Artigo Acesso aberto Revisado por pares

Titan at the edge: 2. A global simulation of Titan exiting and reentering Saturn's magnetosphere at 13:16 Saturn local time

2011; American Geophysical Union; Volume: 116; Issue: A8 Linguagem: Inglês

10.1029/2011ja016436

ISSN

2156-2202

Autores

D. Snowden, R. M. Winglee, A. Kidder,

Tópico(s)

Planetary Science and Exploration

Resumo

Journal of Geophysical Research: Space PhysicsVolume 116, Issue A8 Magnetospheric PhysicsFree Access Titan at the edge: 2. A global simulation of Titan exiting and reentering Saturn's magnetosphere at 13:16 Saturn local time D. Snowden, D. Snowden dsnowden@lpl.arizona.edu Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USASearch for more papers by this authorR. Winglee, R. Winglee Department of Earth and Space Science, University of Washington, Seattle, Washington, USASearch for more papers by this authorA. Kidder, A. Kidder Department of Earth and Space Science, University of Washington, Seattle, Washington, USASearch for more papers by this author D. Snowden, D. Snowden dsnowden@lpl.arizona.edu Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USASearch for more papers by this authorR. Winglee, R. Winglee Department of Earth and Space Science, University of Washington, Seattle, Washington, USASearch for more papers by this authorA. Kidder, A. Kidder Department of Earth and Space Science, University of Washington, Seattle, Washington, USASearch for more papers by this author First published: 31 August 2011 https://doi.org/10.1029/2011JA016436Citations: 10 This is a commentary on DOI:10.1029/2011JA016435 AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] We use a multifluid/multiscale model of Titan inside Saturn's magnetosphere to examine how Titan's induced magnetosphere and ion tail are affected when Titan crosses Saturn's magnetopause at 13:16 Saturn local time (SLT). During the simulation Titan crosses Saturn's magnetopause twice, exiting and reentering Saturn's magnetosphere. The magnetic field in Saturn's magnetosheath is predominately northward. Once inside Saturn's magnetosheath, Titan's connection to Saturn's magnetosphere is removed by slow ionospheric convection. Evidence for reconnection at Titan is not seen. Inside the magnetosheath the plasma flow is not perpendicular to the magnetic field, and magnetic field lines do not strongly drape around Titan. Titan's ionosphere is extended in the magnetosheath because Titan's ionospheric plasma is not stripped away by convecting magnetic field at high altitudes. After Titan crosses back into Saturn's magnetosphere, the magnetospheric plasma and field removes Titan's extended ionosphere, and Titan's induced magnetosphere returns to a “typical” configuration. The simulation is used to determine the time scale of Titan's connection to Saturn's magnetic field lines or magnetosheath magnetic field lines after a magnetopause crossing. In the magnetosheath, slow (∼3 km/s) ionospheric convection removes Titan's connection to Saturn's magnetosphere in ∼1.8 h. After Titan crosses back into Saturn's rapidly rotating magnetosphere, Titan's connection to magnetosheath magnetic field lines is removed through ionospheric convection in ∼50 min. The results of the simulation are also compared to data from Cassini's T32 flyby. Key Points Titan disconnects from Saturn's magnetosphere through ionospheric convection Time scale for Titan to disconnect from Kronian (sheath) field is 108 (50) min In the magnetosheath Titan's induced magnetosphere is irregular 1. Introduction [2] Typically, Titan orbits within Saturn's magnetosphere. However, Saturn's dayside magnetosphere is highly compressible. Achilleos et al. [2008] found that the subsolar distance of Saturn's magnetopause has a bimodal distribution with peaks at 22 and 27 RS and a range from about 18 to 29 Saturn radii (RS, 1 RS = 60,268 km). The data of Achilleos et al. [2008] suggest that at 12:00 Saturn local time (SLT) Titan's orbit is outside of Saturn's magnetosphere ∼5% of the time. Therefore, Titan should occasionally interact with the solar wind and interplanetary magnetic field (IMF). In fact, Titan was observed outside of Saturn's magnetopause during Cassini's T32 flyby on 13 June 2007 when Titan was located at 13:16 SLT [Bertucci et al., 2008]. Cassini was on an outbound trajectory and crossed magnetopause at 17:35 UT, just 20 min before closest approach [Bertucci et al., 2008; Garnier et al., 2009]. [3] During the T32 flyby fossil magnetic field lines were detected within Titan's ionosphere [Bertucci et al., 2008]. The direction of magnetic field observed by Cassini at low altitudes in Titan's ionosphere corresponded to a draped field that was initially southward. However, the interplanetary magnetic field (IMF) direction in the magnetosheath was northward. Bertucci et al. [2008] proposed that magnetic field lines at lower altitudes were frozen into Titan's ionosphere when Titan was still inside Saturn's magnetosphere and were preserved within Titan's ionosphere even after Titan crossed Saturn's magnetopause and entered Saturn's magnetosheath. Bertucci et al. [2008] estimated that the lifetime of the frozen in fields was between 20 min and 3 h. Neubauer et al. [2006] first suggested such fossil fields should exist in Titan's ionosphere and it is likely that fossil fields are observed frequently by Cassini, since the magnetic field configuration is highly variable near Titan [Bertucci et al., 2009; Simon et al., 2010a]. During the T32 flyby the presence of fossil fields was particularly obvious because the IMF was oriented northward, opposite to Saturn's magnetic field. [4] Several models have been used to simulate how crossing Saturn's magnetopause affects Titan's magnetospheric interaction. Ma et al. [2009] simulated Titan crossing the magnetopause into northward IMF using two MHD simulations. In the first simulation, only the magnetic field was changed, it was rotated from −8 to 8 nT. In the second simulation, the magnetic field was rotated from −8 to 8 nT and, simultaneously, the plasma density was increased and temperature was decreased. Real-time magnetic profiles along Cassini's T32 trajectory in both simulations were equivalent and able to reproduce the fossil field signatures in the T32 magnetometer data. The simulations of Ma et al. [2009] predicted reconnection of field lines downstream of Titan and a detachment of Titan's ion tail after Titan crossed into the magnetosheath. Ma et al.'s [2009] simulations determined that the lifetime of fossil field lines in Titan's ionosphere was slightly less than 2 h. [5] Simon et al. [2009] also simulated Titan crossing a simplified magnetopause using a hybrid model. The resolution of the ionosphere in Simon et al.'s [2009] hybrid model was not sufficient to simulate fossil magnetic field lines within Titan's ionosphere, therefore their simulations could not address the lifetime of fossil fields. Although the upstream parameters were similar to the parameters used in the models of Ma et al. [2009], Simon et al.'s [2009] simulation did not result in a large-scale detachment of Titan's ion tail. [6] Müller et al. [2010] simulated Titan crossing Saturn's magnetopause using a hybrid model that had better resolution than the Simon et al. [2009] model and a more physically correct treatment of the conductivity of Titan's ionosphere and interior. They simulated two cases: Titan crossing into a magnetosheath flow oriented parallel to the magnetospheric flow and Titan crossing into a magnetosheath flow oriented antiparallel to the magnetospheric flow. Müller et al. [2010] found the lifetime of Kronian fossil fields in Titan's ionosphere to be about 25 min. Müller et al. [2010] suggested that the treatment of magnetic diffusion through Titan's interior caused the large difference between the lifetime of fossil fields in their simulation and the simulations of Ma et al. [2009]. However, Müller et al. [2010] also noted since ion-neutral collisions that slow the convections of fossil fields to lower altitudes were not included their results should be regarded as a lower limit for the lifetime of fossil fields. Similar to the results of Simon et al. [2009], Müller et al.'s [2010] results also included no large-scale reconnection and detachment of Titan's ion tail. [7] Interestingly, the results of Müller et al. [2010] and Ma et al. [2009] indicate that changing the ambient plasma density, temperature, and velocity does not affect the lifetime of fossil fields in the lower ionosphere. These simulations show that the lifetime of fossil fields is determined by the conductivity of the lower ionosphere rather than the ambient space environment because Titan's upper ionosphere shields the lower ionosphere from changes in ambient conditions. Therefore, the properties of fossil magnetic fields are best addressed with local simulations with good resolution inside the ionosphere. [8] The focus of this paper is on large-scale features of Titan's induced magnetosphere instead of fossil fields within Titan's ionosphere. In particular, a multifluid/multiscale simulation of Titan embedded in Saturn's magnetosphere is used to determine how crossing Saturn's magnetopause affects Titan's ion tail and induced magnetosphere. During the simulation Titan crosses Saturn's magnetopause and enters the magnetosheath, and after a few hours crosses back into Saturn's magnetosphere. The results of the simulation describe how the plasma and field conditions upstream of Titan change as Titan crosses the magnetopause and how those changes affect the properties of Titan's ion tail and induced magnetosphere. The simulation is also used to determine the time scales for Titan's detachment from Saturn's magnetosphere after crossing into the magnetosheath, as well as the time scale for Titan's detachment from magnetosheath fields after Titan crosses back into Saturn's magnetosphere. The simulation results will also be compared with magnetic field and electron density data from Cassini's T32 flyby. 2. Model [9] A detailed description of the multifluid modeling method and initial conditions can be found in work by Snowden et al. [2011, hereinafter part 1]. The main difference between the simulation described in part 1 and this simulation is the location of Titan. In this simulation, Titan is placed in the xy plane at [18.11, 8.48, 0.0] RS corresponding to 13:16 SLT. In the model's coordinate system, the x axis is in the orbital plane and points positively toward the solar direction, the z axis is aligned with the rotational/magnetic axes, and the y axis completes the set. The boundary conditions for Saturn's and Titan's ionosphere are the same as in part 1. Again, Titan is placed in the global Saturn simulation after 48 h of simulated time, after the Saturn simulation has reached quasi steady state. The IMF varies as described in Table 1. The solar wind density is 0.05 cm−3 when Titan is placed in the simulation but rises to 0.14 cm−3, which increases the solar wind dynamic pressure and causes Saturn's magnetopause to move inward of Titan. The solar wind density is then reduced to 0.04 cm−3 which causes Saturn's magnetopause to move radially outward of Titan. The resulting solar wind pressure for each portion of the simulation are listed in Table 1. In order to simulate the tilting of Saturn's rotation and magnetic axis relative to the ecliptic the solar wind is tilted northward by 27°. Table 1. Summary of Noted Events in the Simulation Event Time (h) BIMF (nT) PSW (nPa) RSW (RS) Titan placed in simulation 48.0 [0, 0, 0.05] 0.008 25 Arrival of SW pressure pulse 51.7 [0, 0, 0.2] 0.022 25 Titan crosses into magnetosheath 52.4 [0, 0, 0.2] 0.022 18.75 Titan sheds Kronian magnetic field 54.2 [0, 0, 0.2] 0.022 18.75 Titan crosses into magnetosphere 55.3 [0, 0, 0.2] 0.022 22 Titan sheds magnetosheath field 56.1 [0, 0, 0.2] 0.022 22 SW pressure is reduced 57.0 [0, 0, 0.1] 0.008 27 [10] As noted by Müller et al. [2010] and Ma et al. [2009] the resistance at and below the inner boundary will have an effect on the lifetime of magnetic field calculated by the simulation. In this simulation, a resistive term is set at and below the inner boundary in order to account for the diffusion of magnetic field within Titan's dense neutral atmosphere. The resistivity is set to 55 Ω-m at the inner boundary at an altitude of 1500 km. Inside the boundary the resistivity is increased until it reaches 435 Ω-m at 500 km altitude. Inside of ∼1000 km altitude the bulk plasma velocity is set to zero but the magnetic field is not. Therefore, the magnetic field should be removed from the interior in a time period similar to the diffusion time scale on the order of several hours. 3. Results 3.1. Summary of Simulated Parameters [11] The following sections describe the evolution of Saturn's magnetosphere, Titan's space environment and Titan's induced magnetosphere and ion tail during an increase, and then a decrease, in the solar wind dynamic pressure. Significant events in the simulation are listed in Table 1, where BSW is the magnitude of the IMF, PSW is the dynamic pressure of the solar wind, and RMP is the subsolar distance of the magnetopause. Titan is placed in the simulation at 48 h after the Saturn model has reached equilibrium. The IMF is northward (antiparallel to Saturn's magnetic field) and the initial solar wind pressure at Saturn is 0.008 nPa. The magnetopause distance is ∼25 RS at the subsolar point. At 51.72 h, a pulse of increased solar wind pressure (.022 nPa) arrives at Saturn's magnetosphere and the magnetopause begins to compress. At 52.40 h Titan crosses into the magnetosheath. At its most compressed, the subsolar distance of Saturn's magnetopause is ∼18.75 RS. [12] At 55.30 h Titan crosses back into the magnetosphere when the magnetopause expands outward. At 57 h, Saturn's magnetopause expands further until the subsolar distance of the magnetopause is more than 25 RS due to the reduced solar wind pressure upstream of Saturn. At the end of the simulation Titan is located well inside Saturn's magnetosphere. 3.2. Titan's Exit and Slow Disconnection From Saturn's Magnetosphere [13] Around 52.4 h, Titan crosses Saturn's magnetopause when the magnetopause compresses due to the increase in solar wind dynamic pressure (from 0.008 nPa to 0.022 nPa). Figure 1 shows the three-dimensional changes in Titan's ion tail after Titan has exited the magnetosphere from 3 different perspectives. Titan's ion tail is imaged by a red surface (isosurface) of constant Hvy+ density equal to 0.01 cm−3. At 53.6 h, a full hour after Titan crossed the magnetopause, Titan's ion tail is still inside the magnetosphere. The magnetopause current layer is punched out adjacent to Titan's ion tail, indicating that Titan's ion tail is preventing the inward motion of the magnetopause. This is in agreement with the findings of part 1, where Titan's magnetotail appeared to slow the inward motion of the magnetotail. Titan's ion tail may be able to keep Titan inside the magnetosphere in the prenoon sector, but Figure 1 shows that Titan's ion tail does not prevent Titan from crossing the magnetopause in the postnoon sector. Figure 1Open in figure viewerPowerPoint Titan in Saturn's magnetosheath at 13:16 SLT. Titan's ion tail is depicted with a red isosurface of constant Hvy+ density equal to 0.01 cm−3 and Hvy+ density in the orbital plane. The white line indicates the location of the magnetopause. [14] At 54.44 h, about 2 h after Titan crossed into the magnetosheath, Titan's ion tail has split into two at the location of the magnetopause. Some of the ion tail plasma stays inside Saturn's magnetosphere and some of it appears as a short ion tail inside the magnetosheath. The portion of Titan's ion tail that crossed into the magnetosheath is deformed by the plasma flow in the magnetosheath, as will be discussed further in section 3.5. At 55.28 h, a disconnected clump of plasma from Titan's ion tail moves southward along Saturn's magnetopause. The southward motion of this plasma is due to coupling with southward moving reconnected magnetic field lines at Saturn's magnetopause. Reconnection occurs because the IMF in the simulation is northward, which is antiparallel to Saturn's magnetic field. [15] The fact that Titan's ion tail remains intact for several hours after Titan crosses the magnetopause implies that Saturn's draped magnetic field lines stay frozen into Titan's ionosphere. This can be seen in Figure 2, which shows Hvy+ ion density near Titan in the orbital plane. The red and purple lines are magnetic field lines; the red magnetic field lines are draped within ∼2000 km of Titan's surface. The white lines indicate where BZ is equal to zero and the black line indicates the location of the magnetopause current sheet. As Titan crosses the magnetopause current sheet, shown in black, the current sheet wraps around Titan and Titan's ion tail. The red field lines, which are connected to Titan's ionosphere, appear to swing toward Saturn because Titan is still magnetically connected to Saturn and Titan's ionospheric plasma outflows in the direction of the shifted magnetic field lines. Figure 2Open in figure viewerPowerPoint Close-up of Titan crossing Saturn's magnetopause at 13:16 SLT. Hvy+ density in the orbital plane near Titan. Titan is located in the center of the plot, and Saturn is located toward the right. The thick white line indicates where BZ is equal to zero, and the black line indicates the location of the magnetopause. Both the purple and red lines are magnetic field lines; the red lines go through Titan's ionosphere. [16] Red magnetic field lines near Titan appear magnetically connected to Saturn until about 54.2 h, about 1.8 h after Titan crossed the magnetopause. Titan does not disconnect from Saturn's magnetic field through reconnection of field lines in Titan's tail, as predicted by Ma et al. [2009], as no magnetic null is observed in the tail. Rather the separation from Saturn's magnetic field appears to be controlled by plasma convection at high altitudes in Titan's ionosphere. In the simulation the curvature force () of the frozen in magnetic field causes Titan's ionospheric plasma to convect at the Alfvén speed (∼3 km/s) from the ramside (relative to the magnetospheric flow) to the wakeside of Titan's ionosphere. The distance a field line has to convect from the ramside of Titan's ionosphere to the wakeside, where it can disconnect from Titan's ionosphere, is ∼5 RT. Therefore, the time scale for Titan to disconnect from Saturn's magnetosphere through convection would be ∼1.2 h, which is similar to the disconnection time scale predicted by the simulation. [17] The connection between Titan and Saturn's magnetosphere is slowly eroded away at the edges of Titan's ion tail, until southward oriented field lines are isolated from Titan's dense ionosphere. This is shown in Figure 3. In Figure 3 the BZ component of the magnetic field is mapped onto the field lines near Titan. Purple field lines have southward BZ and are Kronian magnetic field and red field lines have northward BZ and are magnetosheath field lines. To indicate the location of Titan's ionosphere and ion tail there is a translucent isosurface of constant Hvy+ density equal to 0.01 cm−3. Saturn is located out of the plane. At 51.09 h, Saturn's magnetic field (in purple) is draped around Titan. At 53.18 h, Titan has crossed the magnetopause current layer but is very close to the boundary. In this region the magnetic field is not well organized. Magnetosheath field lines (red) are present near Titan but Titan is still draped in Kronian field lines (purple). At 53.39 h, on the Saturn side of Titan's ionosphere, a large bundle of magnetic field lines can be seen. The magnetosheath field eventually piles up on the bundle of field lines causing the bundle to slowly flow away from Titan, finally breaking free at 54.23 h. Once free, this bundle of Kronian magnetic field moves southward taking a clump of Hvy+ plasma from Titan's ion tail along with it, as seen in the global view in Figure 1. Figure 3Open in figure viewerPowerPoint Magnetic field lines located near Titan. The purple field lines have BZ < 0 and indicate southward Kronian magnetic field. The red field lines have BZ > 0 and indicate northward IMF magnetic field lines. Plasma near Titan is imaged with grey translucent isosurface of constant Hvy+ density equal to 10.0 cm−3. [18] Even after Titan has disconnected from Saturn's global field, the white line inside Titan's ionosphere in Figure 2 indicates southward oriented field remain deep within Titan's ionosphere for more than 3 h. This time period is similar to analytical estimates of the lifetime of fossil fields in Titan's ionosphere [Cravens et al., 2009] and the lifetime of fossil fields deep in Titan's ionosphere found by Ma et al. [2009]. It is larger than the lifetime found by Müller et al. [2010], but that lifetime was stated to be a lower limit. However, this time should not be taken as indicative of the lifetime of fossil fields deep in Titan's ionosphere. The lifetime of magnetic field lines below Titan's exobase is controlled by magnetic diffusion terms which depend on ion-neutral and electron-neutral interaction rates. This model is not well suited to make a precise determination of the lifetime of fossil fields below Titan's exobase (altitudes less than ∼1400 km altitude) because the model does not finely resolve Titan's ionosphere or include all the proper diffusion terms. 3.3. Titan's Reentry Into Saturn's Magnetosphere and Shedding of an Expanded Ionosphere and Northward IMF [19] At 55.30 h Titan reenters Saturn's magnetosphere. As indicated in Table 1, Titan reenters Saturn's magnetosphere before the solar wind pressure is reduced upstream of Saturn's magnetopause. The rotation of features at the edge of Saturn's plasma disk causes Saturn's magnetopause to expand. As shown by Kidder et al. [2009] centrifugal interchange in Saturn's magnetosphere causes large features, called fingers, to arise at the edges of Saturn's plasma disk. These fingers can extend into Saturn's outer magnetosphere and sweep over Titan [Winglee et al., 2009]. Figure 4 shows a finger (indicated by an arrow) as it moves from the dawn to dusk side of Saturn's dayside magnetosphere, pushing Saturn's magnetopause radially outward. The rotation of this finger causes Titan to reenter Saturn's magnetosphere. Again this is in agreement with the result of the simulation described in part 1 where the plasma disk was shown to have a large influence on the location of the magnetopause when it is compressed. Figure 4Open in figure viewerPowerPoint Saturn's plasma disk is depicted with a green isosurface of constant O+ density equal to 0.03 cm−3 and O+ density in the orbital plane. The white line marks the location of the magnetopause. [20] Figure 5 shows the global view of Titan and Titan's ion tail, after Titan has reentered Saturn's magnetosphere. At 56.11 h, Titan and a relatively short ion tail are very close to the inside of Saturn's magnetopause and portions of Titan's ion tail are outside of the magnetosphere. The three-dimensional view shows that by 56.95 h Titan appears to be detached from the portion of Titan's ion tail that was left outside of the magnetosphere. The side view of Saturn's magnetosphere (third column) shows how the broken off portion of Titan's tail begins to spread northward along Saturn's magnetopause. Figure 5Open in figure viewerPowerPoint Titan crossing back into Saturn's magnetosphere at 13:16 SLT. Titan's ion tail is depicted with a red isosurface of constant Hvy+ density equal to 0.01 cm−3 and Hvy+ density in the orbital plane. The white line indicates the location of the magnetopause. [21] When Titan enters the magnetosphere it is going from a low-pressure environment to a high-pressure environment with a rotational flow that is perpendicular to the magnetic field. Figure 6 shows a high-resolution view of Titan's reentry into Saturn's magnetosphere with the same convention as Figure 2. The color contour shows Hvy+ ion density near Titan. The white line indicates where BZ is equal to zero and the black line indicates the location of the magnetopause current sheet. As the magnetopause moves toward Titan, the magnetosheath plasma is pushed radially away from Saturn and the IMF field lines drape around Titan. Similar to Titan's first crossing of the magnetopause, at 55.49 h the magnetopause current layer wraps around Titan and Titan's ion tail. Figure 6Open in figure viewerPowerPoint Close-up of Titan crossing back into Saturn's magnetosphere at 13:16 SLT. The color contour is Hvy+ density in the orbital plane near Titan. Titan is located in the center of the plot, and Saturn is located toward the right. The thick white line indicates where BZ is equal to zero, and the black line indicates the location of the magnetopause. Both the purple and red lines are magnetic field lines; the red lines go through Titan's ionosphere. [22] Plasma convection controls the time scale of Titan's connection to magnetosheath fields, similar to what was observed when Titan crossed into the magnetosheath. In this case, the rapidly rotating plasma and perpendicular field inside Saturn's magnetosphere cause plasma convection to occur much more rapidly. Over the next ∼50 min the high-altitude northward field is eroded away through convection in Titan's ionosphere and, starting around 56.12 h a clump of northward field moves downstream. Along with the northward field Titan also sheds the extended ionosphere it obtained inside the magnetosheath. [23] The time scale of Titan's connection to magnetosheath field above Titan's exobase from Titan's excursion into Saturn's magnetosheath is shorter (∼50 min compared to ∼1.8 h) after Titan crosses back into Saturn's magnetosphere than the timescale of Titan's connection to Kronian fields within Saturn's magnetosheath. The main reason for this is that Titan's ionosphere is extended inside the magnetosheath and the magnetosheath magnetic field is mostly frozen into this extended region, which is rapidly stripped off by magnetospheric plasma and field. [24] Titan's induced magnetosphere and ion tail eventually return to a configuration that is much closer to the ideal state. Figure 7 shows the large-scale evolution of Titan's ion tail after Titan has crossed back into the magnetosphere. At 58.62 h, a large clump of plasma near Titan's ion tail can be seen outside of the magnetopause spreading northward. By 60.30 h, Titan has recovered an ion tail similar to the ion tail simulated before Titan exited Saturn's magnetosphere. Figure 7Open in figure viewerPowerPoint Titan's ion tail is depicted with a red isosurface of constant Hvy+ density equal to 0.01 cm−3 and Hvy+ density in the orbital plane. The white line indicates the location of the magnetopause. 3.4. Plasma and Magnetic Field Conditions Upstream of Titan as Titan Crosses In and Out of Saturn's Magnetosphere [25] Here line plots of the plasma and fields 20 RT upstream of Titan are analyzed to determine how Titan's space environment is affected by crossing Saturn's magnetopause. [26] Figure 8a shows the plasma density 20 RT upstream of Titan. From 48 to ∼52 h the plasma density is 0.03 cm−3. The composition is mostly O+ (∼0.02 cm−3) with slightly less H+ (∼0.01 cm−3), which is indicative of Saturn's plasma disk. The plasma density increases to 0.2 cm−3 when Titan enters the magnetosheath around 52 h, consistent with the density observed in the magnetosheath by Cassini's Langmuir probe during the T32 flyby [Garnier et al., 2009]. The density remains higher on average inside the magnetosheath (although it varies from ∼0.1 cm−3 to ∼1.0 cm−3) until Titan reenters the magnetosphere around 55 h. As expected, the plasma is composed entirely of H+ inside the magnetosheath. Figure 8Open in figure viewerPowerPoint Plasma and magnetic field characteristics 20 RT upstream of Titan in Titan-centered (TIIS) coordinates. In the TIIS coordinate system the x axis is in the direction of corotation, the y axis is directed from Titan toward Saturn, and the z axis is perpendicular to the orbital plane. [27] The temperature of the ambient H+, shown in Figure 8b, increases to ∼200 eV near the magnetopause, but inside the magnetosheath the H+ temperature is similar to the temperature inside the magnetosphere (10–50 eV). It is important to note that the dominant plasma near Titan inside the magnetosphere, O+, is much hotter (200–1000 eV) than the plasma inside the magnetosheath and, in general, the plasma disk inside Saturn's magnetosphere is a higher-pressure environment than Saturn's magnetosheath. [28] Plasma velocity upstream of Titan is complex, as seen in Figure 8c. The velocity and magnetic field are shown in the Titan-centered TIIS coordinate system and the x axis points in the direction of corotation, the y axis is directed from Titan toward Saturn, and the z axis points upward perpendicular to the orbital plane. Inside the magnetosphere (48–52.4 h and 55.5–60 h) the plasma velocity is ∼150 km/s in the orbital plane. [29] In the magnetosheath the plasma velocity is much more variable. At first there is a strong decrease in plasma velocity (down from 150 km/s to about 50 km/s) coincident with Titan crossing the magnetopause current layer. Then the plasma velocity sharply increases to about 200 km/s. Finally, the plasma velocity in the magnetosheath settles at ∼150 km/s and is oriented primarily in the positive z direction. Velocity in the positive z direction inside the magnetosheath is

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