a pseudobreakup, or a poleward border intensification. Thus, we claim that Ps 6 disturbances need not be viewed primarily as phenomena of the recovery phase. This produces both the challenge of explaining Ps 6 within a broader context and the opportunity to use Ps 6 observations to better understand magnetospheric phenomenology, including expansive phase onsets. We further examine the position of the causative currents for Ps 6 and find that they may be located at either the equatorward or poleward border of the auroral oval, or within it. In the first case, the relationship of expansive phase onset and time delay to Ps 6 initiation appears to be very short. In the latter case, there is an association with poleward border intensification, but with a measurable time delay. We present HF radar data to discuss how the electric field at onset time favors the growth of Ps 6 current systems.

One example of the response of ionospheric convection and the polar cap boundary to a sudden change in the interplanetary magnetic field (IMF) orientation has been studied by using ground magnetometers, the Super Dual Auroral Radar Network (SuperDARN), and Defense Meteorological Satellite Program (DMSP) particle detectors when the IMF suddenly changed from northward (+6 nT) to strongly southward (−19 nT) at 1716 UT on 5 September 1995. The Bz component was fairly constant for ∼2 hours before and ∼25 min after the sudden IMF change. The convection flow changed almost simultaneously over a global extent. This initial change of the convection pattern can be characterized by a sudden formation of a large flow vortex in the afternoon sector. This agrees with the earlier findings by Ruohoniemi and Greenwald [1998] and Ridley et al. [1998]. On the other hand, the response of the polar cap boundary (or its proxy) is more complicated. The Saskatoon radar, located in the late morning sector, observed an equatorward shift of the cusp scatter region simultaneously with the initial response of the convection flows. The DMSP particle data also showed a simultaneous equatorward expansion of the auroral oval in the 2100 magnetic local time (MLT) sector. The radar and particle data indicate the immediate equatorward expansion of the precipitation regions in the noon and premidnight sectors. About 10–20 min after the initial change, there were changes observed in the dusk region, namely, an equatorward expansion of the current reversal boundary observed by the Greenland magnetometer chain in the dusk sector between 1740 and 1750 UT and an equatorward expansion of the convection reversal boundary detected by the Stokkseyri, Halley, and Syowa radars. The delayed responses were observed 18‐8 min before a substorm onset was recorded at midlatitude stations at 1756 UT. These observations indicate that there were two kinds of ionospheric responses to the southward turning of the IMF; the first response is the formation of the convection vortex and the equatorward shift of the polar cap boundary at noon and at ∼2100 MLT, and the second response is the equatorward expansion of the convection reversal boundary in the dusk sector. We make the case that the first response is associated with the propagation of magnetosonic waves and that the second response is consistent with the Cowley and Lockwood [1992] picture of the redistribution of the newly created open flux in the polar cap region.

SuperDARN observations from the interval 1330 UT to 1500 UT, 17 April 1996, are presented. During this interval, five radars covered 12 hours of magnetic local time on the dayside, including the cusp region, in the Northern hemisphere, and one observed the cusp region in the Southern hemisphere. In response to a southward turning of the interplanetary magnetic field, backscatter appears in all radar fields-of-view, first in the cusp region and then in the dawn and dusk sectors after a delay consistent with an anti-sunward propagation speed of 1.5 km s−1. In the Northern hemisphere cusp region, antisunward plasma drift and poleward-moving forms are observed with a 10 min periodicity, which is typically the radar signature of flux transfer events. Such signatures of transient reconnection are not seen in the Southern hemisphere cusp region, where predominantly azimuthal plasma drift is observed. In the dawn and dusk sectors, the radars observe sunward plasma drift in the convection return flow region, and transient, sunward-moving backscatter features which have a periodicity of approximately 10 mins.

The statistical study of the azimuthal convection flow in the midnight sector, as measured by the Saskatoon and the Stokkseyri SuperDARN radars, reveals the existence of an enhanced eastward convection stream around the poleward boundary of the auroral oval. The stream occupies two to three degrees and is located at geomagnetic latitudes 72–75°, which is indeed significantly poleward of the position of the center of the auroral oval. Poleward of the eastward convection stream, a westward convection stream is detected by the Saskatoon radar though not evident in the Stokkseyri radar measurements. The existence of the eastward convection stream at the poleward edge of the nightside auroral oval is very consistent with earlier results from the Akebono spacecraft. Such plasma flows are the source of possible plasma instabilities in the ionospheric E and F‐regions.

Three SuperDARN coherent HF radars are employed to investigate the excitation of convection in the dayside high-latitude ionosphere in response to transient reconnection occurring in the cusp region. This study demonstrates the existence of transient antisunward-propagating backscatter features at the expected location of the ionospheric footprint of the cusp region, which have a repetition rate near 10 min. These are interpreted as the ionospheric signature of flux transfer events. Moreover, transient sunward-propagating regions of backscatter are observed in the convection return flow regions of both the pre- and post-noon sectors. These patches are observed to propagate towards the noon sector from at least as far around the auroral zone as 07 MLT in the pre-noon sector and 17 MLT in the post-noon sector, travelling with a veloCity of approximately 1.5 to 2 km s−1. These return flow patches have a repetition rate similar to that of the transient features observed at local noon. While providing supporting evidence for the impulsive nature of convection flow, the observation of sunward-propagating features in the return flow region is not consistent with current conceptual models of the excitation of convection.

A sharp latitudinal gradient in the Doppler spectral width is often observed in the nightside data from the SuperDARN HF radars. We use an excellent conjunction between the Halley HF radar and the POLAR satellite to demonstrate that this gradient is associated with the boundary between the central plasma sheet and boundary layer plasma precipitation. We then exploit this identification to examine the CPS/BPS response simultaneously near the dawn, dusk and midnight, to a significant change of IMF By during a period when the magnetosphere is otherwise in a quiescent state. We find that the direction of the boundary motions are consistent with those previously determined by Holzworth and Meng, [1984]. In the morning sector, the magnitude is consistent with previous observations but in the evening sector the displacement is about 3 times greater than expected. Also the response appears to be in the order of minutes near dawn and dusk, faster than previously reported responses to IMF changes in this region.

The passage of an interplanetary magnetic cloud at Earth on January 10-11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about -85 nT. We use a comprehensive set of data collected from space-borne instruments and from ground-based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2-day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the Ε parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross-polar-cap potential drop, hemisphere-integrated Joule heating rate, and hemisphere-integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross-polar-cap potential drop for large AE(>1000 nT), but further studies are needed to confirm this.

The ionospheric convection response to dynamic solar wind and Interplanetary Magnetic Field (IMF) conditions between 1000–1200 UT 23 March 1995 is studied employing data from five northern hemisphere SuperDARN HF radars. The interval studied followed a northward turning of the IMF. There was a ∼3 min delay before the convection pattern began to respond to the new input conditions, followed by a transition time of ∼18 min during which the convection pattern evolved into a pattern which probably consisted of 3 cells and is consistent with previously published models of ionospheric convection under northward IMF. The new cell appeared to evolve between the two existing convection cells.

The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative B z and positive B y , to positive B z and small B y , has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (∼2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within ∼2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of ∼7 and ∼14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further ∼6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change.

A SuperDARN HF radar pair (Saskatoon‐Kapuskasing) has been used to measure Doppler spectra of backscatter from the dayside F‐layer. Normally the spectra are single‐peaked, but a small fraction exhibit a double‐peaked (D‐P) signature. On the 2D convection maps, these D‐P spectra occur in range cells located poleward of the convection reversal. A comparison with DMSP SSJ/4 particle data measurements and their mapping to magnetospheric boundaries shows that the D‐P spectra are concentrated just equatorward of the magnetopause, in regions of spatially/temporally structured soft electron precipitation (about 300 eV) where the highly variable flux can reach 2–5 ergs/cm²/s. The D‐P spectra are most easily explained in terms of scattering from small‐scale vortices of size less than the radar resolution of 45 km. The D‐P spectral measurements are illustrated by SuperDARN and DMSP data for a dayside event on Feb. 20, 1995, when northward IMF conditions prevailed. We conclude that HF radar D‐P observations can be used to map in real time the dayside 2D ionospheric footprint of the outer LLBL.