Effect of the dawn-dusk interplanetary magnetic field B y on the field-aligned current system

Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi:10.1029/2009ja014590, 2010 Effect of the dawn-dusk interplanetary magnetic field B y on the field-aligned current system X. C. Guo 1 and C. Wang 1 Received 22 June 2009; revised 26 August 2009; accepted 4 September 2009; published 16 January 2010. [1] A constant dawn-dusk B y component is set as an interplanetary magnetic field (IMF) condition in our global MHD simulations to investigate the effects of IMF B y on the closure of the field-aligned current (FAC) in the magnetosphere. On the basis of the steady state magnetosphere results, we trace streamlines of FAC from the ionosphere to draw the global geometry of current streamlines in the magnetosphere. Unlike those cases in which the IMF is purely northward or southward, the introduction of the dominant IMF B y significantly changes the topologies of the current streamlines. Cusp and mantle currents arise, and the symmetry of the FAC across the noon-night meridional plane breaks in the ionosphere. In addition to the self-closed currents in the Northern or Southern hemispheres, three more types of current streamlines connecting the two ionospheres are shown from the simulation results. The first current, including the cusp current, originates from the southern ionosphere and flows into the northern ionosphere. The second current, mainly the mantle current, and the tail current are connected to form a single current system, threading most of the magnetosphere along a spiral-like path and closing through the two lobes in the far magnetotail. The third current flowing out of the southern and into the northern ionosphere connects the two ionospheres by finally closing through the bow shock instead of the magnetopause. Quantitative results are presented and discussed for the four types of current streamlines and indicate that for the dominant IMF B y conditions the bow shock current should be included among the magnetosphere-ionosphere current system. Citation: Guo, X. C., and C. Wang (2010), Effect of the dawn-dusk interplanetary magnetic field B y on the field-aligned current system, J. Geophys. Res., 115,, doi:10.1029/2009ja014590. 1. Introduction [2] In the solar wind magnetosphere-ionosphere coupling system, the global field-aligned current (FAC) and convection patterns in the ionospheres are strongly regulated by the orientation of the interplanetary magnetic field (IMF). When the IMF B y component is dominant, the ionosphere exhibits a distorted two-cell convection pattern with significant dawn-dusk and interhemispheric asymmetries determined by the IMF B y polarity. For dawn-dusk IMF B y, the northern (southern) ionosphere has a round cell on the duskside (dawnside) and a crescent-shaped cell on the dawnside (duskside); the dawn-dusk situation reverses for the case of dusk-dawn IMF B y [Burch et al., 1985; Lu et al., 1994; Weimer, 1995]. The ionospheric manifestations of FAC have similar asymmetries with the convection pattern; such basic global FAC and convection patterns can be reproduced by global MHD simulations [Crooker et al., 1998; Tanaka, 2001; Kabin et al., 2003]. 1 State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China. Copyright 2010 by the American Geophysical Union. 0148-0227/10/2009JA014590$09.00 [3] The dominant IMF B y affects the magnetosphere by merging the IMF with the geomagnetic field; its effects on the magnetosphere have been studied using 3-D MHD simulations [Brecht et al., 1981]. It was found that the tail structure and the B x lobe fields are rotated and become asymmetric, and a B y component is generated in the neutral sheet. The simulation results agree with observational data. Kabin et al. [2003] studied the response of the magnetosphere-ionosphere system to a sudden change in the sign of the B y component of the IMF and demonstrated that the magnetosphere responds as a linear system and that the ionospheric potential can be well represented by a linear superposition of the initial and final distributions of the ionospheric potential. [4] Unlike the IMF B z that causes a dusk-dawn or dawndusk electric field across the polar cap, the IMF B y generates an electric field along the z direction. For the positive IMF B y, the electric potential of the southern ionosphere is higher than that of the northern; it drives the interhemispheric FAC between the two ionospheres along closed magnetic field lines [Leontyev and Lyatsky, 1974]. It is commonly believed that in the conjugated hemispheres the ionospheric plasma flows oppositely only within the polar caps where open field lines are connected to the interplanetary space [Burch 1of10

et al., 1985]. However, observations show that the IMF B y related electric field can penetrate into the closed magnetosphere and affect ionospheric convection [Rash et al., 1999]; that is, the opposite directed plasma flows can coexist in the closed magnetic field line regions. Kozlovsky et al. [2003] suggested that the observations of the electric field penetration in the closed field lines can be explained by anomalous resistivity. They estimated the latitudinal width of the region of the interhemispheric field-aligned currents where the IMF B y related contradirected ionospheric plasma flows can coexist in the conjugated ionospheres. [5] Tracing the current streamline in the simulation domain is a direct method for studying the global geometry of the magnetospheric and ionospheric current systems. From a three-dimensional MHD simulation, Tanaka [1995] deduced traditional region 1 and 2 currents in the polar ionosphere for cases of southward and northward IMF B z, and their typical characteristics agree with observational results. The author utilized the terms dynamo or load to depict areas that corresponded to a negative or positive value of the electromechanical energy conversion rate J E; Tanaka found that the region 1 currents are closed mainly in the high-latitude boundary layer and that the region 2 currents are primarily closed in the low-latitude boundary layer. The magnetopause serves as the dynamo for the energy dissipation in the ionosphere. [6] Janhunen and Koskinen [1997] discussed the closure of the region 1 current under a zero IMF condition and gave a simple physical explanation for the results showing that the region 1 current closure path resides more on the dayside than previously thought. In addition to the above IMF B z cases, Siscoe et al. [2000] performed a global MHD simulation to study the currents distribution in the magnetosphere with a constant positive IMF B y (dawn-dusk) input and found that the newly developed cusp and mantle current system threads most of the magnetosphere; the southern mantle current flows into and out of the southern ionosphere as the cusp current, then the cusp current follows dipolar field lines to the northern ionosphere and leaves as the northern mantle current along a spiral-like path into the northern tail lobe. The global current s geometry is then simply constructed when IMF B y dominates. [7] A quantitative analysis of currents based on results from global MHD simulations was first performed by Guo et al. [2008] to demonstrate that the bow shock is another dynamo region for the region 1 currents. In this paper, we carry out the global MHD simulation runs with similar IMF conditions to those of Siscoe et al. [2000], i.e., a constant dawn-dusk IMF B y of 5 nt imposed, to investigate the global geometry of the magnetospheric and ionospheric current systems. We apply a quantitative analysis to trace and classify the current systems with different closing paths in the magnetosphere. A complete picture of the magnetospheric and ionospheric current topology and distribution is presented in detail. Like the cases when IMF turns strongly southward [Guo et al., 2008], the bow shock current becomes a part of the whole current system under a dominant IMF B y condition. In addition, the penetration of the interplanetary electric field into the closed magnetosphere and ionosphere will be discussed on the basis of the simulation results. 2. Model [8] More details of the global MHD model used for this analysis can be found in the paper by Hu et al. [2007]. The numerical simulations are carried out by solving the ideal MHD equations on the basis of an extension of the Lagrangian version of the piecewise parabolic method [Colella and Woodward, 1984]. The GSM coordinate system is used, and the simulation box extends from x =30R E to x = 300 R E along the Sun-Earth line and from 150 to 150 R E in the y and z directions, with a total of 160 162 162 grid points and a minimum mesh grid spacing of 0.4 R E. The interplanetary conditions can be adjusted through the front inflow boundary at x =30R E while the other five outflow boundaries are set to be free. In the inner boundary at a radius of 3 R E, a magnetospheric-ionospheric electrostatic coupling model is imbedded to drive the magnetospheric convection. We sample field-aligned currents near the inner boundary and map them into the ionosphere, where they are used as a source term of a two-dimensional Poisson equation for the electric potential; the potential of the ionospheric regions with low latitude less than 53 is set to be zero as the boundary conditions to solve the Poisson equations. Once the potential is obtained, the corresponding electric field is remapped to the inner boundary to calculate the convection velocity which is used as the inner boundary condition for the simulation. This magnetosphere-ionosphere coupling approach is similar to the electrostatic model mentioned by Janhunen [1998]. Then we construct a three-dimensional simulation of the coupled solar wind magnetosphere-ionosphere. For simplicity, uniform Pedersen conductivity is assumed in the ionosphere, and here a constant conductivity of 5 siemens (S) is used. 3. Numerical Results 3.1. Geometry of Currents [9] First, an MHD simulation run with an input IMF boundary condition of a dawn-dusk B y component of 5 nt is carried out to obtain the steady state of the magnetosphere in the simulation domain; other physical parameters of the solar wind include a velocity of 400 km/s, a numerical density of 5 cm 3, and a temperature of 0.91 10 5 K. The current streamlines can be traced in the domain through the calculated current density according to the normalized equation J = rb. [10] Distributions of the field-aligned currents in both ionospheres (hemispheres) are displayed in Figure 1. The current density can be roughly estimated from either the contour lines, with a contour step of 0.07 ma/m 2, or the color-coded background. The color bar has a unit of ma/m 2. The current flowing into the ionosphere is defined to be positive and is depicted in color from blue to white. The negative values of the current density correspond to the current flowing out of the ionosphere, and their background colors vary between blue and black. Unlike those cases in which the IMF is purely northward or southward, IMF B y 2of10

Figure 1. Distributions of the contours of the FAC in the two ionospheres for a dawn-dusk IMF B y of 5 nt. The three dashed circles indicate the northern (southern) latitudes of 60, 70, and 80. significantly breaks the symmetry across the noon-night meridional plane, and the patterns of the currents differ from the average statistical results of northward IMF B z region 1 and 2 current systems [Iijima et al., 1978; Erlandson et al., 1988]. The round and crescent shapes of the current patterns are displayed in the northern and southern ionospheres, and they appear antisymmetric to each other. Pairs of region 1 FACs that locate at about 70 80 latitude can be identified according to the polarities: flowing into the dawnside and out of the duskside, the density of the region 1 FAC peaks in the dawn sector (north) or the dusk sector (south), and the region 2 FAC adjacent to the region 1 FAC at a lower latitude behaves oppositely. The typical patterns of the currents and the current density match well with the experimental model of FACs derived from high-precision satellite magnetic field data [Papitashvili et al., 2002], which is seen from comparing our simulation results with the southern summer part of their Figure 3. [11] In addition to the region 1 and 2 FACs, two more current systems, named cusp and mantle currents, appear exclusively depending on the IMF B y. Earlier such currents have been identified in the simulation work by Siscoe et al. [2000]. In the northern ionosphere, the negative current expands into the pole region and the morning sector; this newly emerged current across the noon-night meridional plane is distinguished as a mantle current. The cusp current exists at the merging area between the dawnside region 1 current and the duskside region 2 current. In comparison with the synthesis model of the IMF B y dependent midday FACs based on observations [Iijima, 2000], the mantle current based on the simulation result occupies a much larger area. The same cusp and mantle current system exists in the southern ionosphere. Below we will discuss the flowing paths of these currents in the magnetosphere. [12] The dominant IMF B y component makes the geometries of the current streamlines more complicated than in the case of north-south directed IMF, both in the ionosphere and in the magnetosphere. By tracing the current streamlines starting from the ionospheres, we identify four types of current closure paths in the magnetosphere: (1) a self-closed current, starting from and ending in the same ionosphere, flowing through the magnetosphere; (2) a current flowing from the southern ionosphere to the northern ionosphere within the magnetosphere; (3) a current that is closed through the nightside magnetopause; and (4) a current that is closed through the bow shock. The first type of current exists during purely north-south directed IMF, whereas the latter three types emerge because of the IMF B y imposed in the simulation. They are caused by the IMF B y related electric field between the two ionospheres. Meanwhile, current 3, as well as current 4, can be divided into two subclasses of currents according to polarity. Therefore, six typical current closure paths exist. In the following, we will use the terms of currents 1, 2, 3, and 4 for the four types of currents. [13] For simplicity, we trace six colored current streamlines starting from the northern ionosphere to show the six typical current circuits in the magnetosphere, as displayed in Figures 2a and 2b at two different view angles. The solid line represents positive current (entering ionospheres), and the dotted line corresponds to negative current (leaving ionospheres). Currents 3 and 4 are displayed with solid and dashed lines simultaneously. The noon-night meridional plane and the cross-section plane at x = 50 R E are colored by the contours of J E, which indicate the rate of electrical-mechanical energy conversion. The positive value of J E corresponds to the load area, where the electromagnetic energy converts to kinetic energy, which is colored red in Figures 2a and 2b, whereas the negative value of J E corresponds to the dynamo areas, where the kinetic energy converts to the electromagnetic energy, which are colored white. The color bar covers the normalized values of the electrical-mechanical energy conversion rates. First, the self-closed current (current 1) streamline colored yellow appears to be similar to the purely northward or southward IMF cases. In the northern ionosphere, it starts from the duskside ionosphere and is closed across the high-latitude magnetopause, then flows into the dawnside ionosphere and closes the current circuit through the Pedersen current in the northern ionosphere. Nevertheless, the symmetry of the current geometry across the meridional plane is broken. Second, current 2, shown as a solid green streamline, flows 3of10

Figure 2. (a and b) The 3-D configurations of the streamlines of the magnetosphere-ionosphere coupling currents for the dawn-dusk IMF B y viewed from two different angles. The solid and dotted lines represent currents that flow into and out of the northern ionosphere, respectively, and the four colors correspond to the four types of currents (yellow, current 1; green, current 2; white, current 3; red, current 4). The background is color coded showing the distribution of J E in the two planes. 4of10

out of the southern ionosphere, enters the magnetopause circling around the cusp, and forms the converging spiral that corresponds to the circles of the southern Chapman- Ferraro current system. Further, the current diverts into the magnetosheath region and flows across the equatorial plane to connect with the Chapman-Ferraro current system in the northern part of the magnetosphere, and finally, it flows into the northern ionosphere, so the two ionospheres are connected by the current circuit. The streamlines of current 3 are shown as the white solid and dashed lines, indicating the positive and negative currents, respectively. The negative current flows from the northern mantle along a spirallike path into the northern tail lobe. No separated cross-tail current loops are found in our simulation work; all of the cross-tail current seems to be a part of the current 3 system. The positive current originated from the southern magnetosphere, as indicated by the display, which is an interesting result that has never been mentioned in previous work. The white solid streamline indicates that the positive current in the northern ionosphere is directly connected with the southern magnetosphere. All these current systems, except for positive current 3, have been found in previous work by Siscoe et al. [2000], and similar current topologies are obtained in the present work. [14] One more difference from previous works is that in the present simulation the bow shock is involved in the magnetospheric-ionospheric current system. As Figure 2 shows, the solid and dashed red lines representing the positive and negative current streamlines, respectively, are connected with the bow shock. The negative current (red dashed streamline) leaves from the northern ionosphere, flows on the surface of the high-latitude magnetopause, diverts into the magnetosheath in the northern magnetosphere, and finally closes across the bow shock at the dawnside. In contrast, the red solid current streamline represents the current flowing into the northern ionosphere. The current originating from the duskside bow shock diverts into the magnetosheath in the southern magnetosphere, as Figure 2b shows, then flows on the southern magnetopause, crosses through the equatorial plane, and finally enters the northern ionosphere. The two bow shock related current streamlines have opposite polarities in the northern ionosphere, and their connections with the bow shock are different: one links with the bow shock in the north, while the other is in the south. The difference results from the different topologies of the magnetic field in the two hemispheres, which are caused by the magnetic merging presumedly occurring on the magnetopause in the presence of the IMF B y component. For the case of positive IMF B y, the open magnetic field of the magnetotail links with IMF B y at the dawnside in the northern magnetopause but at duskside in the southern magnetopause. Similar to cases when IMF turns southward, the open magnetic field in the magnetotail region provides channels for the currents linkup between the magnetosheath and the magnetosphere. Quantitative discussion will be carried out in detail in section 3.3 about the sources and destinations of the four types of currents. [15] In the simulation, a constant IMF condition is imposed at the front inflow boundary. This treatment leads to zero current density outside of the magnetospheric cavity limited by the bow shock; thus, there is no current streamline connected with the interplanetary space outside of the bow shock. In reality, there may be magnetospheric currents connected to the solar wind, e.g., currents of steady state Alfven waves; a more realistic solar wind condition is necessarily imposed to allow for steady state Alfven waves emitted from the magnetopause to the solar wind, which is beyond the scope of the current work. 3.2. Ionospheric Footprints of the Currents [16] The ionospheric manifestations of the four types of currents are displayed in Figure 3 according to the classification described in section 3.1. Figure 3 shows the footprints of the four types of magnetosphere-ionosphere coupling current systems in the two ionospheres. The currents 1, 2, 3, and 4 are displayed in Figures 3a, 3b, 3c, and 3d, respectively. As in Figure 1, the background is color coded according to the value of current density. The pink dots represent positive currents that flow into the ionosphere, and the green dots indicate the negative currents; the polar cap boundary is portrayed with the white solid curve, which corresponds to the location of the most poleward closed field line. The polar cap boundary is located at a higher latitude in the postnoon from 12 to 18 MLT in the northern ionosphere, which is caused by the presumed magnetic merging occurring on the magnetopause between IMF B y and the magnetospheric magnetic field. [17] Current 1, which is closed in the same ionosphere, still exists when IMF B y is imposed in front of the magnetosphere, similar to the region 1 FAC in those cases when IMF is purely northward or southward. Figure 3a shows that the greater part of the current is closed within the polar cap, corresponding to the open field line region. When leaving the ionosphere, the current does not always flow exactly along the magnetic field lines, depending on the local ratio between rp and jjjjbj in MHD applications [Janhunen and Koskinen, 1997]. In the area where rp is small compared to jjjjbj, the angle between J and B must be small; that is, the current is aligned with the magnetic field. The simulation result shows that footprints of the field-aligned current inside the polar cap boundary in the ionosphere cannot be used as a unique criterion to conclude that the corresponding FAC links with a certain space region in the interplanetary space, such as the bow shock [e.g., Fedder et al., 1997]. [18] The footprints of current 2 in the ionosphere are displayed in Figure 3b; they are mainly located in the prenoon and postnoon regions in the northern and southern ionospheres, respectively, occupying elongated regions just equatorward of the polar cap boundary. A considerable portion of the current is located around the continuation area between the dawnside region 1 and the duskside region 2 currents, near the noon-night meridional plane. This current is identified as the cusp current by Siscoe et al. [2000]. In our analysis, current 2 is defined as the current flowing directly from the southern to the northern ionospheres within the magnetosphere and partially through the cusp and magnetosheath region; thus, the cusp current is a part of current 2. From Figure 3b we can see that the positions of the origins (south) and the destinations (north) of current 2 in the two ionospheres are nearly totally located equatorward of the polar cap boundary, which indicates that current 2 flows within the closed magnetic field lines region. The two distribution patterns of current 2 in the 5of10

Figure 3. Footprints of the four types of current systems in the two ionospheres. The pink and green dots represent FAC flowing into and out of the ionospheres, respectively: (a) current 1, (b) current 2, (c) current 3, and (d) current 4. The white solid closed curve shows the polar cap boundary. 6of10

Table 1. Definition of the Four Types of FACs in the Northern Ionosphere Current Types Ionospheric Expression Magnetospheric Career Current 1 Similar polarity with region 1 FAC. The greater part of the current is located within the polar cap boundary. Current 2 Being positive, the current is located outside of the polar cap boundary and includes the cusp current. Current 3 Mainly negative, located within the scope of the polar cap boundary and occupying the whole polar region. Previously treated as the mantle current. A few positive currents exist. Current 4 The positive current dominates, and the greater part of the positive current resides in the equatorward region of the polar cap boundary. The negative current is mainly located within the polar cap boundary. A self-closed current, starting from and ending with the same ionosphere within the magnetosphere. Flows from the southern to the northern ionosphere within the magnetosphere. Closed across the nightside magnetopause. The positive current flows from the southern magnetopause; the negative one connects with the northern magnetopause. Closed across the bow shock. The positive current originates from the duskside bow shock and diverts into the southern magnetosheath, then flows on the magnetopause and finally enters the northern ionosphere. The negative current leaves from the northern ionosphere, flows on the surface of the northern high-latitude magnetopause, diverts into the magnetosheath, and closes across the bow shock at the dawnside. northern and southern ionospheres are approximately antisymmetric with respect to the noon-night meridional plane from the display. [19] Figure 3c shows that a large portion of the FAC is closed across the magnetopause, and the ionospheric footprints of current 3 are located mainly within the open polar cap, occupying the whole polar region. The current is treated as the mantle current by Siscoe et al. [2000]. It flows out of the northern ionosphere and closes across the magnetopause, as the white dashed streamline shows in Figure 2. In the southern ionosphere, the current flows into the polar region, whereas in the northern ionosphere, it flows out of the polar cap. [20] Figure 3d displays ionospheric distributions of current 4 that are connected with the bow shock. Energy transport occurs between the ionosphere and the bow shock via this kind of current. It is seen that the pink dots mainly reside just equatorward of the polar cap boundary in the northern ionosphere, so the current originating from the bow shock traverses the boundary separating the open and closed magnetic field lines and flows into the dawnside northern ionosphere. The green dots are sparsely distributed near the polar cap boundary. The total positive current (indicated by pink dots) is larger than the negative current (indicated by green dots) in the northern ionosphere. The situation reverses in the southern ionosphere, where the negative current dominates among the two types of currents linked with the bow shock. The currents flowing into and out of the same (northern or southern) ionosphere are unbalanced; therefore, the currents in the two ionospheres should be linked via the bow shock for the current continuity. As we know, IMF B y generates a z direction electric field. Then a voltage between the two ionospheres is formed to drive the interhemispheric FAC between the two ionospheres. From the dotted display of the current s distribution we see that the current flows between the northern and southern ionospheres along both closed and open magnetic field lines. [21] Table 1 presents the definitions of the four types of currents in the northern ionosphere, based on the ionospheric distributions and the magnetospheric paths of the currents. The polarities of the currents are reversed in the southern ionosphere. [22] From the above results, we can see that currents 2 and 4 flow from the southern to the northern ionosphere, and they are associated with the south-north directed electric field caused by the positive IMF B y. The latitudinal width of currents 2 and 4 in the ionosphere may be of the order of 1 5, which can be roughly obtained from the simulation results. The width is larger than the order of 1 2, which was obtained from experimental results by Kozlovsky et al. [2003], who suggested that the latitudinal width of the interhemispheric field-aligned currents associated with the IMF B y should depend on the strength of IMF B y and the ionospheric conductance. In our simulation, we used the conductivity of 5 S and the IMF B y of 5 nt, which is similar to the interplanetary and ionospheric parameters presented by Kozlovsky et al. [2003]. The difference between our analysis and their experimental result is probably caused by two factors. First, as the streamlines of current 2 show, the interhemispheric current is not fieldaligned at large distances from the ionosphere. This current s path is different from the one following from the assumption that the interhemispheric current flows along closed field lines. Second, the simulated FAC patterns in the ionospheres are more complex than the simplified model used by Kozlovsky et al. [2003]. A more realistic ionospheric model of the ionospheric conductivity should be used to study the electric field penetration in the ionospheres, which will be discussed elsewhere in our future work. 3.3. Quantitative Analysis [23] A detailed quantitative analysis is carried out on the basis of the above classification of the FAC according to the current topology and source regions. We trace the current 7of10

Table 2. Total Amount of the Four Types of FACs and the Corresponding Proportions in the Northern Ionosphere I 1 I 2 I 3 I 4 I total I out (MA) 0.24 0.00 0.84 0.11 1.19 I out /I total 0.20 0.00 0.71 0.09 I in (MA) 0.24 0.26 0.06 0.63 1.19 I in /I total 0.20 0.22 0.05 0.53 streamlines originating from the southern or northern ionosphere in a simulation box which is constrained to be [ 100, 20] R E in the x direction and [ 80, 8] R E in the y and and z directions. If the current streamlines are closed in the same ionosphere, we denote them as self-closed current 1; if the current streamlines reach another ionosphere, then they are treated as current 2. When the ending point of the current streamline centers around either the magnetopause or the bow shock, then the current is classified as current 3 or 4, respectively. For simplicity, the bow shock layer is set to be within four mesh points from the shock front, which is defined as the contour of 0.1 of the density enhancement relative to the solar wind density. If the ending point of the current streamline is within the bow shock layer, the corresponding current is considered to be current 4. After excluding the obtained currents 1, 2, and 4, the remaining current is treated as current 3. The ending points of streamlines of current 3 center around the magnetopause, and the latter is defined as the location where the total current density peaks when leaving the inner boundary in the simulation box. Once the ending point of the current streamline is determined, the corresponding current strength is integrated in the ionosphere. Tables 2 and 3 display the integrated currents in the two ionospheres, namely, I 1, I 2, I 3, and I 4, corresponding to currents 1, 2, 3, and 4, respectively (Figure 3). Hence, we can examine the sources and destinations of the currents according to these numerical results, as shown in Figure 3. [24] As we know, a positive B y causes a south-north electric field between the two polar cap ionospheres via the open magnetic field, which consequently results in the interhemispheric current that flows from the southern to the northern ionosphere. After first excluding the self-closed current 1 (20% of the total current), we consider the positive current in Table 2; we identify two current paths including currents 2 and 4 which correspond to 22% and 53% of the total 1.19 MA, thus including a greater part of the total positive current. Then we consider the negative current in the southern ionosphere, and the relative composition of the two currents is displayed in Table 3. We find the same results as those in the north: the contributions of the two currents are 22% and 53% of the total 1.19 MA. Thus, the currents driven by the interhemispheric electric field from the southern ionosphere to the northern one are balanced. Simultaneously, as Figures 3b and 3d show, currents 2 and 4 predominately flow within the closed magnetic field line regions. At some distances from the ionosphere, the currents do not continue to be field aligned but divert into the cusp region or the bow shock, as displayed in Figure 2. [25] Besides the current driven by the interhemispheric electric field, there is another current (current 3) with opposite polarity to complete the current loop through the Pedersen current in the same ionosphere. In the northern ionosphere, the current that flows out of the ionosphere and is closed across the magnetopause, 0.84 MA in total, contains about 71% of the total negative current. It mainly occurs in the open polar cap, as Figure 3c shows. Table 3 shows that the current flowing into the southern ionosphere has nearly the same magnitude, about 0.84 MA, and is closed across the magnetopause. In the Northern Hemisphere, the current flows from the polar cap ionosphere, crosses the magnetopause, and ultimately continues to the solar wind in the northern part of the far magnetotail region. In the Southern Hemisphere, the current flows from the solar wind in the southern part of the far magnetotail region, crosses the magnetopause, and flows into the ionosphere. We can conclude from the quantitative analysis above that the current streamlines of the two hemispheres continue to the solar wind in the far magnetotail region. [26] The sketch in Figure 4 summarizes the main current systems described above. The positive IMF B y related electric field drives currents (currents 2 and 4) to flow from the southern ionosphere to the northern one, with closure paths through the two polar cusp regions or the bow shock. In the near-earth region, the currents flow on the closed magnetic field lines; therefore, their footprints are located equatorward of the polar cap boundary in the two ionospheres. Another current system closed across the magnetopause (current 3) starts from the northern ionosphere within the open polar cap, spirals into the northern magnetosphere, connects with the southern part of the current in the far magnetotail region, and finally flows into the southern ionosphere within the open polar cap. [27] For a dusk-dawn IMF B y of 5 nt, the FAC patterns of the two ionospheres are reversed. In summary, the dominant IMF B y component significantly changes patterns of FACs in both the ionosphere and the magnetosphere; the corresponding positive z direction electric field determines the complex geometries of magnetosphere-ionosphere coupling current systems. 4. Conclusions [28] The topology and the quantitative distribution of magnetosphere-ionosphere coupling current systems for dominating IMF B y of 5 nt are presented in detail. The introduction of the IMF B y component leads to specific patterns of magnetospheric and ionospheric currents, which differ essentially from ones generated by purely northward or southward IMF B z. The south-north direction electric field caused by positive IMF B y drives the interhemispheric currents flowing between two ionospheres, which are identified as the cusp current and the bow shock current. In addition, a current with opposite polarity in the two ionospheres flows within the magnetosphere, linking with the traditional tail current through a spiral-like path. Table 3. Amount of the Four Types of FACs and the Corresponding Proportions in the Southern Ionosphere I 1 I 2 I 3 I 4 I total I out (MA) 0.24 0.26 0.06 0.63 1.19 I out /I total 0.20 0.22 0.05 0.53 I in (MA) 0.24 0.00 0.84 0.11 1.19 I in /I total 0.20 0.00 0.71 0.09 8of10

Figure 4. Sketch of the main current systems between the two ionospheres in the presence of a dawndusk IMF B y of 5 nt. [29] The most significant result is that the bow shock current appears in the magnetosphere-ionosphere coupling current system when a constant dawn-dusk IMF B y is set as the boundary condition. It is an extension to the finding of the bow shock s contributions to region 1 FAC noted by Guo et al. [2008] when the IMF turns extremely southward. The opening of the magnetospheric field lines to the outer space provides channels for energy or momentum exchanges between the bow shock and the magnetosphere-ionosphere coupling system via the current. The bow shock current contains a considerable portion of the currents that flow from the southern to the northern ionosphere, indicating that the solar wind dynamo drives the interhemispheric current through not only the region inside the magnetosphere but also the bow shock in the interplanetary space. [30] The current topology presented in this study is essentially different from the traditional point of view. From our quantitative analysis, field-aligned current leaves from the northern ionosphere along open field lines, flows through the northern magnetospheric lobe along a spirallike path, links with the cross-tail current in the neutral sheet, and closes with the southern magnetospheric current at the far magnetotail region. All current systems seem connected to form a current circuit; no separated cross-tail current loops exist in our simulation. [31] We have shown, on the basis of our simulation results, that the penetration of an interplanetary electric field into the ionosphere results in the interhemispheric FAC flowing equatorward of the polar cap boundary. The latitudinal width of the FAC associated with the interplanetary electric field is estimated to be from 1 to 5 in the ionosphere; this value is larger than the experimental result showing a width of the order of 1 2. [32] Acknowledgments. We acknowledge the constructive suggestions from two reviewers and C. Goodwin, the Editor s Assistant. This work was supported by grants NNSFC 40831060, 40804044, and 40621003 and in part by the Specialized Research Fund for State Key Laboratories of China. [33] Amitava Bhattacharjee thanks James Burch and another reviewer for their assistance in evaluating this paper. References Brecht, S. H., J. Lyon, J. A. Fedder, and K. Hain (1981), A simulation study of east-west IMF effects on the magnetosphere, Geophys. Res. Lett., 8(4), 397 400. Burch, J. L., P. H. Reiff, J. D. Menietti, R. A. Heelis, W. B. Hanson, S. D. Shawhan, E. G. Shelley, M. Sugiura, D. R. Weimer, and J. D. Winningham (1985), IMF B y dependent plasma flow and Birkeland currents in the dayside magnetosphere: 1. Dynamics Explorer observations, J. Geophys. Res., 90(A2), 1577 1593. Colella, P., and P. R. Woodward (1984), The piecewise parabolic method (PPM) for gas-dynamical simulations, J. Comput. Phys., 54, 174 201. Crooker, N. U., J. G. Lyon, and J. A. Fedder (1998), MHD model merging with IMF B y : Lobe cells, sunward polar cap convection, and overdraped lobes, J. Geophys. Res., 103(A5), 9143 9151. Erlandson, R. E., L. J. Zanetti, T. A. Potemra, P. F. Bythrow, and R. Lundin (1988), IMF B y dependence of region 1 Birkeland currents near noon, J. Geophys. Res., 93(A9), 9804 9814. Fedder, J. A., S. P. Slinker, J. G. Lyon, C. T. Russell, F. R. Fenrich, and J. G. Luhmann (1997), A first comparison of POLAR magnetic field measurements and magnetohydrodynamic simulation results for field-aligned currents, Geophys. Res. Lett., 24(20), 2491 2494. Guo, X. C., C. Wang, Y. Q. Hu, and J. R. Kan (2008), Bow shock contributions to region 1 field-aligned current: A new result from global MHD simulations, Geophys. Res. Lett., 35, L03108, doi:10.1029/ 2007GL032713. Hu, Y. Q., X. C. Guo, and C. Wang (2007), On the ionospheric and reconnection potentials of the Earth: Results from global MHD simulations, J. Geophys. Res., 112, A07215, doi:10.1029/2006ja012145. 9of10

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