Confronting Observations and Modeling: The Role of The Interstellar Magnetic Field in Voyager 1 and 2 Asymmetries

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1 Confronting Observations and Modeling: The Role of The Interstellar Magnetic Field in Voyager 1 and 2 Asymmetries M. Opher 1, J. C. Richardson 2, G. Toth 3 & T. I. Gombosi 3 1. Department of Physics and Astronomy, George Mason University, 4400 University Drive, Fairfax, VA, 22030, USA, mopher@physics.gmu.edu 2. Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge MA 3. Center for Space Environment Modeling, University of Michigan, Ann Arbor, MI Magnetic effects are ubiquitous and known to be crucial in space physics and astrophysical media. We have now the opportunity to probe these effects in the outer heliosphere with the two spacecraft Voyager 1 and 2. Voyager 1 crossed, in Dec 2004, the termination shock and is now in the heliosheath. On August 30, 2007 Voyager 2 crossed the termination shock, providing us for the first time in-situ measurements of the subsonic solar wind in the heliosheath. With the recent in-situ data from Voyager 1 and 2 the numerical models are forced to confront their models with observational data. Our recent results indicate that magnetic effects, in particular the interstellar magnetic field, are very important in the interaction between the solar system and the interstellar medium. We summarize here our recent work that shows that the interstellar magnetic field affects the symmetry of the heliosphere that can be detected by different measurements. We combined radio emission and energetic particle streaming measurements from Voyager 1 and 2 with extensive state-of-the art 3D MHD modeling, to constrain the direction of the local interstellar magnetic field. The orientation derived is a plane ~ from the galactic plane. This indicates that the field orientation differs from that of a larger scale interstellar magnetic field, thought to parallel the galactic plane. Although it may take 7-12 years for Voyager 2 to leave the heliosheath and enter the pristine interstellar medium, the subsonic flows are immediately sensitive to the shape of the heliopause. The flows measured by Voyager 2 in the heliosheath indicate that the heliopause is being distorted by local interstellar magnetic field with the same orientation as derived previously. As a result of the interstellar magnetic field the solar system is asymmetric being pushed in the southern direction. The presence of hydrogen atoms tend to symmetrize the solutions. We show that with a strong interstellar magnetic field with our most current model that includes hydrogen atoms, the asymmetries are recovered. It remains a challenge for future works with a more complete model, to explain all the observed asymmetries by V1 and V2. We comment on these results and 1

2 implications of other factors not included in our present model. Keywords: Solar magnetic field, Solar System: Solar Wind, Heliosphere, Termination Shock, Heliosheath 1. Introduction Due to the motion of the solar system through the interstellar medium with a velocity of 26.3±0.4 km/s, the solar wind is deflected by an interstellar wind, producing a comet-like shape with an extended tail. We observe the collision of winds in astrophysical media via remote sensing, such as the bow shocks formed ahead of R Hya (Wareing et al. 2006). Through the study of the interaction of the solar system and the interstellar medium, we can learn with in-situ data, on how two magnetized winds collide. Several boundaries characterize this interaction: the termination shock, where the solar wind become subsonic, and the heliopause, where the two ionized winds are deflected with respect to each other. If the interstellar wind is supersonic, there is a third boundary, the bow shock, where the interstellar wind becomes subsonic. Beyond the termination shock, the solar wind is gradually deflected tailward. As the Sun rotates, the solar magnetic field is carried outward by the solar wind and forms a spiral, becoming almost completely azimuthal in the outer heliosphere. The size and shape of the heliosphere will depend on the properties of both the solar wind and the local interstellar medium. From all the quantities that affect the boundaries of this interaction, the least known quantity is the interstellar magnetic field intensity and direction. The interaction of the solar system with the interstellar medium is a highly complex system. Not only are the two winds are ionized winds, and carry magnetic fields, but they also carry neutral H atoms, neutral He atoms; cosmic rays and pickup ions. Another complication is the solar cycle. The solar cycle affects the solar wind and the magnetic field embedded in it. The heliospheric current sheet (HCS) is known to change its inclination in respect to the solar rotation axis as the solar cycle progresses. The flows produced by the HCS and the solar cycle in the heliosheath has not yet been studied in detail. Also, with the 2

3 solar cycle there will be also more global merger interaction regions (that result of coronal mass ejections) that will disturb this interaction as well. There have been several different techniques trying to model this interaction (Suess 1993, Zank 1999, Izmodenov & Baranov 2006). The challenge is to swift through all the different effects and isolates the more crucial ones. A special difficulty arises from the fact that a lot of the effects go hand in hand. We are fortunate to have the twin spacecraft Voyager 1 (V1) and 2 (V2) probing in-situ the northern and southern hemisphere. Jointly with IBEX (McComas et al. 2004) that will remotely image the three-dimensional structure of the heliosphere, we now have a new view of the outer boundaries of the solar system. V1 crossed the termination shock in December 2004 (Decker et al, 2005; Burlaga et al. 2005; Stone et al. 2005). V2 is the only spacecraft that has the capability to measure in-situ the plasma flows. In August 2007 V2 crossed the Termination Shock (TS) providing us for the first time with in-situ measurements of the subsonic flows in the heliosheath. There is several recent observational evidence from V1 and 2 that there are asymmetries of the solar system. The observational evidence comes from different instruments and locations (termination shock, heliopause and heliosheath), which indicates that V1 and 2 are detecting the signs of a global distortion of the heliosphere. In several recent papers (Opher, Stone, Liewer 2006; Opher, Stone, Gombosi, 2007; Opher, Stone, Richardson, Gombosi 2008) we proposed that the asymmetries are due to a global factor distorting the solar system that is the interstellar magnetic field. Although it is intuitive to expect that interstellar magnetic field will have an effect on heliospheric asymmetries, it is not clear how strong this effect is and that the interstellar magnetic field solely is responsible for them (and not due to temporal or solar cycle effects). We analyzed the effect that the interstellar magnetic field direction has in the termination shock, in the heliopause and in the heliosheath. We found that the orientation of the interstellar magnetic field that matches the observations is in a plane from the galactic plane. This direction differs from the direction in large scale of the magnetic field that is the plane of the galaxy. There are few previous modeling studies that include both the solar magnetic field and the interstellar magnetic field (Linde et al., 1998, Linde 1998, 3

4 Ratkiewicz and Ben-Jaffel 2002, Pogorelov et al., 1998, 2004, 2006, 2007, 2008a,b, Washimi et al. 2001, 2007) Linde et al.1998a,b included the effects of neutral hydrogen atoms treated in a fluid way along with the solar magnetic field and an interstellar magnetic field either parallel or perpendicular to the interstellar wind velocity. Pogorelov et al. 2006, 2008a investigated models with inclined magnetic fields with neutral atoms treated as a separate fluid, while the model by Izmodenov et al included neutrals with a kinetic treatment, but did not include the solar magnetic field. Until recently, results from computer modeling were studies showing qualitative general asymmetries in the solar system due to interstellar magnetic field (such as in Pogorelov, and Matsuda (1998). Ratkiewicz et al. (1998); Ratkiewicz and Ben-Jaffel 2002.). Izmodenov et al show that an interstellar magnetic field can produce the 4 degrees deflection of H atoms from the direction of the pristine interstellar gas flow (with a model that include kinetic neutrals and no interplanetary magnetic field); in agreement with Lallement et al In models such as (Pogorelov and Matsuda 1998; Pogorelov et al. 2006; Zank, 1999) it can be seen that the interstellar magnetic field affect the termination shock; although these pioneer works didn t analyze the asymmetries at the V1 and V2 locations. Works such as Izmodenov, Gloeckler and Malama (2003), Izmodenov et al. 2003; Izmodenov et al and used observations of interstellar H atoms, and modeling of kinetic H and He to constrain the crossing of the termination shock by Voyager 1 and 2. Opher et al was the first study to compare quantitative the computer models with the observations of V1 and V2 both for the TS distances; as with the east-west asymmetry; and the particles produced at the TS (TSP particles). The model described in Opher et al explained the direction of streaming of the TSPs detected by V1 and predicted that V2 will detect TSPs streaming from the opposite direction than V1 what was later observed! Opher et al showed that the interstellar magnetic field can produce the observed asymmetries also in the radio emissions; and that an interstellar magnetic field in the galactic plane wont be consistent with either the TSPs streaming directions or the radio emissions. After these works, Pogorelov et al.; 2007; 2008a,b followed with a more complete 4

5 model (including a self-consistent neutral H atoms) and showed that asymmetries are diminished in comparison with pure MHD calculations. In this paper we reviews our previous work giving special attention to the observations that need to be explained. It remains an open challenge to follow our previous works and with a more complete model reproduce all the heliospheric asymmetries seen. In section 2, we describe in detail the observational evidence of asymmetry and in section 3 the possible directions for the interstellar magnetic field. In Section 4, we describe our model and in Section 5 we discuss our results including the orientation of the interstellar magnetic field direction. In Section 6, we conclude with future work and discussions. 2. Observational Evidences for Asymmetries A. Position of the Termination Shock: V1 crossed the termination shock in 2004 at a distance of 94AU (Stone et al. 2005). V2 on August 30, 2007 crossed the shock at 83.7AU (Stone et al. 2008), indicating that the termination shock is 10AU closer to the Sun in the southern hemisphere. B. Anisotropic streaming of Low Energy Particles: In mid 2002, V1 began observing strong beams of energetic termination shock particles (TSPs) streaming outward along the spiral magnetic field. The strong upstream TSP beams were observed much of the time until V1 crossed the shock at 94 AU in December Jokipii et al and Stone et al suggested that the upstream beaming resulted from a non-spherical shock. For a spherical shock, V1 would have observed upstream TSPs streaming inward along the magnetic field. With a non-spherical shock, V1 could be connected to the termination shock along magnetic field lines that crossed the termination shock (the source of TSPs) and then crossed back into the supersonic solar wind. The north-south asymmetry was also seen by the observations (Stone et al. 2005), where V2 started detecting TSPs 10AU before V1 started. V2 observed TSPs streaming inward (Cummings et al. 2005). C. The distance of the spacecraft to the shock when starting to detect the lowenergy particles from the shock: Although there was no direct indication how distant V1 was from the shock when the upstream episodes of TSPs were 5

6 observed, it is possible to estimate the distance from time-dependent models of the heliosphere (e.g, Izmodenov et al. 2008). MHD models based on V2 solar wind pressure measurements (Richardson and Wang 2005), indicate that the distance was less than 3 to 4 AU; and at the time when V2 (was at 75AU) started measuring the TSPs it was at a distance 5-7AU from the shock (Washimi et al. 2007), indicating that the termination shock is further pushed in, in the southern hemisphere, by 9AU. D. The unrolling of the energetic particle energy spectra: Far from the TS the energetic particle spectra form a peak, essentially corresponding to the classical anomalous cosmic rays. Above the peak the source spectrum is observed since the high energy particles are hardly disturbed in route from the source region to the spacecraft. Below the peak the intensities fall sharply because these lower energy particles are prevented from reaching the spacecraft by transport effects such as scattering or the geometry of the magnetic connection and the relative velocity of the particle and the speed at which it is convected through the shock. As the spacecraft and shock approach one another the peak shifts to lower energies and higher intensities as fewer particles at progressively lower energies are prevented from reaching the spacecraft. This is seen at both Voyager 1 and 2, however for V1 the process is apparent early while at V2 it is only just before crossing the shock that this particular spectral evolution (or "unrolling") takes place (Decker et al. 2005). This indicates that the point of magnetic connectivity is farther away from V2 than from V1. E. Radio Emission at the Heliopause: In the last 20 years, V1 and 2 have been detecting radio emissions in the outer heliosphere at frequencies from 2 to 3 khz (Kurth et al. 1984; Gurnett et al. 1993, Gurnett, Kurth and Stone 2003). The radio emissions were detected each solar cycle: first in during the solar cycle 21 (Kurth et al. 1984), second in during solar cycle 22 (Gurnett et al. 1993), and most recently in solar cycle 23 (Gurnett, Kurth and Stone 2003). The currently accepted scenario is that the radio emissions are generated when a strong interplanetary shock produced by a period of intense solar activity reaches the vicinity of the heliopause and move into the interstellar plasma beyond (Gurnett, Kurth and Stone 2003; Gurnett and Kurth 1995). Radio direction- 6

7 finding measurements from V1 and V2 have been used to determine the positions near the heliopause at which the radio emission are generated (Kurth and Gurnett 2003). The sources lie along a line that passes near the nose of the heliosphere that roughly parallels the galactic plane. The GAL plane is 120 from the ecliptic plane. Based on the fact that the galactic magnetic field is oriented nearly parallel to the galactic plane, (Kurth and Gurnett 2003) suggested the local interstellar magnetic field (in the local neighborhood of the sun) was also parallel to the galactic plane. F. Flows in the Heliosheath: The termination shock separates the supersonic solar wind from the subsonic solar wind. Subsonic flows are sensitive to the obstacle ahead (Opher et al. 2008). Therefore, beyond the termination shock the flows will immediately be sensitive to the shape of the heliopause and start deflecting in response. The shape of the heliopause is affected by the pressure of the local interstellar magnetic field. The measured flows by V2 can probe additional asymmetries. These observations indicate that V1 and 2 are detecting the signs of a global distortion of the heliosphere by the interstellar magnetic field. 3. Interstellar Magnetic Field Direction As mentioned above, among the several physical quantities that describe the interaction of the solar system with the interstellar medium, the least known are the direction and intensity of the local interstellar magnetic field (B ISM ). Models suggest that the strength of B ISM is around few µg (Cox and Helenius 2003). Based on the polarization of light from nearby stars, Frisch (1990) suggested that the magnetic field direction is parallel to the galactic plane (and directed toward l ~ 70 ) (GAL). Voyager 3kHz radio emission data also show preferred source locations in a plane parallel to the galactic plane (Kurth and Gurnett 2003). On the other hand, Lallement et al. 2005, mapping the solar Lyman-α radiation that is resonantly backscattered by interstellar hydrogen atoms, found that the neutral hydrogen flow direction differs from the helium flow direction by 4. The plane of the H deflection is tilted from the ecliptic plane by ~ 7

8 60 and is consistent with an interstellar magnetic field parallel to the H-deflection plane (Izmodenov et al. 2005). We refer to this plane as the H-deflection plane (HDP). However, Gurnett et al recently pointed out that at the earth s bow shock and interplanetary shocks, the radio emission occurs where the magnetic field lines are tangential to the shock surface and suggested that heliospheric radio emissions occur where the local interstellar magnetic field is tangential to the surface of the shock that excites the plasma (or B n=0, where B is the magnetic field and n is the shock normal). They conclude that the condition B n=0 combined with the observed source location by Voyager spacecrafts implies that the local interstellar magnetic field is perpendicular to the galactic plane. This direction differs from the earlier suggestion (Gurnett and Kurth 1995) and is within 16 of the HDP plane. 4. Model The model that we used in Opher et al. 2006, 2007, 2008 is based on the BATS-R-US code, a three-dimensional magnetohydrodynamic (MHD) parallel, adaptive grid code developed by University of Michigan (Powell et al.1999) and adapted by (Opher et al. 2003, 2004) for the outer heliosphere problem. Since we were interested in the global properties of the heliosphere, we used a coarse grid with cells sizes ranging from 1.5AU Figure 1. to 20AU. The inner boundary was set at 30 AU and the outer boundary was from AU to 1500 AU in the y and z direction; and from -800 AU to 800 AU in the x direction. The solar magnetic field axis was aligned with the solar rotation axis with a 26 days solar rotation period. The solar wind was taken as uniform 450 km/s; only the ionized component was included. The parameters of the solar wind at the inner boundary (30AU) are n=7.8x10-3 cm -3, T=1.6x10 3 K and a Parker spiral magnetic field with B=2µG at the equator. For the interstellar wind, we used n=0.07cm -3, and T=10 4 K (Frisch 1996). The interstellar magnetic field (B ISM ) magnitude is taken 8

9 to be B ISM =1.8µG (with the y component of B ISM, B ISM,y <0). The coordinate system has the interstellar velocity direction in the +x direction and the z-axis is parallel to the solar rotation axis, with y completing the right-handed coordinate system. In this coordinate system, V1 is at 29.1 in latitude and in longitude and V2 is at and in longitude. Figure 1 shows the plane for the B ISM in the model coordinate system. The coordinate system has the z-axis as the solar rotation axis of the sun, the interstellar velocity direction in the x direction, with y completing the right handed coordinate system. In this coordinate system β is the angle between the interstellar magnetic field and the solar equator and α the angle is the angle between the interstellar magnetic field and interstellar wind velocity. 5. Squashed Solar System: Global Asymmetries The interstellar magnetic field is frozen into interstellar plasma that is deflected a) b) Figure 2. around the heliopause, causing the field to drape over the heliopause. If the angle between the interstellar magnetic field and interstellar velocity is non zero, it should break the axial symmetry of the heliosphere. This should be seen in the distortion of the heliopause and the termination shock. The shape of the heliopause, is distorted by pressure of the local interstellar magnetic field. For intensities around a few microgauss, the ambient interstellar magnetic pressure is comparable to the gas pressure, with the magnetic pressure increasing further in those regions where the interstellar flow decreases as it approaches the heliopause. 9

10 The heliopause surface will vary with the orientation and strength of the local interstellar magnetic field. Figure 2a indicates that the heliopause is strongly influenced by the interstellar magnetic field direction; the heliopause is asymmetric both north/south and east/west and has a plane of symmetry approximately parallel to the plane of the local interstellar magnetic field. The orange field lines are the interstellar magnetic field lines. The yellow surface is the heliopause and the green iso-surface is the termination shock. The trajectory of V1 and 2 are shown in magenta and red, respectively. At the heliopause due to the slow down of the plasma flow and piling up of the interstellar magnetic field the intensity of the magnetic field outside the heliopause is larger at the southern hemisphere rather that at the northern hemisphere (Figure 2b). In Opher et al. 2006, 2007 we investigated the effect of different interstellar (a) (b) ( c) (d) Figure 3. magnetic field directions on the shape of the heliosphere. We considered several directions of interstellar magnetic field: the hydrogen deflection plane (HDP) 10

11 (β=60 ); the galactic plane (GAL) (β=120 ); and the plane perpendicular to the galactic plane (PPG) (β=44 ). In Opher et al. (2006) we predicted that an interstellar magnetic field parallel to the HDP, distort the TS such that the TSPs will stream inward at V2; which was later confirmed (Cummings et al. 2005, Decker et al. 2008, Stone et al. 2008). The model explained the outward streaming of the TSPs at V1 (Figure 3a (a) (b) Figure 4. and 3b) (Asymmetry B). It also predicted the distance of the TS being closer in the southern hemisphere by 10AU (Figure 1a) (Asymmetry A). We show (Opher et al. 2007) that for an interstellar magnetic field in the GAL the TS is distorted in the opposite direction, such that the TSPs stream inward towards V1 opposite to what was observed (Cummings et al., 2005) (Figure 3c and 3d). Additionally the model also predicted that V1 will be 2-3AU a (a) Figure 5. (b) 11

12 from the shock and V2 will be 7-10AU. This is in agreement with Asymmetry C. The model also explained the difference in unrolling of the energetic particles (Asymmetry D). Figure 4a shows that for B ISM (a) Figure 6. (b) parallel to the HDP, the longitude of the MD (MD stands for the position at which the shock has the minimum radial distance to the Sun) at the northern hemisphere is closer to V1 than at the southern hemisphere where MD is farther away to V2 (in agreement with Asymmetry D). The model also explains the location of the radio sources (Opher et al. 2007) (Asymmetry E). Assuming a spherical interplanetary shock, the tangential field condition for the radio sources translates to B r =0 with B r being the radial component of the interstellar magnetic field. For each modeled direction of the interstellar magnetic field, we compared the expected location of the radio sources (B r =0 at the heliopause) with the observed location of the radio sources detected by V1 and 2. With B ISM parallel to GAL (with α=45, Figure 5b), the region where B r =0 is almost perpendicular to the galactic plane, which is inconsistent with the radio observations. An interstellar magnetic field perpendicular to the galactic plane (PPG plane, with α=30 ) produces the best agreement with the radio observations by Voyager, as suggested by (Opher et al. 2007). The HDP orientation differs from PPG by only 16 and is also in general agreement (Figure 5a). The offset of ~15 between the observations and the region with B r =0 for the model in best agreement indicates that the accuracy of the model is not adequate to distinguish between the PPG and HDP field orientations. Finally, the model is also in agreement with the heliosheath flows (Opher et al. 2008) (Asymmetry F). We used the heliosheath flows from days of 2007, when the heliosheath flows were relatively quiet before a transient arrived on day 320. The flows are mainly radial with the ratios V N /V R =-0.30 and 12

13 V T /V R =0.35. We perform an unweighted average calculation so as to equally weigh each day. The mean angle θ=tan -1 (V N /V T ) for the period is and an uncertainty of 2.1. For B ISM parallel to PPG or HDP and small α (30-45 ), the angle agrees with the observed flows (-28.6 and respectively) 6. Discussion and Conclusions In this paper we discussed the consequences of the interstellar magnetic field on the heliosphere. In particular, we show that the orientation of the local interstellar magnetic field in a plane inclined from the large scale plane of the interstellar magnetic field (with α =30-45 ) can explain the different observational indication that exist a global asymmetry (A-F asymmetries outlined in Section 2). Although the model explains these different observations, future work needs to be done to include and access the effect of additional factors not included in the model. Below we comment briefly on some of these aspects. Neutral H atoms: The model of Opher et al. 2006, 2007 did not include the neutral hydrogen atoms that interact with the ionized component by charge exchange. This can affect the quantitative amount of asymmetry (Pogorelov et al. 2007, 2008a,b). An important aspect is that the neutrals H have a long mean free path and need to be treated kinetically. The inclusion of the neutral H atoms tend to symmetrize the solution and quantitatively affect the degree of asymmetry as seen recently by Pogorelov et al. 2008a,b (see also Izmodenov et al. 2005). The asymmetry in respect to the TS distances are recovered when more intense interstellar magnetic field is included (~ 4-5µG) (Izmodenov 2008; Izmodenov et al. 2008). Recently we implemented a 5 multi-fluid model (1 ionized and 4 neutral H fluids) and reproduced Izmodenov et al results (Opher et al. 2008). We show that with the inclusion of an interplanetary field the asymmetry in respect to the TS is increased (~12AU). Figure 6 show the same view as Figure 3a and 3c for the case with the interstellar magnetic field in the HDP plane with α =20 and intensity B=4.375µG. It can be seen, in a case with strong interstellar field, that even with a presence of neutral H atoms, the east-west asymmetry remains (Opher et al. 2008). 13

14 It remains a challenge for future works to, with a more complete model, explain quantitatively all the observed asymmetries by V1 and V2 (including the deflection of 4 of the H flow in respect to the main interstellar medium flow as seen by the analysis of backscattered solar Lyman-alpha radiation). Solar Cycle: The models discussed above are stationary models. Several previous works investigated also the effect of time-varying heliosphere (e.g., Florinski et al. 2005; Karmesin et al. 1995; Whang et al. 2005; Zank and Muller 2003, Borrman and Fichtner 2005, Izmodenov, Malama & Ruderman (2008); Izmoenov, Malama & Ruderman (2005)). Recently Washimi et al investigated with a 3D-MHD model the effect of the solar cycle in the termination shock position using V2 plasmas data to drive a time-varying inner boundary. This model was a pure MHD model. As noted above, the solar cycle affects the solar wind and the magnetic field imbedded in it. The heliospheric current sheet (HCS) is known to change its inclination in respect to the solar rotation axis as the solar cycle progresses. The flows produced by the HCS and the solar cycle in the heliosheath has not yet been studied in detail. Opher et al. 2003, 2004, 2005 had investigated the effect of the current sheet in the heliosheath. This requires a very high numerical resolution, a challenge for future studies. With the solar cycle there will be also more global merger interaction regions (that result of coronal mass ejections) that will disturb this interaction as well. However, we expect that although details of the heliosheath structure will be affected, a global asymmetry still will remain produced by an external agent, the interstellar magnetic field. Energetic-Particle Mediated Shock: recent observations indicate (Richardson et al. 2008) that the TS is a particle mediated shock. The non-thermal pressure produced by the energetic ions in the heliosheath compare (or exceeds) the thermal and magnetic pressure. The high intensity of the electrons implies that the electron impact ionization can play a comparable (or greater) role in the generation of suprathermal pickup hydrogen than charge exchange ionization (Richardson et al. 2008). The temperature at the sheath was significantly lower than the value expected. The heliosheath plasma has only about 20% of the preshock solar wind energy. The remaining energy must be transmitted to some other components of the heliosheath, possible the pick up protons or other particles and waves. These aspects should be included in future modeling of the interaction of 14

15 the solar system with the interstellar medium (see works of Malama et al. 2006, Heerikhuisen et al. 2008). Our result that the orientation of the local magnetic field differs from the large scale magnetic field could be a result of turbulence in the interstellar medium (Jokipii (2007)) of a consequence of a local distortion at the Local Bubble. The study of Redfield and Linsky (2008) that mapped the threedimensional structure of the warm local interstellar medium within the Local Bubble, indicated that the heliosphere lies in a transition region between the LIC and the G cloud. The interface orientation of the two clouds is also 60 to 90 from the galactic plane; what could indicate the local interstellar magnetic field is parallel to the edges of this interface. Further upcoming missions such as IBEX will further probe this interface with the energetic neutral atoms (ENAs). Preliminary studies show the effect of the interstellar magnetic field (Prested et al. 2008, Izmodenov et al. 2008) will be able to be detected. References Borrmann, T., & Horst Fichtner, Adv. In Space Research, 35, 2091 (2005). Burlaga, L. F., Ness, N. F., Acuna, M. H., Lepping, R. P., Connerney, J. E. P., Stone, E. C., and McDonald, F. B., Science 23, 2027 (2005). Cox, D.P. and L. Helenius, Astrophysical J. 583, 205 (2003). Cummings, A. C., Stone, E. C., McDonald, F. B., Heikkila, B. C., Lal, N., Webber, W. R., AGUFMSH51A-1184 (2005). Decker, R. B., S. M. Krimigis, E. C. Roelof, M. E. Hill, T. P. Armstrong, et al. Science, 309, 2020 (2005). Decker, R., Krimigis, S. M., Roelof, E. C., Hill, M. E., AGUFMSH43B-05 (2005b). Decker et al. Nature in press (2008). Florinki, V., Zank, G. P., Pogorelov, N.V., J. Geophys. Res., 110, Issue A7, A07104 (2005). Frisch, P. C., Physics of Outer Heliosphere; Proceedings of the 1st COSPAR Colloquium, Pergamon Press, 19 (1990). Frisch, P. C., Space Sci. Rev. 78, 213 (1996). Gombosi, T., Powell, K. G. and de Zeeuw, D. L., Journal of Geophysical Res., 99, (1994). Gombosi, Gurnett, D. A., W. S. Kurth, S. C. Allendorf, R. L. Poynter, Science 262, 199 (1993). Gurnett, D. A., W. S. Kurth, Adv. Space Sci. 16, 279 (1995). Gurnett, D. A., W. S. Kurth, E. C. Stone, Geophys. Res. Lett. 30, pp. SSC 8-1 (2003). Gurnett, D. A., W. S. Kurth, I. H. Cairns, J. Mitchell, 5th Annual IGPP International Astrophysics Conference. AIP Conference Proceedings, 858, 129 (2006). 15

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17 Opher, M., E. C. Stone, P.C. Liewer, Astrophysical Journal Letters, 640, L71, (2006). Opher, M., E. C. Stone, T. Gombosi, Science, 316, 875 (2007). Opher, M., E. C., Stone, Toth, G., Izmodenov, V., J. Richardson, T.I.Gombosi, (2008). Pogorelov, N. V., T. Matsuda, J. Geophys. Res. 10, 237 (1998). Pogorolev, N., Zank, G. P., and Ogino, T., Astrophys. J., 614, 1007 (2004). Pogorelov, N. V., G. P. Zank., Astrophys. J., 636, L161 (2006). Pogorelov, N.V., E.C.Stone, V.Florinski, G.P. Zank, Astrophys. J., 668, 611 (2007). Pogorelov, N. V., Heerikhuisen, J. and Zank, G., Astrophys. J. 675, L41 (2008a). Pogorelov, N. V. Zank, G., and Ogino, Tatsuki, Adv. In Space Research. 41, 306 (2008b). Powell, K. G., Roe, P. L., Linde, T. J., Gombosi, T. I., dezeeuw, D. L., J. Comput Physics 154, pp. 284 (1999). Prested, C. et al. JGR, in press (2008). Ratkiewicz, R., Barnes, A., Molvik, G. A., Spreiter, J. R., Stahara, S. S., Vinokur, M., Venkateswaran, S. 1998, Astron. Astrophys. 335, 363 (1998). Ratkiewicz, R., and Ben-Jaffel, L., J. Geophys. Res., 107, 2 (2002). Redfield, S. and Linsky, J.L., Astrophys. J., 673, 383 (2008). Richardson, J. D., Wang, C., AGUFMSH51A-1186 (2005). Richardson J. C., et al. Nature in press (2008) Stone, E. C., Cummings, A. C., McDonald, F. B., Heikkila, B. C., Lal, N., and Webber, W. R., Science 23, 2017 (2005). Stone, E.C., et al. Nature in press (2008) Suess, S., J. Geophys. Res. 93, (1993). Zank, G., Space Science Rev. 89, 413, (1999). Zank, G., and Muller, H.-R., J. Geophys. Res., 108, Issue A6, SSH 7-1 (2003). Wareing, C.J., et al., Monthly Notices of the Royal Astronomical Society: Letters, Volume 372, Issue 1, pp. L63-L67 (2006). Washimi, H. and Tanaka, T., Advances in Space Research, 27, 509 (2001). Washimi, H., Zank, G.P., Hu, Q., Tanaka, T., Munakata, K. Astrophys. J., 670, 139L (2007). Whang, Y. C., Wang, Y.-M., Sheeley, N. R., Burlaga, L. F., J. Geophys. Res., 110, Issue AR., A03103 (2005). Figure 1. Coordinate System used in the Model Figure 2. a. Iso-surface of the Heliopause at logt=11.5 (yellow). The green isosurface is the termination shock and the pink and red are, respectively the trajectory of V1 and V2. The orange field lines are the interstellar magnetic field in the HDP plane. B. Meridional cut of the case (a) showing the contours of the magnetic field intensity. The black streamlines are the interstellar magnetic field lines. 17

18 Figure 3. Streaming of termination shock particles (TSPs) (magenta arrows) from the minimum distance of the termination shock to the Sun (MD) to Voyager 1 (V1) and 2 (V2), for the interstellar magnetic field in the a) hydrogen deflection plane (with α=45 ) and b) galactic plane (with α=45 ). The interplanetary magnetic field is carried radially outward by the solar wind, forming a spiral on a conical surface. The conical surfaces coinciding with the V1 and V2 trajectories are shown. V1 and V2 are first connected to the shock along the spiral magnetic field lines that contact the shock at the point of its minimum distance from the Sun (labeled MD). In the (a)-(d) panels the green line indicates the non-spherical termination shock. The (a) and (b) panels shows the solar magnetic field lines that intersect Voyager 1; the field line intersecting the shock where V1 crosses the shock is labeled 0 AU (black) with red and blue indicating, respectively, magnetic field lines 2.0 AU and 3.0 AU upwind from the 0 AU line. The magenta arrow indicates the streaming direction of the termination shock particles from the shock along the field line to V1. Panels (c) and (d) shows similar plot for V2, showing field lines 3.0 and 5.0 AU upwind of the 0 AU line. Note that in both views the solar magnetic field spirals clockwise with increasing distance outward. Figure 4. Same as figure 3. The panels (a) and (b) summarize the streaming of TSPs from MD back to V1 and V2. The nose direction (diamond) and the galactic plane are indicated. Figure 5. The radio source location as a function of the interstellar magnetic field (B ISM ) direction for B ISM, in the a) HDP plane and b) GAL plane (with α=45 ). Panels (a) and (b) shows the surface of the heliopause converted to ecliptic coordinates. The direction of the nose of the heliosphere (diamond) and the galactic plane (black lines) are indicated for reference. The radio sources detected by V1 and V2 are shown (black points).as viewed from upwind from the interstellar wind point of view. The contours show the strength of the radial component of the interstellar magnetic field, B r, on the heliopause. The green band is the location of the radio sources (at B r =0). Figure 6. Same as figure 3a and 3c for the case of 5 fluid MHD model with B ISM in the HDP plane with α=20 with B=4.375µG. 18

19 Acknowledgements. I would like to thank the conference conveners for organizing an extremely productive conference. I would like to acknowledge the generosity of the International Space Science Institute (ISSI), NASA-Voyager Guest Investigator grant #201351, National Science Foundation CAREER Grant # for support. It is a pleasure to thank E. C. Stone for comments on the manuscript. 19