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Article

Amplified Eastward SAPS Flows Observed in the Topside Ionosphere near Magnetic Midnight

by
Ildiko Horvath
* and
Brian C. Lovell
School of Electrical Engineering and Computer Science, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia
*
Author to whom correspondence should be addressed.
Atmosphere 2025, 16(9), 1076; https://doi.org/10.3390/atmos16091076
Submission received: 12 August 2025 / Revised: 8 September 2025 / Accepted: 9 September 2025 / Published: 11 September 2025
(This article belongs to the Special Issue Atmospheric Impacts of Space Weather and Extreme Meteorological Events)
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Abstract

We report the exceptional observations of amplified eastward subauroral polarization streams (SAPS) made by the F15 spacecraft at ~840 km altitude near magnetic midnight during 2015–2016 in 17 events. The results show the dawn-cell-associated amplified eastward SAPS flows streaming alongside the duskward-extending dawn cell. The amplified eastward SAPS flows maximized at ~3200 m/s within their respective deep plasma density troughs, mimicking the SAPS flows and thus implying positive feedback mechanisms in action, where the electron temperature reached ~7000 K. One set of correlated magnetosphere–ionosphere conjugate observations is also presented. This illustrates the magnetotail-reconnection-related inward-directed cross-tail convection electric field (EC) reaching the near-earth plasmasheet’s tailward end, while the inward-directed SAPS E field was absent on the inner-magnetosphere plasmapause, and the emerging eastward SAPS flow in the conjugate ionosphere. These results provide observational evidence that the earthward-propagating inward-directed dawn–dusk cross-tail E field (1) mapped down to auroral latitudes with an equatorward direction, (2) propagated to subauroral latitudes, and (3) played a key role in the development of the emerging eastward SAPS flow and in the amplification of the fully-developed eastward SAPS flows near magnetic midnight, while positive feedback mechanisms supported further SAPS growth.

Graphical Abstract

1. Introduction

1.1. Subauroral Plasma Flows and Their Generation Mechanisms

Subauroral plasma flows of different types are phenomena of the ionosphere’s subauroral region and include the subauroral polarization streams (SAPS) [1] and subauroral ion drifts (SAID) [2] or polarization jets (PJs) [3]. At subauroral latitudes, the ionosphere plasma is set into motion by the interacting subauroral polarization electric (E) field and geomagnetic (B) field [4,5,6]. At dusk and nighttime, when the subauroral E field is poleward directed, the westward zonal drift drives the broader (3°–5° in magnetic latitude (MLAT)) but weaker (<1200 m/s) westward SAPS flow [7] and the narrower (1°–2°) but stronger (>1200 m/s) westward SAID/JP flow [8,9]. On the dawnside, where the subauroral E field is equatorward directed, the zonal E × B drift is eastward directed and drives the eastward SAPS flow [10,11] and eastward SAID flow [12].
Even the above-mentioned earliest studies reported the development of westward subauroral flows soon (≥10 min and <30 min) after substorm onsets, implying their fast-time development on a short timescale. But controversially, the traditional voltage generator (VG) [13] and current generator (CG) [8,9] theories invoke inherently slow processes unfolding on a timescale of a few hours that are not supported by experimental observations and are also negated by this study. These slow processes are related to the underlying plasma convection in the VG scenario and ring current buildup in the CG scenario. Furthermore, according to these traditional generator theories, SAPS and SAID develop in the same way in a VG or CG setting, which is clearly not supported by experimental observations.
Recent studies conducted by Mishin [14,15] provide correlated multi-point observations and numerical calculations supporting fast-time (FT) SAID and SAPS development in their respective magnetosphere (M) voltage generators (VGs). These are the VGFT related to the short-circuit circle [14,15] and the VGM related to the substorm current wedge (SCW) [16,17] forming a two-loop system (SCW2L) [18].
Overall, fast-time SAPS development is due to the development of the SCW unfolding in the tail region. There, the cross-tail currents become disturbed and diverted permitting dipolarization. Meanwhile, the associated substorm-onset-related particle injections take place on the dawnside or duskside as the substorm onset occurs on the dawnside or duskside [19,20]. Our results show the development of eastward SAPS soon after substorm onset on the dawnside, where the SCW started developing.
First, we focus on fast-time westward SAPS development put forward by Mishin [16,17]. In the magnetosphere, the outward-directed SAPS E field forms at the leading edge of the duskward-expanding SCW. There, the closure of upward region 1 (R1) and downward R2 field-aligned currents (FACs) is being demanded by meridional currents associated with the emerging outward SAPS E field. Via field-line mapping, a poleward-directed SAPS E field appears in the conjugate ionosphere—particularly at the leading edge of the westward travelling surge (WTS) [21,22] formed by the duskward-expanding SCW. At the westward SAPS flow’s poleward edge, poleward Pedersen currents require a poleward subauroral E field to close these duskside large-scale FACs [17].
Continuing with the recently discovered eastward SAPS phenomenon [10,23,24], it has been quite intensively investigated [11,25,26,27,28] since our first report [10]. We investigated fast-time eastward SAPS development in our recent studies [10,11] and applied the duskside SAPS development theory of Mishin [16,17] to the dawnside. Accordingly, in the dawnside magnetosphere, the inward SAPS E field develops at the front of the dawnward-expanding SCW, where large-scale FACs connect via meridional currents associated with the emerging inward SAPS E field. Via field-line mapping into the conjugate ionosphere, the equatorward SAPS E field is located at the front of the dawnward-expanding SCW. We documented the eastward SAPS flow that was newly developed in a VGM scenario and was not accompanied by dawnside upward R2 currents [11]. However, the previously-formed (i.e., old) eastward SAPS flow was accompanied by dawnside upward R2 currents, closing at the eastward SAPS flow’s poleward edge with downward R1 currents [10], since it takes time for the R2 currents to start flowing in a VGM [29].

1.2. Plasma Convection

In the magnetosphere, the plasma convection’s ultimate driver is the electric field generated by the magnetized solar wind velocity’s impact on the Earth’s magnetic (B) field. There, the Dungey convection cycle [30] starts with a subsolar dayside reconnection. It forms open magnetic field lines, under southward IMF conditions (BZ < 0), that are subsequently moved to and closed at the tail region where the cycle ends with nightside magnetotail reconnection often occurring under northward IMF conditions (BZ > 0). Finally, the closed field lines are returned to complete the convection cycle. Magnetospheric convection is driven by tangential stress, via a magnetospheric dawn–dusk E field, which is ~10% of the IEF EY [31]. This dawn–dusk E field is a result of the higher (+) dawnside and lower (−) duskside electrostatic potentials developed in the magnetosphere [32]. Meanwhile, the net effect of the interacting dawn–dusk E field and co-rotation E field define where the inner-magnetosphere plasmapause appears [32,33].
In the ionosphere, the Dungey convection cycle is driven by the polar dawn–dusk E field (EDawn-Dusk) generated by Alfven waves via the magnetospheric dawn–dusk E field’s field-line mapping [34]. Under southward IMF conditions (BZ < 0), the Dungey convection cycle is characterized by a twin-cell pattern formed by a dusk cell and dawn cell, depicting the footprints of convecting magnetic field lines. Between these two convection cells, the ionospheric plasma flows from magnetic midday to midnight across the polar cap, where the magnetic field lines are open. Along the lower-latitude regions of these two convection cells, where the magnetic field lines are closed, the plasma flows sunward from midnight to midday as return flows moving the plasma in the auroral and subauroral regions [35,36,37]. However, the polar convection pattern’s geometry varies because of the IMF BY component’s magnitude and direction. Under purely southward IMF conditions (BZ < 0; BY = 0), the convection pattern is symmetric. An additional IMF BY component tilts the convection axis, since the open flux tubes are added asymmetrically to the tail lobes [38,39]. Under IMF BZ < 0 and BY > 0 orientations in the northern polar cap, the crescent-shaped dawn cell is larger while the round-shaped dusk cell is smaller. An opposite scenario occurs under IMF BZ < 0 and BY < 0 conditions.
Based on E field measurements provided by the Cluster satellites [40,41] or by the high-frequency radar measurements of SuperDARN [42,43,44], the polar potential plots constructed for the northern hemisphere revealed significant asymmetric convection features introduced by the IMF BY component. Figure 1 shows these with the modified Figure 9.9 of Grocott [44] depicting the dusk (in blue) and dawn (in red) cells.
Figure 1a shows the dusk cell’s dawnward intrusion occurring under IMF BZ ≤ 0 and BY < 0 conditions. Characteristic features are the subauroral/mid-latitude plasma flows directed antisunward (in light blue) after magnetic midnight and sunward (in orange) before magnetic midnight.
Figure 1b shows the opposite scenario with the dawn cell’s duskward intrusion occurring under IMF BZ ≤ 0 and BY > 0 conditions. Characteristic features are the subauroral/mid-latitude plasma flows directed antisunward before magnetic midnight and sunward after magnetic midnight.

1.3. Polar Convection EDawn-Dusk Field Driving the Plasma at Subauroral and Mid Latitudes

Since early modeling studies, using experimental models of different techniques [35,36,45,46,47], we know that the polar convection EDawn-Dusk field drives the ionospheric plasma sunward along the flanks of the two convection cells and covers subauroral and mid latitudes. However, these convection models do not include the effects of SAPS. But later and recent studies consider the westward SAPS in the context of both (1) the two-cell convection pattern [7,12,34,48,49,50,51] and (2) the propagating polar convection EDawn-Dusk field of magnetospheric origin [49] reaching subauroral [48,52,53,54] and mid [55] or even lower [56,57] latitudes along with the westward SAPS flows. The propagating polar convection EDawn-Dusk field of magnetospheric origin is known as the prompt penetration E field (PPEF) [58], which also impacts the dynamics of the low-latitude ionosphere on the dayside/nightside with its eastward/westward polarity [59,60].
Lyatsky et al. [61] designed a basic analytical model for the dusk-cell-related westward SAPS. The model considers both the dusk cell’s dawnward extension (see Figure 1a), which is the ionospheric signature of the Harang discontinuity region [62,63,64,65], and the consequential distortion of dusk-cell-related westward subauroral convection flows. Furthermore, the Lyatsky model [61] also specifies the dawnward-extended subauroral flows as westward SAPS flows that can be quite intense during magnetically active times. Moreover, the Lyatsky et al. [61] model scenario was investigated in our recent study [66] based on southern-hemisphere topside-ionosphere observations.
Lyatsky et al. [61] invoked previous statistical experimental studies for supporting observational evidence. According to their statistics, the strongest poleward polarization E field or SAPS E field occurs near magnetic midnight [6,7,67] and the strongest westward SAPS flows occupy the midnight magnetic local time (MLT) sector’s broader region extending up to 3 MLT [7]. However, the direct supporting observational evidence was provided by our recent study [66]. We demonstrated that the westward SAPS flows became amplified in such a convection scenario and that the amplified westward SAPS flows streamed alongside the dawnward-intruding dusk cell. These observational results suggest the polar convection EDawn-Dusk field’s significant role—by propagating equatorward and reaching subauroral latitudes—in the westward SAPS flow’s amplification. Meanwhile, positive feedback mechanisms allowed the poleward-directed SAPS E field’s further growth within the deepened trough’s low-conductivity and increased-recombination region, where the electron temperature became elevated (up to 7000 K).

1.4. Hypothesis and Aim of This Study

Our hypothesis is that if the polar convection EDawn-Dusk field has a significant role in the dusk-cell-related westward SAPS flow’s amplification by propagating equatorward and reaching SAPS/trough latitudes according to the scenario of Lyatsky et al. [61] shown in Figure 1a, then the polar convection EDawn-Dusk field also amplifies the dawn-cell-associated eastward SAPS flow by propagating equatorward and reaching SAPS/trough latitudes in an opposite scenario as shown in Figure 1b.
This study’s aim is to investigate this scenario, when the dawn-cell-associated eastward SAPS flow becomes amplified (≥1200 m/s). This scenario involves both (1) the equatorward- and duskward-expanding dawn cell and (2) the dawn-cell-associated eastward SAPS flow that becomes amplified near magnetic midnight.

2. Materials and Methods

To observe the amplified eastward SAPS flow and its ionosphere plasma environment near magnetic midnight, we used a multi-satellite database. These include the DMSP [68] measurements taken by the F15 spacecraft and the SuperDARN [43,69,70] convection maps [71] depicting the underlying polar convection pattern. SuperDARN convection maps are produced based on data smoothed over the previous 2 min and model predictions made by the TS18 model [72]. We also used SSUSI [73] auroral images showing auroral precipitations tracing the duskward-expanding dawn cell.
Generally, the polar-orbiting DMSP satellites circle the Earth (T ≈ 101 min; I ≈ 98.7°) at an altitude of ~840 km [68]. In 2015 and 2016, F15 crossed the southern-hemisphere polar region from postmidnight to dusk and, therefore, could detect the dawn-cell-associated eastward SAPS flow near magnetic midnight. Since the magnetometer and the particle spectrometer did not work then, we used only a limited set of data provided by F15. These include the ion density (Ni; 1/cm3), electron and ion temperature (Te and Ti; K), and horizontal (HOR) and vertical (VER) ion drifts (VHOR and VVER; m/s).
We note here that all 17 eastward SAPS events observed in 2015 and 2016, depicted by their respective F15 survey plots, SuperDARN convection maps, and SSUSI imageries, are shown in Figures S1–S17 (see the Supplementary Materials).
We also used Geostationary Operational Environmental Satellite (GOES) and GEOTAIL satellite data to observe particle injections. While the GOES satellites are on geostationary orbits (altitude~6.6 RE) and circle the Earth once a day [74], the GEOTAIL satellite is on an elliptical orbit and orbits the Earth with an orbital period of 127 h [75]. For specifying particle injections, we utilized average electron flux values measured by the GOES Energetic Particle Sensor (EPS) at various (40–475 keV) energy channels in nine directions and ion differential flux intensity measured by the GEOTAIL Energetic Particle and Ion Composition (EPIC) instrument [76] at 12 energies within the 67–1361 keV regime.
We could correlate only one topside-ionosphere eastward SAPS event, occurring on 8 September 2015, with one set of inner-magnetosphere observations made by one of the five (TH-A–TH-E) THEMIS satellites [77], TH-E. In September 2015, TH-E completed its Tail Science Phase and orbited the Earth with an apogee of ~12 RE on the nightside. Our TH-E data included the measurements of space craft potential (SC Pot; V) provided by the Electrostatic Analyzer (ESA), EX and EZ (mV/m) in Geocentric Solar Ecliptic (GSE) coordinates measured by the Electric Field Instrument (EFI), and electron and ion pressure tensors (PeXX; PiXX; eV/cm3) measured in the standard X direction (indicated as XX) by the Combined Moments suite in the ground-calculated moments (GMOM).
Orbit data obtained for GOES and TH-E include MLT (Hr), dipole L (RE), spacecraft location (X, Y, Z; RE) in Geocentric Solar Magnetospheric (GSM) coordinates, and footprints in eastern geographic longitude (°E) and northern latitude (°N) and in southern geomagnetic latitude (°S).
Our database also includes IMF BY and BZ (nT) and IEF EY (mV/m) values along with the various indices of AE (nT), Kp, and SYM-H (nT) for monitoring the background conditions. For identifying substorm onset times and locations, we used various substorm lists published by Newell and Gjerloev [78], Forsyth et al. [79], and Ohtani and Gjerloev [80] via SuperMAG.
Our methodology of correctly specifying the dawn-cell-associated amplified eastward SAPS flow, in the absence of magnetometer and particle data, is based on the subauroral region’s six main features listed as 1–6. (1) The plasma trough that is detected frequently with the plasmapause, as shown by the Ni plots. (2) The SAPS flow appearing as an increased VHOR drift (|VHOR| ≥ 1200 m/s) equatorward of the plasmapause and within the plasma trough and streaming antisunward (VHOR < 0) before and at 24 MLT or sunward (VHOR > 0) after 24 MLT. (3) The increased Te marking both the trough minimum and the footprint of the plasmapause. (4) The underlying polar convection mapped by SuperDARN and showing the dawn cell and its duskward extension. (5) The equatorward oval boundary imaged by SSUSI and demonstrating the amplified eastward SAPS location on the oval’s equatorward side. (6) The dawn cell’s signature appearing in the SSUSI imagery and confirming the mapped convection by SuperDARN.
Although the absence of magnetometer and particle data is a major limitation of this study, SAPS can be unambiguously identified based on these six criteria. Importantly, these include the detection of (i) SAPS within (ii) the trough and equatorward of (iii) the plasmapause. These (i–iii) are the well-known critical features of the subauroral geospace [81]. While the plasmapause statistically correlates with the trough [82], wherein the SAPS flow develops [1], the SAPS flow statistically correlates with the trough [7].

3. Results

3.1. Dawn-Cell-Associated Amplified Eastward SAPS Observed by F15 near Magnetic Midnight

We examined the 2015–2016 F15 plots and considered only good-quality data (without any gaps) and SAPS observations. These excluded weak, broad, and multiple SAPS flows and SAPS events when the underlying polar convection was not clear. We found 17 events altogether, when the eastward SAPS flow was amplified (|VHOR| ≥ 1200 m/s). These are listed in Table 1.
These 17 events include 9 events in 2015 and 8 events in 2016. Their geographic and magnetic locations are illustrated in Figure 2.
In Figure 2a,c, the southern geographical map illustrates the magnetic meridians (in blue) and dip equator (in light magenta). Here, we plotted the eastward SAPS locations (as dot symbols in colors). F15 detected the amplified eastward SAPS mostly within the African and South American longitude sectors. These are the longitude regions that are affected by the South Atlantic Magnetic Anomaly (SAMA). There, the magnetic field is anomalously weak [83] and, therefore, the plasma drift is strongest, like in the SAPS channel. Its magnitude is computed as (E × B)/B2 where E is the SAPS E field and B is the geomagnetic field. These are also the longitude regions, where the ring ionospheric trough (RIT) [84] prefers to develop [85,86,87,88] (see details below).
In Figure 2b,d, the southern polar plot of MLT against MLAT shows the amplified eastward SAPS locations (dot symbols in colors). These are situated mostly before magnetic midnight at ~60 MLAT and occasionally after magnetic midnight between 50 and 60 MLAT.
Figure 3 and Figure 4 show the 2015 and 2016 eastward SAPS events, respectively. The plots are constructed with the F15 data of Ni, Te and Ti, and VHOR and show the amplified eastward SAPS flows in their topside-ionosphere plasma environment. We oriented these plots in the equatorward direction and marked the equatorward oval boundary along with the trough (symbol circles in colors) and SAPS (symbol dot in color) locations. Each VHOR plot shows the amplified eastward SAPS flow (shaded interval in yellow; 3200 m/s > |VHOR| > 1200 m/s). In most of the SAPS events, the amplified eastward SAPS flow streamed antisunward (VHOR < 0) before 24 MLT. Only three events (in Figure 3c) show the amplified eastward SAPS flow streaming sunward (VHOR > 0) after 24 MLT.
Overall, these SAPS events in Figure 3 and Figure 4 show that the amplified eastward SAPS flow appeared within the trough that was quite deep (Ni ≤ 3.5 × 103 1/cm3). This trough is the coinciding RIT and flow-stagnation-related main ionospheric trough (MIT) [89]. Because of the descending F15 pass’ alignment, sometimes (as shown in Figure 3b,c) F15 could not track the plasmapause, which is associated with the trough. These two are related phenomena and their positions simultaneously vary with geomagnetic activity [90].
Clearly shown in Figure 3a and Figure 4, the plasmapause (PP) appeared as a steep Ni gradient on the coinciding RIT-MIT’s poleward side. These PP-detections provide observational evidence that both the trough (i.e., RIT-MIT) and the amplified eastward SAPS flow developed equatorward of the plasmapause and on closed magnetic field lines. Possibly, the amplified eastward SAPS played a significant role in moving the ionospheric plasma longitudinally [91]. Due to the decaying ring current, as the hot ring current ions precipitated into the ionosphere, Te became elevated and mostly reached ~6000 K within RIT-MIT.
Such hot ring current ion precipitation occurs most intensively in the SAMA’s longitude region because of the anomalously weak geomagnetic field [74,75,76]. As the ionosphere became heated, leading to both (i) elevated Te and (ii) increased recombination rates, the RIT formed and became further deepened. These (i–ii) occur in the SAMA’s longitude regions according to the statistical investigation of Karpachev [86,87,88] and contribute to the development of positive feedback mechanisms ([15] and the references therein) by further reducing the ionospheric plasma density, and thus conductivity, and therefore further increasing the polarization SAPS E field leading to the further growth of amplified eastward SAPS flows in these events.
Further observational evidence of the active positive feedback mechanisms occurring is provided by the strong similarity shown in each event between the deep trough and the amplified eastward SAPS flow as the trough mimicked ([67] and the references therein) the SAPS flows.

3.2. Underlying Interplanetary and Geophysical Conditions

Figure 5 shows the eastward SAPS’s underlying interplanetary and geophysical conditions. We plotted the IMF BY and BZ and IEF EY components along with the indices of SYM-H, Kp, and AE, as well as the eastward SAPS flows of interest (symbol dots in colors, shaded intervals in yellow) and their respective nearest previous substorm onsets (symbol stars in red; see also Table 2). These plots cover a larger time-period including the previous day and the following day.
Figure 5 shows that the amplified eastward SAPS flows were observed (i) mostly during non-storm substorms, (ii) soon after their respective substorm onsets (see Table 2), and when (iii) the IMF rotated northward (Bz > 0) or southward (Bz < 0), and (iv) the Kp index increased just before or during the eastward SAPS event. These (i-iv) imply that the plasmapause was newly formed and moved equatorward [92]. Furthermore, these (i-iv) are the main underlying conditions required for the propagation of a polar EDawn-Dusk field to subauroral positions. As the IMF BZ component changes suddenly, the shielding layer [93,94,95] is not fully developed yet at southward turning (under BZ < 0 conditions) or breaks at northward turning (under BZ > 0 conditions). Thus, the polar EDawn-Dusk field is able to penetrate from the polar cap to lower latitudes for a shorter (~few minutes; [96]) or longer (several hours; [58,97,98]) time-period. Therefore, the propagating polar EDawn-Dusk field (i.e., PPEF) adds to the already existing polarization SAPS E field and the net E field drives the plasma in the SAPS channel along with the polar convection. These eastward SAPS events were unfolding on the nightside, near magnetic midnight, where the dawn–dusk convection E field is the earthward-directed magnetotail-reconnection-related cross-tail E field. Thus, the resultant PPEF pointed dawnward (or eastward) and equatorward and, therefore, added to the equatorward SAPS E field (driving the plasma eastward/dawnward) and consequently amplified the eastward SAPS flow.

3.3. Sunward Streaming Amplified Eastward SAPS Flow: 7 October 2015 Event

Figure 6 shows the 7 October 2015 eastward SAPS event. Then, the dawn-cell-associated amplified eastward SAPS flow streamed sunward after magnetic midnight, alongside the dawn cell.
In Figure 6a, the southern geographical map depicts the F15 pass (in light red) and the GOES-13 footprints (in red). While F15 detected the eastward SAPS flow along its descending pass in the African longitude sector, the ascending F15 pass was located over the South Pacific. During a larger 12 h time interval of the eastward SAPS event, the GOES-13 footprints crossed the F15 pass in the South American sector.
In Figure 6b, the plots cover the postmidnight subauroral region, the midnight and dusk auroral oval, and the duskside subauroral region. From top to bottom, these plots are constructed with the F15 data of Ni, Te and Ti, and VHOR and VVER. The main features shown are as follows. The amplified eastward SAPS flow (dot symbol in light red; shaded interval in yellow) streamed sunward (+) after magnetic midnight and reached VHOR ≈ 3200 m/s within RIT-MIT. Here, Te peaked at ~6400 K and the upward drift increased to VVER ≈ 1500 m/s. Within the auroral oval, the near-constant and close-to-zero drifts (VHOR and VVER) can be explained with the alignment of the F15 pass (see details below). However, as the F15 pass crossed the auroral arc (located along the dusk auroral zone’s poleward edge; see Figure 6d), the plasma drifts showed the weak signatures of Alfven waves (marked as shaded intervals in light blue) since auroral arcs are generated by FACs carried by Alfven waves [17].
In Figure 6c, the southern polar convection map shows the prevailing polar convection pattern operating under IMF BZ > 0 and BY > 0 conditions during 00:10–00:12 UT. This is the closest match with the SSUSI imagery recorded at 00:07 UT (shown in Figure 6d). We plotted the F15 pass (in light red) and the GOES-13 location (dot symbol in red) appearing within the dusk cell, and the location of the nearest substorm onset (star symbol in red), which occurred at the end of the previous day (at 23:45 UT) in the postmidnight MLT sector and near the eastward SAPS channel (dot symbol in light red). Based on the SuperDARN convection map generated, the dusk cell (in blue) and dawn cell (in red) are depicted by their respective lowest equipotential contours. Furthermore, the Heppner–Maynard (H-M) convection boundary (in dark green), defining the equatorward boundary of the auroral oval [47], was located at ~60 MLAT. This two-cell convection pattern developed after a longer-lasting southward IMF BZ orientation, when the IMF just turned northward and the shielding layer broke—allowing the propagation of polar EDawn-Dusk field near midnight on the dawnside (as a dawnward/eastward and equatorward-directed PPEF) to subauroral latitudes. Then, the convection pattern appeared with a bigger dusk cell and slightly smaller dawn cell, where the convection axis was tilted mostly in the postnoon–postmidnight direction. Furthermore, dawn-cell-associated amplified eastward SAPS flow was observed after midnight and thus streamed sunward alongside the dawn cell. Thus, the dawnward/eastward and equatorward-directed PPEF extended to subauroral latitudes and enhanced the dawnside eastward subauroral convection flows in the SAPS channel and, therefore, amplified the eastward SAPS flow.
Figure 6d shows the SSUSI imagery. On each side, it illustrates the auroral arc along the oval’s poleward edge and the auroral precipitation signatures of the convection cell. Due to the postnoon–postmidnight tilt of the convection axis, these signatures appeared on the dawnside. By plotting over the F15 pass and the eastward SAPS location, we can view the F15 pass’ alignment. As the F15 pass mostly followed the boundary of the auroral zone–polar cap, the drifts (both VHOR and VVER) were close to zero. All this provides observational evidence and demonstrates further that the postmidnight eastward SAPS flow (i) was tracked equatorward of the auroral oval and (ii) was associated with the dawn cell, and (iii) the F15 pass crossed the auroral arc along the oval’s poleward edge.
Figure 6e shows the GOES-13 electron flux time series covering a 12 h time-period from 18 UT on 6 October to 6 UT on 7 October 2015. Here, the substorm onset (star symbol in red) and eastward SAPS (dot symbol in light red; shaded interval in yellow) are marked. As shown, the amplified eastward SAPS flow was observed soon after the substorm onset and after dipolarization. This is when the magnetic field reconfigures from stretched (tail-like) to bipolar (dipole-like) after substorm onset [16]. Although GOES-13 was located on the duskside (see Figure 6c) and the substorm onset occurred on the dawnside (see Figure 6c), these time series still illustrate quite clearly the electron flux decrease (just before the substorm onset) characteristic to a tail-like magnetic field and the following electron flux increase created by the following particle injections (just after the substorm onset) characteristic to a dipole-like magnetic field.

3.4. Antisunward Streaming Amplified Eastward SAPS Flow: 3 March 2015 Event

Figure 7 is like Figure 6 but illustrates the 3 March 2015 eastward SAPS event and shows GEOTAIL data. Then, the dawn-cell-associated eastward SAPS flow became amplified and streamed antisunward before magnetic midnight, alongside the duskward-extending dawn cell.
In Figure 7a, the southern geographical map shows the F15 passes (in cyan). F15 detected the amplified eastward SAPS flow in the African longitude sector along the descending F15 pass. Meanwhile, the ascending F15 pass was located over the South Pacific. Since footprints are not available for GEOTAIL, we could not map them.
In Figure 7b, the F15 data plot set covers the premidnight subauroral and auroral zones and the dusk region. Before magnetic midnight within RIT-MIT, Te peaked at ~7400 K and the amplified eastward SAPS flow streamed antisunward (-) and reached 2600 m/s in strength. Meanwhile, the upward (+) drift locally maximized in the SAPS channel and reached 3000 m/s. Within the premidnight auroral zone, the plasma drift was constant and close to zero (see details below).
In Figure 7c, the southern polar map shows the prevailing polar convection operating during 01:08–01:10 UT, which is the closest match with both the F15 eastward SAPS observation (01:07 UT) and the SSUSI imagery (01:57 UT). Then, the polar convection was characterized by a convection axis tilted in the prenoon–premidnight direction and by a duskward-extending dawn cell. Meanwhile, the H-M convection boundary (in dark green) was located at ~68 MLAT. Again, we plotted the previously occurring (at 01:00 UT) substorm onset (star symbol in red) situated on the dawnside. As shown by the mapped F15 pass and eastward SAPS location, the F15 pass crossed the dusk cell’s equatorward edge, where VHOR and VVER were close to zero (see Figure 7b). Furthermore, the eastward SAPS channel was associated with the duskward-extending dawn cell. Therefore, the dawn-cell-associated amplified eastward SAPS flow streamed antisunward before magnetic midnight, alongside the duskward-extending dawn cell.
In Figure 7d, the SSUSI imagery depicts the signatures of the dusk cell and duskward-extending dawn cell appearing on the duskside because of the prenoon–premidnight tilt of the convection axis. Also, we mapped the location on the dawn-cell-associated amplified eastward SAPS flow situated on the duskside and equatorward of the oval boundary.
In Figure 7e, the line plots cover a 7 h time-period (from 20 UT on 2 March to 3 UT on 3 March 2015) and illustrate the GEOTAIL-measured ion differential flux intensity variations at 12 energy channels. During that time-period, GEOTAIL was traveling on the dawnside and from the dayside to the nightside by crossing the x axis at 00:30 UT on 3 March. Then, a series of particle injections occurred on the nightside and during the previous substorms. These are shown by the dispersionless differential ion flux increases and decreases. F15 observed the amplified eastward SAPS on 3 March, just after the last dawnside substorm onset (at 01:00 UT) of the substorm onset series occurring in the postmidnight MLT sector. Because of the stored energy released during the substorm onset, enhancing the transpolar voltage, the dawn cell became enlarged and extended duskward; see [98,99] and the references therein.

3.5. Correlated F15 and TH-E Observations of 8 September 2015 Event

Figure 8 shows the two consecutive SAPS events observed by F15 on 8 September 2015 at 6:32 UT and 8:13 UT (see also Figure 3c). We could provide further illustrations with the emerging eastward SAPS flow detected earlier at 4:50 UT (shown in Figure 8b) that we matched with TH-E measurements (made at 4:45 UT).
Figure 8a shows the southern geographical map. Here, we plotted the three descending F15 passes (in colors) and the three eastward SAPS flow locations (dot symbols in colors) F15 observed on the dawnside in the South American longitude sector. Also, the three ascending F15 passes on the duskside in the Australian sector and the GOES-15 footprints (in red) over the South Pacific are shown. Here, we focus on the earliest observed eastward SAPS channel (dot symbol in light gray) and the nearby TH-E-observed large-scale E field (symbol diamond in orange) that we specified as an earthward-propagating inward (i.e., earthward)-directed cross-tail convection E field (EC; in orange).
Figure 8b shows three sets of F15 line plots, constructed the same way as Figure 3 and Figure 4. These three consecutive F15 observations show the eastward SAPS flows that streamed sunward after magnetic midnight on 8 September 2015. The first set depicts the earliest (i.e., emerging) eastward SAPS flow (dot symbol in light gray; shaded interval in yellow; VHOR ≈ 800 m/s) within the shallower (Ni ≈ 2 × 103 cm−3) coinciding RIT-MIT. There, Te was not locally increased yet (Te ≈ 4500 K). The next two plots show (see also Figure 3c) the amplified eastward SAPS flows later on at 6:32 UT (symbol dot in light blue; VHOR ≈ 2800 m/s) and at 8:13 UT (symbol dot in dark yellow; VHOR ≈ 2200 m/s). Their respective troughs (i.e., coinciding RIT-MIT), where Te became locally elevated to ~7000 K, were deeper (Ni ≈ 1×103 cm−3) and mimicked the eastward SAPS flows implying positive feedback mechanisms in action [15].
Figure 8c illustrates the electron flux recorded by GOES-15. These line plots cover a larger 10 h time-period of the above-described eastward SAPS events. We marked (shaded intervals in yellow) the published substorm onset times (star symbols in red) along with the eastward SAPS flow detections (symbol dots in colors) and the TH-E-observed EC (symbol diamond in orange). Demonstrating the amplified eastward SAPS flows’ fast-time development, a series of quite intense dipolarization events occurred after their respective magnetotail reconnections and associated cross-tail convection E field development. While the earlier dipolarization events covered the first two substorm onsets, the following weaker dipolarization events occurred during the time of correlated EC-SAPS recordings.
Figure 8d shows the two-cell polar convection operational during the time-period of 06:32–06:34 UT when F15 observed the first amplified eastward SAPS flow (dot symbol in light blue). In the midnight MLT sector, the prevailing polar convection was characterized by a stronger dusk cell confined to the duskside and a weaker dawn cell confined to the dawnside. Furthermore, the TH-E footprints were taken on the dawnside, where the earthward-propagating cross-tail convection E field (EC; symbol diamond in orange) was located poleward of both the emerging eastward SAPS flow (symbol dot in light gray) and the amplified eastward SAPS flow (symbol dot in light blue). These eastward SAPS flows commonly streamed sunward alongside the dawn cell. The TH-E-observed plasmapause location (dotted line in blue) was situated at ~59 MLAT and the H-M convection boundary at ~61 MLAT. All this provides observational evidence that the inward-directed cross-tail convection E field (developed earlier, before the previous dipolarization event) propagated earthward and mapped down to the auroral oval as an equatorward-directed PPEF (driving the plasma eastward) and possibly reached subauroral/trough latitudes. This triggered (i) the eastward SAPS flow’s emergence (symbol dot in light gray) within RIT-MIT and (ii) the further amplifications of eastward SAPS flows developed on a short timescale (symbol dot in light blue in Figure 8d and symbol dot in dark yellow in Figure 8e).
In Figure 8e, the polar map shows a more intense polar convection later on (at 08:20 UT) along with the H-M convection boundary at ~63 MLAT. Mapped over, the second amplified eastward SAPS flow (symbol dot in dark yellow) streamed sunward alongside the dawn cell. Consistent with Figure 8d, these Figure 8e-related observations imply the eastward SAPS flow’s further amplification as described above.
In Figure 8f, the X vs. Z and X vs. Y orbit plots (in GSM coordinates) show the TH-E orbit completed during 4.5–5.5 UT on 8 September 2015. Then, TH-E traveled tailward on the nightside and dawnside and observed the earthward-propagating cross-tail convection E field (EC) in the inner magnetosphere (at 4.72 UT). In these orbit sections, the positive directions are outward and sunward.
Figure 8g is constructed with the TH-E data of spacecraft potential (SC Pot), E field components (EX; EZ), and the high-energy ion and electron pressure components (PiXX, PeXX). These depict the earthward-propagating inward (earthward)-directed cross-tail convection E field (EC) observed on the dawnside and its inner-magnetosphere environment. Between the two potential drops detected, the plasmasheet’s earthward edge (marked as shaded interval in gray) was observed. This is the region of trapped high-energy electrons [100] appearing in a bipolar magnetic field after dipolarization (triggered by magnetotail reconnection) when the stretched (or tail-like) magnetic field became bipolar (or dipole-like). The smaller potential drop depicts the inner-magnetosphere plasmapause (PP) after magnetic midnight, when the PP coincides [100] with the stable trapping boundary (STB) at the earthward end of the plasmasheet’s earthward edge [101]. The other potential drop depicts the trapping boundary (TB) [100] at the tailward end of the plasmasheet’s earthward edge. This is where TH-E observed the sunward EX and inward EZ components (~18 mV/m; in magnitude) of the earthward-propagated cross-tail convection E field (EC; diamond symbol in orange, shaded interval in yellow). TH-E made these E field component observations near magnetic midnight, when the dawn–dusk EY component was zero and, therefore, we did not plot it. Furthermore, at that time, the dawnside inward SAPS E field was not developed yet on the PP and, therefore, was not observed by TH-E. Next, the pressure tensor line plots (Pe and Pi measured in the standard X direction indicated as XX) illustrate that the high-energy (up to 700 keV) electron pressure peaked more earthward than the high-energy (up to 4000 keV) ion pressure. As shown, PeXX peaked at the earthward edge of EC while PiXX peaked at its tailward edge. Thus, EC developed where these Pe and Pi peaks separated: near the TB. Furthermore, this Pe-Pi peak separation represents the separation of high-energy/hot electron (-) and ion (+) populations (charges) under reduced shielding (see details in Section 5). Such charge separation allows the establishment of the earthward-propagating cross-tail convection E field closer to Earth, at the TB, in this case. This implies the field-line mapping of the TH-E-observed dawnside inward-directed EC to the diffuse-discrete oval boundary. This is where the TB maps down to [101], in this case, as an equatorward-directed convection E field (or PPEF). Although we could not provide any ionospheric E field observational evidence, we speculate that the above-mentioned equatorward-directed convection E field (i.e., mapped-down earthward-propagating cross-tail convection E field) propagated equatorward (from the diffuse-discrete oval boundary to subauroral latitudes as an equatorward-directed PPEF) and triggered the emergence of the eastward SAPS flow within the coinciding RIT-MIT (observed by F15; see Figure 8d). This suggests that the emerging eastward SAPS flow developed mainly due to the equatorward-directed PPEF, since TH-E did not observe the inward SAPS E field on the inner-magnetosphere PP (only the inward EC at the TB). According to the TH-E orbit data, EC was observed at ~1 MLT and ~3.6 RE.

4. Summary of New Results

In this study, we report exceptional observational results revealing the dawn-cell-associated eastward SAPS flow’s amplification because of the extension of the polar equatorward- and eastward-directed EDawn-Dusk field to subauroral/trough latitudes (as an equatorward- and eastward-directed PPEF) on the nightside and dawnside.
Observed in 17 events by F15 in 2015 and 2016, the dawn cell became enhanced, expanded equatorward and duskward, and thus extended to the duskside (as shown in Figure 1b). Under such increased convection conditions, the dawn-cell-associated eastward SAPS flow amplifications observed suggest that the polar EDawn-Dusk field, by propagating equatorward and reaching subauroral/trough latitudes, had a significant role (1) in the equatorward and duskward expansion of the dawn cell, (2) in the enhancement of the dawn-cell-associated convection flows, and (3) in the amplification of the dawn-cell-associated eastward SAPS flow that (4) was detected soon after substorm onset, implying its fast-time development, and (5) streamed alongside the duskward-extended dawn cell: sunward after magnetic midnight and antisunward before magnetic midnight. The amplified eastward SAPS flow (6) appeared in the coinciding RIT-MIT, mimicking the amplified eastward SAPS flow, where (7) Te maximized and sometimes reached ~7000 K, implying (8) positive feedback mechanisms supporting SAPS flow maintenance and further growth. Correlated TH-E—F15 observations demonstrate (9) the cross-tail E field reaching the tailward boundary of the plasmasheet’s earthward edge, while the outward SAPS E field was absent on the inner-magnetosphere plasmapause, and (10) the emergence of the eastward SAPS flow in the conjugate ionosphere.

5. Discussion

In our more recent study [66], we investigated the dusk-cell-associated amplified westward SAPS flow in an opposite scenario (shown in Figure 1a). We provided observational evidence of the polar EDawn-Dusk field’s active role (A) in the dusk cell’s equatorward and dawnward extension, (B) in the dusk-cell-associated convection flows’ enhancement, and (C) in the dusk-cell-associated westward SAPS flow’s amplification within RIT-MIT.
In agreement with the Lyatsly et al. [61] model linking stronger SAPS flows to enhanced plasma convection near magnetic midnight, we obtained consistent observational results for the dusk-cell scenario (in our more recent study) and dawn-cell scenario (in this study). These dawn-cell-related results (listed as 1–3) and dusk-cell-related results (listed as A-C) provide convincing evidence of the polar EDusk-Dawn field’s key role in the amplification (and possibly even in the development) of SAPS in such scenarios, before the shielding E field builds up (see details below).
We note here that in our SAPS studies, we could not separate the underlying physical mechanisms related to the amplified SAPS flow’s development (on a short timescale and/or due to the polar EDawn-Dusk field’s equatorward propagation). This is the major shortfall of our studies because of the limited F15 data worked with. But the results obtained are consistent and show good agreement with the Lyatsky et al. [61] model. Although the proposed underlying generation mechanism put forward by Lyatksy et al. [61] for the amplification of westward SAPS flow is problematic, their model highlights the key role of polar EDawn-Dusk field. This is what we demonstrated with experimental observations covering the dawn-cell-associated eastward SAPS events (in this study) and the dusk-cell-associated westward SAPS events (in our more recent study [66]).
Regarding the underlying generation mechanisms, the magnetospheric convection is driven by the dawn–dusk E field. However, its main regulator in the magnetic equatorial region is the magnetospheric ring current [102,103]. According to the Dungey cycle [30], the magnetospheric convection is earthward in the nightside equatorial magnetosphere and the dawn–dusk cross-tail E field drives the hot plasma away from the reconnection site. On the earthward side in the magnetotail, the earthward (or inward) moving hot plasma leads to the generation of a ring current that is made up of trapped hot ions circling the Earth. However, the hot plasmasheet ions are more energetic than the hot plasmasheet electrons. Therefore, the hot plasmasheet ions’ contribution to the total plasma pressure is significantly larger. Consequently, the large-scale R2 FACs are generated only at the hot plasmasheet ions’ earthward edge. There, the pressure gradients are significant. Furthermore, increased earthward magnetospheric convection (a) moves the ring current’s earthward edge closer to Earth, (b) therefore modifies the amount of R2 FAC-flux passing through the new location, and also (c) polarizes the ring current’s earthward edge by applying a dawn–dusk potential drop to the ring current’s earthward edge. This (i.e., c) leads to the polarization of the ring current’s earthward edge by the primary dawn–dusk convection E field [58,102,103]. This, in turn, leads to the development of an opposing (dusk–dawn) secondary E field, called the shielding E field (shielding EDusk-Dawn field), providing shielding [104] by reducing or cancelling out the primary dawn–dusk E field [58].
Importantly, the primary dawn–dusk E field can transiently extend closer to Earth during the overall time-period of actions (listed above as a–c), before shielding builds up. In such a scenario, the boundaries of ring current ions and plasmasheet electrons separate closer to Earth, while the shielding effect is absent or decreases. Consequently, this allows the primary dawn–dusk E field’s transition closer to Earth [58]. On the nightside and dawnside, the primary dawn–dusk E field is the magnetotail-reconnection-related cross-tail dawn–dusk E field. We could demonstrate this scenario with TH-E observations. As shown in Figure 8g, TH-E observed the earthward-propagating inward- and dawnward-directed cross-tail E field (marked as EC) appearing near the trapping boundary (TB; i.e., tailward boundary of the plasmasheet’s earthward edge). Possibly, this inward EC mapped down along the magnetic field lines from the TB to the diffuse-discrete aurora boundary and then propagated to subauroral/trough latitudes as an equatorward- and eastward-directed PPEF triggering the emergence of eastward SAPS flow in the conjugate ionosphere (as observed by F15; see first line-plot set in Figure 8b). This suggestion is supported by the absence of a dawnside inward SAPS E field on the inner-magnetosphere plasmapause (as observed by TH-E, see Figure 8g). On the duskside, the primary dawn–dusk E field is the dayside-reconnection-related dawn–dusk E field directed outward and duskward. By mapping down to the ionosphere as a poleward- and westward-directed PPEF and reaching subauroral/trough latitudes, a scenario modeled by Lystsky et al. [61] can be created, where the dusk-cell-associated westward SAPS flows become amplified. This scenario was demonstrated in our recent study [66]. Califf et al. [105] documented the dayside-reconnection-related outward- and duskward-directed convection E field propagating earthward. Since in our recent study [66] we demonstrated the amplifications of dusk-cell-related westward SAPS flows in the region of the Harang discontinuity, our THEMIS observations did not show the dayside-reconnection-related outward- and duskward-directed convection E field propagating earthward.
This study is particularly relevant to the previous study of Makarevich et al. [12] investigating for the first time the scenario of an increased duskward-extending dawn cell (as shown in Figure 1b). There, the authors observed increased narrow eastward subauroral flows (developed after the second substorm onset) along the dawn cell’s equatorward edge. Makarevich et al. [12] specified the narrow eastward subauroral flow as eastward SAID and reported, for the first time, the narrow eastward SAID flow being associated with the duskward-extending dawn cell.

6. Conclusions

Based on the observational results (listed in Section 4 as 1–10) obtained for the 17 SAPS events investigated, we conclude the polar convection EDawn-Dusk field’s important role in impacting the dawn-cell-associated eastward SAPS flows investigated before shielding built up. These impacts include the eastward SAPS flow’s (i) amplification (evidenced by results 1–5) along with (ii) the positive feedback mechanisms (evidenced by results 6–7) and the eastward SAPS flow’s (iii) emergence (evidenced by results 9–10).
Lastly, we highlight the significance of these new observational results (listed in Section 4). By focusing on the field-line mapping of the magnetosphere dawn–dusk electric field and the resultant polar EDawn-Dusk field in the conjugate ionosphere, these new results provide new insights into the eastward SAPS flow’s amplification and generation. Their documentation with experimental observations advances our current understanding of SAPS/SAID development. It is described by various generation theories, which are controversial and still highly debated, and the underlying generation mechanisms are still not completely clear.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/atmos16091076/s1, Figure S1: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 22 February 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 10-22 MLT di-rection and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S2: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 22 February 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT di-rection and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S3: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 3 March 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S4: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 4 March 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 10-22 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S5: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 6 March 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S6: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 10 March 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell reaching the magnetic midnight sector. Bottom right panel: The SSUSI auroral image shows the signature of dawn cell in the particle precipitation. Figure S7: Top panel: The DMSP F15 survey plot shows the sunward streaming eastward SAPS flow after magnetic midnight on 8 September 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direc-tion and with the dawn cell reaching the magnetic midnight sector. Bottom right panel: The SSUSI au-roral image shows the signature of dawn cell in the particle precipitation. Figure S8: Top panel: The DMSP F15 survey plot shows the sunward streaming eastward SAPS flow after magnetic midnight on 8 September 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 10-22 MLT direc-tion and with the dawn cell reaching the magnetic midnight sector. Bottom right panel: The SSUSI au-roral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S9: Top panel: The DMSP F15 survey plot shows the sunward streaming eastward SAPS flow after magnetic midnight on 7 October 2015. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 13-1 MLT direction and with the dawn cell before magnetic midnight. Bottom right panel: The SSUSI auroral image shows the signatures of the dusk and dawn cells in the particle precipitation. Figure S10: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 28 January 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S11: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 25 February 2016. Bottom panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 13-1 MLT direction and with the dawn cell slightly intruding duskward. Figure S12: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 29 February 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT di-rection and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S13: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 10 March 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S14: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 10 March 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S15: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 11 March 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S16: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 12 March 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation. Figure S17: Top panel: The DMSP F15 survey plot shows the antisunward streaming eastward SAPS flow before magnetic midnight on 22 March 2016. Bottom left panel: The SuperDARN convection map shows the underlying two-cell polar convection pattern with a convection axis tilted in the 11-23 MLT direction and with the dawn cell intruding duskward. Bottom right panel: The SSUSI auroral image shows the signature of the duskward-intruding dawn cell in the particle precipitation.

Author Contributions

Conceptualization, I.H.; methodology, I.H.; software, I.H. and B.C.L.; validation, I.H. and B.C.L.; formal analysis, I.H.; investigation, I.H.; resources, B.C.L.; data curation, I.H.; writing—original draft preparation, I.H.; writing—review and editing, B.C.L. and I.H.; visualization, I.H.; supervision, B.C.L.; project administration, B.C.L.; funding acquisition, B.C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the United States Office of Naval Research under award number N62909-23-1-2057.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The DMSP data set can be accessed online through https://cedar.openmadrigal.org/list (accessed on 29 July 2025). The SSUSI auroral images can be accessed online through https://ssusi.jhuapl.edu/ (accessed on 2 February 2025). The SuperDARN convection maps can be accessed online: https://superdarn.ca/convection-maps (accessed on 29 July 2025). The THEMIS, GOES, and GEOTAIL data sets can be accessed online through https://cdaweb.gsfc.nasa.gov/cdaweb/istp_public/ (accessed on 29 July 2025). The Kp, AE, and SYM-H indices and the IEF EY and IMF BY and BZ components can be accessed online from the OMNI database through https://cdaweb.gsfc.nasa.gov/cdaweb/istp_public/ (accessed on 29 July 2025). The SuperMAG provided various types of substorm lists that can be accessed online from the SuperMAG substorm lists through https://supermag.jhuapl.edu/ (accessed on 29 July 2025).

Acknowledgments

We gratefully acknowledge that this material is based upon research supported by the United States Office of Naval Research under award number N62909-23-1-2057. We acknowledge the CEDAR Archival Madrigal Database for the DMSP data. The DMSP particle detectors were designed by Dave Hardy of Air Force Research Laboratory (AFRL). We also gratefully acknowledge the SSUSI auroral images and the THEMIS, GOES, and GEOTAIL data. We acknowledge the World Data Center for Geomagnetism at Kyoto (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html (accessed on 29 July 2025)) for providing the Kp AE indices and the use of SuperDARN convection maps. SuperDARN is a collection of radars funded by national scientific funding agencies of Australia, Canada, China, France, Italy, Japan, Norway, South Africa, the United Kingdom, and the United States of America. We also acknowledge the SuperMAG provided various types of substorm lists determined from the SML index.

Conflicts of Interest

The authors declare no conflicts of interest.

Glossary

SAPS E fieldDevelops in the inner magnetosphere and drives the subauroral polarization streams (SAPSs) in the ionosphere.
Dawn-dusk convection E fieldRefers to the large-scale electric field within the magnetosphere, driven by the solar wind’s interaction with the Earth’s magnetic field, which causes charged particles to convect or move.
Polar convection E fieldRefers to the electric field in Earth’s polar regions ultimately driven by the solar wind’s interaction with the magnetosphere.
Equatorward polar convection E fieldRefers to the mapped-down inward-directed magnetotail-reconnection-related cross-tail convection E field.
Polerward polar convection E fieldRefers to the mapped-down outward-directed magnetopause-reconnection-related convection E field.
Prompt penetration electric field (PPEF)Refers to an almost instantaneous, rapid penetration of electric fields from the Earth’s high-latitude ionosphere to the equatorial ionosphere.

Abbreviations

The following abbreviations are used in this article:
B fieldMagnetic Field
CGCurrent Generator
DMSPDefense Meteorological Satellite Program
E fieldElectric Field
ECConvection Electric Field
EDawn-DuskDawn-to-Dusk E Field
EDusk-DawnDusk-to-Dawn E Field
EPICEnergetic Particle and Ion Composition
EPSEnergetic Particle Sensor
EFIElectric Field Instrument
ESAElectrostatic Analyzer
FACsField-Aligned Currents
GLATGeographic Latitude
GLONGeographic Longitude
GMOMGround-Calculated Moments
GOESGeostationary Operational Environmental Satellites
GSEGeocentric Solar Ecliptic
GSMGeocentric Solar Magnetospheric
H-MHeppner–Maynard
LL Shell
M-IMagnetosphere–Ionosphere
MITMain Ionospheric Trough
MLATMagnetic Latitude
MLTMagnetic Local Time
NeElectron Density
PeElectron Pressure
PiIon Pressure
PJPolarization Jet
PPPlasmapause
PPEFPrompt Penetration Electric Field
RITRing-Current-Related Ionospheric Trough
R1Region 1
R2Region 2
SAIDSubauroral Ion Drifts
SAPSSubauroral Polarization Streams
SC PotSpacecraft Potential
SCWSubstorm Current Wedge
SCW2LSubstorm Current Wedge 2-Loop
SSUSISpecial Sensor Ultraviolet Spectrographic Imager
SuperDARNSuper Dual Auroral Radar Network
TeElectron Temperature
THEMISTime History of Events and Macroscale Interactions during Substorms
TBTrapping Boundary
TiIon Temperature
VeElectron Drift
VGVoltage Generator
VGFTFast-Time Voltage Generator
VGMMagnetospheric Voltage Generator
VHORCross-Track Horizontal Drift Velocity
VVERCross-Track Vertical Drift Velocity
WTSWestward Traveling Surge

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Figure 1. The schematic diagrams depict the two-cell convection pattern characterized by (a) a dawnward-extending dusk cell (shown in blue) and (b) a duskward-extending dawn cell (shown in red). The arrows in light blue and orange indicate plasma flow directions.
Figure 1. The schematic diagrams depict the two-cell convection pattern characterized by (a) a dawnward-extending dusk cell (shown in blue) and (b) a duskward-extending dawn cell (shown in red). The arrows in light blue and orange indicate plasma flow directions.
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Figure 2. The southern-hemisphere maps depict the 17 SAPS detections (dot symbols in colors) made by F15 in the African and South-American longitude sectors (a) in 2015 and (c) in 2016 and before and after magnetic midnight (b) in 2015 and (d) in 2016.
Figure 2. The southern-hemisphere maps depict the 17 SAPS detections (dot symbols in colors) made by F15 in the African and South-American longitude sectors (a) in 2015 and (c) in 2016 and before and after magnetic midnight (b) in 2015 and (d) in 2016.
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Figure 3. Observed in 2015 by F15, the amplified eastward SAPS flow (dot symbol in color, shaded interval in yellow) streamed (a,b) antisunward before magnetic midnight and (c) sunward after magnetic midnight within the deep coinciding troughs at ~840 km altitude. These are the RIT (circle in red) and the MIT (circle in blue), where Te became elevated. The plasmapause is marked as PP.
Figure 3. Observed in 2015 by F15, the amplified eastward SAPS flow (dot symbol in color, shaded interval in yellow) streamed (a,b) antisunward before magnetic midnight and (c) sunward after magnetic midnight within the deep coinciding troughs at ~840 km altitude. These are the RIT (circle in red) and the MIT (circle in blue), where Te became elevated. The plasmapause is marked as PP.
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Figure 4. Observed in 2016 by F15, the amplified eastward SAPS flow (dot symbol in color, shaded interval in yellow) streamed (a,b) antisunward before magnetic midnight within the deep coinciding troughs at ~840 km altitude. These are the RIT (circle in red) and the MIT (circle in blue), where Te became elevated. The plasmapause is marked as PP.
Figure 4. Observed in 2016 by F15, the amplified eastward SAPS flow (dot symbol in color, shaded interval in yellow) streamed (a,b) antisunward before magnetic midnight within the deep coinciding troughs at ~840 km altitude. These are the RIT (circle in red) and the MIT (circle in blue), where Te became elevated. The plasmapause is marked as PP.
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Figure 5. The time series cover the (a) 2015 and (b) 2016 eastward SAPS events depicting the underlying interplanetary and geomagnetic conditions. Each SAPS event of interest is marked (dot symbol in color; shaded interval in yellow) along with the preceding substorm onset (star symbol in red).
Figure 5. The time series cover the (a) 2015 and (b) 2016 eastward SAPS events depicting the underlying interplanetary and geomagnetic conditions. Each SAPS event of interest is marked (dot symbol in color; shaded interval in yellow) along with the preceding substorm onset (star symbol in red).
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Figure 6. (a,c,d) The different types of maps illustrate the 7 October 2015 eastward SAPS event’s location. (b) The F15 plots show the amplified eastward SAPS flow (shaded interval in yellow) and its plasma environment (e) soon after the last substorm onset.
Figure 6. (a,c,d) The different types of maps illustrate the 7 October 2015 eastward SAPS event’s location. (b) The F15 plots show the amplified eastward SAPS flow (shaded interval in yellow) and its plasma environment (e) soon after the last substorm onset.
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Figure 7. (a,c,d) The different types of maps illustrate the 3 March 2015 eastward SAPS event’s location. (b) The F15 plots show the amplified eastward SAPS flow (shaded interval in yellow) and its plasma environment (e) soon after the previous substorm onset.
Figure 7. (a,c,d) The different types of maps illustrate the 3 March 2015 eastward SAPS event’s location. (b) The F15 plots show the amplified eastward SAPS flow (shaded interval in yellow) and its plasma environment (e) soon after the previous substorm onset.
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Figure 8. (a,d,e) The various maps illustrate the 8 September 2015 eastward SAPS events’ locations. (b) The F15 plots show the amplified eastward SAPS flows (shaded intervals in yellow) and their plasma environment (c) after a series of substorm onsets. (f,g) The TH-E observations depict the earthward-propagating inward-directed cross-tail convection E field (EC) reaching the trapping boundary (TB).
Figure 8. (a,d,e) The various maps illustrate the 8 September 2015 eastward SAPS events’ locations. (b) The F15 plots show the amplified eastward SAPS flows (shaded intervals in yellow) and their plasma environment (c) after a series of substorm onsets. (f,g) The TH-E observations depict the earthward-propagating inward-directed cross-tail convection E field (EC) reaching the trapping boundary (TB).
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Table 1. Amplified eastward SAPS flows and associated plasma variables recorded by F15.
Table 1. Amplified eastward SAPS flows and associated plasma variables recorded by F15.
Eastward SAPS EventsPlasma Variables
Event
Number		Event
Date		UT (Hr:Mn)MLAT (°S)MLT (Hr:Mn)Ne
103 (cm−3)		VHOR (m/s)Te
(K)
122 February 201500:0860.2223:363.5−22005500
222 February 201523:5259.8823:452.8−20006600
33 March 201501:0757.3623:482.5−26007400
44 March 201502:3456.7623:483.5−12006400
56 March 201500:2161.4923:173.2−13006000
610 March 201523:1265.4522:362.5−31007000
78 September 201506:3255.5701:471.1+28006000
88 September 201508:1357.4602:301.1+22007000
97 October 201500:2354.5700:471.0+32006400
1028 January 201622:4162.3223:293.5−14006100
1125 February 201621:5763.2323:372.8−20006700
1229 February 201600:3560.7123:122.5−14005500
1310 March 201601:1758.4823:362.1−20007500
1410 March 201603:0658.3423:363.0−20005500
1511 March 201602:4457.6623:392.5−25006000
1612 March 201600:4558.7623:402.8−15005800
1722 March 201603:1360.7023:302.5−22006400
Table 2. Amplified eastward SAPS flows and their underlying conditions and preceding substorm onsets. # Newell and Gjerloev [78], * Forsyth et al. [79], ♦ Ohtani and Gjerloev [80].
Table 2. Amplified eastward SAPS flows and their underlying conditions and preceding substorm onsets. # Newell and Gjerloev [78], * Forsyth et al. [79], ♦ Ohtani and Gjerloev [80].
Eastward SAPS EventsIMF (nT)SYM-H
(nT)		KpAE
(nT)		Substorm Onsets
DateUT (Hr:Mn)BYBZUT (Hr:Mn)MLT (Hr)
22 February 201500:08−5.26−4.41−182+23023:10 #1.28
22 February 201523:52−6.48−4.43−84−17123:34 ♦1.29
3 March 201501:070.37−3.00−18328200:06 *#1.76
4 March 201502:341.342.72−724300:54 *0.21
6 March 201500:213.862.86−1.201+79--
10 March 201523:123.583.515333--
8 September 201506:32−6.2013.47−343−29--
8 September 201508:130.8015.32−273−3607:02 *9.39
7 October 201520:574.48−5.8−1057+63420:07 #7.71
28 January 201622:413.670.97−131−26--
25 February 201621:578.204.425242--
10 March 201601:17−1.55−2.42−2323900:12 *1.31
10 March 201603:06--−223−75--
11 March 201602:44−4.541.97−141+20800:29 *2.48
12 March 201600:45−4.34−3.44−17324300:13 ♦1.81
22 March 201603:130.45−3.65−162+10702:47 #0.57
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Horvath, I.; Lovell, B.C. Amplified Eastward SAPS Flows Observed in the Topside Ionosphere near Magnetic Midnight. Atmosphere 2025, 16, 1076. https://doi.org/10.3390/atmos16091076

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Horvath I, Lovell BC. Amplified Eastward SAPS Flows Observed in the Topside Ionosphere near Magnetic Midnight. Atmosphere. 2025; 16(9):1076. https://doi.org/10.3390/atmos16091076

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Horvath, Ildiko, and Brian C. Lovell. 2025. "Amplified Eastward SAPS Flows Observed in the Topside Ionosphere near Magnetic Midnight" Atmosphere 16, no. 9: 1076. https://doi.org/10.3390/atmos16091076

APA Style

Horvath, I., & Lovell, B. C. (2025). Amplified Eastward SAPS Flows Observed in the Topside Ionosphere near Magnetic Midnight. Atmosphere, 16(9), 1076. https://doi.org/10.3390/atmos16091076

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