Background Convection Research Articles

Auroral Bead Propagation: Explanation Based on the Conservation of Vorticity

AbstractThe beading of auroral arcs often takes place at substorm onset. It is known that auroral beads propagate more often eastward than westward at several km/s, which is difficult to explain by existing models. We investigate this issue observationally and theoretically. First, based on previous research and additional statistical analysis, we suggest that (a) auroral beads often propagate eastward in the presence of westward background convection, and (b) background ionospheric convection may be better represented by large‐scale convection for westward propagation, and by meso‐scale convection for eastward propagation. Then we model auroral beads as vortices of ionospheric flow, and consider the longitudinal propagation of their meridional displacement based on the conservation of vorticity. Here it is crucial that the background zonal flow has vorticity (i.e., flow shear) changing with latitude. It is found that the wave propagates either parallel or anti‐parallel to the background flow depending on whether the background vorticity increases or decreases in latitude, and if its latitudinal scale is significantly smaller than the longitudinal wavelength, the phase velocity exceeds the background flow speed. The result suggests that the latitudinal structure of the background flow is crucial for the bead propagation. More specifically, the aforementioned feature (a) implies that the zonal flow associated with eastward propagation is confined in latitude, which may correspond to the preonset approach of mesoscale flows. In contrast, the large‐scale ionospheric flow suggested for westward propagation as described in (b) may correspond to the global convection of the conventional growth phase.

Pacific Decadal Oscillation Modulates the Relationship Between Pacific Meridional Mode and Tropical Cyclone Genesis in the Western North Pacific

AbstractPacific Meridional Mode (PMM) is known to be significantly correlated with tropical cyclone (TC) genesis over the western North Pacific (WNP), while the stability of their relationship remains unknown. Here we found that their relationship is nonstationary, which depends on the phase of Pacific Decadal Oscillation (PDO). During the PDO warm phases, the PMM‐emanated cyclonic circulation and ascending motion can propagate to the entire WNP due to the enhanced background convection. In contrast, during the PDO cold phases, the PMM‐resulted cyclonic circulation and ascending motion are confined to the eastern WNP, while the compensated descending motion prevails in the western WNP. Accordingly, the PMM‐induced consistent (inconsistent) changes in large‐scale conditions across the western and eastern WNP act to strengthen (weaken) the relationship between the PMM and WNP TC genesis during the PDO warm (cold) phases. The result provides further guidance for improving seasonal prediction of TC genesis.

Diversity of the Madden–Julian Oscillation: Initiation Region Modulated by the Interaction between the Intraseasonal and Interannual Variabilities

AbstractAs one of the aspects of the diversity of the Madden–Julian oscillation (MJO), the modulation of initiation regions of the boreal-winter MJO is studied in terms of the relationship between intraseasonal and interannual variabilities. MJOs are categorized as those initiating in the Indian Ocean (IO), Maritime Continent (MC), and western Pacific (WP), referred to herein as IO-MJOs, MC-MJOs, and WP-MJOs, respectively. The composite analyses for each MJO category using observational data reveal that the diversity of MJO initiation regions directly results from the modulation of areas where horizontal advective premoistening efficiently occurs via intraseasonal/synoptic-scale winds. This is supported by the difference in the zonal location of equatorial intraseasonal circulations established before MJO initiation, which is related to a spatial change in background convection and associated Walker circulations forced by interannual sea surface temperature (SST) variability. Compared to IO-MJOs (favored in the climatological background on average), MC-MJOs tend to be realized under the eastern-Pacific El Niño–like condition, as a result of eastward-shifted intraseasonal convection and circulation patterns induced by background suppressed convection in the eastern MC. WP-MJOs are frequently initiated under the central-Pacific El Niño–like and positive IO dipole–like conditions, in which the WP is selectively moistened with the aid of background enhanced (suppressed) convection over the WP (the southeastern IO and the central-to-eastern Pacific). This major tendency derived from sample-limited observations is verified by a set of 15-yr numerical experiments with a global nonhydrostatic MJO-permitting model under a perpetual boreal-winter condition where observation-based SSTs are prescribed.

Second-Harmonic Contactless Method for Measurement of RMS Current Using a Standard Infrared Camera

This article presents a new method for measuring rms values of alternative current (ac) in power lines using an infrared (IR) camera. An IR camera registers thermal images in time with an appropriate frame rate in order to calculate the spectrum of the temperature signal of a power cable. For the European standard, the amplitudes of the harmonics for $f \approx ~100$ Hz correlate to the power dissipated in a conductor. The fast Fourier transform (FFT) and rms analysis for different current values were performed. The obtained results allow plotting the calibration curve of temperature changes versus the rms values of the current flowing in wire. They confirmed the linear relation between the 100-Hz spectral component of the temperature and the square value of the rms current. This method has unique advantages. It is noncontact and makes it possible to estimate the rms current regardless of the ambient background radiation and convection cooling conditions having a significant impact on overhead lines, in particular.

Kinetics of motile solitons in nematic liquid crystals

The generation of spatially localized, soliton-like hydrodynamic disturbances in microscale fluidic systems is an intriguing challenge. Herein, we introduce nonequilibrium solitons in nematic liquid crystals stimulated by an electric field. These dynamic solitons are robust as long as the electric field is maintained. Interestingly, their kinetic behaviours depend on the field condition—Tuning of the amplitude and frequency of the applied electric field alters the solitons to self-assemble into lattice ordering like physical particles or to command them to various dynamic states. Our key property to the realisation is the electrohydrodynamic instability due to the coupling between the fluid elasticity and the background convection. This paper describes a new mechanism for realising dynamic solitons in fluid systems on the basis of the electrohydrodynamic phenomena.

Polar Ionospheric Large‐Scale Structures and Dynamics Revealed by TEC Keogram Extracted From TEC Maps

AbstractA method named total electron content (TEC) keogram is introduced for surveying the large‐scale irregularities continuously in the polar ionosphere. The TEC keogram is developed from a movie of TEC maps along various meridian lines from the dayside to the nightside across the magnetic pole, trying to identify and track several types of ionospheric structures. Through two examples, a clear train of polar cap patches are identified from TEC keogram and confirmed by SuperDARN radar observations. The motion speed of these patches estimated from this tool agrees with SuperDARN radar measurements. Then, the motions of patches relative to the background convection through the whole polar cap are statistically studied for the first time. Moreover, the occurrence dependence of fully tracked patches on months, UT hours, and interplanetary magnetic field conditions is generally consistent with previous reports. These results suggest that the TEC keogram offers a power tool for continuous monitoring and studying of large‐scale plasma irregularities in the polar ionosphere.

Spatial Characteristics of Mesoscale Plasma Flow Perturbations and Accompanying Electron Precipitation in the High‐Latitude Ionosphere

AbstractThe spatial characteristics of mesoscale plasma perturbations in the high‐latitude ionosphere are important considerations for a more complete description of the energy deposition from the magnetosphere. In this study, ion drift and particle precipitation measurements from the Defense Meteorological Satellite Program F17 satellite during local summer seasons in 2012 are used to examine the flow perturbations and their closure paths and the particle precipitation associated with them. The closed circulation associated with a flow perturbation is identified from the potential distribution and can be described by a single‐cell or a two‐cell configuration with characteristic spatial scale sizes between 100 and 600 km and 200 and 1,200 km, respectively. Observations suggest that an asymmetry in space and flow speed is frequently seen with faster flows in a narrower region and slower return flows in wider adjacent regions. For a single‐cell configuration, the faster flows are preferentially sunward and in the same direction as the background convection, and the weaker return flows are preferentially duskward/dawnward of the potential maxima/minimum. For a two‐cell configuration, the faster flow is seen in the central region and also tends to follow the background convection with weaker return flows on the two sides. Except in the region poleward of the convection reversal boundary, 0.5–3 keV electron precipitation, displaying the same spatial asymmetry as the flow perturbation, is frequently seen in a single cell around a potential minimum on the dusk side.

Kinetic Analysis of the Energy Transport of Two‐Flow Components in the Plasma Sheet

AbstractSatellite observations indicate that multiple components often exist in the plasma sheet, particularly during impulsive fast flow events. In this paper, we perform a kinetic analysis of the energy transport of plasma sheet ion flow using a model of two‐component plasma sheet ion flows (Background convection flow represented by subscript “b” and Fast flow represented by subscript “f”) and compare the energy transport calculated by kinetic approach Qk with those obtained from magnetohydrodynamic (MHD) approach QMHD. The ratio of Qk/QMHD is always larger than unity and is positively proportional to the ratios of Vf/Vb and Tf/Tb. The maximum values of Qk/QMHD occur in the low‐speed ranges (i.e., small density ratio Nf/N). When Nf/N exceeds 0.4, the ratio of Qk/QMHD is almost the same for a wide parameter ranges of Vf, Tf, and Vb. Heat flux is important in low‐speed range and is neglectable in the high‐speed range. The adiabatic polytropic index 5/3 cannot correctly describe energy transport rate. A density ratio Nf/N of 0.3% of high‐speed ion flow can make the effective polytropic index obviously deviate from adiabatic polytropic index (5/3). The above theoretic results can well explain previously reported satellite in situ observations.

Velocity Rotation Events in the Outer Magnetosphere Near the Magnetopause

AbstractWe report velocity rotation events with amplitudes more than a few tens of kilometers per second observed by the Magnetospheric Multiscale mission. The events are detected in the outer magnetosphere near the magnetopause at approximately noon magnetic local time. When we examine one event, velocity and magnetic field variations are correlated. Accelerated electrons >100 eV are present. The azimuthal propagation direction is opposite to that of the background convection, implying that the structure and the background have different sources. The magnetic field variation can be approximately fitted to that of the line current. It is inferred that bundles of magnetic field lines with the same helicity propagate with the Alfvén velocity along the background magnetic field. One possible source is a flux transfer event. In addition, the variation in the solar wind dynamic pressure may also affect the velocity rotation. We have found 18 events during the first 2 years of Magnetospheric Multiscale operations. The derived properties are generally similar to those from the event study. For some events, we have identified a flux transfer event as a possible source. Again, the dynamic pressure variation may be the cause as well. The events reported resemble in some respects crewcuts and midday auroral brightening sequence, as measured by ground‐based optical instruments.

Mesoscale Plasma Convection Perturbations in the High‐Latitude Ionosphere

AbstractAn investigation of flow perturbations with spatial scale sizes between 100 and 500 km in the high‐latitude ionosphere is presented. These localized flow perturbations are deviations from the large‐scale background convection, expected to give us new insights into the magnetosphere‐ionosphere coupling process. Ion drift measurements from the Defense Meteorological Satellite Program F17 are utilized to identify these mesoscale flow perturbations. Our intent is to discover the properties of these perturbations in terms of perturbation flow speeds, location, scale size, and occurrence frequency as well as their dependence on the interplanetary magnetic field (IMF) and underlying large‐scale convection pattern. Observation suggests that flow perturbation locations strongly depend on the IMF orientation as does the occurrence frequency of the flow perturbations. For southward IMF, more flow perturbations occur in regions of sunward background flow than in regions of antisunward background flow. For flow perturbations with speeds over 300 m/s, an asymmetry in the preferred direction and scale size is seen for those embedded in sunward and antisunward background flows. Significantly less asymmetry is present for flow perturbations with speeds between 100 and 300 m/s. The flow perturbations exceeding 300 m/s are most likely closed locally with lower magnitude return flows or with adjacent flows across the convection reversal boundary and representing additional sources of frictional heating and momentum transfer to the thermosphere. The perturbation flow speed is almost independent of the scale size and underlying convection speed, but the largest speeds are preferentially seen at scale sizes between 200 and 300 km.

Statistical Properties of Mesoscale Plasma Flows in the Nightside High‐Latitude Ionosphere

AbstractThe field of space weather has made great strides in understanding and simulating large‐scale, global responses of the Earth's upper atmosphere to various inputs from the Sun; however, the mesoscale phenomena (~30–500 km wide) that are much more dynamic and powerful in the coupled system have remained uncharacterized. We therefore present key parameters of mesoscale ionospheric plasma flows in order to evaluate their relationship within the coupled system and to inform global models. We characterize mesoscale plasma flow properties in the nightside polar cap and auroral oval using nine years of Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight velocity data from the stations at Rankin Inlet and Saskatoon. We quantify their width, velocity, occurrence rates, duration, and orientation, and how these characteristics depend on latitude, magnetic local time (MLT), season, substorm activity (AL), solar cycle (F10.7), and interplanetary magnetic field (IMF) clock angle. Measuring the ionospheric footpoint of magnetospheric fast flows, our analysis technique from the ground also provides a 2‐D picture of flows and their characteristics that spacecraft alone cannot provide. Results include a characteristic equatorward flow width of ~180 km in the polar cap and ~140–150 km in the auroral oval. Flow velocities vary under the different conditions we studied. Equatorward polar cap flows and poleward flows everywhere have a postmidnight preference, suggesting a connection to polar cap arcs. Equatorward auroral oval flows have a premidnight preference, suggesting a connection to substorm‐related phenomena. The location and orientation of mesoscale flows dependent on IMF clock angle suggests that mesoscale flows follow the large‐scale background convection.

RCM‐E simulation of bimodal transport in the plasma sheet

Plasma sheet transport is bimodal, consisting of both large-scale adiabatic convection and intermittent bursty flows in both earthward and tailward directions. We present two comparison simulations with the Rice Convection Model—Equilibrium (RCM-E) to investigate how those high-speed flows affect the average configuration of the magnetosphere and its coupling to the ionosphere. One simulation represents pure large-scale slow-flow convection with time-independent boundary conditions; in addition to the background convection, the other simulation randomly imposes bubbles and blobs through the tailward boundary to a degree consistent with observed statistical properties of flows. Our results show that the bursty flows can significantly alter the magnetic and entropy profiles in the plasma sheet as well as the field-aligned current distributions in the ionosphere, bringing them into much better agreement with average observations.

Implications of a heuristic model of auroral Farley Buneman waves and heating

The global implications, particularly with respect to altitude dependence, of the heuristic model of Farley Buneman waves put forward initially by Milikh and Dimant (2002) are studied. This model prescribes a relationship between the background convection electric field that excites the waves and the transverse electric fields of the waves that grow in response. It also prescribes the magnetic aspect angle of the waves, which is related to their ability to heat the auroral E region. The prescription is based on the condition of marginal stability. We reformulate the basic model, which is local, and embed it in the SAMI2 ionospheric model, which includes wave and Joule heating, heat transport, cooling, temperature‐dependent collisions, and related chemistry. Within the limits of its underlying assumptions, the combined model can be used to predict the phase‐speeds and magnetic‐aspect widths of Farley Buneman waves in the auroral zone and the heating they can cause, all as functions of altitude. Model predictions are compared with experimental results, and the efficacy of the model assessed. This modeling exercise highlights the importance of the thickness of the layer in which Farley Buneman waves exist, the strong variations in wave characteristics across the layer, and the consequences this has for coherent scatter radar measurements of the phenomenon.

Two‐dimensional ionospheric flow pattern associated with auroral streamers

We present direct two‐dimensional flow observations and auroral images from the Super Dual Auroral Radar Network (SuperDARN) radar and Wideband Imaging Camera onboard Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE/WIC) imager to investigate the ionospheric flow pattern associated with auroral streamers for four events. Using the SuperDARN observations allows us to observe the flow associated with the streamers in the context of the larger‐scale background convection. For all four cases studied, streamers developed from preexisting east‐west (EW) arcs that initiated from bead‐like or small‐scale auroral intensifications around the auroral poleward boundary. We find a vortex‐like flow pattern with clockwise sense surrounding both the initial auroral forms and the ensuing streamers. We also found in three cases signatures of flow bifurcations at the equatorward end of the streamers, which suggests a double vortex‐like flow pattern with the counterclockwise vortex east of the clockwise one. Our results are consistent with the double‐vortex flow structure that results from interchange convection associated with a plasma sheet bubble predicted by different numerical simulations. This supports several previous studies and offers more complete observational evidence that the observed flow pattern may be the ionospheric manifestation of interchange instability in the plasma sheet.

Climatology and Interannual Variability of Convectively Coupled Equatorial Waves Activity

Abstract Climatology and interannual variability of convectively coupled equatorial wave (CCEW) activity, including the mixed Rossby–gravity (MRG), tropical-depression-type (TD-type), equatorial Rossby (ER), and Kelvin waves, are investigated using the satellite-observed brightness temperature data from the Cloud Archive User Service. The monthly activity of CCEWs is represented by the root mean square of the daily filtered convections in each month based on the Wheeler–Kiladis filtering method. More precise seasonal cycles of CCEW activity are obtained from the meridional and zonal mean climatology. Interannual variance of CCEW activity is further investigated. Kelvin wave activity has maximum interannual variance over the eastern Pacific, while the other three waves are most variable in the intertropical convergence zone. The four active CCEWs all have significant correlation with the background convection and local sea surface temperature (SST) over the central and eastern Pacific, but they are not significantly correlated over other regions. The El Niño events may induce more trapped and active CCEWs over the central and eastern Pacific but weaker MRG and TD-type waves over the warm pool. In contrast, the El Niño Modoki has much weaker correlation with CCEW activity. CCEW activity over the southeastern Indian Ocean is negatively correlated with the Indian Ocean dipole, while that over the western and northern Indian Ocean may be determined by atmospheric internal disturbances. The tropical southern Atlantic mode is the strongest Atlantic SST anomaly mode correlated with the Atlantic CCEW activity.

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

[1] Fast azimuthal auroral expansion and poleward expansion are characteristic features of the expansion phase of substorms. In the first study of its kind, we have investigated the azimuthal auroral expansion and its magnetospheric counterpart using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers and multiple spacecraft. During the tail season in 2008–2009, we found 16 events of azimuthally expanding aurora that passed near the magnetic footprints of the multiple spacecraft operating in the near-Earth plasma sheet. In the magnetosphere, these events commonly showed fast azimuthal and earthward flows associated with intense electric fields and magnetic dipolarization. The speed of the propagating structure, which was estimated from the time difference of the depolarization observed by the multiple spacecraft, was close to the measured azimuthal plasma flow velocity. We also found that this azimuthal plasma transport was dominated by the E × B drift speed associated with the enhanced electric field. In a statistical analysis, the averaged speeds of the leading edge of the westward and eastward auroral expansion were 8.8 and 5.3 km/s, respectively. When mapped onto the equatorial magnetosphere, these speeds (267 and 162 km/s) were comparable to the averaged azimuthal plasma (E × B) flow speeds observed by the spacecraft, which were 193 (239) km/s in the westward direction and 112 (139) km/s in the eastward direction. Our events showed that E × B flows and auroral expansion predominantly propagated westward, indicating an effect of westward background convection in the Harang flow shear. From these results, we concluded that the azimuthal auroral expansion was closely related to magnetic dipolarization which expanded azimuthally at the E × B drift speed. On the basis of the abrupt formation of the fast E × B flows and their propagation away from the onset location, we suggest that the effects of the intense large-scale electric fields, which are possibly generated through substorm onset turbulence, propagate to the ionosphere along the magnetic field lines and lead to azimuthal expansion of an auroral arc.

On the relationship between thin Birkeland current arcs and reversed flow channels in the winter cusp/cleft ionosphere

In this paper we study reversed flow events (RFEs) that seem regulated by Birkeland current arcs in the winter cusp ionosphere above Svalbard. An RFE is a longitudinally elongated, 100–200 km wide channel, in which the flow direction is opposite to the background convection, persisting for 10–20 min. The RFE onset occurs with the brightening of a discrete arc near the open‐closed boundary. The auroral arc is situated exactly at a sharp clockwise flow reversal, consistent with a converging electric field and an upward field‐aligned current. One category of RFEs propagates into the polar cap in tandem with poleward moving auroral forms, while another category of RFEs moves with the cusp/cleft boundary. The RFE phenomenon is addressed to a region void of electron precipitation, and in lack of direct sunlight the E‐region conductivity will be very low. We propose two possible explanations: (1) the RFE channel may be a region where two MI current loops, forced by independent voltage generators, couple through a poorly conducting ionosphere and (2) the reversed flow channel may be the ionospheric footprint of an inverted V‐type coupling region. Electron beams of <1 keV will not give rise to significant conductivity gradients, and the form of a discontinuity in the magnetospheric electric field will be conserved when mapped down to the ionosphere, although reduced in amplitude. These two explanations may be related in the sense that the boundary discontinuity in the magnetospheric electric field in (1) may be the driver for the inverted V in (2).

A 2-D comparison of ionospheric convection derived from SuperDARN and DMSP measurements

Ion drift vectors measured by the DMSP satellites are compared with plasma convection vectors obtained by the SuperDARN HF radars through the standard Map Potential algorithm of Ruohoniemi and Baker [Ruohoniemi, M., Baker, K.B. Large-scale imaging of high-latitude convection with super dual auroral radar network HF radar observations. J. Geophys. Res. 103, 20797–20811, 1998]. Despite significant data spread, the agreement can be qualified as reasonable for a data set comprising of 149 satellite passes over the Northern Hemisphere at high latitudes. The slope of the best-fit line relating SuperDARN and DMSP velocity magnitudes is of the order of 0.3 with a tendency for the SuperDARN velocities to be smaller. The agreement between the azimuths of the ion drift and convection is better with the slope of the best-fit line being close to 1. It is shown that consistency between the radar and satellite measurements is much better if the SuperDARN line-of-sight velocities are compared with the DMSP cross-track ion drifts for events showing slow spatial and temporal variations of the convection. If areas of strong convection changes are included into comparison, the degree of agreement deteriorates drastically. This result implies that differences in the spatial and temporal resolutions of DMSP and SuperDARN measurements are crucial factors contributing to the observed discrepancies. In addition, some differences are introduced when the SuperDARN line-of-sight velocities are filtered and reprocessed into vectors with the application of a background convection model.

Ballooning instability at the plasma sheet–lobe interface and its implications for polar arc formation

Huang et al. (1987, 1989) reported hot filaments of plasma sheet origin filling the magnetospheric lobes during northward interplanetary magnetic field (IMF). On the other hand, cold plasma transients of presumably lobe origin are often observed in the plasma sheet. These features can be interpreted in terms of plasma exchange at the plasma sheet–lobe interface (PSLI) proceeding in a filamentary manner. We present a description of this process within ballooning destabilizing of the near‐Earth curved segment of the PSLI. Although the basic ballooning instability condition is not typically met inside the plasma sheet, it is satisfied at this segment. The PSLI always separates the cold lobe population from the hot plasma sheet, thus providing a pressure gradient favorable for the instability; its near‐Earth part has a nonnegligible magnetic curvature. A solution for the least stable ballooning harmonics is found, which satisfies the finite conductivity boundary condition in the ionosphere and the outgoing Alfvén wave condition at the tailward end of the near‐Earth curved segment of the plasma sheet boundary. We show that this part of the PSLI may be a generator region launching filamentation. The background convection is imposed on the ballooning motions. The large‐scale convection associated with southward IMF Bz suppresses hot filament progression into the lobes, while promoting penetration of lobe transients into the plasma sheet. However, during northward IMF, the convection favors at certain magnetic local times the extension of plasma sheet filaments into the lobes and their subsequent protrusion toward noon. This process is signified in the ionosphere by the occurrence of nightside originating polar arcs. Several polar arc events are shown that develop from the nightside oval boundary into the polar cap on timescales of approximately 10–15 min, consistent with the growth rates of the studied instability.

Auroral poleward boundary intensifications (PBIs): Their two‐dimensional structure and associated dynamics in the plasma sheet

Auroral poleward boundary intensifications (PBIs) are typically seen both in ground meridian scanning photometers (MSP) and in ground and spacecraft auroral images. They appear as a localized increase in intensity at or near the magnetic separatrix. This increase is often seen to extend equatorward through the ionospheric mapping of the plasma sheet. PBIs are associated with plasma sheet flow bursts and are thus important for the remote monitoring of plasma sheet dynamics. From the study of simultaneous ground MSP observations, IMAGE FUV auroral images, and Geotail plasma sheet data, we find that PBIs correlate well with plasma sheet fast flows observed within the local time sector of the PBIs and that there can be several PBIs over the longitudinal range of fast flows in the tail. We infer that every north‐south PBI is the ionospheric signature of a fast flow channel in the plasma sheet and that many fast flow channels exist simultaneously over a width of the plasma sheet that can comprise the whole width of the plasma sheet or only a part of it. Also, we find that there is a local time dependence on the type of PBI structure. Most PBIs are narrow auroral structures that are not strictly north‐south oriented. Instead, PBIs are tilted counterclockwise away from the north‐south direction, leading to a preferred orientation that is approximately aligned with a line going from 0300 magnetic local time (MLT) to 1700 MLT. This results in PBIs that are closer to north‐south (NS) structures in the postmidnight sector and closer to east‐west (EW) near the dusk sector. In the premidnight sector (2200–0000 MLT), PBIs start as EW arcs and then tilt and become primarily NS structures. We further found for one event that the PBI fast flows have a large Vy component resulting in tail convection that is both earthward and dawnward in the region of Geotail. We suggest that the continuous and strongly positive interplanetary magnetic field (IMF) By may order the two‐dimensional convection as observed, thus offering a possible explanation for the alignment direction of PBIs in the ionosphere under the assumption that fast flow channels align themselves with the background convection. However, the projection of the PBI structures into the tail using the T96 model suggests that all PBIs, both EW and NS, map to radially stretched channels in the tail that do not have a significant dawnward component. Future work is needed to clarify this apparent contradiction. Finally, frequency analysis indicates that the PBI/bursty bulk flow (BBF) period is characterized by oscillations in the velocity and magnetic field with frequencies of ∼0.6 mHz and ∼1.3–1.5 mHz. This oscillation in velocity is superposed on the background strong convection.