Available online at www.sciencedirect.com Planetary and Space Science 51 (2003) 295–307 www.elsevier.com/locate/pss A simulation study of currents in the Jovian magnetosphere Raymond J. Walkera;∗, Tatsuki Oginob aInstitute of Geophysics and Planetary Physics, and Department of Earth and Space Science, University of California, Los Angeles, Los Angeles, CA 90095-1567, USA bSolar Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi, Japan Received 23 April 2002; accepted 8 January 2003 Abstract We have used a global magnetohydrodynamic simulation of the interaction of Jupiter’s magnetosphere with the solar wind to investigate the e/ects of the solar wind on the structure of currents in the jovian magnetosphere. A thin equatorial current sheet with currents 2owing around Jupiter dominates Jupiter’s middle magnetosphere. However, in our simulations this current is not uniform in azimuth. It is weaker on the day side than the night side with local regions where the current density decreases by more than 50%. In addition to this ring current the current sheet contains strong radial “corotation enforcement” currents. Outward radial currents are found at most local times but there are regions with currents directed toward Jupiter. The current pattern is especially complex in the local afternoon and evening regions. In the near equatorial magnetosphere the ÿeld-aligned current pattern also is complex. There are regions with currents both toward and away from Jupiter’s ionosphere. However, when we mapped the currents from the inner boundary of the simulation to the ionosphere we found a pattern more like that expected for the ionosphere to drive corotation with currents away from Jupiter at lower latitudes and currents toward Jupiter at higher latitudes. Since upward ÿeld-aligned currents are associated with aurora at the Earth they may be associated with aurora at Jupiter. The upward ÿeld-aligned currents map to larger distances on the night side (40RJ to 60 RJ) than on the day side (20RJ to 30RJ). In the simulations changing the solar wind dynamic pressure did not make major changes in the current sheet or ÿeld-aligned currents (both were slightly stronger for higher pressures). The interplanetary magnetic ÿeld had a stronger e/ect on the currents with the strongest currents for northward IMF. However, it took a very long time for the magnetosphere to respond to the changes in the IMF. ? 2003 Elsevier Science Ltd. All rights reserved. Keywords: Jupiter’s magnetosphere; Field-aligned currents; Current sheet; MHD simulation 1. Introduction from about 20RJ to 60RJ or 70RJ is frequently called the mid- dle jovian magnetosphere. Plasma originating at the moon A thin equatorial azimuthal current sheet characterizes Io that is energized by planetary rotation is thought to carry Jupiter’s magnetosphere. This current sheet is found at all the equatorial currents (Hill et al., 1983; Vasyliunas, 1983). local times and on the day side extends from near Io’s or- The current sheet has been estimated to be 2–8RJ (Jupiter bit to within a few tens of RJ of the magnetopause (Smith radii) in thickness. Jupiter’s intrinsic magnetic ÿeld de- et al., 1974; Ness et al., 1979; Balogh et al., 1992; Kivelson creases as r−3 while the current sheet ÿeld decreases as r−1 et al., 1997). On the night side the current sheet merges or r−2 so the magnetic ÿeld from the current sheet is domi- with the magnetotail current system. The current sheet is nant in the middle magnetosphere. (See Bunce and Cowley, located near the magnetic equatorial plane and undergoes (2001a) for a recent and detailed review of the properties of a quasi-sinusoidal north–south motion as Jupiter rotates the current sheet.) Connerney et al. (1981) suggested that because Jupiter’s magnetic dipole is tilted 10◦ from the the current sheet is thicker on the day side than the night spin axis (see Khurana (1992) and references therein). The side while Jones et al. (1981) and more recently Bunce and region containing the current sheet is dominated by rotat- Cowley (2001a) and Khurana (2001) have argued that the ing 2ows. That part of the magnetosphere that contains the current sheet ÿeld is weaker on the day side than the night current sheet, plasma sheet and rotating 2ow and extends side. The observed day night asymmetry in the azimuthal cur- ∗ Corresponding author. rent implies a radial equatorial current with ÿeld-aligned E-mail address: [email protected] (R.J. Walker). currents carrying the current north and south away from the 0032-0633/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-0633(03)00018-7 296 R.J. Walker, T. Ogino / Planetary and Space Science 51 (2003) 295–307 current sheet in order to maintain current continuity. Goertz the largest Jupiterward currents were on the dawn side led (1978) noted that on the day side rotating 2ux tubes are Khurana (2001) to conclude that solar wind e/ects were constrained by the solar wind dynamic pressure but on the important well inside the jovian magnetosphere. night side they are free to expand. As the 2ux tubes move Jovian aurorae have been observed in infrared, ultravio- from the day side to the night side they are stretched result- let and visible light (e.g. Clarke et al., 1996; 1998; Satoh ing in stronger azimuthal currents. Radial currents that 2ow et al., 1996, PrangÃe et al., 1998; Vasavada et al., 1999). By away from Jupiter on the dawn side of the magnetosphere analogy with the Earth’s aurorae, we would expect discrete and toward Jupiter on the dusk side will maintain current jovian aurorae to be associated with ÿeld-aligned currents continuity. An outward radial current bends the magnetic directed away from Jupiter since precipitating electrons ÿeld lines in the direction of corotation “lag” (see Fig. 9 cause discrete aurorae. Southwood and Kivelson (2001) and of Khurana (2001)). On the dawn side corotation lag bends Cowley and Bunce (2001) have presented theories on the the magnetic ÿeld in the same direction as the solar wind relationship between jovian currents and the aurorae. They interaction while on the dusk side the bending is opposite to argue that the strength of the ionospheric ÿeld-aligned cur- that expected from the solar wind. Corotation lag has been rents and the luminosity of the aurora depend on the struc- observed near Jupiter at all local times, however, nearer ture of the magnetic ÿeld in the middle magnetosphere and the dusk side magnetopause the observed bending is in the on the angular velocity proÿle of the equatorial plasma. In opposite direction (Dougherty et al., 1993; Khurana, 2001; particular, they argue that the auroral oval maps to the part Kivelson et al., 2002). of the middle magnetosphere where the velocity of the In the jovian ionosphere collisions between ions and neu- rotating 2ows falls below the corotation velocity. Both tral particles in the Pedersen-conducting layer create a fric- papers conclude that aurorae will respond mainly to changes tional torque on the magnetic 2ux tubes that accelerates them in solar wind dynamic pressure in contrast to the Earth toward corotation. Hill (1979) showed that this ionospheric where changes in the interplanetary magnetic ÿeld (IMF) torque was suLcient to maintain near-corotation in the mid- and reconnection are dominant. For instance they argue that dle magnetosphere. The ionospheric torque is transmitted if the magnetosphere is compressed by the solar wind, the to the magnetosphere via ÿeld-aligned currents. These cur- 2ow in the magnetosphere will increase toward rigid coro- rents close in the ionosphere and the equatorial current sheet. tation thereby decreasing the lag in the ÿeld and the corota- The resulting ˜J × ˜B force in the ionosphere is directed to tion enforcement current. This will lead to dimming of the balance the viscous torque and slow the rotation of Jupiter’s aurorae. Conversely a decrease in solar wind dynamic pres- ionosphere (or atmosphere) while in the equatorial magne- sure will lead to increased ÿeld-aligned currents and more tosphere the ˜J × ˜B force is in the direction of corotation intense aurorae. Both Southwood and Kivelson (2001) and and accelerates the plasma (Hill, 1979; 2001; Vasyliunas, Cowley and Bunce (2001) argue that these changes in the 1983). The ÿeld-aligned currents are away from Jupiter at rotation and currents should hold during dynamic changes lower latitudes and toward Jupiter at higher latitudes clos- in the pressure while Cowley and Bunce (2001) argue that ing through equatorward Pedersen currents in the ionosphere they should hold for the steady-state magnetosphere as well. and outward 2owing radial currents in the equatorial mag- In this paper, we use our three-dimensional global mag- netosphere. The outward radial current frequently is called netohydrodynamic simulation of the interaction of Jupiter’s the corotation enforcement current. magnetosphere with the solar wind (Ogino et al., 1998; Recently, Bunce and Cowley (2001b) and Khurana Walker et al., 2001) to model the jovian current structure (2001) have used near-equatorial magnetic ÿeld observa- and its dependence on solar wind parameters. We mainly tions to infer the ÿeld-aligned currents. Bunce and Cowley concentrate on the formation and structure of the equatorial (2001b) used Pioneer and Voyager observations in the mid- current sheet and the ÿeld-aligned currents that connect it dle magnetosphere (20RJ–50RJ) and found currents away to the jovian ionosphere. In Section 2 we brie2y discuss the from Jupiter along the early morning trajectories of Voyager global MHD code. In Section 3 we examine the structure 1 and 2 outbound and currents toward Jupiter along the Pi- of the jovian currents in detail by considering the simple oneer 10 outbound trajectory at ∼ 0500 LT and the Pioneer case in which an unmagnetized solar wind interacts with the 11 inbound trajectory at ∼ 0900 LT.