Polar cap bifurcation during steady-state northward interplanetary magnetic field with j B Y j B Z

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003ja009944, 2004 Polar cap bifurcation during steady-state northward interplanetary magnetic field with j B Y j B Z Masakazu Watanabe, George J. Sofko, and Dieter A. André Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Canada Takashi Tanaka Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Fukuoka, Japan Marc R. Hairston William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA Correction published 29 March 2005 Correction published 12 December 2007 Received 17 March 2003; revised 5 September 2003; accepted 22 October 2003; published 24 January [1] In this paper, for steady-state northward interplanetary magnetic field (IMF) with jb Y jb Z, we describe a new merging sequence that results in polar cap bifurcation and accompanying paired exchange cells in the ionospheric convection pattern. Although the IMF is northward, it reconnects with the closed geomagnetic field on the dayside highlatitude magnetopause, creating two types of open geomagnetic field lines. For the first type the neutral point and the foot point are in the same hemisphere; for the second type the neutral point and the foot point are in opposite hemispheres. The latter type of field lines slips on the magnetopause in the azimuthal direction opposite to the normal B Y - associated flux transport and forms an overdraped tail lobe. The ionospheric signature of this overdraped lobe is the appearance of an open magnetic flux island inside the dawn/ dusk plasma sheet (i.e., polar cap bifurcation). For B Y > 0 the island emerges in the duskside (dawnside) plasma sheet in the northern (southern) ionosphere and conversely for B Y < 0. The overdraped field lines which have slipped on the magnetopause then reconnect with closed geomagnetic field lines in the opposite hemisphere to the foot points, thereby transferring the open magnetic flux to the nightside convection system and maintaining the steady-state magnetic flux circulation. As a result, paired ionospheric convection cells form which exchange magnetic flux. For B Y > 0 the pair is located in the noon-dusk and midnight-dawn (dawn-noon and dusk-midnight) quadrants of the northern (southern) ionosphere; for B Y < 0 a mirror image with respect to the noon-midnight meridian applies to the convection pattern. We demonstrate observational evidence that supports this model. INDEX TERMS: 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2776 Magnetospheric Physics: Polar cap phenomena; 2463 Ionosphere: Plasma convection; 2475 Ionosphere: Polar cap ionosphere; 2760 Magnetospheric Physics: Plasma convection; KEYWORDS: convection, magnetic topology, northward IMF, theta aurora, internal reconnection, exchange cell Citation: Watanabe, M., G. J. Sofko, D. A. André, T. Tanaka, and M. R. Hairston (2004), Polar cap bifurcation during steady-state northward interplanetary magnetic field with jb Y jb Z, J. Geophys. Res., 109,, doi: /2003ja Introduction [2] The polar cap, a region of open geomagnetic field lines, is a mirror of solar wind-magnetosphere interactions. When the interplanetary magnetic field (IMF) is southward, the polar cap shape is circular and its equatorward edge is clearly identified by precipitating particles or optical auroras. In contrast, the northward IMF situation is more complicated and sometimes controversial. A commonly observed configuration is a teardrop or horse-collar polar cap [Meng, 1981; Hones et al., 1989]. In this configuration the polar cap is rather shrunken, in particular, across Copyright 2004 by the American Geophysical Union /04/2003JA the dawn-dusk meridian. Auroral arcs on the dawnside/ duskside are still excluded from the central high-latitude region constituting the remaining open magnetic flux. In addition to this normal polar cap, another special configuration exists for northward IMF. Frank et al. [1982] discovered a q aurora consisting of a ring of auroral luminosity (auroral oval) and a transpolar arc extending contiguously from the dayside auroral oval to the nightside oval. The plasma population in the transpolar arc strongly resembles that of the plasma sheet boundary layer [Peterson and Shelley, 1984; Frank et al., 1986], showing that the arc is on closed magnetic field lines bounded by adjacent open field line regions (i.e., a bifurcated polar cap). A double-q aurora associated with two transpolar arcs occasionally 1of20

2 B Y spikes by Cumnock et al. [1997] and Chang et al. [1998]. A similar scenario to the B Z spike case occurs when the sign of B Y changes temporarily, and the dawn/dusk plasma sheet is isolated to create a q aurora. Thus the q aurora configuration seems to occur most commonly during the reconfiguration of the magnetosphere associated with temporal IMF changes. This idea is supported by recent global MHD simulations. In their case study, for example, Slinker et al. [2001] showed that the input of IMF changes to their simulation model can quantitatively reproduce the polar cap reconfiguration that was delineated from ground optical observations. [4] Thus for mainly northward IMF, IMF perturbations can cause open magnetic flux to intrude into the dawn/dusk plasma sheet and complicate the polar cap configuration. As summarized above, a temporal change in the IMF is one of the causes of polar cap bifurcation. However, this need not be the only mechanism for plasma sheet isolation. In fact our understanding of the magnetospheric response to northward IMF is still developing. In this paper we show that a different type of polar cap bifurcation occurs for a steadystate IMF condition with jb Y j B Z > 0. This special configuration of the magnetosphere was predicted by Tanaka [1999] using an MHD simulation. The purpose of this paper is to demonstrate Tanaka s prediction observationally. However, although the observations are basically consistent with Tanaka s simulation, the detailed geometry (e.g., the spatial scale of convection cells) deviates somewhat from that predicted. Therefore we start with a model modified from Tanaka s simulation results. Figure 1. Sketches showing dayside magnetopause reconnection between closed geomagnetic field lines and IMF lines when B Y = B Z > 0, viewed from the magnetotail: (a) before the reconnection, and (b) after the reconnection. In Figure 1a the reconnection in the southern hemisphere occurs at X p between the geomagnetic field C 1 and the IMF. Similarly, the reconnection in the northern hemisphere occurs at X 0 p. The open arrows in Figure 1b indicate the motion of the newly reconnected field lines. occurs [e.g., Newell et al., 1999], and in such cases the topology of the polar cap is further complicated. [3] Newell and Meng [1995] proposed a theory of q aurora formation. A q aurora occurs in association with a southward IMF B Z spike during a prolonged or intense northward B Z period. Prior to the IMF spike, the polar cap has a horse-collar configuration and the dawn/dusk plasma sheets are expanded poleward. For a B Z spike with B Y >0,a new open magnetic flux region is created at lower latitudes in the postnoon (prenoon) sector in the northern (southern) hemisphere and transported into the dawnside (duskside) plasma sheet. This addition of new open magnetic flux isolates the dawnside (duskside) plasma sheet in the northern (southern) ionosphere and creates the q aurora configuration. For B Y < 0 the mirror image of above-mentioned dawn/dusk relation occurs. This formation mechanism of q aurora configuration has been extended to the cases of IMF 2. Model 2.1. Magnetopause Merging Between the IMF and a Closed Geomagnetic Field [5] Tanaka [1999], using a numerical MHD model, simulated steady-state configuration of the magnetosphereionosphere system during periods of IMF B Z = jb Y j = 3.5 nt. The model we propose here is based on his simulation results. Figure 1 depicts the dayside merging geometry for B Z = B Y > 0 viewed from the magnetotail (see Figure 3 of Tanaka [1999]). (Here we consider the B Y > 0 case. For the B Y < 0 case a mirror image with respect to the noonmidnight meridian applies to the following description.) In this IMF condition, the IMF reconnects with both open and closed geomagnetic fields at the high-latitude magnetopause. Reconnection with the open geomagnetic field produces lobe cells [Reiff and Burch, 1985]. We do not discuss lobe cells in this paper; their physics was studied in detail by Crooker et al. [1998]. Here we consider only reconnection with the closed geomagnetic field. Figure 1a shows field line geometry just before the reconnection. The curved IMF (more precisely, magnetosheath magnetic field) lines in Figure 1a are partly a result of draping of field lines against the magnetopause and partly a result of distortion when they approach the diffusion region. Both geomagnetic and IMF field lines become distorted as they move into the diffusion region so that they are antiparallel within the diffusion region. Another important consequence of this field line distortion is that the closed geomagnetic field lines just inside the magnetopause become tilted so that the field lines emanating from the southern prenoon ionosphere are 2of20

3 is a normal way of flux transport. In contrast, the type B field lines cannot be directly transported to the magnetotail, because they strongly drape the magnetosphere. Figure 2. The motion of NB field lines after the dayside magnetopause reconnection at t = t 0, viewed from north. The dashed portion indicates the part of the field line that is in the southern hemisphere. The reconnection occurs at X p. The solid circles show the rotational discontinuity (i.e., the magnetopause). mapped into the northern postnoon ionosphere. An intuitive explanation of this tilt is a simple superposition of the dipole field and the duskward IMF (see Figure 3 of Cowley et al. [1991]). [6] For northward IMF with appreciable positive B Y, reconnection occurs on the duskside high-latitude magnetopause in the northern hemisphere (X 0 p in Figure 1a) and on the dawnside high-latitude magnetopause in the southern hemisphere (X p in Figure 1a). After the reconnection, there emerge two types (A and B) of open geomagnetic field lines from the two diffusion regions (Figure 1b). For type A, the neutral point (which becomes a rotational discontinuity) and the foot point are in the same hemisphere; for type B, the neutral point and the foot point are in opposite hemispheres. We also label field lines with N or S, depending on whether their foot points are in the northern ionosphere or southern ionosphere. For example, NA means type A field lines with their foot points in the northern ionosphere. After the reconnection, both type A and type B field lines are transported to the magnetotail by the solar wind. We should note here that there is a significant difference in the method of flux transport between type A and type B field lines. The NA (SA) field lines first move dawnward (duskward) by magnetic tension and then turn tailward by magnetosheath flow. After that the NA (SA) field lines are directly transported to the northern (southern) magnetotail, which 2.2. Field Line Motion and the Resulting Ionospheric Convection Subsequent to Magnetopause Merging [7] After the magnetopause merging, the NB field lines become an overdraped tail lobe. Note that Earthward of the rotational discontinuity, the NB field lines are within the magnetopause. The field lines cannot penetrate deep into the magnetosphere because they are pressed against rigid closed field lines. They slip on the magnetopause as the solar wind drags the magnetosheath end. As Figure 1b indicates, the NB field lines inside the magnetopause lie mostly on the duskside of the Y = 0 plane, and the NB field lines outside the magnetopause lie southward of the Z = Y plane. Therefore the NB field lines inside the magnetopause slip eastward with their antisunward motion, draping the magnetosphere all the way around. This geometry is essential when we consider the magnetospheric configuration for IMF B Y B Z >0.IfIMFB Y B Z > 0 and NB field lines are northward of the Z = Y plane, then the field lines slip westward, resulting in normal flux transport. [8] Figure 2 depicts the motion of NB field lines after the magnetopause reconnection at t = t 0. The reconnection occurs at X p, forming a rotational discontinuity. The dashed part of each field line shows the portion that is in the southern hemisphere. As noted above, the NB field lines just after the reconnection drape the duskside magnetosphere, and therefore they cannot slip westward when the solar wind drags the magnetosheath end. Instead they slip eastward as in Figure 2. The ionospheric foot points also drift eastward with this eastward slipping. As time lapses from t = t 0, the location of the rotational discontinuity moves tailward away from the original reconnection point (X p ). The NB field lines cannot penetrate into the inner magnetosphere because they are pressed against closed field lines. The overdraped lobe and the closed field line region would be separated by a weak tangential discontinuity. [9] Figure 3 shows expected convection patterns in the ionosphere (schematic interpretation of Figure 1 of Tanaka [1999]), with Figure 3a for the northern hemisphere and Figure 3b for the southern hemisphere. The solid lines with arrows show streamlines, while the dashed curves represent the open/closed boundary. Each pair of the same labeling numbers (1 and 1 0, 2 and 2 0, etc.) indicates that the two positions are geomagnetically conjugate at a certain time during the magnetospheric convection. The formation of two different types of open field lines results in a unique feature in the ionosphere. The magnetopause reconnection in the southern hemisphere (reconnection between the IMF and C 1 in Figure 1a) is mapped into point 1 in Figure 3a (NB line) and point 1 0 in Figure 3b (SA line), while the magnetopause reconnection in the northern hemisphere is mapped into point 5 in Figure 3a (NA line) and point 5 0 in Figure 3b (SB line). In the open magnetosphere, the ionospheric projection of the cusp becomes a line. The cusp in the northern (southern) ionosphere includes the line connecting points 1 and 5 (1 0 and 5 0 ). The small round cell circulating within the open magnetic flux region in the middle of the polar cap represents the lobe cell, which we do not discuss in this paper. In the northern ionosphere 3of20

4 Details will be described later. A topological change is required for the further tailward flux transport to maintain the steady-state magnetic flux circulation. In accordance with the model of Tanaka [1999], the NB field lines can undergo another internal reconnection when they come to point 2 in Figure 3a. Figure 4 depicts this second reconnection. The numbers labeled on field lines (1, 1 0,2,2 0, etc.) correspond to those labeled on foot points in Figure 3. Figure 3. Convection patterns and polar cap configurations in the ionosphere during B Y = B Z > 0 periods for (a) the northern hemisphere and (b) southern hemisphere. The dashed lines indicate open/closed boundaries. The points with the same number (1 and 1 0, 2 and 2 0, etc.) are geomagnetically conjugate at a certain time during the magnetic flux circulation. (Figure 3a), the NA field lines resulting from the northern magnetopause reconnection move dawnward from point 5, constituting part of the round merging cell that circulates in the circumpolar region. On the other hand, as illustrated in Figure 2, the NB field lines resulting from the southern magnetopause reconnection move duskward from point 1 until they reach point 2. As a result, an island of open magnetic flux (the region bounded by the dashed contour around streamline 1 NB 2 in Figure 3a) appears in the duskside plasma sheet and expands antisunward, as shown in Figure 3a. Note here that topologically, completely isolated islands cannot exist. The dashed line connecting points 1 and 5 shows that the island is topologically linked to the main polar cap Internal Merging Between the Overdraped Lobe Field and a Closed Geomagnetic Field [10] The NB field lines cannot be transported directly to the magnetotail because their tailward motion conflicts with the sunward motion of another type of field lines. Figure 4. Sketches depicting the second internal reconnection during the tailward transport of NB field lines, viewed from the dawnside: (a) before the reconnection, and (b) after the reconnection. The internal reconnection occurs at X i in Figures 4a and 4b. The dayside magnetopause reconnection occurs at X p in Figure 4a, while the magnetotail reconnection occurs at X t in Figure 4b. The open arrows indicate the direction of field line motion. The dayside dashed line within the magnetopause (Figure 4a) shows the closed geomagnetic field line C 1 just before the magnetopause reconnection. The nightside open dashed line (Figure 4b) shows the field line SA created by the magnetopause reconnection and transported to the southern tail lobe. The nightside closed dashed line (Figures 4a and 4b) indicates the closed geomagnetic field line C 2 resulting from the magnetotail reconnection between NB 0 and SA. The labeling numbers of field lines correspond to those in Figure 3. 4of20

5 Figure 4a shows the field line geometry before the internal reconnection. The magnetopause reconnection described earlier occurs at point X p in the southern hemisphere between the IMF and the closed field line C 1 (see also Figure 1a), producing SA field lines (not shown in Figure 4a) and NB field lines. The NB field lines move antisunward, draping the duskside magnetosphere, and reach foot point 2 in the northern hemisphere. In the southern hemisphere, however, open line NB can merge with closed field line C 2, which is moving sunward along the dawnside from the magnetotail. The foot points of C 2 just before the internal reconnection are points 3 and 3 0 in Figure 3. When the NB field lines reach foot point 2, they reconnect with C 2 at point X i in Figure 4a in the dawnside southern hemisphere, creating new open field lines (denoted NB 0 ) and new closed field lines as depicted in Figure 4b. In Figure 3, the foot point of the new open field line NB 0 is at point 3 on the dawnside in the northern ionosphere, while the foot points of the new closed field line (labeled C 1 because it replaces the former C 1 line involved in the original magnetopause merging at X p )are at point 2 on the duskside in the northern ionosphere and point 2 0 on the dawnside in the southern ionosphere. Note that the total amount of magnetic flux in the NB 0 flux tube is the same as that in the NB tube. In Figure 3b, point 2 0 /3 0 is on the open/closed boundary. Therefore the round merging cell meets the open/closed boundary at three points in one cycle. This feature is different from the normal merging cell which intersects the open/closed boundary only twice in one cycle. We should also remark here that in the simulation results of Tanaka [1999] the internal reconnection occurs on the dayside flank soon after the magnetopause reconnection (see his Figure 7). In our model, however, the internal reconnection occurs on the nightside flank (Figure 4). This is a modification of Tanaka s model based on observations (section 4), which show that the open magnetic flux island in the ionosphere extends beyond the dawn-dusk meridian as depicted in Figure Field Line Motion and the Resulting Ionospheric Convection Subsequent to Internal Merging [11] After the internal reconnection, the new closed field lines C 1 are transported sunward, becoming part of the dayside circulation system. In the northern ionosphere, their motion constitutes the sunward convection portion (2 C 1 1) of the duskside crescent-shaped convection cell in Figure 3a (labeled 1 NB 2 C 1 1), which we shall call a primary exchange cell. In the southern ionosphere the field line motion corresponds to the dayside part of the sunward convection (2 0 C ) of the large round cell labeled 1 0 SA 4 0 C C in Figure 3b. Thus the closed magnetic flux lost in the magnetopause reconnection ultimately returns to foot points 1 and 1 0. In other words, the internal reconnection at X i reproduces the closed field line C 1 on which the original merging took place at X p. The newly created C 1 field lines move sunward as depicted in Figure 4b. When the C 1 foot points reach 1 and 1 0,C 1 can again reconnect with the IMF on the dawnside southern magnetopause and become SA and NB as sketched in Figure 1. [12] After the internal reconnection at X i in the southern hemisphere, the NB 0 field lines are transported to the magnetotail in the usual fashion and take part in the nightside circulation system. Their motion constitutes the antisunward convection portion (3 NB 0 4) of the dawnside crescent-shaped convection cell in the northern ionosphere (labeled 3 NB 0 4 C 2 3), which we shall call a secondary exchange cell Magnetotail Merging Between the Northern and the Southern Lobe Field [13] In the deep magnetotail, the neutral sheet lies roughly in the Z = Y plane. The NB 0 field lines are transported to the dawnside magnetotail northward of the neutral sheet. The ionospheric projection of their motion exhibits an eastward turn before the magnetotail reconnection as in Figure 3a (see the polar plot in Figure 6 of Tanaka [1999], but the reader must flip the figure with respect to the noon-midnight meridian). Owing to the IMF B Y effect, the NB 0 field lines are twisted about the X axis. This twist must be relaxed before the magnetotail reconnection. The eastward turn of the NB 0 field lines in the ionosphere corresponds to this relaxation process in the magnetotail. Further discussion on this topic would require detail beyond the scope of the present paper. On the other hand, the SA field lines are transported to the dawnside magnetotail southward of the neutral sheet. The ionospheric projection of their motion exhibits a sunward turn before the magnetotail reconnection as in Figure 3b (see the polar plot in Figure 5 of Tanaka [1999]). Note that sunward motion in the ionosphere does not necessarily correspond to sunward motion in the magnetotail. In the cross section of the magnetotail, NB 0 and SA become neighboring antiparallel lines in the vicinity of the neutral sheet (compare the tail cross sections in Figures 5 and 6 of Tanaka [1999], but the reader must flip his Figure 5 with respect to Z = 0 and his Figure 6 with respect to Y =0). From this geometry, in the quadrant of Y < 0 and Z <0, the NB 0 field lines reaching foot point 4 in Figure 3a reconnect with the SA field lines reaching foot point 4 0 in Figure 3b. This magnetotail reconnection occurs at X t in Figure 4b (remember that the total amount of magnetic flux carried by the SA and NB 0 flux tubes is the same). As a result, a new closed field line C 2 is created, and it subsequently moves along the dawnside. We label this line C 2 because it replaces the former C 2 line involved in the internal reconnection at X i. [14] Similarly, the SB 0 field lines (the counterpart of NB 0, see Figure 3b) are transported to the duskside magnetotail southward of the neutral sheet, while the NA field lines are transported to the duskside magnetotail northward of the neutral sheet. The SB 0 field lines reconnect with the NA field lines in the quadrant of Y > 0 and Z > 0. We show below that the magnetotail reconnection must occur between SA and NB 0 and between NA and SB 0, for otherwise the steady-state magnetic flux circulation is not maintained. [15] We should remark here that in the simulation results of Tanaka [1999] the reconnection point 4 0 in Figure 3b is located sunward of the dawn-dusk meridian (see his Figure 5). In our model, however, we place point 4 0 (and its counterpart in the northern hemisphere) antisunward of the dawn-dusk meridian. This is a modification of Tanaka s model based on observations (section 4), which show that the sunward convection associated with the round merging 5of20

6 cell lies largely on closed field lines near the dawn-dusk meridian Field Line Motion and the Resulting Ionospheric Convection Subsequent to Magnetotail Merging [16] The closed field lines C 2 created from NB 0 and SA start shrinking mainly by magnetic tension. In the northern ionosphere their motion constitutes the sunward convection portion (4 C 2 3) of the dawnside crescent-shaped convection cell in Figure 3a labeled 3 NB 0 4 C 2 3 (the secondary exchange cell introduced in section 2.4). In the southern hemisphere it corresponds to the near-dawn portion (4 0 C ) of the sunward convection of the large round cell labeled 1 0 SA 4 0 C C in Figure 3b. Thus the closed magnetic flux lost in the internal reconnection at X i ultimately returns to foot points 3 and 3 0. In other words, the magnetotail reconnection at X t between NB 0 and SA reproduces the closed field line C 2 originally involved in the internal reconnection at X i. The newly created C 2 field lines move sunward in the magnetotail as depicted in Figure 4a. At foot points 3 and 3 0 they can again reconnect with NB and become NB 0 and C 1, as sketched in Figure 4b. Thus the steady-state magnetic flux circulation is completed globally Global Topology of the Magnetic Field [17] It is instructive to understand the topology of the type of magnetic field lines in this paper. Figure 5 shows how all the field line types are topologically related with each other. Note that Figure 5 is drawn for the explanation of the topology, and it does not necessarily represent the geometry. The solid circle represents the ionosphere as seen from the magnetotail. Heavy solid lines inside the circle denote the convection cells, while the light solid and dashed lines outside the circle represent the magnetic field lines. When a dashed and a solid field line intersect, the former is behind the latter. (Inside the magnetopause, the dashed field lines are mainly on the dayside, while the solid field lines are on the nightside.) On the flanks of the magnetosphere, the solid field lines are inside the dashed field lines. The heavy dots on the field lines show three pairs of points at which the subsequent reconnections occur. In the southern hemisphere, we labeled the points X p,x i, and X t, using the same notation as Figures 1 4. The equations at the bottom of Figure 5 represent the three reconnection processes, with the left side showing the field lines before the reconnection and the right side showing the field lines after the reconnection. [18] C 1 is the closed field line connecting the duskside crescent-shaped cell (the primary exchange cell) in the northern hemisphere with the round merging cell in the southern hemisphere. The dayside magnetopause reconnection occurs between C 1 and the IMF at point X p, producing NB and SA field lines. C 2 is the closed field line connecting the dawnside crescent-shaped cell (the secondary exchange cell) in the northern hemisphere with the round merging cell in the southern hemisphere. The internal reconnection occurs between C 2 and NB at X i, producing NB 0 and C 1 field lines. Finally, the magnetotail reconnection occurs between SA and NB 0 at X t, producing C 2 and the IMF (unconnected field lines). Thus the magnetic field topology returns to its initial state. We learn from Figure 5 that the magnetic flux circulation model proposed in this paper can Figure 5. The topology of magnetic field lines viewed from the magnetotail. The solid circle in the middle shows the ionosphere. The arrowed lines inside the circle represent the ionospheric convection, with PE, SE, and RM denoting, respectively, the primary exchange cell, the secondary exchange cell, and the round merging cell. The light solid and dashed lines outside the circle represent magnetic field lines. C 1 (C 0 1) is the closed field line connecting the PE cell in the northern (southern) hemisphere with the RM cell in the southern (northern) hemisphere. C 2 (C 0 2) is the closed field line connecting the SE cell in the northern (southern) hemisphere with the RM cell in the southern (northern) hemisphere. When a dashed and a solid field line intersect, the former is behind the latter. The crosses (bullets) in the equatorial region show the sunward (tailward) motion of the field line. The three reconnection processes in the southern hemisphere occur at X p (magnetopause reconnection), X i (internal reconnection), and X t (magnetotail reconnection), as shown in the reconnection equations at the bottom. maintain its steady-state topology without tangling of magnetic field lines. [19] So far we have described the magnetic flux circulation mainly by tracking the NB field lines. The SB field lines follow the same processes as the NB field lines. The reconnection relevant to SB field lines occurs in the northern hemisphere. This is the counterpart of the three reconnection processes described above and corresponds to the six heavy dots in the upper part of Figure 5. In the duskside equatorial region, the NB field lines moving tailward meet the C 0 2 (the counterpart of C 2 ) field lines 6of20

7 Figure 6. Interplanetary data on 25 December (a e) ACE observations of the solar wind dynamic pressure, the IMF clock angle (q = atan(b Y /B Z )), and the three IMF components in the GSM coordinates 85 min after the observation, with the time axis for the ACE observations at the top. (f ) Geotail observations of the magnetosheath magnetic field B Z component in GSM coordinates, with the time axis (shifted 85 min from the ACE values) at the bottom. The vertical dashed lines indicate the time interval discussed in this paper. moving sunward on the flank of the magnetosphere. At this meeting place, a pileup of magnetic flux occurs. This is the reason why the NB field lines cannot be transported directly to the magnetotail without changing their topology. The pileup of magnetic flux is canceled by the internal reconnection in both hemispheres. That is, the NB field lines moving tailward become C 1 moving sunward by the southern hemisphere reconnection, and the C 0 2 field lines moving sunward become SB 0 (the counterpart of NB 0 ) moving tailward by the northern hemisphere reconnection. The similar magnetic flux pileup which would occur on the dawnside flank is also canceled by internal reconnection. The SB field lines moving tailward become C 0 1 (the counterpart of C 1 ) moving sunward by the northern hemisphere reconnection, and the C 2 field lines moving sunward become NB 0 moving tailward by the southern hemisphere reconnection. Thus we can conclude that the role of the internal reconnection is to prevent the pileup of magnetic flux on the flanks of the magnetosphere and to maintain the steady-state magnetic flux circulation. From the consideration above, it is clear that the reconnection processes in both hemispheres are closely related to each other, although both are topologically independent. [20] Another point to be noted in Figure 5 is that the magnetotail reconnection between NB 0 and SA and between SB 0 and NA must occur in order for the steady-state circulation to be maintained. If reconnection were to occur first between NB 0 and SB 0 (or between NA and SA) and then between NA and SA (NB 0 and SB 0 ), the unconnected field lines resulting from NB 0 and SB 0 (NA and SA) would stick through the closed field line loop resulting from NA and SA (NB 0 and SB 0 ). Accordingly, the unconnected field lines could not be removed from the magnetotail, and the steadystate magnetic flux circulation could not be maintained Global Ionospheric Convection [21] In summary, except for the lobe cells circulating exclusively in the open regions, a total of six convection cells in both hemispheres (two crescent-shaped cells and one large round cell in each hemisphere) are working together in maintaining the steady-state magnetic flux circulation. They are all merging cells according to the nomenclature by Reiff and Burch [1985]. Although we cannot isolate the function of any one of the six convection cells, from the point of view of magnetic flux transport, we can say that the two crescent-shaped cells in the northern hemisphere (Figure 3a) exchange magnetic flux with each other, with the help of the round merging cell in the opposite hemisphere (Figure 3b). Note that the foot points of the open field lines (NB and NB 0 ) jump from point 2 to 3, conserving the open magnetic flux. At the same time the foot points of closed field lines (C 2 and C 1 ) jump from point 3 to 2, conserving the closed magnetic flux. Of course, these open and closed magnetic fluxes are equal. That is why we refer to the two crescent-shaped cells in Figure 3a as a pair of exchange cells. Similarly, the two southern hemisphere crescent-shaped cells in Figure 3b are also a pair of exchange cells. Of the two exchange cells in pair, we call the one in the postnoon/prenoon sector the primary exchange cell and the other in the postmidnight/premidnight sector the secondary exchange cell. As noted above, exchange cells are a subset of merging cells. [22] For B Y < 0 the dawn-dusk relation described above is inverted (i.e., Figure 3a applies to the southern ionosphere and Figure 3b to the northern ionosphere). In summary, for B Y > 0 the exchange cell pair appears in the postnoon and postmidnight (prenoon and premidnight) sectors in the northern (southern) hemisphere, while for B Y < 0 the pair appears in the prenoon and premidnight (postnoon and postmidnight) sectors in the northern (southern) hemisphere. In section 4 we demonstrate observations which support this model, focusing on primary exchange cells. 3. Instruments [23] In this paper we present two main data sets. One is global ionospheric plasma flow determined by Super Dual 7of20

8 Auroral Radar Network (SuperDARN) data [Greenwald et al., 1995]. The radars operate at frequencies 8 20 MHz and measure the coherent backscattered power and Doppler spectral characteristics of decameter-range field-aligned irregularities. At F region altitudes the line-of-sight Doppler velocity of the irregularities gives a measure of the electric field drift of the plasma. The other main data set is precipitating particle and ion drift data obtained by Defense Meteorological Satellite Program (DMSP) satellites. These carry electrostatic analyzers designed to measure the flux of precipitating electrons and ions in the energy range kev in 19 logarithmically spaced steps [Hardy et al., 1984]. The satellites also carry ion drift meters that measure angles of ion arrival [Greenspan et al., 1986]. The measured angles are converted into the two drift components perpendicular to the spacecraft s velocity vector. Both data sets are presented in magnetic latitude (MLAT) and local time (MLT) coordinates determined from Altitude Adjusted Corrected Geomagnetic (AACGM) coordinates based on the International Geomagnetic Reference Field Epoch In addition to the two main data sources, we use interplanetary data by ACE and Geotail satellites in order to monitor the solar wind and IMF conditions. 4. Observations [24] In this section we focus on primary exchange cells, because, as shown in Figure 3, they exhibit a unique feature in the polar cap topology that is easily identified from observations B Y < 0 Case Interplanetary Context [25] We first show an example of the B Y < 0 case. Figure 6 demonstrates interplanetary observations by ACE and Geotail satellites on 25 December Figures 6a 6e show, respectively, the solar wind dynamic pressure, IMF clock angle, and three components of the IMF (in timeshifted Geocentric Solar Magnetospheric (GSM) coordinates as described below) observed by ACE located near the Lagrangian point 234 R E ahead of the Earth (R E being the mean radius of the Earth). The time axis is plotted at the top of Figure 6. Figure 6f shows the B Z component of the magnetic field observed by Geotail in GSM coordinates, with the time axis at the bottom. Geotail was in the dawnside magnetosheath at (X, Y, Z ) ( 3.5, 29.7, 0.9) R E in GSM coordinates. The B Z profile observed by Geotail is similar to that by ACE. The timelag between the two spacecraft determined from correlation analysis is 85 min. Thus we assume that the IMF conditions that actually affected the magnetosphere were those observed by ACE with an 85-min delay. The ACE observations in Figure 6 are shifted forward 85 min with respect to Geotail observations. During the 85-min propagation of the solar wind, the GSM coordinates rotate several degrees about the X axis. Since we are investigating phenomena sensitive to the IMF clock angle, we corrected this timelag effect. The data in Figures 6b 6e are presented in the GSM coordinates 85 min after the observation. Hereafter ACE observations are referenced with the shifted UT at the bottom of Figure 6. The IMF clock angle (q) in this paper is defined as q = atan(b Y /B Z ), i.e., the angle measured from the Z axis and positive duskward. [26] The time interval bounded by the vertical dashed lines ( UT) indicates the period we discuss in this paper. By 1445 UT the IMF became stably northward. At 1515 UT, the B Y component turned dawnward (negative), and after that the B Y < 0 and B Z > 0 condition continued more than 4 hours. The solar wind dynamic pressure (Figure 6a) was nearly constant during the northward B Z period. The IMF clock angle was 30 at first and then slightly decreased on average, reaching 45 during the period of interest. This stable IMF clock angle continued until 1815 UT when a B Y spike hit the magnetosphere SuperDARN Observations [27] According to the scenario in section 2, the primary exchange cell is expected to appear in the dawnside plasma sheet in the northern ionosphere (i.e., the convection pattern of Figure 3b). (In section 4.1 for the B Y <0 case we refer to Figure 3 by applying a different caption: Figure 3a is for the southern hemisphere and Figure 3b is for the northern hemisphere.) One salient feature of this configuration is that a region of antisunward flow with open magnetic flux is embedded in the dawnside plasma sheet in the northern ionosphere. We can use this characteristic as an identifier of the primary exchange cell. On this day the radars operated with a 1-min scan mode. The SuperDARN radars in the northern hemisphere provided enough ionospheric echoes during this event to derive global potential maps [Ruohoniemi and Baker, 1998] determined mainly by the measured data. Figure 7 shows selected potential maps determined from 5-min averaged data. The dots show the points of radar observations, so in regions of dense dots the fitted potentials are data-driven. Note that equipotentials are equivalent to streamlines, and the denser the contours, the faster the flow. [28] Figure 7a shows a potential map for UT. The dawnside convection cell is expanded across the noonmidnight meridian. As a result, the duskside cell is crescentshaped, while the dawnside cell is round, which is a typical pattern for B Y <0. [29] Figure 7b shows a potential map for UT. Comparing Figures 7a and 7b, we notice that an antisunward flow has emerged in the dawnside high (>80 ) latitudes (marked by the arrow), forming a small counterclockwise convection cell centered at 0700 MLT and 80 MLAT. Figure 7c is a potential map for UT. The antisunward flow has strengthened and consequently the counterclockwise convection cell has further enhanced compared to Figure 7b. From convection maps with 1-min resolution, together with individual radar data, we determined that the antisunward flow in the dawn sector started at 1628 UT. The left dashed line in Figure 6 shows this commencement time. From ACE observations we learn that the onset of the antisunward flow roughly corresponds to the time when the IMF clock angle became 45. We thus infer that the dawnside counterclockwise cell is the primary exchange cell in Figure 3b. [30] After 1628 UT, the primary exchange cell in the dawn sector was continuously observed until 1818 UT. Figures 7d and 7e show two examples of the convection pattern observed after the exchange cell establishment. The 8of20

9 Figure 7. Selected potential maps on 25 December 2000, determined from SuperDARN data in the northern hemisphere. Equipotentials are plotted every 3 kv (labeled in kv) in MLAT-MLT coordinates. Dots show points of radar observations. The dashed concentric circles indicate MLAT values of 80 and 70, and the outermost circle corresponds to 65 MLAT. 9of20

10 Figure 8. DMSP F13 observations for 1755: :30 UT on 25 December 2000 over the northern ionosphere: (top) cross-track horizontal ion drifts, and (middle and bottom) precipitating electrons and ions. The heavy horizontal arrowed bars in the top panel denote the polar cap and the overdraped lobe (OL). Figure 7d subinterval ( UT) was chosen to show the global pattern clearly using the best quality data from the radars, while the Figure 7e subinterval ( UT) was chosen simply because DMSP overflight data were available. The overall convection pattern in Figures 7d and 7e is well interpreted by the three-cell model in Figure 3b, except for a few questionable cells appearing where there are almost no radar observations (e.g., the postmidnight cell in Figure 7d). A note should be added here. Although the IMF conditions were quasi-stationary during the event period, the ionospheric convection was not always stationary. The shape, location, and strength of each cell changed significantly with time, showing some deviations from Figures 7d and 7e. However, the basic pattern of Figure 3b was retained throughout the period. [31] The antisunward flow in the dawn sector started to weaken rapidly at 1818 UT, and the counterclockwise cell completely disappeared in 10 min. Figure 7f shows the potential map for UT. We can clearly see the disappearance of the primary exchange cell. The right dashed line in Figure 6 marks 1818 UT. Figures 6b and 6d indicate that at that time there was a temporal increase of the IMF clock angle associated with a B Y spike. After this the clock angle decreased again down to 55, but the antisunward flow (and equivalently the primary exchange cell) in the dawn sector did not recover. Thus we infer that formation of the exchange cell is sensitive to the IMF clock angle DMSP Observations [32] Our model predicts that the antisunward flow in the dawn sector is associated with open magnetic field lines. To verify this, we utilized DMSP satellite data. Figure 8 demonstrates a dusk-to-dawn overflight of DMSP F13 in the northern hemisphere corresponding to Figure 7e. The middle and bottom panels show energy-versus-time spectrograms of precipitating electrons and ions, respectively, in the energy range kV, while the top panel shows the cross-track component of horizontal ion drifts (positive sunward). In the middle of the spectrograms, we can clearly identify a region of very weak particle flux (in particular, for ions) as marked by an arrowed bar in the top panel. This plasma regime is the polar cap threaded by open magnetic field lines. In the dawnside plasma sheet, at about 0603 MLT, we can find a region characterized by lowenergy (<1 kev) electrons and ions with weaker flux compared to its surrounding regions (marked with OL 10 of 20

11 exchange cell depicted in Figure 3a. Thus at times, there are multiple open magnetic flux islands. Figure 9. The superposition of the potential data in Figure 7e and the drift data in Figure 8. The potential contours (dotted lines) are the same as Figure 7e. The dashed concentric circles indicate MLAT values of 80 and 70, and the outermost circle corresponds to 65 MLAT. The satellite positions at 1805 UT and 1808 UT are shown by ticks. (overdraped lobe) in the top panel). We suspect this is a region of open magnetic field lines. The ion drift data indicate that this region is associated with antisunward convection, while the rest of the dawnside plasma sheet is associated with sunward convection. This antisunward flow corresponds to the antisunward flow in the dawn sector found in SuperDARN data. Figure 9 shows the superposition of the SuperDARN potential data in Figure 7e and the DMSP drift data in Figure 8. The two measurements relatively agree well. Thus we conclude that the antisunward flow embedded in the dawnside plasma sheet is the tailward open flux transport associated with the primary exchange cell (i.e., foot points of NB field lines). [33] In the southern hemisphere, we expect a counterpart of Figure 8, i.e., a region of antisunward flow with open magnetic flux embedded in the duskside plasma sheet (Figure 3a). Figure 10 shows a dawn-to-dusk overflight of DMSP F13 in the southern hemisphere, in the same format as Figure 8. The particle data (in particular, in ions) indicate a clear signature of the polar cap as denoted in the top panel. In the duskside plasma sheet, we can identify at least three portions of weak flux both in electrons and ions (denoted as OL ). We infer that these regions are on open field lines. The top panel indicates that these weak particle flux regions are all associated with or very near to the antisunward convection, while the rest of the duskside plasma sheet is associated with sunward convection. The overall precipitation and flow pattern is also a mirror image of Figure 8. Therefore we conclude that the regions labeled OL are foot points of SB field lines and manifest the primary 4.2. B Y > 0 Case Interplanetary Context [34] We next show an example of the B Y > 0 case. Figure 11 shows interplanetary observations by ACE and Geotail satellites on 7 November 2000, in nearly the same format as Figure 6. Figures 11a 11e demonstrate the solar wind dynamic pressure, IMF clock angle, and three components of the IMF in time-shifted GSM coordinates observed by ACE near the Lagrangian point 234 R E ahead of the Earth, with the time axis at the top. Figure 11f shows the IMF B Y component in GSM coordinates observed by Geotail, with the time axis at the bottom. Geotail was skimming the dawnside bow shock at (X, Y, Z ) (6.1, 27.0, 7.2) R E. Figure 11f shows only the upstream IMF data, i.e., the magnetosheath data are removed. The B Y profiles observed by Geotail and ACE are very similar, and both IMF data show a sharp dawnward turn (at 1819 UT for Geotail). The timelag between the two spacecraft determined by the dawnward turn of B Y is 54 min. We found that the IMF clock angles determined by ACE and Geotail (when available) were essentially the same. Therefore we postulate that the IMF conditions observed by ACE affected the magnetosphere with a delay of 54 min. The ACE observations in Figure 11 is shifted forward 54 min with respect to Geotail observations. As in the previous event, the coordinates in Figures 11b 11e are the GSM coordinates 54 min after the observation. Hereafter ACE observations are referenced with the shifted UT at the bottom of Figure 11. [35] The B Y component of the IMF was strongly positive (B Y > 10 nt) from the beginning of this day. At 1230 UT the B Z component turned northward. As seen in Figure 11, the IMF was very stable after 1603 UT with a clock angle of 35. This stable IMF continued until 1800 UT, when the clock angle started to decrease gradually down to 15 by 1815 UT. Then B Y turned dawnward sharply at 1819 UT. The time interval bounded by the vertical dashed lines ( UT) indicates the period of interest when convection patterns consistent with Figure 3a were observed in the northern hemisphere by SuperDARN radars. The solar wind dynamic pressure (Figure 11a) was constant for the first half of the period, but for the second half there were two appreciable increases at 1715 UT and 1753 UT SuperDARN Observations [36] On this day all the radars operated with a 2-min scan mode. Therefore the total number of ionospheric echoes was not as large as in the previous case (in particular, echoes on the dawnside were scarce), but overall the radars provided enough echoes for the generation of useful large-scale potential maps for this study. Figure 12 shows selected potential maps determined from 5-min averaged data, as in the same format in Figure 7. [37] Figure 12a is a potential map for UT. The duskside convection cell is expanded across the noonmidnight meridian and the convection pattern consists of the duskside round cell and the dawnside crescent-shaped cell. We should note here that there are few observations on the dawnside and therefore the dawnside cell comes not from observations but from a statistical model used to support the 11 of 20

12 Figure 10. DMSP F13 observations for 1703: :30 UT on 25 December 2000 over the southern ionosphere, in the same format as Figure 8. potential fitting. However, the duskside cell is based on measurements. The convection pattern is typical of that for B Y >0. [38] Figure 12b shows a potential map for UT. Although the dawnside potentials are unreliable for scarce observations, the duskside potentials at latitudes <80 MLAT are reliable. By comparing Figures 12a and 12b, we see that an antisunward flow has appeared in the postnoon region at MLAT (marked by the arrow), forming a small clockwise convection cell with a central potential of about 18 kv and an outer contour of 12 kv. (The three small vortices at >80 MLAT with central potentials of 0 kv, 9 kv, and 15 kv are dubious, because there are few observations in the inter-vortex regions.) Figure 12c shows a convection map for UT. The clockwise cell in Figure 12b has grown and strengthened (the potential contours go from a central potential of 15 kv to an outer value of 3 kv), and a strong antisunward flow is observed in the dusk sector. (As in Figure 12b, the three small vortices at >80 MLAT with central potentials of 6 kv, 6 kv, and 9 kv are dubious.) From the sequence of convection maps of 2-min resolution, as well as individual radar data, we determined that the antisunward flow in the postnoon sector started at 1600 UT (the left vertical dashed line in Figure 11). From Figure 11 we notice that this onset time roughly corresponds to the beginning (1603 UT) of the interval of stable IMF clock angles. These results strongly indicate that the clockwise cell in the dusk sector is the primary exchange cell shown in Figure 3a. The 3-min difference lies within the uncertainties arising from the Geotail-ionosphere timelag and the time resolution of radar observations (2 min). In addition, the ACE-Geotail timelag determined from the IMF change at 1819 UT may not be applicable to the entire period of interest. [39] After 1600 UT the duskside clockwise cell was present continuously. In this event, however, a reliable global convection pattern was not obtained until 1750 UT because of scarce radar observations on the dawnside. Figures 12d and 12e show two examples of convection maps when a wide spatial coverage of radar data was available. Again, the Figure 12d subinterval ( UT) was chosen to show the global convection pattern clearly using the best quality data from the radars, while the Figure 12e subinterval ( UT) was chosen just because DMSP overflight data were available. The overall convection pattern in Figures 12d and 12e is successfully interpreted by Figure 3a. We thus conclude that the clockwise convection cell on the duskside is the primary exchange cell. 12 of 20

13 Figure 11. Interplanetary data on 7 November 2000, in nearly the same format as Figure 6. (a e) ACE observations of the solar wind dynamic pressure, the IMF clock angle, and the three IMF components in the GSM coordinates 54 min after the observation, with the time axis for the ACE observations at the top. (f ) Geotail observations of the IMF B Y component in GSM coordinates (magnetosheath data are removed), with the time axis (shifted 54 min from the ACE values) at the bottom. The vertical dashed lines indicate the time period discussed in this paper. [40] At 1819 UT, IMF B Y switched from positive to negative. Figure 12f shows the potential map just after the IMF B Y switch. In response to the B Y change, the ionospheric convection in the noon meridian promptly switched from dawnward to duskward. That is, the new open flux is now exclusively transported to the duskside. In addition, if we follow the stream line, the duskward flow in the noon meridian joins the duskside antisunward flow associated with the primary exchange cell. As a result, intrusion of open magnetic flux into the duskside plasma sheet enhanced greatly, and the duskside plasma sheet was detached completely to form a q aurora configuration. An optical q aurora corresponding to this plasma sheet isolation was indeed observed by the Polar satellite. This sequence of plasma sheet detachment to form a q aurora is consistent with Cumnock et al. [1997] and Chang et al. [1998]. We return to this topic later in terms of DMSP observations DMSP Observations [41] As in the previous case, it is expected that the antisunward flow in the dusk sector observed by Super- DARN is associated with open magnetic field lines. We confirmed this using DMSP data. Figure 13 demonstrates a dusk-to-dawn overflight of DMSP F13 in the northern hemisphere 30 min after the exchange cell appearance, in the same format as Figures 8 and 10. In the central part of the overflight, we can find the polar cap in which ion precipitation is almost void (denoted by an arrowed bar in the top panel). The diffuse electron precipitation extending up to 3 kev would be the kev polar rain [e.g., Newell and Meng, 1990]. In the middle of the duskside plasma sheet, at about 1800 MLT, we can identify a region of very weak flux both in ions and electrons (labeled as OL in the top panel). We infer that this weak precipitation region is on open field lines. The ion drift data show that this open magnetic flux region is largely associated with antisunward flow, although sunward flow is seen at its poleward potion. Thus as in the previous example, we conclude that the region labeled OL is the manifestation of the overdraped lobe (i.e., foot points of NB field lines) depicted in Figure 3a. During this DMSP overflight, SuperDARN did not receive enough ionospheric echoes to produce global potential maps. [42] Figure 14 shows DMSP F13 data one revolution after Figure 13, and corresponds to Figure 12e. The IMF B Y polarity change occurred in the middle of the overflight at 1819 UT, so we exclusively focus on the duskside observations. The precipitation and flow pattern is basically the same as Figure 13. The polar cap is clearly seen in the center of the spectrograms. On the duskside we can find at least three regions of weak precipitation embedded in the plasma sheet, as labeled OL in the top panel. We infer that these regions are on open field lines. The drift data show that the three regions are all roughly associated with antisunward convection. These regions are suggested to be foot points of NB field lines. Thus, as in Figure 10, there can be multiple open magnetic flux islands. Figure 15 shows the superposition of the SuperDARN potential data in Figure 12e and the DMSP drift data in Figure 14. From Figures 12e and 15, we know that there are no Super- DARN observations along the satellite track on the duskside. Nevertheless, the potentials are naturally exterpolated and the two observations are basically consistent. The locations of the duskside antisunward flow around 82 MLAT roughly coincide with each other. [43] Another thing to be noted here is the way in which the detachment of the duskside plasma sheet took place after the IMF B Y switch at 1819 UT. As described in section 4.2.2, the duskward flow in the noon meridian joins the duskside antisunward flow associated with the primary exchange cell. In the dusk meridian, the antisunward flow is observed at MLATs (Figure 12f ). By comparing Figures 12f and 14, we infer that the new open magnetic flux intruded into the middle and the equatorwardmost overdraped lobe in Figure 14. In other words, the plasma sheet denoted as A in Figure 14 was 13 of 20

14 Figure 12. Selected potential maps on 7 November 2000, determined from SuperDARN data in the northern hemisphere, in the same format as Figure of 20

15 Figure 13. DMSP F13 observations for 1630: :00 UT on 7 November 2000 over the northern ionosphere, in the same format as Figure 8. detached to form a q aurora configuration. This inference is consistent with the thickness of the theta bar observed by DMSP F13 one revolution after Figure 14. This morphology indicates that the preexisting open flux islands guided the new open flux associated with the B Y change. [44] In the southern hemisphere, a counterpart of Figures 13 and 14 should be observed, i.e., we expect an open magnetic flux region associated with antisunward convection in the dawnside plasma sheet. Figure 16 shows a dawn-to-dusk overflight of DMSP F13 in the southern hemisphere. In this pass the satellite trajectory shifted slightly sunward off the dawn-dusk meridian, but we can still identify the polar cap as denoted in the top panel. The polar cap is characterized by an absence of >1 kev ions. In the dawnside plasma sheet, we can identify a region of reduced particle flux (denoted as OL in the top panel) with a tiny subregion of significant particle flux embedded in the middle. We infer that this region labeled OL is mostly on open field lines except for the subregion. This subregion may consist of fragments of the plasma sheet. Otherwise, the drift data indicate that the region OL is an open magnetic flux region associated with antisunward convection. Thus we again conclude that an open magnetic flux island, located at the foot points of SB field lines, appeared in the dawnside plasma sheet as depicted in Figure 3b. 5. Discussion 5.1. Discrepancy Between MHD Simulation and Observations [45] In this paper we used a deductive approach to the morphology, postulating the existence of the counterintuitive exchange cells deduced from the MHD simulation results by Tanaka [1999]. This approach seems to be well supported by the observations, which show that the MHD simulation results may underestimate the spatial extent of the primary exchange cells. In Figure 1 of Tanaka [1999] the primary exchange cell is confined to a small region in the forenoon or afternoon sector, while in the observations it extends beyond the dawn-dusk meridian and into the nightside. Here we suggest two possible explanations of this discrepancy. First of all, in the MHD simulations, numerical viscosity inevitably obscures the fine structure of the solution. Second, the strength of the IMF is considerably different. In Tanaka s simulation, the IMF B Z value was set to 3.5 nt, while in the 15 of 20

16 Figure 14. DMSP F13 observations for 1811: :30 UT on 7 November 2000 over the northern ionosphere, in the same format as Figure 8. See text for the description of region A. observations of this paper, B Z was 8 nt for the B Y <0 case and 18 nt for the B Y > 0 case. It seems plausible that the extent of the primary exchange cell is positively correlated with the IMF magnitude Relationship to Preexisting Models Internal Reconnection [46] The magnetospheric configuration proposed in this paper is characterized by the internal reconnection in the magnetospheric boundary layer that occurs subsequent to dayside magnetopause reconnection. The concept of internal reconnection was first discussed by Crooker [1992], following the concept of sequential reconnection by Cowley [1983]. Using superposition of the Earth s dipole field and a uniform IMF, Crooker [1992] considered the magnetospheric configuration for northward IMF when dipole tilt or IMF B X effects are dominant. Under these conditions, the magnetopause reconnection with closed geomagnetic field lines occurs exclusively in the summer or B X - favored hemisphere. The open field lines created by the magnetopause reconnection become closed again by the internal reconnection in the magnetospheric boundary layer in the opposite hemisphere. The newly closed field lines created by the internal reconnection then return to the tail via the flanks of the magnetosphere. As a result, a pair of reverse merging cells appears in the ionosphere with a finite size polar cap in both hemispheres. Figure 17f sketches the ionospheric convection in the winter or B X -unfavored hemisphere predicted by Crooker [1992]. The magnetopause reconnection occurs at the bullets, and the internal reconnection occurs at the open circles. Cowley s [1983] concept of sequential reconnection and Cooker s [1992] concept of internal reconnection are incorporated into the model in this paper. We explore this point more in detail in the next section Relationship to Normal and Reverse Merging Cells [47] It would be instructive to see how exchange cells are related to the normal merging cells for southward IMF and the reverse merging cells for due northward IMF. The magnetospheric configuration discussed in this paper occurs only when the IMF clock angle (q) is near ±45. Figure 17 depicts the transition of the ionospheric convection pattern in the northern hemisphere when q is away from 45 toward 90 (upper row) and toward 0 (lower row). The dashed lines indicate the open/closed boundary. The bullets show the foot point of NB just after the magnetopause reconnection in the southern hemisphere, and the open circles show the location of the internal reconnection. Only merging cells are plotted 16 of 20

17 Figure 15. The superposition of the potential data in Figure 12e and the drift data in Figure 14. The potential contours (dotted lines) are the same as Figure 12e. The dashed concentric circles indicate MLAT values of 80 and 70, and the outermost circle corresponds to 65 MLAT. The satellite positions at 1812 UT and 1815 UT are shown by ticks. in Figure 17 for simplicity. Figure 17a is the basic exchange cell configuration when q = 45. [48] When q rotates from 45 toward 90, the magnetopause reconnection site in the southern hemisphere moves dawnward, equatorward, and sunward [Luhmann et al., 1984]. As a result, the NB field line becomes more tilted in the Y-Z plane than in Figure 1. From this simple geometry, the overdraping of NB tends to be cancelled when the solar wind drags the magnetosheath end tailward. Thus at a certain value of q, the slipping direction of the NB field line after the magnetopause reconnection changes from eastward to westward. Roughly speaking, NB slips eastward (westward) if it passes to the east (west) of the subsolar point. Figure 17c shows the situation when all the NB field lines move westward. In this case, the primary exchange cell on the duskside disappears, and a large crescent-shaped cell appears on the dawnside extending across the noon meridian (hereafter we call this merging cell the crescent cell and distinguish it from the secondary exchange cell). Similarly, all the SB field lines slip eastward, so the round merging cell in the northern hemisphere becomes a normal round cell which crosses the open/closed boundary only twice in one cycle (hereafter we call this merging cell the normal round cell and distinguish it from the round merging cell associated with internal reconnection). The configuration in Figure 17c is the normal two-cell convection pattern when IMF B Y is positive, consisting of the normal round cell on the duskside and the crescent cell on the dawnside. Figure 17b shows the transition between Figures 17a and 17c. During the transition, some of the NB field lines form the crescent cell, while others form the primary exchange cell. Therefore the primary exchange cell becomes smaller compared to Figure 17a. Accordingly, the location of the internal reconnection would move sunward as depicted in Figure 17b. During the transition, the crescent cell and the secondary exchange cell coexist on the dawnside and form one crescent-shaped cell. These two cells are essentially indistinguishable. Similarly, the normal round cell and the round merging cell associated with internal reconnection coexist and form one round cell. Again the two cells are essentially indistinguishable. [49] When q rotates from 45 toward 0, the magnetopause reconnection sites in both hemispheres move toward the noon-midnight meridian poleward of the usual cusp location [Luhmann et al., 1984]. In this case, the magnetosphere switches to Crooker s [1992] configuration described in section For simplicity, we assume that the magnetopause reconnection is active only in the southern hemisphere, that is, we consider the case of boreal winter or positive IMF B X. Figure 17d shows the rather shrunken exchange cells when q is a little smaller than 45. Note that, since magnetopause reconnection is assumed to be absent in the northern hemisphere, the round merging cell disappears in the northern ionosphere. At a certain IMF orientation, the ionospheric convection switches from Figure 17d to Figure 17e. We suggest that this transition occurs when the NB field line ceases to overdrape the dayside magnetosphere but instead drapes the nightside magnetosphere. In this configuration, the foot point of NB moves sunward in the ionosphere when NB moves antisunward in the magnetosphere. On the other hand, SA drapes the dayside magnetosphere and becomes an overdraped lobe. The subsequent internal reconnection occurs between NB and SA in the northern hemisphere. The new closed field line created by the internal reconnection returns to the nightside via the dawnside flank (because SA drapes the dawnside magnetosphere), thus forming a single reverse merging cell. As q becomes nearer to 0, the closed flux also returns via the duskside flank, forming a twin reverse merging cell as shown in Figure 17f. In the reverse merging cell configuration (Figures 17e and 17f ), the foot point of NB (the bullet) in the winter or B X - unfavored hemisphere should be located deep into the nightside, because the electric field associated with the magnetopause reconnection is not mapped into the ionosphere opposite to the reconnection site [Crooker, 1992]. From our present knowledge, it is not clear how the transition occurs from Figure 17d to Figure 17e. Since the geometry is totally different, it is unlikely that the exchange cells in Figure 17d and the reverse merging cell in Figure 17e coexist. When one mode is switched on, the other mode would be switched off. [50] In summary, the exchange cells have a relatively clear relationship to the normal merging cells for southward IMF, while their relationship to the reverse merging cells for due northward IMF is not necessarily clear. This is convincing if we consider the basic role of the exchange cells: transport of open magnetic flux from the dayside to the magnetotail. Although the concept of internal reconnection can be applied to both the exchange cell case and the reverse merging cell case, these cases are essentially different Relationship to the Theta Aurora Configuration [51] The ionospheric configuration discussed in this paper would be categorized not into the theta aurora configuration 17 of 20

18 Figure 16. DMSP F13 observations for 1718: :30 UT on 7 November 2000 over the southern ionosphere, in the same format as Figure 8. but into the horse-collar configuration, in that the plasma sheet is not completely detached. However, it plays an important role in the formation of the theta aurora configuration. In the B Y > 0 example (section 4), IMF B Y switched from positive to negative sharply at 1819 UT. After that, in the northern hemisphere, the duskside plasma sheet denoted as A in Figure 14 was completely isolated and a theta aurora was formed. This formation process is the one proposed by Cumnock et al. [1997] and Chang et al. [1998]. As described in section 4, the newly created open magnetic flux intruded into the preexisting open magnetic flux islands in the duskside plasma sheet. In other words, the intrusion was guided by the open flux islands. We think this example provides a prototype of the theta aurora formation associated with an IMF B Y change. The seeds of the theta aurora are actually sowed before the IMF change Previous Observations of Exchange Cells [52] To the best of our knowledge, there is no published work that has shown observations of the polar cap configuration in Figure 3. However, there is some observational work which may be relevant to this topic. During periods of strong IMF B Y (including southward IMF periods), the latitudinal profile of the dayside field-aligned current (FAC) system in the northern ionosphere exhibits a four-sheet FAC pattern in the postnoon (prenoon) sector for B Y > 0 (B Y < 0); in the southern ionosphere the prenoon-postnoon relation is inverted [Taguchi et al., 1993; Ohtani et al., 1995; Watanabe et al., 1996]. Ohtani and Higuchi [2000] performed a statistical survey of the occurrence of the four-sheet FAC system and found that it tended to occur for northward IMF. The four-sheet FAC structure indicates that the system lies in a sunward convection region with a weak antisunward convection region embedded in the middle. Ohtani et al. [1995] and Ohtani and Higuchi [2000] interpreted the weak antisunward convection as a viscous cell on closed field lines. In light of the results of this paper, however, we suggest that the viscous cell proposed by Ohtani and his colleagues might be a primary exchange cell on open field lines, at least for northward IMF. 6. Conclusions [53] Upon the basis of MHD simulation results for northward IMF and jb Y j = B Z [Tanaka, 1999], we modeled a new merging sequence that results in polar cap bifurcation and accompanying paired exchange cells in the ionospheric convection pattern. The sequence consists of three 18 of 20