Dusk Cell Research Articles

SuperDARN Observations of the Two Component Model of Ionospheric Convection

AbstractWe use a 20 years database of Super Dual Auroral Radar Network (SuperDARN) observations to investigate the two component model of ionospheric convection. A convection pattern is included in the database if it is derived from at least 250 radar vectors and has a distribution of electric potential consistent with Dungey‐cycle twin vortex flow (a negative potential peak in the dusk cell and a positive potential peak in the dawn cell). We extract the locations of the foci of the convection cells from the SuperDARN convection patterns, and compare their dependencies on the north‐south component of the interplanetary magnetic field, IMF BZ, and the SuperMAG auroral electrojet index, SML. We use these parameters to define intervals of expected dayside or nightside dominated reconnection. Our results show that, under conditions favorable for dominant dayside reconnection, the dawn and dusk foci are shifted toward the dayside and that, under conditions favorable for dominant nightside reconnection, the dawn and dusk foci are shifted toward the nightside.

Duskside Sub‐Auroral Polarization Streams (SAPS) and Dawnside Subauroral Flows During the Magnetically Quiet 24 November and Moderately Active 25–27 November 2008

AbstractThis study investigates the coupled solar wind‐magnetosphere‐ionosphere (SW‐M‐I) system during the magnetically quiet (Kp < 3) 24 November and moderately active (Kp ≤ 4‐) 25–27 November 2008. Its main characteristics specified include the (a) antisunward propagating solar‐wind Alfven waves, (b) oscillating Interplanetary Magnetic Field (IMF) leading to undershielding at southward IMF and to overshielding and a three‐cell (dusk, dawn, reverse) polar convection pattern at northward IMF, (c) Kelvin‐Helmholtz (K‐H) waves on the magnetopause, and (d) subauroral plasma flows on the duskside and dawnside. These include the westward Sub‐Auroral Polarization Streams (SAPS) streaming sunward along the dusk cell and the eastward subauroral flows streaming sunward along the dawn cell and antisunward along the dawnside reverse cell. These subauroral flows were driven by the combination of their underlying polarization electric (E) field and unshielded penetration E field (PEF) or shielding E field. While the subauroral flows became mostly enhanced by the combined polarization and PEFs, the duskside SAPS flows were suppressed by the shielding E field and structured by the K‐H waves. From these findings we conclude that (i) the above‐describe conditions (a–d) promoted the development of duskside and dawnside subauroral polarization E fields, (ii) the resultant duskside SAPS and dawnside subauroral flows followed the convection streams evidencing a close association with the polar convection pattern, and (iii) the SAPS flows became structured by the K‐H waves.

Average Ionospheric Electric Field Morphologies During Geomagnetic Storm Phases

AbstractWe utilize principal component analysis to identify and quantify the primary electric potential morphologies during geomagnetic storms. Ordering data from the Super Dual Auroral Radar Network (SuperDARN) by geomagnetic storm phase, we are able to discern changes that occur in association with the development of the storm phases. Along with information on the size of the patterns, the first six eigenvectors provide over ∼80% of the variability in the morphology, providing us with a robust analysis tool to quantify the main changes in the patterns. Studying the first six eigenvectors and their eigenvalues shows that the primary changes in the morphologies with respect to storm phase are the convection potential enhancing and the dayside throat rotating from pointing toward the early afternoon sector to being more sunward aligned during the main phase of the storm. We find that the ionospheric electric potential increases through the main phase and then decreases once the storm phase begins. The dayside convection throat points toward the afternoon sector before the main phase and then as the potential increases throughout the main phase, the dayside throat rotates toward magnetic noon. Furthermore, we find that a two‐cell convection pattern is dominant throughout and that the dusk cell is overall stronger than the dawn cell.

Modeled IMF By Effects on the Polar Ionosphere and Thermosphere Coupling

AbstractThere is still an inadequate understanding of how the interplanetary magnetic field (IMF) east‐west component (By) affects thermospheric composition, and other ionospheric and thermospheric fields in a systematic way. Utilizing the state‐of‐art first‐principles Coupled Magnetosphere Ionosphere Thermosphere (CMIT) modeling and TIMED/Global Ultraviolet Imager (GUVI)‐observed ΣO/N2 covering an entire solar cycle (year 2002–2016), as well as a neutral parcel trajectory tracing technique, we emphasize that not only the direction of By, but also its strength relative to the IMF north‐south component (Bz) that has important effects on high latitude convection, Joule heating, electron density, neutral winds, and neutral composition patterns in the upper thermosphere. The Northern Hemisphere convection pattern becomes more twisted for positive By cases than negative cases: the dusk cell becomes more rounded compared with the dawn cell. Consequently, equatorward neutral winds are stronger during postmidnight hours in negative By cases than in positive By cases, creating a favorable condition for neutral composition disturbances (characterized by low ΣO/N2) to expand to lower latitudes. This may lead to a more elongated ΣO/N2 depletion area along the morning‐premidnight direction for negative By conditions compared with the positive By conditions. Backward neutral parcel trajectories indicate that a lower ΣO/N2 parcel in negative By cases comes from lower altitudes, as compared with that for positive By cases, leading to larger enhancements of N2 in the former case.

Scintillation and irregularities from the nightside part of a Sun‐aligned polar cap arc

AbstractIn this paper we study the presence of irregularities and scintillation in relation to the nightside part of a long‐lived, Sun‐aligned transpolar arc on 15 January 2015. The arc was observed in DMSP UV and particle data and lasted at least 3 h between 1700 and 2000 UT. The arc was more intense than the main oval during this time. From all‐sky imagers on Svalbard we were able to study the evolution of the arc, which drifted slowly westward toward the dusk cell. The intensity of the arc as observed from ground was 10–17 kR in 557.7 nm and 2–3.5 kR in 630.0 nm, i.e., significant emissions in both green and red emission lines. We have used high‐resolution raw data from global navigation satellite systems (GNSS) receivers and backscatter from Super Dual Auroral Radar Network (SuperDARN) radars to study irregularities and scintillation in relation to the polar cap arc. Even though the literature has suggested that polar cap arcs are potential sources for irregularities, our results indicate only very weak irregularities. This may be due to the background density in the northward IMF polar cap being too low for significant irregularities to be created.

Dynamic effects of restoring footpoint symmetry on closed magnetic field lines

AbstractHere we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive y component of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large‐scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside.

The Harang reversal and the interchange stability of the magnetotail

AbstractThe present study addresses steady convection in the plasma sheet in terms of the interchange stability with special attention to the Harang reversal. The closure of the tail current with a field‐aligned current (FAC) results from the divergence/convergence of the pressure gradient current. If the magnetotail is in a steady state, the associated change of local plasma pressure p has to balance with its advective change. Accordingly, for adiabatic transport, the flux tube entropy parameter pVγ increases and decreases along the convection path in regions corresponding to downward and upward FACs, respectively. This requirement, along with the condition for the interchange stability imposes an important constraint on the direction of convection especially in the regions of downward FACs. It is deduced that for the dusk cell, the convection in the downward R2 current has to be directed azimuthally duskward, which follows the sunward, possibly dawnward deflected, convection in the region of the premidnight upward R1 current. This duskward turn of convection takes place in the vicinity of the R1‐R2 demarcation, and it presumably corresponds to the Harang reversal. For the dawn cell the convection in the postmidnight downward R1 current has to deflect dawnward, and then it proceeds sunward in the upward R2 current. The continuity of the associated ionospheric currents consistently reproduces the assumed FAC distribution. The proposed interrelationships between the convection and FACs are also verified with a quasi‐steady plasma sheet configuration and convection reproduced by a modified Rice Convection Model with force balance.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector

Abstract. The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

Large plasma density enhancements occurring in the northern polar region during the 6 April 2000 superstorm

AbstractWe focus on the ionospheric response of northern high‐latitude region to the 6 April 2000 superstorm and aim to investigate how the storm‐enhanced density (SED) plume plasma became distributed in the regions of auroral zone and polar cap plus to study the resultant ionospheric features and their development. Multi‐instrument observational results combined with model‐generated, two‐cell convection maps permitted identifying the high‐density plasma's origin and the underlying plasma transportation processes. Results show the plasma density feature of polar cap enhancement (PCE; ~600 × 103 i+/cm3) appearing for 7 h during the main phase and characterized by increases reaching up to 6 times of the quiet time values. Meanwhile, strong westward convections (~17,500 m/s) created low plasma densities in a wider region of the dusk cell. Oppositely, small (~750 m/s) but rigorous westward drifts drove the SED plume plasma through the auroral zone, wherein plasma densities doubled. As the SED plume plasma traveled along the convection streamlines and entered the polar cap, a continuous enhancement of the tongue of ionization (TOI) developed under steady convection conditions. However, convection changes caused slow convections and flow stagnations and thus segmented the TOI feature by locally depleting the plasma in the affected regions of the auroral zone and polar cap. From the strong correspondence of polar cap potential drop and subauroral polarization stream (SAPS), we conclude that the SAPS E‐field strength remained strong, and under its prolonged influence, the SED plume provided a continuous supply of downward flowing high‐density plasma for the development and maintenance of PCEs.

Different responses of northern and southern high latitude ionospheric convection to IMF rotations: a case study based on SuperDARN observations

Abstract. We use SuperDARN data to study high-latitude ionospheric convection over a three hour period (starting at 22:00 UT on 2 January 2003), during which the Interplanetary Magnetic Field (IMF) flipped between two states, one with By>>|Bz| and one with Bz>0, both with negative Bx. We find, as expected from previous works, that day side ionospheric convection is controlled by the IMF in both hemispheres. For strongly northward IMF, we observed signatures of two reverse cells, both in the Northern Hemisphere (NH) and in the Southern Hemisphere (SH), due to lobe reconnection. On one occasion, we also observed in the NH two viscous cells at the sides of the reverse cell pair. For duskward IMF, we observed in the NH a large dusk clockwise cell, accompanied by a smaller dawn cell, and the signature of a corresponding pattern in the SH. On two occasions, a three cell pattern, composed of a large clockwise cell and two viscous cells, was observed in the NH. As regards the timings of the NH and SH convection reconfigurations, we find that the convection reconfiguration from a positive Bz dominated to a positive By dominated pattern occurred almost simultaneously (i.e. within a few minutes) in the two hemispheres. On the contrary, the reconfiguration from a By dominated to a northward IMF pattern started in the NH 8–13 min earlier than in the SH. We suggest that part of such a delay can be due to the following mechanism: as IMF Bx<0, the northward-tailward magnetosheath magnetic field reconnects with the magnetospheric field first tailward of the northern cusp and later on tailward of the southern cusp, due to the IMF draping around the magnetopause.

IMF dependence of high-latitude thermospheric wind pattern derived from CHAMP cross-track measurements

Abstract. Neutral thermospheric wind pattern at high latitudes obtained from cross-track acceleration measurements of the CHAMP satellite above both North and South polar regions are statistically analyzed in their dependence on the Interplanetary Magnetic Field (IMF) direction in the GSM y-z plane (clock angle). We compare this dependency with magnetospheric convection pattern obtained from the Cluster EDI plasma drift measurements under the same sorting conditions. The IMF-dependency shows some similarity with the corresponding high-latitude plasma convection insofar that the larger-scale convection cells, in particular the round-shaped dusk cell for ByIMF+ (ByIMF−) conditions at the Northern (Southern) Hemisphere, leave their marks on the dominant general transpolar wind circulation from the dayside to the nightside. The direction of the transpolar circulation is generally deflected toward a duskward flow, in particular in the evening to nighttime sector. The degree of deflection correlates with the IMF clock angle. It is larger for ByIMF+ than for ByIMF− and is systematically larger (~5°) and appear less structured at the Southern Hemisphere compared with the Northern. Thermospheric cross-polar wind amplitudes are largest for BzIMF−/ByIMF− conditions at the Northern Hemisphere, but for BzIMF−/ByIMF+ conditions at the Southern because the magnetospheric convection is in favour of largest wind accelerations over the polar cap under these conditions. The overall variance of the thermospheric wind magnitude at Southern high latitudes is larger than for the Northern. This is probably due to a larger "stirring effect" at the Southern Hemisphere because of the larger distance between the geographic and geomagnetic frameworks.

On the MLT distribution of F region polar cap patches at night

Eight years of meridian scanning photometer data from Ny‐Ålesund, Svalbard have been analyzed to study the occurrence of F region polar cap patches at night. In total 333 patches in 43 days were observed to hit the poleward boundary of nighttime auroras which is a unique signature of ongoing tail reconnection. The MLT distribution of patches is smooth and exhibit a bell shaped function symmetric around 23:25 MLT. The symmetry of the patch distribution about midnight indicates that patches populate the morning cell and the dusk cell of polar cap convection at the same probability. About 60% of the patches exit the polar cap from 22–01 MLT, but the entire distribution span from 18:30–04:50 MLT, i.e., nearly the full MLT span where tail reconnection may occur. The patch occurrence statistics presented here is an important new result of relevance to phenomena related to the presence and transport of patches.

High latitude convection based on long-term incoherent scatter radar observations in North America

High latitude ionospheric convection patterns and models are obtained based on a composite analysis of incoherent scatter radar (ISR) observations in the North America sector at Millstone Hill and Sondrestrom (MHS). These observations span a combined latitude range of 55 ∘ ⩽ Λ ⩽ 82 ∘ over time periods starting from 1978 for Millstone Hill and from 1995 for Sondrestrom. These models, regional to the America sector, represent average and steady patterns of the convection as driven by the interplanetary magnetic field (IMF) By and Bz or by Kp and the By direction, and update the existing MHS models. This paper describes the ISR and IMF data processing and modeling technique and discusses convection changes under a variety of conditions. In particular, we have examined By and Bz effects on the electrostatic potential at dawn and dusk, the location of the cells, and the shape of the cells; we also present the convection change in response to magnetic activity. This long-term data set has enabled us to study seasonal variations of the convection. The cross polar cap potential is on average higher in equinox than in solstice. With the advance of season from winter to summer, the convection cells appear to move antisunward, and the axis across the two cells rotates toward earlier local times; meanwhile, for By negative, the crescent-shaped dawn cell and circular dusk cell become well defined with a reduced cross polar cap potential, while for By positive, the cross polar cap potential tends to be higher. The dusk–dawn cell similarity in summer is found to be high for By negative and low for By positive, supportive of the theory of lobe cell generation in summer. Because of the asymmetry of the convection response to By negative and positive, and of seasonal differences in the convection, it is speculated that there exist perceptible hemispheric differences in the cross polar cap potential drop.

IMF effect on ionospheric trough occurrence at equinoxes

Previous observations have shown that there is a relationship between the F region trough and both B z and B y components of the interplanetary magnetic field (IMF). Since IMF governs the polar cap convection, we investigate here if this relationship can be explained by means of polar cap convection. The study is limited to equinox seasons. The poleward and equatorward edges of the trough are determined from satellite tomographic observations and their locations are plotted in magnetic coordinates together with the convection pattern given by Papitashvili and Rich [Papitashvili, V.O., Rich, F.J. High-latitude ionospheric convection models derived from DMSP ion drift observations and parameterized by the IMF strength and direction. J. Geophys. Res. 107, 2002, doi:10.1029/2001JA000264] using IMF measurements coincident with trough observations. The results indicate a close relationship between the troughs and convection. Most of the troughs are seen within the dusk cell and the pattern of trough observations rotates with the convection pattern, when B y changes its sign. More dayside troughs are observed when B z is negative than in the opposite case, i.e. fast convective flow favours the dayside trough occurrence. Nightside troughs are observed more frequently when B y is negative. In both evening and morning sectors the trough is situated close to the edges of convection cells, which partly contradicts previous results showing that the troughs are associated with the convection reversal. It is concluded that plasma convection has an important role in trough generation, although the effect of a strong electric field and other mechanisms like precipitation certainly have a role of their own.

EISCAT observations of plasma patches at sub-auroral cusp latitudes

Abstract. A sequence of 3 patches of high-density (1012 m−3) cold plasma on a horizontal scale-size of 300–700 km was observed near magnetic noon by the EISCAT VHF radar above Svalbard on 17 December 2001. The patches followed a trajectory towards the cusp inflow region. The combination of radar and all-sky observations demonstrates that the patches must have been segmented equatorward of the cusp/cleft auroral display, and hence their properties had not yet been influenced by cusp particle showers and electrodynamics on open flux tubes. The last patch in the sequence was intersected by radio tomography observations, and was found to be located adjacent to a broader region of the same high electron density further south. The patches occurred under moderately active conditions (Kp=3) and the total electron content (TEC) of the high-density plasma was 45 TEC units. The train of patches appeared as a segmentation of the tongue of ionization. The sequence of patches occurred in association with a sequence of flow bursts in the dusk cell return flow. It is proposed that reconnection driven pulsed convection is able to create sub-auroral patches in the region where high density mid-latitude plasma is diverted poleward toward the cusp. It is the downward Birkeland current sheet located at the equatorward boundary of the flow disturbance that represents the actual cutting mechanism.

Dependence of neutral winds on convection E‐field, solar EUV, and auroral particle precipitation at high latitudes

We use the Global Ionosphere Thermosphere Model (GITM) to investigate the thermospheric neutral winds and their relationship to the high‐latitude ionospheric convection, auroral precipitation, and solar activity. Each simulation is run for 24 hours, allowing an approximate steady state to be reached. Our results show that the duskside vortex and the antisunward polar cap neutral winds are prominent. The interplanetary magnetic field (IMF) southward Bz control of the polar cap winds is evident in both dusk cell and polar cap. When the IMF Bz changes from −1 to −10 nT, at the auroral region both the root of mean square (RMS) of Vn and Vi increase more than 50% at 300 km altitude, the ratio of R2 (RMS(Vn · (E × B))/RMS(E × B)2) increases, while R1 (RMS(Vn)/RMS(E × B)) changes little. In the polar cap region, since the ion flow is in the same direction as the neutral pressure gradient, above around 160 km altitude, ∣Vn∣ is larger than ∣Vi∣. When the F10.7 increases from 70 to 250, in the auroral region, the magnitude of ∣Vn∣ increases close to 20% at 300 km altitude. At lower latitudes, where ∣Vn∣ > ∣Vi∣ and the ions act as a load to the neutral winds, as a consequence of increases in F10.7, the drag on the neutrals becomes greater and therefore slows the neutrals down. The altitude dependent change of R2 is associated with the multiple effects of the F10.7 to the neutral winds, including the increased day‐to‐night pressure gradient and ion drag. The impact of hemispheric power (HP) to the low thermosphere is pronounced. At 150 km altitude, the neutral winds increase 30% in magnitude at auroral latitudes. When HP is raised from 10 to 100 GW, a dawn cell forms, concurrent with an enhancement in the dusk cell. The dawn and dusk cells show an obvious asymmetry both in the average neutral winds and the sensitivity of neutral winds to the HP. It is shown that at 250 km altitude, the magnitude of the southward IMF Bz has a positive correlation with the magnitude of the polar cap winds and the sensitivity increases with F10.7. A negative correlation between the magnitude of Vn and F10.7 with small southward IMF Bz becomes positive when the southward IMF Bz increases. This change is presumably due to the momentum balance between ion drag and advection. The dawn (dusk) convection cell is enhanced when By is negative (positive). In response to the effect of the Coriolis force, the By negative winds are not exactly mirror reflections of the By positive winds. For northward IMF Bz, the neutral winds flow sunward at the highest latitudes near noon. When the northward Bz increases from 10 nT to 20 nT, the distribution of the polar cap potential becomes more symmetric and the sunward neutral flow is enhanced and expanded.

A case study of relationship between substorm expansion and global plasma convection

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.

Maps of precipitation by source region, binned by IMF, with inertial convection streamlines

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 By. This asymmetry matches the SuperDARN convection flows, in which, irrespective of the sign of By, 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. The mantle shape thus matches convection pattern variations. Only ∼25–35% of the dayside open‐closed field line conversion occurs within the particle cusp, with the lower number appropriate to northward IMF. Most closed‐to‐open field line conversion occurs away from noon. Merging is thus active throughout the frontside magnetosphere. Field lines that merge well away from noon do not experience enough particle inflow against the solar wind velocity to produce anything more than a weak, deenergized (mantle) precipitation. The boundary between the dusk cell and dawn cells consistently coincides with one edge of the cusp. IMF By also controls where most of the nightside reconnection occurs. For positive Bz and 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 By < 0 (By > 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.

Astrid-2 and ground-based observations of the auroral bulge in the middle of the nightside convection throat

Abstract. Results concerning the electrodynamics of the nightside auroral bulge are presented based on simultaneous satellite and ground-based observations. The satellite data include Astrid-2 measurements of electric fields, currents and particles from a midnight auroral oval crossing and Polar UVI images of the large-scale auroral distribution. The ground-based observations include STARE and SuperDARN electric fields and magnetic records from the Greenland and MIRACLE magnetometer network, the latter including stations from northern Scandinavia north to Svalbard. At the time of the Astrid-2 crossing the ground-based data reveal intense electrojet activity, both to the east and west of the Astrid-2 trajectory, related to the Polar observations of the auroral bulge but not necessarily to a typical substorm. The energetic electron fluxes measured by Astrid-2 across the auroral oval were generally weak being consistent with a gap observed in the auroral luminosity distribution. The electric field across the oval was directed westward, intensifying close to the poleward boundary followed by a decrease in the polar cap. The combined observations suggests that Astrid-2 was moving close to the separatrix between the dusk and dawn convection cells in a region of low conductivity. The constant westward direction of the electric field across the oval indicates that current continuity was maintained, not by polarisation electric fields (as in a Cowling channel), but solely by localized up- and downward field-aligned currents in good agreement with the Astrid-2 magnetometer data. The absence of a polarisation electric field and thus of an intense westward closure current between the dawn and dusk convection cells is consistent with the relatively weak precipitation and low conductivity in the convection throat. Thus, the Cowling current model is not adequate for describing the electrodynamics of the nightside auroral bulge treated here.Key words. Ionosphere (auroral ionosphere; electric fields and currents; plasma convection)

The UV aurora and ionospheric flows during flux transfer events

Abstract. Far Ultra Violet (FUV) signatures in the polar ionosphere during a period of magnetopause reconnection are compared with ionospheric flows measured in the cusp ‘throat’ and dusk cell by the CUTLASS Hankasalmi HF radar. Regions of peak FUV emission in the 130.4 nm and 135.6 nm range, observed by the Polar spacecraft’s VIS Earth Camera, consistently lie at the turning point of the flows from the dusk cell, poleward into the throat, and at the equatorward edge of the region of high and varied radar spectral-width associated with the cusp. The Equator-S spacecraft was near the magnetopause at the time of the ionospheric observations and geomagnetically conjugate with the region of ionosphere observed by the radar. Flux transfer events (FTEs), suggestive of bursty reconnection between the IMF and geomagnetic fields, were observed by Equator-S prior to and during the periods of high FUV emission. Enhanced poleward ionospheric flow velocities in the polar cusp region, previously shown to be associated with bursty reconnection, consistently lie poleward of the enhanced FUV optical feature. The enhanced optical feature is consistent with the expected position of the largest upward region 1 field-aligned current, associated with electron precipitation, on the dusk edge of the merging gap. The optical feature moves duskward and equatorward during the course of the reconnection sequence, consistent with expansion of the merging line and the polar cap with newly added open magnetic flux by the FTEs. The DMSP F14 spacecraft passed through the enhanced FUV region and measured strong, structured electron precipitation far greater than in the adjacent regions.Key words. Magnetospheric physics (current systems; magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)