Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region


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Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region

  1. 1. Reports / / 27 June 2013 / Page 1 / 10.1126/science.1235451 Voyager 1 (V1) crossed the termination shock and entered the heli- osheath on ≈ December 16, 2004, moving in the northern hemisphere ~34.5° above the solar equatorial plane in the general direction of the nose in the heliosphere. V1 moved radially from 94 AU to 121 AU at 34.5°N in the heliosheath since it crossed the termination shock (1–3). Recent estimates of the position of the heliopause (the boundary of the heliosheath and the interstellar medium) along the V1 trajectory range from ≈110 – 150 AU (4–6). During 2010 the radial component of the velocity at V1 was near 0 km/s (7), and from day 2010/126 through 2011/308 the average northward component of the velocity was 28 ± 3 km/ s (8). The speed slowed during 2011 to form a quasi-stagnation region extending from 113 to beyond 119 AU (8, 9) suggesting that V1 may be approaching the heliopause. We present V1 magnetic field observations from 2012/150 through 2012/270 [day of year (DOY) 1 = January 1] in Fig. 1. The particles >0.5 MeV/nuc are discussed in more detail in (10, 11), and the magnetometer and data are described in (12, 13) and in the supplementary materials). During this interval, V1 was at 34.5°N moving from 120.7 - 121.9 AU radially away from the Sun. From 2000/150 to 2012/210, there is no correlation between B (which varies from 0.072 nT to 0.36 nT) and the counting rate of particles >0.5 MeV/nuc (14) (which remains nearly constant). In contrast, from 2012/210 to 2012/270 there is a strong anti- correlation between B and the particle counting rates. Figure 1 shows a series of jumps in B starting on 2012/210 and end- ing on 2012/240, labeled B1, B2, B3, B4, and B5 (Table 1). The jumps indicate multiple crossings of a boundary unlike anything observed pre- viously by V1. On 2012/210 (B1), B increased abruptly from 0.17 nT to 0.43 nT (the strongest magnetic fields observed by V1 in the heliosheath since crossing the termination shock in 2004) and the particle counting rate dropped by a factor of ≈2 (from 23.6 to 12.2 count/s) at the same time. The energetic particle data show that jump B3 did not correspond to a complete entry into the region beyond the boundary, because the counting rates did not drop to near the minimum value and B did not rise to the level observed after B1 and B5. At the last jump on ~2012/238 (B5) B increased to ≈0.43 nT and it remained at that value until at least 2012/270, while the particle counting rate dropped from ≈25 counts/s to ≈2 counts/s (background) until at least 2012/270. In the heliosheath, the average magnetic B between the termination shock crossing at the end of 2004 and the beginning of 2011 was 0.1 nT, cor- responding to a magnetic pressure B2 /8π = 0.04 × 10−12 dyn cm−2 = 0.004 pPa. In the heliosheath depletion region (HDR), B is (0.44 ± 0.01) nT and the magnetic pressure is B2 /8π = 0.8 × 10−12 dyn cm−2 = 0.08 pPa, nearly 20 times greater than observed for five years following the termination shock cross- ing. The enhancements of B between B1 and B2 and between B3 and B4 and following B5 are possibly largely the response of B to the decrease in pres- sure caused by loss of the energetic particles, in order to maintain pressure balance normal to B and equilibrium in the region. In this case the boundaries are pressure balanced structures (15), which correspond to MHD tangential discontinuities such as stream interfac- es. It is generally assumed that the helio- pause is a pressure balanced structure (or tangential discontinuity in the MHD approximation) which is possi- bly rippled by waves and turbulence generated by instabilities (16, 17) and punctuated by reconnection events (6). Because the observations above suggest that the boundaries observed by V1 during 2012 are pres- sure balanced structures, one must consider the hypothesis that the boundaries represent multiple crossings of the heliopause and that V1 has entered the interstellar medium. Due to the rotation of the Sun, the solar magnetic field forms the Parker spiral field as it is carried radially outward by the solar wind (18) which is observed to have an east-west orientation at V1. In contrast, the 10-AU difference in the location of the termination shock in the northern and southern hemisphere implies that the interstellar magnetic field must have a component in the north-south direction (19–21) and is not parallel to the east-west direction of the solar magnetic field in the heliosheath. Consequently, the magnetic field direction should change when V1 crosses the heliopause (fig. S1). The magnetic field direction could re- main constant across the heliopause only if the interstellar magnetic field were nearly parallel to the solar ecliptic plane and tangent to the helio- spheric magnetic field. Such a configuration is highly improbable and would have to be a remarkable coincidence, because the interstellar magnetic field has no causal relation to the solar magnetic field (22, 23). Higher resolution magnetic field observations from 2012/210 to 2012/270 (Fig. 2) suggests that V1 did not observe a significant change in the direction of B at any of the five crossings of the boundary. Table 1 shows the angles on the low field (subscript L) and high field (subscript H) sides of each boundary crossing as well as the absolute value of the differences of these angles. The changes in the direction of B for each of the 5 boundary crossings are indeed very small. The weighted averages of the changes in direction angles are <Δλ> = <| λH – λL|> = (1.8 ± 1.9)° and <Δδ> = <|δH – δL |> = (1.8 ± 1.5)°, consistent with no change in the direction of B. During the last boundary crossing (Fig. 3) the strength of B increased from 0.272 nT to 0.438 nT during an interval of ≈18.4 hours centered at day 237.7. The changes in the angles across B5 are Δδ = (0 ± 2)°, Δλ = (1 ± 3)°. Because the uncertainties refer to differences of angles within one day, they probably represent statistical uncertainties, relatively unaf- Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region L. F. Burlaga,1 * N. F. Ness,2 E. C. Stone3 1 NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA. 2 The Catholic University of America, Washington, DC 20064, USA. 3 California Institute of Technology, Pasadena, CA 91125, USA. *Corresponding author. E- mail: Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈ 0.2 nT to ≈0.4 nT, and B remained near 0.4 nT until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 MeV/nuc particles. The direction of B did not change significantly across any of the 5 boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath (“the heliosheath depletion region”), rather than the interstellar medium. onJune28,2013www.sciencemag.orgDownloadedfrom
  2. 2. / / 27 June 2013 / Page 2 / 10.1126/science.1235451 fected by drifts and other systematic errors. Because there was no change in the direction of B with a high degree of certainty, it is very unlikely that the boundary B5 is the heliopause. The magnetic properties of the HDR from 2012/238 to at least 2012/270 define the region, because they differ from all previous obser- vations within the heliosheath. The average magnetic field strength is (0.436 ± 0.010) nT. An interstellar magnetic field strength of this magni- tude or greater has been ruled out as being too high to explain the IBEX ribbon (24), which adds support to our conclusion that the HDR is asso- ciated with the heliosheath rather than the interstellar medium. The mag- netic field components in this region are BR = (0.126 ± 0.008) nT, BT = (-0.400 ± 0.010) nT and BN = (0.120 ± 0.013) nT. The uncertainties in these average values are the standard deviations, and their values are close to the digitization level and RMS noise of the instrument, 0.004 nT and 0.003 nT, respectively. Thus, the fluctuations in the components of B are extremely small in the HDR. The region is not turbulent. The average direction of the magnetic field in the HDR is λA = (287° ± 1°) and δA = (14° ± 2°). The average magnetic field direction is close to the Parker spiral magnetic field direction (Fig. 2), but there is a statis- tically significant difference from the spiral field direction in the HDR, namely λA – λP = (17 ± 1)° and δA - δP = (14 ± 2)° as shown in Fig. 2 . The magnetic polarity of the magnetic field in the HDR indicates that it has moved from the southern hemisphere to the position of V1 in the northern hemisphere. The small departure from the spiral field direction might be the result of a flow that carried the magnetic field northward in the heliosheath to the location of V1. It has been suggested that such a flow moves northward in the heliosheath between a “magnetic wall” or “magnetic barrier” and the heliopause at the latitude of V1 (5, 25). Increasingly strong magnetic fields from the middle of 2010 until at least the middle of 2011 (possibly extending up to 2012/150 as shown in this paper) were reported in (26), where it was suggested that these strong magnetic fields might be related to a magnetic wall or magnetic barrier. Thus, it is conceivable that the HDR corresponds to this north- ward heliosheath flow near the heliopause, and the boundary of the HDR represents a boundary of material that was moving radially closer to the Sun. The strong magnetic fields observed from mid-2010 to 2012/270 could be an interaction region that extends into the HDR, produced by the collision of these two flows. The stronger magnetic field in the HDR might be produced in response to the reduction of pressure owing to the absence of energetic particles. The absence of energetic particles could indicate that magnetic lines passing V1 were no longer connected to their source (the blunt termination shock), because V1 crossed a topolog- ic boundary in the magnetic field of the inner heliosheath beyond the last magnetic connection point to the termination shock (27). Alternatively, the energetic particles could have escaped into interstellar space, if the heliosheath magnetic field reconnected with the interstellar magnetic field beyond the position of V1. References and Notes 1. E. C. Stone, A. C. Cummings, F. B. McDonald, B. C. Heikkila, N. Lal, W. R. Webber, Voyager 1 explores the termination shock region and the heliosheath beyond. Science 309, 2017–2020 (2005). doi:10.1126/science.1117684 Medline 2. L. F. Burlaga, N. F. Ness, M. H. Acuña, R. P. Lepping, J. E. Connerney, E. C. Stone, F. B. McDonald, Crossing the termination shock into the heliosheath: Magnetic fields. Science 309, 2027–2029 (2005). doi:10.1126/science.1117542 Medline 3. R. B. Decker, S. M. Krimigis, E. C. Roelof, M. E. Hill, T. P. Armstrong, G. Gloeckler, D. C. Hamilton, L. J. Lanzerotti, Voyager 1 in the foreshock, termination shock, and heliosheath. Science 309, 2020–2024 (2005). doi:10.1126/science.1117569 Medline 4. N. V. Pogorelov, S. N. Borovikov, G. P. Zank, L. F. Burlaga, R. A. Decker, E. C. Stone, Radial velocity along the VOYAGER 1 trajectory: The effect of solar cycle. Astrophys. J. Lett. 750, L4 (2012). doi:10.1088/2041-8205/750/1/L4 5. H. Washimi, G. P. Zank, Q. Hu, T. Tanaka, K. Munakata, H. Shinagawa, Realistic and time-varying outer heliospheric modelling. Mon. Not. R. Astron. Soc. 416, 1475–1485 (2011). doi:10.1111/j.1365-2966.2011.19144.x 6. M. Opher, J. F. Drake, M. Swisdak, K. M. Schoeffler, J. D. Richardson, R. B. Decker, G. Toth, Is the magnetic field in the heliosheath laminar or a turbulent sea of bubbles? Astrophys. J. 734, 71 (2011). doi:10.1088/0004-637X/734/1/71 7. S. M. Krimigis, E. C. Roelof, R. B. Decker, M. E. Hill, Zero outward flow velocity for plasma in a heliosheath transition layer. Nature 474, 359–361 (2011). doi:10.1038/nature10115 Medline 8. E. C. Stone et al., Proc. 32nd International Cosmic Ray Conference, 12, 29, doi:10.7529/ICRC2011/V12/I06 (2011). 9. R. B. Decker, S. M. Krimigis, E. C. Roelof, M. E. Hill, No meridional plasma flow in the heliosheath transition region. Nature 489, 124– 127 (2012). doi:10.1038/nature11441 Medline 10. E. C. Stone et al., Science (2012). 11. W. R. Webber, F. B. McDonald, in press, doi: 10.1002/grl.50383 (2013). 12. K. Behannon et al., Space Sci. Rev. 21, 235 (1997). 13. D. B. Berdichevsky, Voyager mission, detailed processing of weak magnetic fields; constraints to the uncertainties of the calibrated magnetic field signal in the Voyager missions (2009); VOY_sensor_opu090518.pdf. 14. E. C. Stone et al., Space Sci. Rev. 21, 355 (1977). 15. L. F. Burlaga, N. F. Ness, Current sheets in the heliosheath: Voyager 1, 2009. J. Geophys. Res. 116, (A5), A05102 (2011). doi:10.1029/2010JA016309 16. V. Florinski, G. P. Zank, N. V. Pogorelov, Heliopause stability in the presence of neutral atoms: Rayleigh-Taylor dispersion analysis and axisymmetric MHD simulations. J. Geophys. Res. 110, (A7), A07104 (2005). doi:10.1029/2004JA010879 17. S. N. Borovikov, N. V. Pogorelov, G. P. Zank, I. A. Kryukov, Consequences of the heliopause instability caused by charge exchange. Astrophys. J. 682, 1404–1415 (2008). doi:10.1086/589634 18. E. N. Parker, Interplanetary Dynamical Processes (Interscience Publishers, New York, 1963). 19. M. Opher, F. A. Bibi, G. Toth, J. D. Richardson, V. V. Izmodenov, T. I. Gombosi, A strong, highly-tilted interstellar magnetic field near the Solar System. Nature 462, 1036–1038 (2009). doi:10.1038/nature08567 Medline 20. N. V. Pogorelov, J. Heerikhuisen, J. J. Mitchell, I. H. Cairns, G. P. Zank, Heliospheric asymmetries and 2-3 kHz radio emission under strong interstellar magnetic field conditions. ApJ 695, L31–L34 (2009). doi:10.1088/0004-637X/695/1/L31 21. V. Izmodenov, Y. G. Malama, M. S. Ruderman, S. V. Chalov, D. B. Alexashov, O. A. Katushkina, E. A. Provornikova, Kinetic- gasdynamic modeling of the heliospheric interface. Space Sci. Rev. 146, 329–351 (2009). doi:10.1007/s11214-009-9528-3 22. J. Heerikhuisen, N. V. Pogorelov, G. P. Zank, G. B. Crew, P. C. Frisch, H. O. Funsten, P. H. Janzen, D. J. McComas, D. B. Reisenfeld, N. A. Schwadron, Pick-up ions in the outer heliosheath: A possible mechanism for the interstellar boundary explorer ribbon. Astrophys. J. Lett. 708, L126–L130 (2010). doi:10.1088/2041- 8205/708/2/L126 23. P. C. Frisch, Physics of the Heliosphere: A 10 Year Retrospective: Proc. 10th Annual Internat. Astrophys. Conf., AIP Conf. Proc., 1436, 239 (2012). 24. G. P. Zank, J. Heerikhuisen, B. E. Wood, N. V. Pogorelov, E. Zirnstein, D. J. McComas, Heliospheric structure: The bow wave and the hydrogen wall. Astrophys. J. 763, 20 (2013). doi:10.1088/0004- 637X/763/1/20 25. H. Washimi, T. Tanaka, 3-D magnetic field and current system in the heliosphere. Space Sci. Rev. 78, 85–94 (1996). doi:10.1007/BF00170795 26. L. F. Burlaga, N. F. Ness, Heliosheath magnetic fields between 104 AND 113 AU in a region of declining speeds and a stagnation region. onJune28,2013www.sciencemag.orgDownloadedfrom
  3. 3. / / 27 June 2013 / Page 3 / 10.1126/science.1235451 ApJ 749, 13 (2012). doi:10.1088/0004-637X/749/1/13 27. D. J. McComas, N. A. Schwadron, Disconnection from the termination shock: the end of the Voyager paradox. Astrophys. J. 758, 19 (2012). doi:10.1088/0004-637X/758/1/19 28. Acknowledgments: T. McClanahan and S. Kramer provided support in the processing of the data. D. Berdichevsky computed correction tables for the 3 sensors on each of the two magnetometers. N. F. Ness was partially supported by NASA Grant NNX12AC63G to the Catholic University of America. L. F. Burlaga was supported by NASA Contract NNG11PN48P. The data are available at NASA’s Virtual Heliospheric Observatory maintained within the Heliospheric Physics Laboratory at NASA’s Goddard Space Flight Center. Supplementary Materials Supplementary Text Fig. S1 21 January 2013; accepted 30 May 2013 Published online 27 June 2013; 10.1126/science.1235451 onJune28,2013www.sciencemag.orgDownloadedfrom
  4. 4. / / 27 June 2013 / Page 4/ 10.1126/science.1235451 Table 1. Changes in B at the boundaries of the heliosheath depletion region. Angles Angle changes Parameters for B(t) to (days) λL° λH° δL° δH° |λH -λL|° |δH - δL|° BL (nT) BH (nT) w (hours) B1 210.6 275 ± 7 282 ± 1 5 ± 9 12 ± 1 7 ± 7 7 ± 9 0.170 0.425 5.3 B2 215.6 295 ± 11 282 ± 2 19 ± 5 11 ± 2 13 ± 11 8 ± 5 0.236 0.416 8.6 B3 225.7 285 ± 3 285 ± 2 13 ± 3 15 ± 3 0 ± 4 2 ± 4 (0.249) (0.372) <10.7 B4 233.5 284 ± 4 286 ± 1 13 ± 3 17 ± 2 2 ± 4 4 ± 4 0.271 0.425 35.4 B5 237.7 286 ± 3 287 ± 1 12 ± 2 12 ± 1 1 ± 3 0 ± 2 0.272 0.438 18.4 AVG 285.8 ± 1.8 284.8 ± 0.5 12.8 ± 1.4 12.5 ± 0.6 1.8 ± 1.9 1.8 ± 1.5 0.237 0.426 11.9 ____________________________________________________________________________________________________________________ Fig. 1. Hour averages of magnetic field strength B (A). The counting rate of energetic particles >0.5 MeV/nuc to ~30 MeV (B). onJune28,2013www.sciencemag.orgDownloadedfrom
  5. 5. / / 27 June 2013 / Page 5 / 10.1126/science.1235451 Fig. 2. 48-s averages of the magnetic field strength B (A), azimuthal angle λ (B), and elevation angle δ (C), as a function of time measured from DOY 150 to DOY 270, 2012. The angles are in RTN coordinates (28). Prior to 2012/210, V1 observed magnetic fields characteristic of the heliosheath (26). The elevation and azimuthal angles are close to the Parker spiral direction, δP ≈ 0° and λP ≈ 90° or 270°, respectively. A magnetic sector in which B was directed sunward along the Parker spiral angle was observed between 2012/171 and 2012/208. The magnetic field strength varied from 0.07 nT to 3.36 nT prior to the boundary crossings. onJune28,2013www.sciencemag.orgDownloadedfrom
  6. 6. / / 27 June 2013 / Page 6 / 10.1126/science.1235451 Fig. 3. 48-s averages of the magnetic field profile during the fifth crossing of the boundary into the heliosheath depletion region (see Fig. 2). The solid curve is a sigmoid function, B(t) = B2 + [B1 – B2]/[1 + exp(t – to)/(w/4.4)], which provides an excellent fit to the data (coefficient of determination R 2 = 0.98). The parameter w gives the time required for B to change from 10% to 90% of the way to the asymptotic values (15). onJune28,2013www.sciencemag.orgDownloadedfrom