Magnetosphere–Ionosphere Conjugate Harang Discontinuity and Sub-Auroral Polarization Streams (SAPS) Phenomena Observed by Multipoint Satellites

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.

Atmosphere2024,15(12), 1462;https://doi.org/10.3390/atmos15121462

Submission received: 30 October 2024 /Revised: 3 December 2024 /Accepted: 4 December 2024 /Published: 7 December 2024

(This article belongs to the Special IssueCoupling between Plasmasphere and Upper Atmosphere)

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Abstract

It is well understood that near midnight, the Harang Discontinuity separates the auroral duskside eastward electrojet (EEJ) and dawnside westward electrojet (WEJ) and associated plasma flows driven by enhanced magnetospheric convections via Magnetosphere–Ionosphere (M–I) coupling. There are conflicting reports regarding the significance of Region1 (R1) and R2 currents and the enhancement of Sub-Auroral Polarization Streams (SAPS) in the Harang region. We investigate the M–I conjugate Harang and SAPS phenomena using multipoint satellite observations. Results show the inner-magnetosphere (1) Harang region at midnight (between the plasmapause and the closed/open field-line boundary) with (2) a strong SAPS electric field (E_X ≈ 30 mV/m; in magnitude) in a fast-time voltage generator (VG_FT) near the plasmapause and the topside ionosphere (3) Harang Discontinuity (where R1 and R2 currents flow along) with (4) an enhanced SAPS flow (~1800 m/s) in the underlying VG_FT system (requiring no R2 currents). From these (1–4) findings we conclude (i) the significance of both R1 and R2 currents in the observed M–I conjugate Harang phenomenon’s development, (ii) the different development of the reversing EEJ–WEJ compared to the regular auroral EEJ and WEJ in the topside ionosphere R1–R2 system, and (iii) the R2 currents’ absence in the enhanced SAPS flow newly formed in the VG_FT system.

Keywords:

plasmasphere;upper atmosphere;ionosphere;magnetosphere–ionosphere coupling;energy and particle transport;hot zone;Harang discontinuity;SAPS;plasmapause;SAR arc

1. Introduction

Under southward interplanetary magnetic field (IMF) conditions (B_Z < 0), the polar convection electric (E) field of magnetospheric origin convects the ionospheric plasma in a two-cell convection pattern in which the plasma drifts (E×B) antisunward (from midday to midnight) across the polar cap (between the dusk and dawn convection cells) and then, sunward (forming return flows) along the flanks of the dusk and dawn cells [1].

As shown inFigure 1, the azimuthal (eastward–westward) Hall currents (J_H) flow (dominantly below ~125 km altitude) in the negative E×B drift direction along the auroral zone, and their concentrated flow forms the auroral electrojet: the eastward electrojet (EEJ) on the duskside and the westward electrojet (WEJ) on the dawnside. Across the oval, the meridional (poleward–equatorward) Pedersen currents (J_P) flow (dominantly above ~125 km altitude) [2] and connect the higher-latitude Region 1 (R1) and lower-latitude R2 field-aligned currents (FACs), as was first described by Iijima and Potemra [3]. On the duskside and within the EEJ’s regime, the poleward J_P connects the downward (↓) R2 FACs with the upward (↑) R1 FACs. Oppositely, on the dawnside and within the WEJ’s regime, the equatorward J_P connects the ↓R1–↑R2 FACs [4,5,6]. Since auroral electrojet currents are the main cause of magnetic (B) field disturbances, the magnetic disturbance computed for the horizontal (H) geomagnetic B field component (as ΔH = H_disturbed − H_quiet) is a good indicator of the auroral electrojet [7]: EEJ (ΔH > 0) and WEJ (ΔH < 0).

First described by Harang [8], the Harang Discontinuity (HD), as called by Heppner [9], but also known as Harang Reversal (HR), was depicted by the mapped ΔH contours and appeared as a ΔH reversal or discontinuity within a narrow magnetic local time (MLT) sector near magnetic midnight. Within a narrow region of the discontinuity (marked as HD inFigure 1), a reversal in the auroral electrojet appeared where the dominating EEJ became westward-directed [8]. This led to a scenario where the EEJ (situated equatorward of the discontinuity) and the WEJ (situated poleward of the discontinuity) appeared together within a narrow longitude region or MLT sector (marked with the shaded region in yellow inFigure 1) where often, polar substorms (i.e., substorms located at polar latitudes in the 19–23 MLT sector) were recorded [10,11]. The reversal was also depicted by the J_P-related meridional E field as a gradual direction change where the dominating poleward E field became equatorward-directed [9,12,13,14] and also appeared in the azimuthal E×B plasma convection, where the reversal separated oppositely directed convection flows [15] while velocity shear developed along the discontinuity [13]. However, during polar substorms, when the EEJ represented the equatorward part of the substorm current wedge (SCW) [16,17], the reversal was associated with increased ↑FACs and was located along the interface of two magnetic vortices [10,11].

Figure 1. The schematic diagram modified fromFigure 1 of Koskinen and Pulkkinen [13] shows the Harang Discontinuity (HD; in red), appearing as an eastward/westward electrojet (EEJ/WEJ) reversal (shaded interval in yellow), and the various currents and E×B drifts in the auroral zone.

Since the early study of Maynard [12], it is well understood that the Harang region is magnetosphere–ionosphere (M–I) conjugate. While the discontinuity’s poleward region maps well into the plasmasheet, its equatorward region maps close to the plasmasheet’s inner edge [18]. In the magnetic midnight sector, where the Harang region maps into the magnetotail, the discontinuity can also be interpreted as a fault line separating the inflated and collapsed magnetic fields during substorms [12,19], while substorm injection occurs near the Harang discontinuity [20].

Based on theoretical considerations and Rice Convection Model (RCM) simulations, the M–I conjugate Harang Discontinuity’s development in the magnetosphere was explained by Erickson et al. [18], with the dawn–dusk pressure asymmetry developed in the near-Earth plasmasheet. Due to the domination of the magnetic pressure gradient and curvature drift over the E×B drift, causing ions to drift westward and electrons eastward, a dawn–dusk ion pressure gradient forms, since hot ions build up the plasma pressure in the plasmasheet. Consequently, the dawnside depletion of magnetospheric hot ions provides the primary mechanism for the large-scale ↑FACs to map down from the plasmasheet to the center of the auroral zone in the midnight MLT sector. These large-scale ↑FACs include the ↑R1 and ↑R2 FACs (shown inFigure 1), as were first defined by Iijima and Potemra [5].

However, there are conflicting results reported by previous studies regarding the large-scale FACs’ locations with respect to the Harang region and their roles played in the development of the Harang Discontinuity. Some studies documented the Harang region’s collocations with sufficiently strong R1 FACs causing converging E fields [18,21,22], while others documented collocations with R2 FACs because of the energy-dependent plasma intrusion [23]. There were also collocations with subauroral flows [19,21,24,25,26] and even with both R1 and R2 FACs, where the R1–R2 demarcation line marks the Harang Discontinuity [27]. Subauroral flows include the Sub-Auroral Polarization Streams (SAPS) [28] that are often associated with the Stable Auroral Red (SAR) arc [29], which is a 630 nm redline emission [30] and is generated by the same heating mechanism that fuels the inner-magnetosphere hot zone [31].

Understanding and modeling the M–I conjugate Harang region and SAPS are significant because of the associated space weather phenomena [32]. These include the magnetotailreconnection-related particle injections, occurring during substorms that are more transient, and therefore less predictable, than storms and causing substantial damage to satellites [33] and also the M–I conjugate SAPS, of which development often starts near the Harang region, when the plasma convection increases in the substorm growth phase [34]. Then, the SAPS flows and associated abrupt changes in the subauroral ionosphere’s plasma environment [35], along with the plasma turbulence and instabilities developed [36], significantly affect space-based technology and impact radio wave propagations [37].

Although these above-mentioned studies demonstrate the Harang region’s complex nature in the coupled M–I system and greatly increase our understanding, there are still significant gaps in our knowledge and understanding. (i) Experimental E field observations depicting the entire Harang E field phenomenon in the inner magnetosphere are still missing, along with (ii) M–I conjugate Harang region and SAPS observations, and (iii) the large-scale FAC pattern in the Harang region is still not clear.

Motivated by these (i–iii) existing gaps in our knowledge and understanding, we conducted a detailed study investigating the M–I conjugate Harang and SAPS phenomena based on multipoint satellite observations covering two events that occurred on 3 and 6 September 2014. From our significant results, we conclude for the two events investigated that (a) both the R1 and R2 FACs were significant in the development of the M–I conjugate Harang phenomenon observed, (b) the mapped down and collocated strong SAPS E field drove strong plasma flows in the SAPS channel near the Harang region, and (c) the reversing EEJ–WEJ across the Harang Discontinuity developed differently than the regular auroral EEJ and WEJ away from the Harang Discontinuity.

2. Materials and Methods

We used data from two (TH-A and TH-D) of the five THEMIS (Time History of Events and Macroscale Interactions during Substorms) satellites [38]. Since 2011, when TH-B and TH-C were maneuvered into their respective orbits around the Moon [39], only three THEMIS satellites (TH-A, TH-D and TH-E) orbit the Earth on their highly elliptical orbits in the magnetosphere. During the two events investigated, all these three THEMIS satellites were traveling close to the equatorial plane in the inner-magnetosphere tail region, but only TH-A and TH-D observed the Harang and SAPS phenomena in the inner magnetosphere. For observing the large-scale E fields, we used the E field components (E_X, E_Y, E_Z; mV/m) measured in Geocentric Solar Magnetospheric (GSM) coordinates and provided by the EFI (Electric Field Instrument) suite. For observing the inner-magnetosphere plasma environment, we used the data of hot ion and electron density (Ni, Ne; cm⁻³), ion temperature (Ti; eV), and ion and electron flux (eV/cm²-s-sr-eV) from the ESA (Electrostatic Analyzer) suite, along with the high-energy Ti measured in the perpendicular (⟂) and parallel (‖) directions (Ti_⟂, Ti_‖; eV) and hot ion and electron pressure (Pi, Pe; eV/cm³) from the MOM (On Board Moment) and GMOM (Ground Calculated Particle Moment) suites. Magnetic (B) field components (B_X, B_Y, B_Z; nT) were provided by the SCM (Search Coil Magnetometer) suite.

For observing the Harang Discontinuity and SAPS phenomena in the topside ionosphere (at ~840 km altitude), we used multi-instrument DMSP (Defense Meteorological Satellite Program) [40] data collected by spacecraft F17 and F18 along their ~98.7° inclined and 110 min polar orbits. These data include electron density (Ne; 1/cm³) plus electron and ion temperature (Te, Ti; K), depicting the topside-ionosphere plasma environment, cross-track horizontal (HOR) ion drift (V_HOR; m/s), depicting the sunward SAPS flow and the auroral sunward EEJ and antisunward WEJ flows, and magnetic deflection components (δB_X, δB_Y, δB_Z; nT), from which we inferred the locations and directions of the Hall currents (J_H from δB_X) and large-scale R1 and R2 FACs (from δB_Y, δB_Z).

For observing magnetotail-reconnection-related particle injections, we used electron flux measurements taken by the Geostationary Operational Environmental Satellites (GOES) orbiting the Earth once a day at 6.6 R_E. GOES-15 observed the magnetic midnight sector and provided average electron flux values measured at various channels: 40, 75, 150, 275, and 475 keV.

Orbit data provided by THEMIS, GOES, and DMSP include spacecraft location in GSM coordinates, magnetic local time (MLT; Hr), Northern- and SouthernHemisphere footprints in geographic [longitude (GLON), °E; latitude (GLAT), °N] coordinates, geomagnetic latitude (MLAT, °N), and L shell (R_E).

We also employed a small collection of imageries, including the DMSP SSUSI (Special Sensor Ultraviolet Spectrographic Imager) [41] images, for observing the auroral oval, and the Redline Geospace Observatory (REGO) [42] images, for observing the SAR arc.

For specifying the substorm onset times of interest, we utilized the substorm list of Newell and Gjerloev [43] published by SuperMAG along with the auroral electrojet indices of AE (nT) and AL (nT).

3. Results

3.1. The Harang Reversal Observed in the Inner Magnetosphere by THEMIS in Events 1–2

Figure 2 andFigure 3 are constructed with TH-E and TH-D data, respectively, and depict the inner-magnetosphere Harang and SAPS phenomena observed near magnetic midnight, close to the magnetic equatorial plane, during the 3 and 6 September 2014 events investigated.

InFigure 2a andFigure 3a, the orbit plots show that during these events, THEMIS was travelling tailward, close to the magnetic equatorial plane, and observed the Harang region from its earthward edge (marked as symbol dot) on the duskside to its tailward edge (marked as symbol diamond) on the dawnside.

InFigure 2b andFigure 3b, we show again the previously (inFigure 1) illustrated schematic diagram that we simplified and modified for the event of interest based on the THEMIS observations made in the inner magnetosphere. This modified diagram depicts the Harang Discontinuity (marked as HD in red) mapped down to the auroral zone near magnetic midnight. As shown, the Harang region’s equatorward edge (marked as shaded region in magenta) is located on the duskside, where the duskside ↓R2 and dawnside ↑R2 FACs are separated by the discontinuity. Oppositely, the Harang region’s poleward edge (marked as shaded region in cyan) is located on the dawnside, where the duskside ↑R1 and dawnside ↓R1 FACs are separated by the discontinuity.

InFigure 2c andFigure 3c, the MLT versus MLAT polar plot shows the THEMIS footprints that crossed the auroral oval from the duskside to the dawnside near magnetic midnight. We also marked the Harang region’s equatorward and poleward edges (shaded regions in colors) in order to highlight the similarities between the schematic diagram and the polar plot. We also mapped the substorm onsets (as symbol stars in red and magenta), which occurred. During Event 1, four substorm onsets occurred on the duskside, just poleward of the mapped-down THEMIS-observed Harang region. During Event 2, two substorm onsets occurred near the mapped-down THEMIS-observed Harang region’s edges.

Figure 2d andFigure 3d are constructed with a set of THEMIS time series that cover 2.5 and 3.0 universal time (UT) hours, respectively, in the tailward direction and depict the entire Harang region (from earthward edge to tailward edge) and its plasma environment in the inner magnetosphere.

By progressing from top to bottom, first, we show with the hot (30–300 keV) Ne plot how the density of hot electrons varied in the inner magnetosphere. Based on the classification of Frank [44] and Burrows and McDiarmid [45], we marked the various regimes and boundaries observed by THEMIS. In the tailward direction, the hot Ne decreased significantly, from a maximum near the plasmapause (PP) to a local minimum at the trapping boundary (TB). According to the studies of Roederer [46,47], the region between the PP and TB is known as the plasmasheet’s earthward edge (marked as shaded interval in gray), where the electrons are trapped and able to drift around the Earth. Progressing tailward, the region between the TB and closed–open (C/O) field-line boundary is a region, specified by Roederer [46,47], where the particles are quasi-trapped and able to drift less [46,47]. There, the THEMIS-observed reduced hot Ne became further decreased in the tailward direction.

Next, the Efield line-plot series depict how the E field components (E_X, E_Y, E_Z) varied in the Harang region. Here, we marked (as red arrows) the associated oppositely directed FACs (as also shown by the schematic diagram inFigure 2b andFigure 3b). These include the ↓–↑ R2 FACs (shaded interval in magenta), marking the Harang region’s earthward edge, and the ↓–↑ R1 FACs (shaded interval in cyan), marking the Harang region’s tailward edge. These Efield line-plot series depict two sets of oppositely directed E fields. One set is located at the Harang region’s earthward edge, near the PP, where the (antisunward, duskward, outward) SAPS E field developed within the ↓R2 FACs and the (sunward, dawnward, inward) convection E field (E_C) within the ↑R2 FACs. The other set is located at the Harang region’s tailward edge, across the C/O field-line boundary, where the convection E field (E_C) reversed from antisunward–duskward–outward (located within the ↓R1 FACs) to sunward–dawnward–inward (located within the ↑R1 FACs). Between the Harang region’s two edges, the absence of a strong E field provides observational evidence that THEMIS mainly followed the discontinuity in the tail region.

Since we could not compute the numerical values of large-scale R1 and R2 FACs, we used the THEMIS-provided hot (30–300 keV) ion pressure (Pi_XX) and electron pressure (Pe_XX) measurements in order to get an indication of the large-scale FACs’ locations and directions. Centered over the PP, the steep positive and shallow negative Pi_XX gradients depict the signatures of oppositely directed (↓–↑) R2 FACs in the Harang region’s earthward edge and imply that the ↑R2 FACs are located within the regime of trapped electrons. Thus, the Pi_XX line plot depicts a strong duskside–dawnside asymmetry, first explained by Erickson et al. [18], and shows that the hot ion pressure became enhanced on the duskside within the regime of trapped particles and became depleted on the dawnside within the regime of quasi-trapped particles. Across the C/O field-line boundary, the positive and negative Pe_XX gradients depict the signatures of oppositely directed (↓–↑) R1 FACs at the Harang region’s tailward edge and imply that the ↓R1 FACs are located within the regime of quasi-trapped electrons and that the interface of the duskside ↑R2 and dawnside ↓R1 FACs is located at the TB [4].

Next, the Pi_XY and Pe_XY line plots depict the oscillating amplitudes, which are the signatures of shear flows developed along the discontinuity, mostly in the regime of quasi-trapped particles.

Finally, the MLT and L line plots show that THEMIS observed the Harang region’s earthward edge at ~23 MLT and ~4 R_E and the tailward edge just after magnetic midnight at ~7 R_E. Here, we also plotted the GOES-15-observed substorm onset time (as symbol star in magenta) that occurred just before the SAPS E field observation. For Event 1, we also marked the following substorm onset times (listed as 2nd and 3rd; marked as symbol stars in red) that occurred when TH-E travelled along the discontinuity and observed the strong shear flows.

3.2. Inner-Magnetosphere SAPS and the Hot Zone Observed by THEMIS in Events 1–2

Now, we focus on the Harang region’s earthward edge, where the SAPS E field developed soon after the GOES-observed substorm onset. InFigure 4, we demonstrate with THEMIS observations that the inner-magnetosphere SAPS developed on a short timescale, via the process of short circuiting, during the unfolding magnetotail-reconnection-related particle injections [48,49]. Meanwhile, the hot zone developed within the regime of trapped electrons and became most enhanced near the SAPS channel.

Figure 4a,b illustrate Event 1 and Event 2, respectively, with the electron flux line-plot sets, E_X component, hot (6 eV–30 keV) Ne and hot (5 eV–25 keV) Ni, B_Y component, high-energy (30–300 keV) field-aligned Ti_‖, and lower-energy (5 eV–25 keV) Ti, along with the previously described regions (indicated by the shaded intervals) and boundaries (marked).

As shown by the electron flux line-plot sets, THEMIS observed the Harang region during the unfolding magnetotail-reconnection-related particle injections. Then, the strongest dispersionless particle injections occurred across the trapping boundary (TB) in Event 1 and across the soft electron (0.1–1.0 keV) boundary (SEB) in Event 2. These imply that the trapped and quasi-trapped electrons and the hot ions had been injected from the tail region [46,47]. Meanwhile, the strongest dispersed particle injection occurred across the C/O field-line boundary in Event 1.

As observed by THEMIS, these reconnection-related plasma injections led to the development of the outward SAPS E field on a short timescale via the process of short circuiting. Originally, the short-circuiting theory was put forward by Mishin and Phul-Quinn [50] and, later on, was further evolved by Mishin [48,49] specifying the fast-time voltage generator (VG_FT) [49]. The short-circuiting theory defines the earthward-injected plasma (made up of hot ions, which are the earthward-injected hot ring-current ions, and hot electrons [48,49]) as mesoscale plasma flows (MPFs; ≥1 keV) [51] that are also known from previous studies as bursty bulk flows (BBF) [52].

According to the short-circuiting theory, the MPFs become shorted out (or neutralized) across the plasmapause. Consequently, the hot electrons and ions become separated as they become stopped at their respective stopping points; the hot electrons are stopped (by the neutralized MPFs) at the plasmapause, but the hot ions are allowed moving across the plasmapause and further inward (or earthward) and are stopped (by the emerging outward SAPS E field) further earthward. Such hot electron–ion separation generates a VG_FT [49], located between the hot electron and ion stopping points, wherein the outward SAPS E field develops. Within the VG_FT, where the hot electrons are absent, the mixture of hot ions and cold plasmasphere electrons creates a turbulent plasmasphere boundary layer (TPBL) [50].

The signatures of short-circuiting and stopping points are shown by the hot Ne and Ni line plots. The hot Ne line plot depicts the steep Ne gradient formed tailward of the plasmapause and the narrow Ne peak (i.e., stopping point) upon the Ne pileup. The hot Ni plot depicts the narrow Ni peak (i.e., stopping point) upon the Ni pileup formed earthward of the plasmapause. Between the two stopping points (i.e., in the VG_FT), the E_X line plot shows the outward SAPS E field developed. These narrow Ne and Ni peaks create their respective diamagnetic electron and ion drifts, which, in turn, drive their respective diamagnetic electron and ion currents [53].

InFigure 4b, Event 2 demonstrates some additional aspects of the short-circuiting phenomenon with the more earthward-moving ring-current ions, a phenomenon appearing in ion spectrograms as a “nose structure” [54]. Because of the “nosestructure” developed, the TPBL was wider in this Event 2, since the hot ions (i.e., ring-current ions) kept moving earthward, leading to the development of a larger Ni peak (marked as shaded interval in light orange).

InFigure 4a,b, we marked the region of the TPBL (as dotted line in dark red) developed between the narrow Ne and Ni peaks driving their respective increased diamagnetic electron and ion currents, and thus providing energy for the excitation of various waves, including electromagnetic ion cyclotron (EMIC) waves [53]. This is demonstrated with the SCM B_Y line plot depicting a sudden and localized B_Y increase: within the SAPS E field channel where the narrow Ni peak developed in Event 1 and earthward of the SAPS channel where the larger narrow Ni peak (i.e., “nose structure”) developed in Event 2. A subset of the SCM B_Y line plot illustrates the structured periodic pulsations, which are the typical signatures of EMIC waves [55]. Both the EMIC waves (generated in the ring-current core region) and the plasma turbulence (developed in the TPBL) produced rapid, bulk plasma heating that fueled the hot zone [51].

InFigure 4a,b, the line plot of high-energy (30–300 keV) field-aligned Ti_‖ shows the hot zone appearing with a steep gradient on its earthward side and peaking within the TPBL where the EMIC waves developed, both (TPBL and EMIC) providing bulk plasma heating [56]. Meanwhile, the line plot of lower-energy (5 eV–25 keV) Ti shows that the hot zone’s tailward boundary was located at the trapping boundary. Thus, a larger region of the hot zone developed in the regime of trapped electrons, where the trapped particles’ higher energy provided the bulk plasma heating, as was first suggested by the early study of Serbu and Maier [57]. As marked, the region of the cold zone [31] was also observed in the regime of minimum Ti located earthward of the hot zone.

3.3. The Harang Reversal Observed in the Duskside Topside Ionosphere by DMSP in Event 1

InFigure 5a, the schematic diagram depicts the Harang region in a scenario that DMSP F17 observed in Event 1 in an earlier MLT sector (than TH-E in Event 1). Then, F17 travelled in the dawn–dusk direction and crossed the auroral zone within the Harang region. F17 crossed the discontinuity (marked as HD in red) and observed that the auroral electrojet reversed from a duskside EEJ (marked as shaded region in magenta) to a dawnside WEJ (marked as shaded region in cyan).

InFigure 5a, two sets of MLT versus MLAT polar plots are shown. In each polar plot, we mapped the F17 pass (in green), along with the locations of EEJ (marked as symbol square) and WEJ (marked as symbol diamond) within the Harang region and SAPS (observed by F17 and THEMIS; marked as symbol dots in colors) near the Harang region. The first plot depicts the two-cell convection pattern drawn based on the SuperDARN convection map generated for 06:20 UT, which is the time of the F17 SAPS detection (6.40 UT or 06:24 UT). Here, we also plotted the 3 substorm onset locations (marked as symbol stars in red) situated near the auroral electrojet reversal in the Harang region. The convection pattern depicts the weaker dawn cell (in red) and the stronger dusk cell (in blue), which (1) became quite thin on the duskside (where the Harang region developed) and (2) intruded into the dawnside across the magnetic midnight meridian. The second plot shows the auroral precipitation pattern with the DMSP/SSUSI imagery and depicts the above-described (1–2) dusk-cell signatures appearing in the auroral precipitation pattern. The phenomenon of an unusually thin auroral oval in the Harang region was first reported by Nishimura et al. [58] and further investigated by Lyons et al. [59] and was explained with the large reduction of the electron-precipitation energy flux occurring during substorm onset.

InFigure 5b, the southern hemisphere map shows the mapped F17 ground track (in green) and the loosely correlated (in MLT) TH-E footprints (in orange) depicting the mapped-down location of the M–I conjugate Harang region that was observed (in Event 1) over the South–Eastern Pacific.

InFigure 5b, the DMSP F17 time series are constructed with the plasma density (Ne), temperature (Te and Ti), zonal drift (V_HOR), and magnetic deflection (δB_X and δB_Y) data. These line plots cover a larger region of the Harang reversal and depict its plasma environment. In the duskside subauroral region, the main ionospheric trough (MIT) developed poleward of the ring ionospheric trough (RIT) where Te peaked at ~4200 K. Within the MIT, the sunward SAPS flow (shaded interval in yellow) reached ~1800 m/s in the vicinity of the Harang region. In the duskside auroral zone, F17 observed the regular auroral EEJ appearing as a sunward flow (~800 m/s; shaded interval in light blue) and, by crossing the discontinuity, the reversal of EEJ (sunward flows, ~1000 m/s; shaded interval in magenta) to WEJ (antisunward flows, ~−1700 m/s; shaded interval in cyan). In the duskside polar cap region, the zonal plasma flow was minimal. Since we could not compute the R1 and R2 FACs’ values, we used the δB_X and δB_Y gradients in order to mark the underlying current flows’ locations and directions. As shown, no currents were flowing into the SAPS channel, implying that the SAPS flow was newly formed at that time (~25 min after the TH-E SAPS observation) and developed in a fast-time voltage generator [49] where the mapped-down poleward subauroral E field drove the plasma (in the SAPS channel) westward that was streaming sunward, in concert with the dusk cell (as marked in both polar plots). As shown by the schematic diagram (inFigure 5a) and depicted here by the δB_X line plot, the Hall currents (J_H) were flowing antisunward (or eastward) in the two EEJs and sunward (or westward) in the WEJ and changed directions (from antisunward to sunward) across the Harang Discontinuity. Depicted by the δB_Y line plot, the ↓R2–↑R1 FACs connected via the poleward-directed Pedersen currents (J_P) within the regular duskside auroral EEJ. Based on the pre- and postmidnight THEMIS observations, we suggest that the electrojet reversal occurred in the duskside Harang region because of the flow of ↑R2 FACs (from equatorward) and ↑R1 FACs (from poleward) into the oval along the discontinuity. We note here that TH-E observed ↓R1 FACs on the dawnside, but these ↓R1 FACs become upward-directed (i.e., ↑R1 FACs) on the duskside. Therefore, because of the flow of ↑R2 (from equatorward) and ↑R1 (from poleward) into the oval along the discontinuity, the demarcation of ↑R2–↑R1 FACs marks the discontinuity’s [27] center, where the antisunward Hall currents reversed to sunward (as shown by the δB_X line plot). Thus, in the duskside Harang region, the same types of region currents became oppositely directed and connected via the Pedersen currents (J_P): ↓–↑R2 connected within the EEJ via the poleward J_P, and ↑–↓R1 connected within the WEJ via the equatorward J_P. Consequently, these reversing electrojets (from EEJ to WEJ) developed in the Harang region differently from their respective regular auroral duskside EEJ and dawnside WEJ (where the oppositely directed R1 and R2 FACs connect).

3.4. The Harang Reversal Observed in the Topside Ionosphere by DMSP F18 in Event 2

Figure 6 is constructed the same way asFigure 5 and shows the F18 observations made in the Harang region in Event 2.

InFigure 6a, the schematic diagram shows that the Harang region developed in a scenario that was described above (inSection 3.3), but in this event, the Harang region was observed near magnetic midnight in the Australian longitude sector.

Figure 6a shows the polar convection plot drawn based on the SuperDARN convection map generated for 09:26 UT, which is similar to the respective UTs of the F18 Harang observation (made at 09:27 UT) and DMSP/SSUSI imagery (generated for 09:24 UT). These polar plots show that the Harang region was observed within the dusk cell and that the substorm onset mapped to the tip of the dusk cell, reaching the magnetic midnight meridian. Depicted by the DMSP/SSUSI imagery, the WEJ developed across an auroral arc (where the convection flow was antisunward) and the EEJ within the auroral oval (where the convection flow was sunward). Meanwhile, the TH-D-observed SAPS E field mapped down to the equatorward region of the dusk cell.

InFigure 6b, the F18 line-plot sets reveal that because of the alignment of the F18 pass, the duskside plasma density troughs (MIT and RIT) and SAPS, along with the regular auroral EEJ, were not observed. However, in the pre-midnight Harang region, F18 observed the reversal of EEJ (sunward flows, ~1200 m/s; shaded interval in magenta) to WEJ (antisunward flows, ~−1200 m/s; shaded interval in cyan), wherein Te locally peaked at ~4500 K. Depicted by the δB_X line plot, the Hall currents (J_H) reversed across the discontinuity from antisunward (or eastward) in the EEJ to sunward (or westward) in the WEJ. Meanwhile, the δB_Z plot shows that the discontinuity developed (in the same way as in Event 1) because of the flow of ↑R2 (from equatorward) and ↑R1 (from poleward) into the oval along the discontinuity (as described inSection 3.3).

3.5. The SAR Arc Developed Within the SAPS Channel

InFigure 7 andFigure 8, the global map shows the Northern–Southern Hemisphere conjugate THEMIS footprints made during the event of interest and the mapped-down northern–southern conjugate SAPS E field locations. In the Northern Hemisphere, THEMIS observed the Harang region along with the SAPS E field channel over the North American continent, close to the Gillam REGO station. Situated near the Harang region, the substorm onset locations (symbol stars in red) and the GOES-15 footprints (in magenta) are also mapped.

InFigure 7 andFigure 8, the REGO imagery recorded from Gillam shows the unstructured diffuse aurora formed as a result of the trapped plasmasheet electrons’ (30–300 keV; shown inFigure 2 andFigure 3) pitch angle scattering into the loss cone by the interactions of electron cyclotron harmonic and whistler-mode chorus waves resonating with the trapped electrons in that energy range [60,61,62]. The image of the diffuse aurora appears predominantly in the greenline (557.7 nm) and redline (630.0 nm) emissions from the atomic oxygen (O) but also in the blueline (470.9 nm) emission from the ionized molecular nitrogen (N₂⁺) [63].

As depicted by the 630 nm imagery, a series of SAR arc events occurred, commencing at the substorm onsets (marked as symbol stars in colors). At the event onset, the SAR arc developed at the equatorward edge of the diffuse aurora, which maps to the earthward edge of the plasmasheet, where the underlying heat source igniting the SAR arc is located [64]. Later on, the SAR arc deviated from the diffuse aurora’s equatorward edge and moved equatorward during the unfolding substorm (as shown in the bottom panel) when the auroral oval moved equatorward and because of the nature of the underlying M–I coupled interactions [65].

Based on the close correlation obtained in Event 1, the SAR arc developed because of the downward heat flux produced by (i) the hot ring current and cold plasma density interaction generating EMIC waves and because of (ii) the plasma turbulence developed in the SAPS E field channel (TPBL), as shown inFigure 4a. Thus, the downward heat flux (igniting the SAR arc) was provided by the same heating mechanisms (i.e., TPBL and EMIC waves located in the SAPS E field channel) that fueled the hot zone. Although the SAPS E field channel–SAR arc correlation obtained for Event 2 is quite loose, we speculate that the development of “nose structure” and the consequential earthward extension of the TPBL and EMIC waves could be the cause of the strong deviation of the SAR arc from the diffuse aurora by generating bulk plasma-heating (more earthward) and downward heat-flux (more equatorward).

In the bottom panel ofFigure 7 andFigure 8, the GOES-15 electron flux line-plot sets, along with the AE and AL indices, show that a series of dispersionless particle injections occurred during the substorm onsets (marked as shaded intervals in yellow). THEMIS observed the SAPS E field soon after the nearest substorm onset, when the SAR arc was already positioned away from the diffuse aurora.

4. Discussion

In this study, we have investigated the M–I conjugate Harang and SAPS phenomena based on the experimental inner-magnetosphere THEMIS and topside-ionosphere DMSP observations made in two events. Significantly, these events of interest occurred during magnetotailreconnection-related plasma injections, when the outward SAPS E field develops on a short timescale by short circuiting [48,49]. Our investigation included also the development of the SAR arc (in the F region) unfolding in the vicinity of the Harang region.

With our new findings, we add to the previous studies highlighted inSection 1. While these previous studies focused on specific regions of the Harang Reversal, this study investigates the entire M–I conjugate Harang phenomenon based on experimental observations covering the entire Harang region. In the two events investigated, the THEMIS satellite followed the Harang Discontinuity along the equatorial plane in the tail region and thus, observed the discontinuity from its earthward edge along with the SAPS phenomenon to its tailward edge in the inner magnetosphere. Meanwhile, the DMSP spacecraft observed the discontinuity across the auroral oval from its equatorward boundary, along with the nearby SAPS channel, to its poleward boundary in the topside ionosphere. These unique observational results reported here set this study apart from the previous studies and provide new insights.

Significantly, by crossing the magnetic midnight meridian, the THEMIS orbit was dusk–dawn aligned and, therefore, THEMIS was able to observe the duskside–dawnside hot ion-pressure asymmetry, providing the primary mechanism for the mapping down of large-scale FACs to the auroral zone, as was first reported by Erickson et al. [18].

THEMIS also observed the Harang Discontinuity’s basic inner-magnetosphere characteristics: its earthward edge on the duskside (observed just before magnetic midnight) and its tailward edge on the dawnside (observed just after magnetic midnight).

These THEMIS observations provide observational evidence that in the Harang region’s earthward edge, located in the regime of trapped electrons, the R2 FACs reversed from ↓R2 within the outward SAPS E field to ↑R2 within the inward convection E field. Our findings are in good agreement with the results of Gkioulidou et al. [23], reporting the necessity of the longitudinal overlap between the ↓R2 and ↑R2 FACs in the Harang Reversal’s development.

We demonstrate also that in the Harang region’s tailward edge, the R1 FACs reversed across the closed–open field-line boundary from ↓R1 to ↑R1 in concert with their respective inward and outward convection E fields. These findings are in good agreement with the study of Yang et al. [22], reporting the Harang Reversal’s collocation with the ↑R1 FACs.

Importantly, THEMIS observed the Harang region’s earthward edge during short circuiting, creating a fast-time voltage generator (VG_FT) where the SAPS E field channel developed [50]. In the premidnight sector, at ~23 MLT, the dominating SAPS component was E_X. In magnitude, the SAPS E_X component reached ~20 mV/m in Event 1 and ~30 mV/m in Event 2 in the antisunward (outward) direction. By mapping down to the ionosphere, the large SAPS E field drove unusually strong SAPS flows that we could not demonstrate with closely correlated M–I conjugate THEMIS-DMSP observations. However, our results are in good agreement with the studies of Zou et al. [25,26] and Gkioulidou et al. [23], reporting enhanced SAPS flows in the low-conductivity subauroral region situated equatorward of the Harang region.

Our new results also include that within the discontinuity (located between the earthward and tailward edges of the Harang region), the trapping boundary separated the duskside ↑R2 FACs (located within the regime of trapped electrons) and the dawnside ↓R1 FACs (located within the regime of quasi-trapped electrons). These provide observational evidence of the significance of both the R2 and R1 FACs in the Harang Reversal’s development in the inner magnetosphere.

The trapping boundary maps down to the diffuse–discrete oval boundary separating the R2 FACs in the diffuse auroral zone and the R1 FACs in the discrete auroral zone [66]. We applied the inner-magnetosphere large-scale FAC configuration observed by THEMIS to the topside-ionosphere Harang region observations made by DMSP on the duskside. We specified the flow of ↑R2 from equatorward and ↑R1 from poleward into the oval along the discontinuity and provided observational evidence that the switch from ↑R2 to ↑R1 occurred where the Hall currents reversed from eastward (or antisunward) to westward (or sunward). Consequently, within the Harang region, the ↓–↑R2 FACs connected via the poleward-directed Pedersen currents within the EEJ while the ↑–↓R1 FACs connected via the equatorward-directed Pedersen currents within the WEJ. These provide observational evidence that within the Harang region, the EEJ and WEJ (separated by the discontinuity) developed differently from the regular auroral duskside EEJ and dawnside WEJ (where the oppositely directed R2–R1 FACs connect via the Pedersen currents).

Finally, we also demonstrated that the bulk plasma heating, fueling the hot zone, occurred due to both the increased plasma turbulence generated in the TPBL and the EMIC waves developed in the core region of the ring current [53]. Consequently, the hot zone peaked within the TPBL/SAPS E field channel and led to SAR arc development. We provided direct observational evidence, with one close correlation when the outward SAPS E field mapped down to near Gillam observing the SAR arc in Event 1. Although the correlation obtained for Event 2 was loose, we speculate that the inner magnetosphere “nose structure” developed because of the more earthward-moving ring-current ions, led to the intense deviation of the strong SAR arc observed over Gillam, since the “nose structure” has a significant role in the inner-magnetosphere heating mechanism, like widening the TPBL and enhancing the EMIC waves in Event 2, as shown inFigure 4b and as reported by the recent studies of Mishin and Streltsov [56,67].

5. Summary of New Results

From this study’s significant findings, we obtained the following new results for the inner-magnetosphere Harang region observed in the tail region by THEMIS during the two events investigated:

(1)

Aligned in the dusk–dawn direction, the inner-magnetosphere Harang region was characterized:

(a): at its earthward edge: by the outward-directed SAPS E field developed on a short timescale in a fast-time voltage generator (VG_FT) system across the plasmapause and by the inward-directed convection E field tailward of the plasmapause
(b): at its tailward edge: by the reversal of the convection E field across the closed–open field-line boundary from outward- to inward-directed.

(2)

Large-scale FACs reversed:

(a): from ↓R2 to ↑R2 near the plasmapause,
(b): from ↑R2 to ↓R1 across the trapping boundary, and
(c): from ↓R1 to ↑R1 across the closed-open field-line boundary.

(3)

Along the discontinuity:

(a): ↑R2 FACs occupied the regime of trapped electrons located between the plasmapause and the trapping boundary, and
(b): ↓R1 FACs occupied the regime of quasi-trapped electrons located between the trapping boundary and the closed-open field-line boundary.

(4)

The hot zone developed within the regime of trapped electrons and peaked within the turbulent plasmaspheric boundary layer.

From the two DMSP events, loosely correlated with the two THEMIS events in terms of MLT, we obtained the following results for the topside-ionosphere Harang region observed on the duskside:

(i): The duskside discontinuity developed because of the flow of ↑R2 from equatorward and ↑R1 from poleward into the auroral oval.
(ii): Across the discontinuity, the electrojet reversal from lower-latitude EEJ to higher-latitude WEJ occurred because of the reversal of the Hall currents from eastward (or antisunward) to westward (or sunward).
(iii): Within the lower-latitude EEJ: the ↓R2–↑R2 FACs connected via the poleward-directed Pedersen currents.
(iv): Within the higher-latitude WEJ: the ↑R1–↓R1 FACs connected via the equatorward-directed Pedersen currents.
(v): Away from the Harang region, in the regular duskside auroral EEJ: the Hall currents were flowing eastward (or antisunward) and the ↓R2–↑R1 FACs connected via the poleward-directed Pedersen currents.
(vi): Located equatorward of the discontinuity, the newly-formed SAPS flow developed in a fast-time voltage generator (VG_FT) system where the subauroral ↓R2 FACs were absent.

6. Conclusions

These new results listed above (as 1–4 and i–vi) help explain previously published results, which are only seemingly conflicting according to our increased understanding. These previous results include the Harang region’s collocation with sufficiently strong R1 FACs (explained by results listed as 2b–2c and 3b) causing converging E fields (explained by result listed as 1b), with R2 FACs because of the energy-dependent plasma intrusion (explained by results listed as 2a and 3a), with subauroral flows (explained by results listed as 1a and vi), and even with both R1 and R2 FACs (explained by results listed as 3a and 3b), where the R1–R2 demarcation line marks the Harang Discontinuity (explained by results listed as i and ii).

From these new results listed above (as 1–4 and i–vi), we conclude for the two events investigated that:

(a): Both the R1 and the R2 FACs were collocated with the M–I conjugate Harang phenomenon and were essential to its development.
(b): In the duskside ionosphere, the Harang Discontinuity was associated with ↑R2 (at lower altitudes) and with ↑R1 (at higher latitudes) and the ↑R2–↑R1 demarcation corresponded to the Harang Discontinuity.
(c): In the Harang region, the reversing EEJ–WEJ developed differently than their respective regular auroral EEJ and WEJ away from the Harang region.
(d): In the topside ionosphere, the newly-formed SAPS flow became enhanced near the Harang region because of the mapped-down large innermagnetosphereoutward-SAPS E field developed at the Harang Region’s earthward edge in a fast-time voltage generator.
(e): Therefore, the ↓R2 FACs were not essential to the enhancement of the newly-formed SAPS flow developed in a fast-time voltage generator at the Harang region.

Finally, we note that the RCM was able to reproduce both the strong innermagnetosphere–outward-SAPS E field and the resultant strong E×B plasma convection generating the strong SAPS flows in the duskside ionosphere near the Harang region [24]. Furthermore, our new results fill some of the existing gaps in our knowledge and understanding with experimental E field observations and with observations implying the large-scale FACs’ locations and directions within the entire inner-magnetosphere Harang phenomenon in the midnight MLT sector. However, only loosely correlated (in MLT) experimental observations were used to investigate the M–I conjugate Harang and SAPS phenomena. How the large-scale FACs vary in the M–I conjugate Harang and SAPS phenomena still needs to be investigated with accurately computed FACs. Furthermore, how the M–I conjugate Harang and SAPS phenomena develop on the dawnside is still unexplored and remains an open question.

In future studies, we plan to conduct more detailed studies based on observations that are closely correlated and M–I conjugate and that cover various MLT sectors.

Author Contributions

Conceptualization, I.H.; methodology, I.H.; software, B.C.L. and I.H.; 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 Unites States Office of Naval Research, grant 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 throughhttp://cedar.openmadrigal.org (accessed on 10 September 2024). The SSUSI auroral images can be accessed online throughhttps://ssusi.jhuapl.edu/ (accessed on 5 September 2024). The REGO images can be accessed online throughhttps://data-portal.phys.ucalgary.ca/archive/?datestring=2018%2F02%2F22&site_selected=all (accessed on 5 September 2024). The SuperDARN convection maps can be accessed online athttps://superdarn.ca/convection-maps (accessed on 10 September 2024). The THEMIS and GOES data sets can be accessed online throughhttps://cdaweb.gsfc.nasa.gov/cdaweb/istp_public/ (accessed on 8 September 2024). The AE and AL indices can be accessed online from the OMNI database: throughhttps://cdaweb.gsfc.nasa.gov/cdaweb/istp_public/ (accessed on 23 October 2024). The SuperMAG-provided various types of substorm lists can be accessed online from the SuperMAG substorm lists throughhttps://supermag.jhuapl.edu/substorms/ (accessed on 16 October 2024).

Acknowledgments

We gratefully acknowledge that this material is based upon research supported by the U. S. Office of Naval Research under award number N62909-23-1-2057. We also 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 acknowledge the REGO and SSUSI images and the THEMIS and GOES data. We gratefully acknowledge the World Data Center for Geomagnetism at Kyoto (https://wdc.kugi.kyoto-u.ac.jp//wdc/Sec3.html accessed on 10 September 2024) for providing the AE and AL 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 alsoacknowledge the SuperMAG-provided various types of substorm lists determined from the SML index.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Illustrating Event 1 observed by TH-E in the inner magnetosphere. (a) The orbit plots depict the earthward and tailward edges of the Harang region, (b) the schematic diagram of the auroral scenario along with the (c) actual TH-E footprints and substorm onset locations (symbol stars), and (d) the Harang region’s E field and plasma environment.

Figure 3. Illustrating Event 2 observed by TH-D in the inner magnetosphere. (a) The orbit plots depict the earthward and tailward edges of the Harang region; (b) the schematic diagram of the auroral scenario along with (c) the actual TH-D footprints and substorm onset locations (symbol stars); and (d) the Harang region’s E field and plasma environment.

Figure 4. Illustrating the Harang region’s inner-magnetosphere plasma environment, the THEMIS line plots depict the signatures of short circuiting, TPBL and EMIC waves, and the hot zone during (a) Event 1 and (b) Event 2. As indicated by ∗10¹⁶, the flux values are multiplied by 10¹⁶ for better presentations.

Figure 5. Illustrating Event 1 observed by DMSP F17 in the topside ionosphere. (a) The schematic diagram of the auroral scenario is shown, along with the underlying polar convection and auroral precipitation pattern, where the F17 passes are plotted with the SAPS, EEJ, WEJ, and substorm onset locations, and (b) the F17 line plots depict the Harang region’s plasma environment and underlying currents. As indicated by ∗10³, the Ne values are multiplied by 10³ for a better presentation.

Figure 6. Illustrating Event 2 observed by DMSP F18 in the topside ionosphere. (a) The schematic diagram of the auroral scenario is shown, along with the underlying polar convection and auroral precipitation pattern, where the F18 passes are plotted with the SAPS, EEJ, WEJ, and substorm onset locations, and (b) the F18 line plots depict the Harang region’s plasma environment and underlying currents. As indicated by ∗10³, the Ne values are multiplied by 10³ for a better presentation.

Figure 7. Illustrating SAR arc development in Event 1 observed over Gillam, near the TH-E-observed SAPS location and substorm onset locations; the REGO image shows a series of SAR arc events occurring during the magnetotail-reconnection-related particle injections unfolding during a series of substorms (marked by the symbol stars in colors).

Figure 8. Illustrating SAR arc development in Event 2 observed over Gillam, away from the TH-D-observed SAPS location but close to one of the substorm onset locations; the REGO image shows a series of SAR arc events occurring during the magnetotail-reconnection-related particle injections unfolding during a series of substorms (marked by the symbol stars in colors).

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Horvath, I.; Lovell, B.C. Magnetosphere–Ionosphere Conjugate Harang Discontinuity and Sub-Auroral Polarization Streams (SAPS) Phenomena Observed by Multipoint Satellites.Atmosphere2024,15, 1462. https://doi.org/10.3390/atmos15121462

AMA Style

Horvath I, Lovell BC. Magnetosphere–Ionosphere Conjugate Harang Discontinuity and Sub-Auroral Polarization Streams (SAPS) Phenomena Observed by Multipoint Satellites.Atmosphere. 2024; 15(12):1462. https://doi.org/10.3390/atmos15121462

Chicago/Turabian Style

Horvath, Ildiko, and Brian C. Lovell. 2024. "Magnetosphere–Ionosphere Conjugate Harang Discontinuity and Sub-Auroral Polarization Streams (SAPS) Phenomena Observed by Multipoint Satellites"Atmosphere 15, no. 12: 1462. https://doi.org/10.3390/atmos15121462

APA Style

Horvath, I., & Lovell, B. C. (2024). Magnetosphere–Ionosphere Conjugate Harang Discontinuity and Sub-Auroral Polarization Streams (SAPS) Phenomena Observed by Multipoint Satellites.Atmosphere,15(12), 1462. https://doi.org/10.3390/atmos15121462

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