Click Here JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A11319, doi:10.1029/2006JA011976, 2006 for Full Article Electrodynamics of a split-transpolar aurora S. Eriksson,1 G. Provan,2 F. J. Rich,3 M. Lester,2 S. E. Milan,2 S. Massetti,4 J. T. Gosling,1 M. W. Dunlop,5 and H. Rème6 Received 14 July 2006; accepted 21 September 2006; published 23 November 2006. [1] We report new results on the relation between drift velocity shear, average inverted V energy, and the separation of bifurcated high-latitude Sun-aligned polar arcs within a global transpolar auroral arc system. The two parallel arcs appeared at the location of converging v B electric field of two sunward flow channels that were separated by an intermediate region of much weaker antisunward flow. The flow shear consisted of differences mainly in the magnitude of the sunward drift velocity. The cross-track disturbance magnetic field was in phase with the drift velocity fluctuations in such a way that two upward field-aligned currents coincided with the peak average energy of the electron precipitation that generated the two polar arcs while a downward field-aligned current was observed in the intermediate region. The observed transverse arc separation is consistent with a reported linear dependence between the average energy of the precipitating electrons of the two arcs and the arc separation. We propose that this relation mainly follows from a local Pedersen current closure requirement. The greater the upward field-aligned current density of the two arcs, the wider the transverse region of the intermediate downward field-aligned current needs to be in order to supply a sufficient amount of current carriers to the bifurcated arc current circuit and thus to balance the Pedersen current. Citation: Eriksson, S., G. Provan, F. J. Rich, M. Lester, S. E. Milan, S. Massetti, J. T. Gosling, M. W. Dunlop, and H. Rème (2006), Electrodynamics of a split-transpolar aurora, J. Geophys. Res., 111, A11319, doi:10.1029/2006JA011976. 1. Introduction [2] Single and multiple auroral arcs are often observed at very high latitudes during northward interplanetary magnetic field (IMF) conditions [Lassen and Danielsen, 1978; Burke et al., 1982; Carlson et al., 1988; Valladares et al., 1994]. Such arcs are generally aligned in the Sun-Earth direction and they are typically caused by precipitating electrons with 2 keV average energy. In the literature they are referred to as Sun-aligned arcs, polar cap arcs, transpolar arcs (TPA), or theta aurora. Burch et al. [1979] first reported that polar arcs occur at flow shears where the corresponding dawn-dusk convective electric field points toward the center of the electron acceleration region. Subsequent reports have confirmed that the arcs typically coincide with a localized upward field-aligned current (FAC) sheet at a flow shear with a converging electric field [Burke et al., 1982; Carlson et al., 1988; Obara et al., 1993; Carlson and Cowley, 2005]. 1 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA. 2 Department of Physics and Astronomy, University of Leicester, Leicester, UK. 3 Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts, USA. 4 Istituto di Fisica dello Spazio Interplanetario, Rome, Italy. 5 Space Sciences Division, Rutherford Appleton Laboratory, Chilton, UK. 6 Centre d’Etude Spatiale des Rayonnements, Toulouse, France. Copyright 2006 by the American Geophysical Union. 0148-0227/06/2006JA011976$09.00 [3] Obara et al. [1996] reported 17 events of pairs of adjacent inverted V electron precipitation in the high-latitude northern polar region using Akebono satellite measurements. Each inverted V pair was assumed to correspond to a pair of polar arcs on the basis of the electron energy flux alone. Akebono generally confirmed that the two arcs coincided with upward FAC sheets within regions of converging electric field that were separated by a downward FAC. Obara et al. [1996] also found a linear relation between the dawn-dusk transverse separation of the two arcs (30 – 100 km at 120 km altitude) and the mean of the average energy of the electron precipitation (0.3 – 1.1 keV). A larger average energy was observed to result in a greater separation. An indirect dependence is thus expected between the arc separation and the Hall-to-Pedersen SH/SP conductance ratio, since this ratio depends on the average energy [e.g., Spiro et al., 1982; Robinson et al., 1987]. This result was indeed reproduced in a magnetosphereionosphere (M-I) coupling simulation of multiple polar arcs [Zhu et al., 1994; Obara et al., 1996]. However, no satisfactory explanation has been proposed for the original average energy dependence of the observed dualarc separation. [4] The Zhu et al. [1993] M-I coupling model of multiple polar arcs assumes an initial magnetospheric flow velocity shear [e.g., Lyons, 1980; Sojka et al., 1994] that generates an initial upward FAC and a primary arc in the ionosphere. Zhu et al. [1993] proposed that the adjacent downward FAC and a second upward FAC (the secondary arc) would result A11319 1 of 7 A11319 ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA A11319 on the average energy, a local self-consistent Pedersen current closure requirement, and the Zhu et al. [1994, 1996] M-I coupling model results. 2. Observations Figure 1. (a – b) Solar wind (ACE) and magnetosheath (Cluster C1) IMF GSM components (X:black, Y:green, Z:red) observed during the 2013 – 2140 UT period on 14 February 2003. (c) The ACE data were shifted by 45 min 35 s to match its IMF clock angle q = arctan(By/Bz) with q measured at Cluster C1 (red line). (d) Alfvén Mach number of the magnetosheath plasma flow measured by Cluster C1. A clear TPA was observed by Polar UVI during the time period marked in gray. The vertical red line marks when DMSP F13 encountered this TPA. due to the conductivity enhancement caused by the primary arc as the M-I system approaches steady-state. [5] Large-scale 200 – 400 km wide TPAs may occasionally also bifurcate along the nightside section or even along their entire noon-midnight extension into two parallel polar arcs separated by a region of much lower luminosity [Craven et al., 1991; Eriksson et al., 2005]. This paper examines the electrodynamics of such a split-TPA feature observed by the Polar UVI instrument [Torr et al., 1995] and recorded by the DMSP F13 satellite in the Northern Hemisphere (NH) on 14 February 2003. We discuss a possible physical explanation of the arc separation based [6] Figure 1 displays the IMF conditions (GSM components and YZGSM clock angle) in the solar wind (ACE) and in the magnetosheath (Cluster C1) during the 2013 – 2140 UT interval on 14 February 2003. The X, Y, Z components are color coded as black, green, and red, respectively, showing that the IMF was northward after 2017 UT with IMF By > 0 and IMF Bx < 0. The bottom panel shows that the upstream magnetosheath flow was mostly sub-Alfvénic along this Cluster C1 trajectory from (X, Y, Z) = (4.4, 0.5, 9.1) to (6.3, 0.7, 9.4) RE. These conditions are favorable for antiparallel lobe reconnection poleward and duskward of the NH cusp and the generation of sunward E B convection [e.g., Gosling et al., 1991; Eriksson et al., 2005] in the vicinity of the reconnection site. A flow shear region should develop between such a sunward flow and the magnetosheath flow. The gray area in Figure 1 corresponds to a period when the Polar UVI instrument observed a clear signature of a duskside TPA in the NH that eventually bifurcated while the vertical red bar at 2111 – 2112 UT marks when the DMSP F13 satellite encountered this split-TPA. [7] Figure 2 displays the Polar UVI observation of the bifurcated TPA at 2110:44 UT (red color indicates higher photon flux). The Polar s/c was at 1.88 RE altitude when the field-of-view (FOV) of the UVI instrument captured this section of the split-TPA over northern Greenland. The auroral oval was not in the UVI FOV due to the low s/c altitude. The dual-arc UVI observation was confirmed at 2110 UT by the ITACA2 all-sky camera in Daneborg, northeastern Greenland (not shown). [8] Figure 2 also shows the 2110– 2112 UT interval of the global SuperDARN convection pattern displayed here as equipotential contours [Ruohoniemi and Baker, 1998]. There was sufficient radar backscatter to confirm a sunward E B drift over the split-TPA at this time. The DMSP F13 cross-track E B drift velocity vectors are also shown along this 2107:08 – 2119:42 UT dusk-to-dawn satellite trajectory. The SuperDARN and DMSP F13 drift information suggest that the split-TPA occurred at a structured sunward flow section of a clockwise lobe cell that covered most of the high-latitude polar cap. Colored along-track F13 segments correspond to the various magnetospheric source regions as identified from the measured DMSP F13 particle precipitation shown in Figure 3. [9] Figure 3 shows the cross-track magnetic field perturbation (DBz, solid line), particle precipitation, and crosstrack drift velocity (solid line) recorded by DMSP F13 along the duskside 2107 – 2114 UT interval. A positive (negative) DBz slope along the direction of motion corresponds to a downward (upward) FAC sheet. The electron and ion energy flux were measured by a low-energy (32 eV to 1 keV) and a high-energy (1 keV to 32 keV) pair of electrostatic particle analyzers of the DMSP F13 SSJ/4 plasma instrument [Hardy et al., 1984] whose entrance aperture points toward local zenith. We note that the lowenergy ion detector has degraded over time and therefore that low fluxes of low-energy ions may be undetected. 2 of 7 A11319 ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA A11319 scale FACs as a structured NBZ current system [Iijima and Shibaji, 1987] offset toward the duskside of the polar cap in agreement with the positive IMF By and northward IMF [Eriksson et al., 2005, and references therein]. [12] Detailed electron precipitation observations during the 2111:05 – 2112:05 UT interval are shown in Figure 4. The average energy of the electrons is defined as E = JE/JNE where JE is the energy flux (see Figure 4a) and JNE is the number flux (not shown) of the electrons. The peak of the average energy was E = 2.26 keV and E = 1.19 keV at the 2111:24 and 2111:51 UT times of the two arcs, respectively, corresponding to an average E = 1.72 keV of this bifurcated arc system. The intermediate region between the two arcs was characterized by a low average Pedersen conductance SP 0.9 mho on the basis of Spiro et al. [1982] pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1:6 1012 pJE SP ¼ 20E=4 þ E2 ð1Þ while the Robinson et al. [1987] expression pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1:6 1012 pJE SP ¼ 40E=16 þ E 2 Figure 2. The duskside oval-aligned split-TPA as observed by the Polar UVI instrument at 2110:44 UT is rendered over the 2110– 2112 UT SuperDARN equipotential contours [Ruohoniemi and Baker, 1998] in geomagnetic latitude and magnetic local time. The 2107 –2120 UT DMSP F13 E B cross-track drift is also shown and the along-track color-coded segments correspond to periods of proposed magnetospheric source regions. [10] Several plasma source populations are indicated between the vertical dotted lines in Figure 3 such as a central plasma sheet (CPS) and a boundary plasma sheet (BPS). The most poleward region is identified as the polar rain of the polar cap. DMSP F13 observed the center of two 1 – 3 keV inverted V electron precipitation regions at 2111:24 and 2111:51 UT that generated the two visual polar arcs shown in Figure 2. The two arcs were separated by 201 km along the F13 orbit in a boundary region labeled TPA between the poleward part of the BPS and a region that we identify as a mantle population (MTL) on the basis of low-energy 1 keV ion precipitation coincident with polar rain-type electron precipitation [Newell et al., 1991]. [11] There was a clear correlation between the cross-track E B drift velocity and DBz (solid lines). The two arcs appeared on the poleward side of two distinct sunward flow channels in a converging electric field region that was dominated by a sunward flow shear. The corresponding DBz indicated an individual upward FAC at each arc and a downward FAC separated the arcs. This system of mesoscale upward and downward FACs related to the two polar arcs occurred within a large-scale upward FAC region (dark gray area in Figure 3) that reached poleward into the polar rain. A large-scale downward FAC (light gray) was found equatorward of the split-TPA. We identify this pair of large- ð2Þ resulted in an average SP 0.5 mho. [13] Obara et al. [1996] illustrated a dependence between the estimated dual-arc separation and the observed mean of the average electron energy of the two inverted Vs. We derived a linear fit E ¼ 10:6y 83:7 ð3Þ on the basis of their Figure 4, where y is the transverse separation of the two arcs in km (mapped to 120 km altitude) and E is the mean of the average energy of both arcs in eV. The wider the arc separation, the greater the mean average energy. This Akebono relation is consistent with the observed mean E = 1.72 keV and the Dt = 27 s along-track arc separation on 14 February 2003 which corresponds to a 162 km perpendicular distance at 848 km DMSP F13 altitude (vsc = 7.43 km/s) or 146 km at 120 km altitude. We used an a = 36° angle of incidence between the DMSP F13 trajectory and the TPA normal (see Figure 2). The top panel of Figure 5 illustrates this comparison between the DMSP F13 event (open circle) and the Akebono data (black dots) that we reproduced from Figure 4 by Obara et al. [1996]. The solid line corresponds to equation (3). [14] The bottom panel of Figure 5 compares the arc separation and the SH/SP conductance ratio using equation (3) in the Spiro et al. [1982] expression SH =SP ¼ E5=8 ð4Þ where E is in keV. The general nonlinear form of the quantitative M-I coupling model result of Zhu et al. [1994] is confirmed. We note, however, that the predicted range of the conductance ratio using either equation (4) or the Robinson et al. [1987] result 3 of 7 SH =SP ¼ 0:45E 0:85 ð5Þ ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA A11319 A11319 Figure 3. The panels from top to bottom display the 2107 – 2114 UT DMSP F13 along-track (dotted) and cross-track (solid, positive in tailward direction) disturbance magnetic field (DB = B BIGRF), energy-time spectra for precipitating electrons and ions, and the cross-track E B drift (solid line) measured by an ion drift meter [Rich and Hairston, 1994]. Vertical dashed lines indicate the proposed magnetospheric source regions while red vertical lines mark the TPA interval. The origin of the particle precipitation within the interval marked ‘‘BPS?’’ is unclear and could be related to BPS or LLBL mixed with polar rain. A downward (NBZ-D) and an upward (NBZ-U) NBZ current are suggested within light gray and dark gray intervals, respectively. where E is in keV, is lower than the 1 – 2 range predicted by Zhu et al. [1994] for a corresponding 50– 100 km dual-arc separation. 3. Discussion [15] The two polar arcs that DMSP F13 intersected were related to a flow shear corresponding to a converging electric field directed toward the center of each upward FAC which was carried by the earthward accelerated electrons of the two inverted Vs [Burch et al., 1979; Burke et al., 1982; Carlson and Cowley, 2005]. Moreover, the Obara et al. [1996] relationship for the 0.3 – 1.1 keV average energy double arcs appeared to hold for even higher energy and greater separation. The question is how this relation may be explained. [16] We first examine whether the high-latitude bifurcated arc system poleward of the duskside region 2 current is a locally balanced current circuit as suggested by Zhu et al. [1993]. We assume that the region 2 FAC density is negligible as indicated by the DMSP F13 DBz observations (see Figure 3). A closer inspection of the along-track FAC budget between the CPS and the polar rain is summarized in Table 1 using an infinite current sheet approximation. This is a valid assumption, since the two horizontal disturbance magnetic field components (see Figure 3) are either in phase or out-of-phase. There were no bipolar along-track signatures which are expected at local current carrying flux tubes. Table 1 lists the estimated time intervals Dt of each significant FAC segment and the FAC densities using jk = Dby/(m0Dx), where Dby is the difference in cross-track magnetic field (by = DBz), Dx = vscDt, and vsc = 7.43 km/s is the DMSP F13 speed. [17] Current continuity [e.g., Lyons, 1980; Milan et al., 2003] relates the height-integrated Pedersen current JP and jk by JP ¼ Z x2 jk dx ð6Þ x1 where x is the DMSP F13 along-track distance (x2 > x1), j = jk^z, and ^z is positive in the upward direction or antiparallel to the NH magnetic field. JP > 0 corresponds to a Pedersen current directed along the direction of motion ^ x. The alongtrack electric field Ex that drives this Pedersen current is Ex = JP/SP while the cross-track drift velocity corresponding to Ex is Vy = Ex/B0 under the assumption that the total field is 4 of 7 A11319 ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA A11319 [18] Assume now that the magnitude of jk increases at the equatorward or primary 2111:24 UT arc and for simplicity that nothing else changes. This will result in a nonbalanced Pedersen current. It is well-known that density cavities develop after some finite time period beneath downward current regions. This is caused by a depletion of ions that carry the Pedersen current away from the downward FAC and the upward escaping electrons that carry the downward FAC [Karlsson et al., 2005]. We therefore propose that the intermediate downward current region between the two arcs needs to expand in order to supply a sufficient amount of current carriers to the bifurcated arc current circuit and thus to balance the Pedersen current. [19] Figure 7 demonstrates this fact on the basis of the observed and derived electrodynamic quantities on 14 February 2003. There is a linear relation between the Figure 4. DMSP F13 electron precipitation data from 2111:05 to 2112:05 UT. (a) Energy flux JE in eV/(cm2 s sr). (b) Average energy E = JE/JNE where JNE is the number flux in cm2 s1 sr1. (c) Conductance ratios SH/SP using equation (4) (dots) and equation (5) (crosses). (d) Spiro et al. [1982] Pedersen conductance in mho (c.f. equation (1)). vertical B = B0^z and B0 = 50 mT. A positive Vy corresponds to a sunward drift velocity. The bottom panels of Figure 6 display the resulting along-track JP, Ex, and Vy [see also Milan et al., 2003, Figure 4] during a 5-min section of the DMSP F13 orbit from 2108:35 UT using the observed jk (see Figure 6c and Table 1). An effective SfP = SP + SbP, where SbP = 1 mho is an additional background conductance, was employed in order to retrieve realistic Ex and Vy. We have also assumed that the average energy E of the electron precipitation (see Figure 6a) may be used directly in equation (1) (see Figure 6b). We conclude two important results from Figure 6. First, we find that the Pedersen current J? P = JP cos a transverse to the split-TPA (thin line) is balanced to zeroth order. Second, it is clear that the enhanced SP below the two polar arcs suppresses Ex = JP/SfP and that this contributes to the structured sunward ionospheric drift velocity Vy related to JP. The resulting drift Vy shown in Figure 6f compares favorably with the observed cross-track drift (c.f. Figure 3). Figure 5. (a) Black dots reproduce the average inverted V energy versus transverse arc separation of two parallel polar arcs on the basis of Akebono satellite observations [see Obara et al., 1996, Figure 4]. The bifurcated polar arc event observed by DMSP F13 is shown as an open circle. The best fit to the Akebono events is shown as a solid line. (b) The arc separation versus SH/SP conductance ratio using equation (3) in equations (4) – (5) [Spiro et al., 1982; Robinson et al., 1987] is compared with the Zhu et al. [1994] model results (their Figure 5). The data are shown along the Spiro et al. formula. 5 of 7 A11319 ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA Table 1. Estimated DMSP F13 FAC Density for Each Time Interval Poleward of the R2 Current System on 14 February 2003a Start Time, UT Stop Time, UT jk, mA/m2 2108:35 2109:49 2110:09 Structured R1 2109:49 2110:09 2110:31 0.20 0.61 0.85 2110:31 NBZ Downward 2111:20 0.79 2111:20 2111:28 2111:44 2111:58 2112:25 NBZ Upward With Polar Arcs 2111:28 2111:44 2111:58 2112:10 2112:49 1.60 0.80 0.91 0.40 0.38 a Positive values mark upward currents. A11319 magnitude of the primary upward FAC density and the required transverse width of the intermediate downward FAC in order to balance the bifurcated arc system by Pedersen currents alone. Lyons [1980] showed in his numerical solution results of a single auroral arc (his Figure 4) that an increased field-aligned potential drop fk typically corresponds to an increased upward FAC density jk [Knight, 1973] as well as an enhanced precipitating electron energy flux JE [Lundin and Sandahl, 1978], since both jk and JE (see equation (3) and equation (10) as stated by Lyons [1980]) are assumed to depend on fk in a similar form. Figure 7 thus provides a possible explanation for the arc separation dependence on the average energy E = JE/JNE under the assumption that the upward FAC density jk is proportional to the average energy E of the accelerated electrons that carry the upward FAC. This scenario is in agreement with the observed and modeled linear dependence between the average energy of the accelerated electrons of a bifurcated arc system and the arc separation [Zhu et al., 1994; Obara et al., 1996]. 4. Summary and Conclusions [20] The split-TPA event that was observed by DMSP F13, SuperDARN, and Polar UVI on 14 February 2003 displayed a transverse dual-arc spacing of 146 km at 120 km altitude with an average E = 1.72 keV inverted V energy distributed between the two polar arcs. The bifurcation of the original TPA is in agreement with the linear dependence that Obara et al. [1996] reported between the average energy and the arc separation. We propose that this relation is caused primarily by a local Pedersen current closure requirement and related to the dynamics of the intermediate downward FAC and the density cavity that develops beneath it. Our hypothesis is that the width of this downward FAC region is determined by a need to supply Figure 6. Electron precipitation data and estimated electrodynamic quantities are shown along the 2108:35 – 2113:35 UT section of the DMSP F13 orbit. (a) Observed average energy E. (b) SP on the basis of E and the Spiro et al. [1982] equation (1). (c) Derived jk (see Table 1). (d) Alongtrack Pedersen current JP (thick line, equation (6)) and its component transverse to the TPA (thin line) using an a = 36° angle of incidence of the F13 trajectory. (e) Along-track electric field Ex = JP/SPf. (f) Cross-track drift velocity Vy = Ex/B0 where Vy > 0 is a mostly sunward drift. The horizontal bar in Figure 6b indicates the dual-arc separation. Figure 7. The relation between the magnitude of the primary upward FAC and the transverse width of the intermediate downward FAC of the 14 February 2003 event using a balanced transverse Pedersen current requirement at DMSP F13 altitude. All other FAC density magnitudes and widths were held constant (see Table 1), including the secondary upward FAC (jk = 0.91 mA/m2) of the bifurcated arc system. 6 of 7 A11319 ERIKSSON ET AL.: ELECTRODYNAMICS OF A SPLIT-TPA a sufficient ion source population to balance the upward FAC system of the two adjacent polar arcs via Pedersen currents. [21] The split-TPA feature also appears to be consistent with the physics contained in the Zhu et al. [1993] M-I coupling model of multiple polar arcs. However, the main difference between this model and the 14 February 2003 split-TPA event is related to the initial state and the subsequent evolution of the dual-arc system. The splitTPA and the mesoscale structure observed by DMSP F13 evolved as a bifurcation of a preexisting large-scale TPA [see also Craven et al., 1991] and the transverse width of the split-TPA was comparable to the width of the original TPA. The Zhu et al. [1993] model, however, gradually evolves from one mesoscale primary arc into multiple arcs distributed over an increasingly wider region until a steady state width is achieved. In summary, although a local Pedersen current closure requirement probably determines the width of the intermediate downward current region, we still need to understand what initiates the development of a downward current at the center of the primary TPA in the first place if we assume that the TPA coincided with an initial large-scale upward FAC. [22] The two arcs of the split-TPA presented here occurred at a drift velocity shear which was dominated by a sunward component with a narrow intermediate region of weak antisunward flow. It may be argued that the enhanced Pedersen conductance SP (caused by the downward accelerated electrons of the two arcs) eventually contributed to the deceleration of the sunward flow at the two arcs and resulted in a more severe and localized sunward flow shear in the ionosphere than what was perhaps originally imposed on it from the magnetosphere. 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