Bursts of flows with an equatorward component are seen within the nightside auroral oval (Sergeev et al., 1990; Kauriste et al., 1996; Yeoman & Lühr, 1999), these being the ionospheric counterpart of earthward flow bursts in the plasma sheet (Angelopoulos et al., 1992). Within flow bursts, flux tubes with lower total entropy than the surrounding plasma are brought earthward via interchange motion as seen in both theory (e.g., Pontius & Wolf, 1990; Yang et al., 2011) and observations (e.g., Angelopoulos et al., 1994; Dubyagin et al., 2010; Panov et al., 2010; Xing et al., 2010; Sergeev et al., 2012).

Flow bursts in the ionosphere (often referred to “meso-scale” flow channels) drive convergence of ionospheric currents to the right of their the flow direction. This convergence is related via current continuity to upward field-aligned currents (FACs) that can require magnetic-field-aligned electric fields that energize electrons and form discrete aurora (e.g., Lyons, 1981). This gives rise to intensifications along the poleward boundary of the auroral oval (de la Beaujardière et al., 1994; Lyons et al., 1999; Zesta et al., 2000) that are the most common auroral disturbance and have become referred to as poleward boundary intensifications (PBIs). PBIs can extend equatorward from the poleward boundary of the auroral oval along with the meso-scale flow bursts and become elongated in very roughly the north-south direction forming what are referred to as streamers (Rostoker et al., 1987; Nakamura et al., 1993; Gallardo-Lacourt et al., 2014).

Flow bursts in the ionosphere have also been observed to lead to a variety of other important geomagnetic disturbances as the reach to near the equatorward boundary of the auroral oval, which maps to the inner plasma sheet. These disturbances include thin substorm growth phase aurora arc formation or intensification (Nishimura et al., 2011), substorm onset (e.g., Nishimura et al., 2010), and omega bands (Henderson et al., 2002; Liu et al., 2018). Flow bursts moving earthward/equatorward are also important because they can give particle injections (Gabrielse et al., 2014; Liu et al., 2016), including into the storm-time ring current e.g., Gkioulidou et al. (2014) and radiation belts (e.g., Kim et al., 2021).

Thus the physics of flow bursts as they move earthward within the plasma sheet is of critical importance to understanding geomagnetic activity. Evaluation of flow bursts from an ideal MHD perspective e.g., Birn and Hesse (2013), Birn and Hesse (2014) has yielded much insight into flow burst dynamics in the more distant magnetotail, but is not appropriate for investigation of what happens as the flowing plasma encounters the inner plasma sheet where magnetic drift becomes significant. In ideal MHD, flow bursts come to rest where the flux tube integrated entropy of the plasma within the flow burst equals that of the background plasma (Wolf et al., 2012). However, the energy dependence of magnetic drift should dramatically change this result. As predicted by the Rice Convection Model (RCM), the low entropy plasma should spread azimuthally, be accompanied by substantial azimuthal drift (Yang et al., 2014; Yu et al., 2017), and lead to a substantial reduction of flux tube integrated entropy as plasma moves earthward (Lyons et al., 2009; Wang et al., 2018a; Wang et al., 2020).

This paper provides an observational test of the RCM predictions of the fate of the reduced entropy, flow burst plasma as it enters the near-Earth plasma sheet. We base our test on the Wang et al. (2018) RCM results. The model gives predictions of azimuthal flow relative to upward FAC regions when a localized plasma perturbation is introduced into the outer plasma sheet and leads to a reduced entropy plasma bubble. The model also shows important differences in response when these perturbations are introduced at three different MLTs relative to the near midnight model boundary between the large-scale dusk and dawn convection cells. Our test uses observations of upward FACs and flows within the ionosphere. The upward FACs are inferred from the aurora obtained primarily from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) white-light all sky imager (ASI) array, which offers continent-scale coverage of the North American auroral zone with high temporal and spatial resolution (Mende et al., 2008). Poker Flat 557.7 nm ASI observations are included for events over Alaska. Flows are identified from Super Dual Auroral Radar Network (SuperDARN) observations, and presented as maps of flow vectors obtained with the Bristow et al. (2016) technique. This technique gives spatial resolution set by that of the underlying measurements and not by the smoothing from the global fits that are typically used for global SuperDARN convection maps. The resulting vectors are determined by the local line-of-sight (LOS) observations and the divergence-free condition. They are not strongly influenced by information beyond a distance of a few grids, so that localized steep gradients can be represented without influencing the remainder of the domain. Flow vector maps are available every 2 min.

The RCM (Harel et al., 1981; Toffoletto et al., 2003) computes species- and energy-dependent particle transport and convection electric field self-consistently using the bounce–averaged electric drift and magnetic curvature/gradient drift. Drift due to the induced electric field, which is associated with magnetic field changes, are included in computing motions in the magnetosphere. Particles are released from the RCM outer/tail boundary, proton loss by charge exchange and electrons loss by precipitation are considered, and the RCM computes electric field potential in the ionosphere self-consistently through the requirement for current continuity. Conductance includes Solar-EUV-generated conductance and Hall and Pedersen auroral conductance computed from the simulated particle spectra and precipitation loss rates based on (Robinson et al., 1987). The cross polar-cap potential is the external driver of the large-scale convection.

In the RCM simulations presented in this study (Wang et al., 2018b), the UCLA version of the RCM, UCLA-RCM, is used. Three modules developed separately have been incorporated in the UCLA-RCM: (1) observation-based tail particle boundary conditions (Wang et al., 2011), (2) force-balanced magnetic field (Liu et al., 2006; Gkioulidou et al., 2011), and (3) observation-based electron loss rates (Chen & Schulz, 2001; Gkioulidou et al., 2012).

Figure 1 shows UCLA-RCM results in the equatorial plane and ionosphere from Run one of Wang et al. (2018b) for times 3, 9, and 15 min after a meso-scale perturbation with a 1 h in MLT width and centered at 24 MLT was imposed on the RCM outer boundary, which is near X = −20 R_E at midnight. A cross-polar cap potential drop of 90 kV was used. The perturbation had a density decrease by a factor of six and a temperature increase by a factor of 6, so that the bubble was created by the heat flux vector divergence due to the higher magnetic drift speeds of the hotter particles relative to those of the cooler background. As the bubble moves earthward, the lower energy ions tend to follow the electric field drift (along equipotentials, which are shown in the ionosphere as the black contours in the right column), while the higher energy ions magnetic drift more duskward. As a result, the bubble spreads in longitude, consistent with magnetotail observations (Liu et al., 2015), and its entropy decreases and as it moves earthward. The spreading can very clearly be seen in the left column of Figure 1, which shows the ratio of flux tube integrated entropy S to the integrated entropy S_o at t = 0 (S = PV^5/3, where P is plasma pressure and V is flux tube volume). A crimson curve outlines the approximate boundary of the expanding bubble for each of the three times. S typically has a tailward gradient in the plasma sheet. However, the bubble spreading leads to an approximately azimuthal-aligned decrease in the tailward gradient in S at t = 15 min. As shown in Lyons et al. (2021a), there are enhancements of the Region 1 and 2-sense FACs as the bubble moves earthward (t = 9 min), and these FACs become longitudinally broad Region 1 and Region 2-sense currents as the bubble moves earthward to the inner plasma sheet. The enhanced Region 1-sense FACs are located approximately within the bubble itself, whereas the enhanced Region 2-sense FACs are earthward of the bubble, consistent with magnetotail observations (Liu et al., 2013; Sun et al., 2013).

The electric field changes as the FACs develop to maintain current continuity in both the ionosphere and magnetosphere. These changes can be seen in the middle row of Figure 1 as azimuthal turnings of the electric field drift vector (V_E) as the flow channel moves earthward (t = 9 min). By t = 15 min, the electric field changes give significant enhancements in the subauroral polarization stream (SAPS) and dawnside polarization stream (DAPS) Liu et al. (2020) flows in the regions of the enhanced downward FACs, where conductivities are substantially lower than in the upward FAC regions. These enhancements can be seen by comparing the equatorial-plane electric field drift velocities in the middle column and the spacing of ionospheric equipotentials in the right column at t = 3 and t = 15 min. Red and white rectangles in Figure 1 are drawn to help identify the locations for making this comparison, and comparison with the crimson curve shows that the DAPS increase is within the plasma sheet bubble, while the SAPS increase is adjacent to the equatorward boundary of the bubble. (The nightside auroral oval and plasma sheet can have both the well-known SAPS region and an analogous DAPS’ region as discussed in Liu et al. (2020). SAPS are strong westward flows in the low conductivity region of downward (duskside) Region two field-aligned currents (FACs) that lies just equatorward of the electron auroral oval. DAPS consist of strong eastward flows in the more poleward, downward (dawnside) Region one FAC portion of the dawnside auroral oval, where auroral precipitation Lyons and Fennell (1986), Zou et al. (2009) and thus conductivities are much lower than within the more equatorward dawnside upward Region two FAC portion of the dawnside auroral oval.)

The bubble in Figure 1 was approximately centered between the dawn and dusk large-scale convection cells. Run 2, shown in Figure 2, is for the same conditions as Run 1 except the perturbation leading to the bubble was centered at 23 MLT and thus further toward the dusk convection cell. The important differences from Run 1 are that the bubble expands further towards dusk and less towards dawn, leading to a stronger SAPS region flow enhancement and a substantially smaller and weaker DAPS region for enhancement. Run 3, shown in Figure 3 is also for the same conditions as Run 1 except the perturbation leading to the bubble was centered at 01 MLT and thus further within the dawn convection cell. This bubble expanded further dawnward than did the bubble in Run 1, and led to a substantial DAPS region flow enhancement but no discernable enhancement of SAPS.

We started with 14 nights that were selected for having good auroral viewing from Poker Flat, Alaska that we have used in previous studies [most in Lyons et al. (2021a); Lyons et al. (2021b)], and identified six flow channel events having good auroral viewing conditions and good radar coverage of flows moving to the equatorward portion of the auroral oval and of their connection to azimuthal flows. We first show two examples with a flow channel that appears to be near the boundary between the dusk-side and dawn-side convection cells, approximately corresponding to the RCM model run in Figure 1. We then show one example each of a flow channel that appears to be more within the dusk-side or dawn-side convections cells, resembling the RCM model runs in Figures 2, 3, respectively. This is followed by examples with two or more flow channels.

We only examine the two-dimensional structure of the flow channels, its location relative to upward FACs as indicated by bright auroral arcs, and the dependence on whether the flow channel is between the two large-scale convection cells or more within the dusk or dawn cell. We do not have the combination of time resolution with consistent echo coverage to consistently see the evolution of flow channels as they move equatorward within the auroral oval. We do, however, show sequences of flow and ASI measurement to show that the two-dimensional pattern is maintained for several minutes and is not just a coincidental pattern seen during a single measurement interval, and to also show that the flow channel appears to come in from higher latitudes (probably the polar cap).

While flow bursts are a major component of plasma sheet and auroral oval transport, and lead to a variety of other geomagnetic disturbances when they reach the inner plasma sheet (equatorward portion of the auroral oval), their two-dimensional structure as they approach the inner plasma sheet has not received much attention beyond that which is seen in ideal MHD simulations. We have examined this structure using both the RCM and ground-based radar and ASI observations. Specifically, we first presented the RCM predictions, and then tested these predictions using upward FACs identified from ASI images of the aurora and SuperDARN observations presented as maps of flow vectors obtained with the Bristow et al. (2016) technique. We used only observations for cases having good coverage of flow channels as they approach the equatorward portion of the auroral oval and the regions of possible connection to the azimuthal flows of SAPS and DAPS.

The energy dependence of magnetic drift becomes crucial within the inner plasma sheet, and as predicted by the RCM, the low entropy plasma of a flow burst should spread azimuthally and be accompanied by substantial azimuthal drift. We have taken advantage of the RCM results of Wang et al. (2018b) who ran three RCM simulations that were identical except for the MLT of a plasma perturbation applied at the model outer boundary that gives rise to a low-entropy plasma bubble. The results gave specific predictions of azimuthal flow enhancements in the SAPS and DAPS regions and their relation to flow-channel FACs that are associated with the azimuthal spreading of flow channels’ low entropy plasma as the flows reach to near the equatorward boundary of the auroral oval. We find consistency between the RCM pattern and the patterns seen in the observation results when a single flow channel is observed. The results also show clear response distinctions based on a flow channel’s location relative to the dusk-side and dawn-side large-scale convection cells, as seen in the RCM results of Figures 1–3 and the observations in Figures 4–6. The flow channels in Figures 1, 4, 5 were approximately centered between the dawn and dusk large-scale convection cells and gave significant flow enhancements of both SAPS and DAPS. The flow channels in Figure 2 and the left side of Figure 6 were further toward the dusk convection cell, and showed a far more significant enhancement of SAPS than for DAPS. The flow channels in Figure 3 and the right side of Figure 6 were further within the dawn convection cell. The low entropy plasma within these flow channels expanded further dawnward than did the between cell flow channels, and led to a substantial DAPS region flow enhancement but no discernable enhancement of SAPS.

It needs to be remembered that there is an infinite variety of flow channels plasma parameters and of background plasma sheet and auroral oval conditions. That the observations show the general trends predicted by the RCM simulations with the idealized parameters of Wang et al. (2018a) gives strong support that the azimuthal spread of a low-entropy plasma sheet plasma and its associated FAC and flow responses gives a realistic two-dimensional physical description of the structure of plasma sheet flow bursts (auroral oval flow channels) as they reach the inner plasma sheet (near the equatorward edge of the auroral oval).

That the structure of the flow bursts becomes approximately azimuthally aligned within the inner plasma sheet (equatorward portion of the auroral oval) may be quite important. For example, this alignment is likely critical for understanding the disturbances of the magnetosphere-ionosphere system that have east-west alignment, such as growth-phase auroral arcs, substorm onset, and dawnside auroral omega bands. Based on the few cases presented here, the auroral oval flow channels have a typical duration of several minutes. Also, based on the observations in Figure 7, the character of a flow channel can evolve with time, and more than one flow channel can significantly affect the plasma sheet and auroral oval at any specific time. Such evolution and the effects of multiple flow channels warrant further consideration, as does other effects of MLT location of bubble injection such as seen in (Nishimura et al., 2020).