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N F Ness - One of the best experts on this subject based on the ideXlab platform.
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voyager 1 observations of the Interstellar Magnetic field and the transition from the heliosheath
Journal of Physics: Conference Series, 2015Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) has been observing Interstellar Magnetic Fields (ISMF) for more than one year, from 2012/209 to at least 2013.6. From 2013.0 to 2013.6 the difference between the azimuthal angle of the ISMF and the Parker spiral angle at the latitude 34.6° of V1 was (22 ± 3)° and the corresponding difference of the elevation angle was (0 ± 8)°. During 2012 the deviation from the Parker spiral angle was somewhat smaller. The Interstellar Magnetic field has a West to East polarity, opposite to the direction of planetary motions. The magnitude of the ISMF varied smoothly in the range 0.38 nT to 0.59 nT with an average strength 0.49nT. The strongest Interstellar Fields were observed behind a shock at 2012/297 that was preceded by 2.2 KHz plasma oscillations, which implies an Interstellar electron density ne = 0.05/cm−3. The ISMF was observed after V1 crossed a current sheet CS0 having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an Interstellar Magnetic field draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed earlier in the heliosheath at 2012/167 and 2011/276 with high inclinations (99 +/−10)° and (89 ± 10)°, respectively). The transition from heliosheath to Interstellar Magnetic Fields is related to a two-step increase in the cosmic ray intensity observed by V1 from 2012.30 to 2012.65. The first step-increase began near the end of an unusual away-polarity sector, and it reached a plateau when V1 moved into a toward-polarity sector that ended at CS0. The second step-increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.
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Interstellar Magnetic Fields OBSERVED BY VOYAGER 1 BEYOND THE HELIOPAUSE
The Astrophysical Journal, 2014Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) was beyond the heliopause between 2013.00 and 2014.41, where it was making in situ observations of the Interstellar Magnetic field (ISMF). The average azimuthal angle and elevation angle of the Magnetic field B were (λ) = 292.°5 ± 1.°4 and (δ) = 22.°1 ± 1.°2, respectively. The angles λ and δ varied linearly at (1.°4 ± 0.°1) yr{sup –1} and (–1.°1 ± 0.°1) yr{sup –1}, respectively, suggesting that V1 was measuring the draped ISMF around the heliopause. The distributions of hourly averages of λ and δ were Gaussian distributions, with most probable values 292.°5 and 22.°1, and standard deviations (SDs) 1.°3 and 1.°1, respectively. The small SD indicates little or no turbulence transverse to B . An abrupt decrease in B from 0.50 nT on 2013/129.9 to 0.46 nT on 2013/130.6 was observed, possibly associated with a weak reverse shock or magnetoacoustic pressure wave following a burst of electron plasma oscillations. Between 2013/130.6 and 2013/365.3, (B) = 0.464 ± 0.009 nT, (λ) = 292.°6 ± 0.°8, and (δ) = 22.°1 ± 1.°1. The corresponding distribution of hourly averages of B was Gaussian with the most probable value 0.464 nT and σ = 0.009 nT. Since the uncertaintymore » σ corresponds to the instrument and digitization noise, these observations provided an upper limit to the turbulence in the ISMF. The distributions of the hourly increments of B were Gaussian distributions with σ = 0.05 nT, 0.°4, and 0.°4, respectively, indicating that the V1 did not detect evidence of ''intermittent bursts'' of Interstellar turbulence.« less
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voyager 1 observations of the Interstellar Magnetic field and the transition from the heliosheath
The Astrophysical Journal, 2014Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) has been observing Interstellar Magnetic Fields for more than one year beginning ≈2012/209, when V1 crossed a current sheet, a "CS0" having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an Interstellar Magnetic field B draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed at ≈2012/167 and ≈2011/276 with high inclinations (99° ± 10° and 89° ± 10°, respectively). From 2013.0 to ≈2013.6, the difference between the azimuthal angle λ of B from the Parker spiral angle at the latitude 346 of V1 was λ – λP = 22° ± 3° and the corresponding difference of the elevation angle δ was δ – δP = 23° ± 8°. During 2012, the deviation from the Parker spiral angle was somewhat smaller. The Interstellar Magnetic field has a "west to east polarity," opposite to the direction of planetary motions. The magnitude of B varied smoothly in the range 0.38-0.59 nT with an average B = 0.486 ± 0.045 after 2012/237.7. The transition from heliosheath to Interstellar Magnetic Fields is related to a "two-step" increase in the cosmic ray intensity observed by V1 from ≈2012.30 to ≈2012.65. The first step increase began near the end of an unusual "away-polarity" sector, and it reached a plateau when V1 moved into a "toward-polarity" sector that ended at CS0. The second step increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.
Thiem Hoang - One of the best experts on this subject based on the ideXlab platform.
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ParaMagnetic alignment of small grains: a novel method for measuring Interstellar Magnetic Fields
The Astrophysical Journal, 2014Co-Authors: Thiem Hoang, A. L. Lazarian, P. G. MartinAbstract:We present a novel method to measure the strength of Interstellar Magnetic Fields based on ultraviolet (UV) polarization of starlight, which is in part produced by weakly aligned, small Interstellar grains. We begin with calculating degrees of alignment of small (size $a\sim 0.01\mu$m) and very small ($a\sim 0.001\mu$m) grains in the Interstellar Magnetic field due to the Davis-Greenstein paraMagnetic relaxation and resonance paraMagnetic relaxation. We compute the degrees of paraMagnetic alignment with the ambient Magnetic field $B$ using Langevin equations. In this paper, we take into account various processes essential for the dynamics of small grains, including infrared (IR) emission, electric dipole emission, plasma drag and collisions with neutral and ionized species. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and IR wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model requires a higher degree of alignment of small grains for the case with the peak wavelength of polarization $\lambda_{\max}
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paraMagnetic alignment of small grains a novel method for measuring Interstellar Magnetic Fields
The Astrophysical Journal, 2014Co-Authors: A. L. Lazarian, Thiem Hoang, P. G. MartinAbstract:We present a novel method to measure the strength of Interstellar Magnetic Fields using ultraviolet (UV) polarization of starlight that is in part produced by weakly aligned, small dust grains. We begin with calculating the degrees of the paraMagnetic alignment of small (size a ∼ 0.01 μm) and very small (a ∼ 0.001 μm) grains in the Interstellar Magnetic field due to the Davis-Greenstein relaxation and resonance relaxation. To calculate the degrees of paraMagnetic alignment, we use Langevin equations and take into account various interaction processes essential for the rotational dynamics of small grains. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and infrared (IR) wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model for the case with the peak wavelength of polarization λ{sub max} < 0.55 μm requires a higher degree of alignment of small grains than for the typical case with λ{sub max} = 0.55 μm. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e., of p(6 μm{supmore » –1})/p{sub max}) with λ{sub max}{sup −1} for cases of low λ{sub max} by appealing to the higher degree of alignment of small grains. We utilize the correlation of the paraMagnetic alignment of small grains with the Magnetic field strength B to suggest a new way to measure B using the observable parameters λ{sub max} and p(6 μm{sup –1})/p{sub max}.« less
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paraMagnetic alignment of small grains a novel method for measuring Interstellar Magnetic Fields
arXiv: Astrophysics of Galaxies, 2013Co-Authors: A. L. Lazarian, Thiem Hoang, P. G. MartinAbstract:We present a novel method to measure the strength of Interstellar Magnetic Fields based on ultraviolet (UV) polarization of starlight, which is in part produced by weakly aligned, small Interstellar grains. We begin with calculating degrees of alignment of small (size $a\sim 0.01\mu$m) and very small ($a\sim 0.001\mu$m) grains in the Interstellar Magnetic field due to the Davis-Greenstein paraMagnetic relaxation and resonance paraMagnetic relaxation. We compute the degrees of paraMagnetic alignment with the ambient Magnetic field $B$ using Langevin equations. In this paper, we take into account various processes essential for the dynamics of small grains, including infrared (IR) emission, electric dipole emission, plasma drag and collisions with neutral and ionized species. We find that the alignment of small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and IR wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model requires a higher degree of alignment of small grains for the case with the peak wavelength of polarization $\lambda_{\max}<0.55\mu$m, which exhibits an excess UV polarization relative to the Serkowski law, compared to the typical case $\lambda_{\max}=0.55\mu$m. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e. of $p(6\mu m^{-1})/p_{\max}$) with $\lambda_{\max}^{-1}$ by appealing to the higher degree of alignment of small grains. We identify paraMagnetic relaxation as the cause of the alignment of small grains and utilize the dependence of the degree of alignment on the Magnetic field strength $B$ to suggest a new way to measure $B$ using the observable parameters $\lambda_{\max}$ and $p(6\mu m^{-1})/p_{\max}$.[Abridged]
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alignment of dust with Magnetic inclusions radiative torques and superparaMagnetic barnett and nuclear relaxation
The Astrophysical Journal, 2008Co-Authors: A Lazarian, Thiem HoangAbstract:We consider grains with superparaMagnetic inclusions and report two new condensed matter effects that enhance the internal relaxation of the energy of a wobbling grain, namely, superparaMagnetic Barnett relaxation, and an extension of the range where nuclear relaxation is important to higher frequencies. In addition, we report a new effect related to grain rotation: in situations where grains otherwise rotate subthermally subject to radiative torques, they get into a state of fast rotation in the presence of superparaMagnetic inclusions. This effect enhances alignment, as fast rotating grains, unlike slowly rotating ones, exhibit the perfect alignment of grain axes with respect to the ambient astrophysical, e.g., Interstellar, Magnetic Fields. Thus measurements of the polarization degree provide a possibility of testing whether grains have superparaMagnetic inclusions.
Jongsoo Kim - One of the best experts on this subject based on the ideXlab platform.
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amplification of Interstellar Magnetic Fields and turbulent mixing by supernova driven turbulence ii the role of dynamical chaos
The Astrophysical Journal, 2005Co-Authors: Dinshaw S Balsara, Jongsoo KimAbstract:In this paper we further advance the study of Magnetic field amplification in the Interstellar medium that we started in 2004. We show that the flux growth rate is comparable to the rate of Magnetic energy growth found in the earlier paper. We also demonstrate the role of intermittency in field amplification. The density shows a double-peaked PDF, consistent with the cooling curve that was used. The PDF of the Magnetic field shows a high-end tail, providing a telltale signature of the operation of the small-scale dynamo. The Magnetic field strength correlates with the density as ~ ρα, with α = 0.386. As a result, the field amplification takes place more vigorously in the lower temperature, denser gas. The Lagrangian chaos in the simulated turbulent flows is studied in substantial detail. It is shown that the stretching rate of material lines, as well as the Lyapunov exponents, can be used to gain important insights into the growth of Magnetic field. The cancellation exponent for the small-scale supernova-driven dynamo is derived, and it is shown that constructive folding of field lines in the dynamo is very inefficient. We also show that our Lagrangian approach can yield actual measures of the turbulent diffusivity in the simulated ISM. The turbulent diffusivity provides insights into the mixing of elements from supernova ejecta on macroscopic scales. The high rates of line stretching in Interstellar turbulence suggests that the eventual diffusion of elements at the molecular level is very efficient. Many of the diagnostics of turbulence that are presented here can be used to make direct connections between simulations and observations.
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amplification of Interstellar Magnetic Fields by supernova driven turbulence
The Astrophysical Journal, 2004Co-Authors: Dinshaw S Balsara, Jongsoo Kim, Mordecaimark Mac Low, G J MathewsAbstract:Several lines of evidence suggest that Magnetic Fields grow rapidly in protogalactic and galactic environments. However, mean field dynamo theory has always suggested that the Magnetic Fields grow rather slowly, taking of order a Hubble time to reach the observed values. The theoretical difficulties only become worse when the system has a high Magnetic Reynolds number, as is the case for galactic and protogalactic environments. The discrepancy can be reconciled if fast processes for amplifying the Magnetic field could operate. Following the 2001 work of Balsara and coworkers, we show that an Interstellar medium that is dominated by realistic energy input from supernova explosions will naturally become a strongly turbulent medium with large positive and negative values of the kinetic helicity. Even though the medium is driven by compressible motions, the kinetic energy in this high Mach number flow is mainly concentrated in solenoidal rather than compressible motions. These results stem from the interaction of strong shocks with each other and with the Interstellar turbulence they self-consistently generate in our model. Moreover, this interaction also generates large kinetic helicities of either sign. The turbulent flow that we model has two other characteristics of a fast dynamo: Magnetic energy growth independent of scale and a growth time that is comparable to the eddy turnover time. This linear phase of growth permits the field to grow rapidly until the Magnetic energy reaches about 1% of the kinetic energy. At that stage, other astrophysical processes for producing Magnetic Fields can take over. Energetics, power spectra, statistics, and structures of the turbulent flow are studied here. Shock-turbulence interaction is shown to be a very general mechanism for helicity generation and Magnetic field amplification, with applicability to damped Ly? systems, protogalaxies, the Galaxy, starburst galaxies, the intracluster medium, and molecular clouds.
Dinshaw S Balsara - One of the best experts on this subject based on the ideXlab platform.
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amplification of Interstellar Magnetic Fields and turbulent mixing by supernova driven turbulence ii the role of dynamical chaos
The Astrophysical Journal, 2005Co-Authors: Dinshaw S Balsara, Jongsoo KimAbstract:In this paper we further advance the study of Magnetic field amplification in the Interstellar medium that we started in 2004. We show that the flux growth rate is comparable to the rate of Magnetic energy growth found in the earlier paper. We also demonstrate the role of intermittency in field amplification. The density shows a double-peaked PDF, consistent with the cooling curve that was used. The PDF of the Magnetic field shows a high-end tail, providing a telltale signature of the operation of the small-scale dynamo. The Magnetic field strength correlates with the density as ~ ρα, with α = 0.386. As a result, the field amplification takes place more vigorously in the lower temperature, denser gas. The Lagrangian chaos in the simulated turbulent flows is studied in substantial detail. It is shown that the stretching rate of material lines, as well as the Lyapunov exponents, can be used to gain important insights into the growth of Magnetic field. The cancellation exponent for the small-scale supernova-driven dynamo is derived, and it is shown that constructive folding of field lines in the dynamo is very inefficient. We also show that our Lagrangian approach can yield actual measures of the turbulent diffusivity in the simulated ISM. The turbulent diffusivity provides insights into the mixing of elements from supernova ejecta on macroscopic scales. The high rates of line stretching in Interstellar turbulence suggests that the eventual diffusion of elements at the molecular level is very efficient. Many of the diagnostics of turbulence that are presented here can be used to make direct connections between simulations and observations.
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amplification of Interstellar Magnetic Fields and turbulent mixing by supernova driven turbulence part ii the role of dynamical chaos
arXiv: Astrophysics, 2005Co-Authors: Dinshaw S Balsara, J S KimAbstract:In this paper we further advance the study of Magnetic field amplification in the Interstellar medium that was started in Balsara et al (2004, Paper I). We show that the flux growth rate is comparable to the rate of Magnetic energy growth found in Paper I. We also demonstrate the role of intermittency in field amplification. The density shows a double-peaked PDF, consistent with the cooling curve that was used. The PDF of the Magnetic field shows a high-end tail, providing a tell-tale signature of the operation of the small scale dynamo. The Magnetic field strength correlates positively with the density. As a result, the field amplification takes place more vigorously in the lower temperature, denser gas. The Lagrangian chaos in the simulated turbulent flows is studied in substantial detail. It is shown that the stretching rate of material lines as well as the Lyapunov exponents can be used to gain important insights into the growth of Magnetic field. The cancellation exponent for the small scale supernova-driven dynamo is derived and it is shown that constructive folding of field lines in the dynamo is very inefficient. We also show that our Lagrangian approach can yield actual measures of the turbulent diffusivity in the simulated ISM. The turbulent diffusivity provides insights into the mixing of elements from supernova ejecta on macroscopic scales. The high rates of line stretching in Interstellar turbulence suggests that the eventual diffusion of elements at the molecular level is very efficient. Many of the diagnostics of turbulence that are presented here can be used to make direct connections between simulations and observations.
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amplification of Interstellar Magnetic Fields by supernova driven turbulence
The Astrophysical Journal, 2004Co-Authors: Dinshaw S Balsara, Jongsoo Kim, Mordecaimark Mac Low, G J MathewsAbstract:Several lines of evidence suggest that Magnetic Fields grow rapidly in protogalactic and galactic environments. However, mean field dynamo theory has always suggested that the Magnetic Fields grow rather slowly, taking of order a Hubble time to reach the observed values. The theoretical difficulties only become worse when the system has a high Magnetic Reynolds number, as is the case for galactic and protogalactic environments. The discrepancy can be reconciled if fast processes for amplifying the Magnetic field could operate. Following the 2001 work of Balsara and coworkers, we show that an Interstellar medium that is dominated by realistic energy input from supernova explosions will naturally become a strongly turbulent medium with large positive and negative values of the kinetic helicity. Even though the medium is driven by compressible motions, the kinetic energy in this high Mach number flow is mainly concentrated in solenoidal rather than compressible motions. These results stem from the interaction of strong shocks with each other and with the Interstellar turbulence they self-consistently generate in our model. Moreover, this interaction also generates large kinetic helicities of either sign. The turbulent flow that we model has two other characteristics of a fast dynamo: Magnetic energy growth independent of scale and a growth time that is comparable to the eddy turnover time. This linear phase of growth permits the field to grow rapidly until the Magnetic energy reaches about 1% of the kinetic energy. At that stage, other astrophysical processes for producing Magnetic Fields can take over. Energetics, power spectra, statistics, and structures of the turbulent flow are studied here. Shock-turbulence interaction is shown to be a very general mechanism for helicity generation and Magnetic field amplification, with applicability to damped Ly? systems, protogalaxies, the Galaxy, starburst galaxies, the intracluster medium, and molecular clouds.
L F Burlaga - One of the best experts on this subject based on the ideXlab platform.
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voyager 1 observations of the Interstellar Magnetic field and the transition from the heliosheath
Journal of Physics: Conference Series, 2015Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) has been observing Interstellar Magnetic Fields (ISMF) for more than one year, from 2012/209 to at least 2013.6. From 2013.0 to 2013.6 the difference between the azimuthal angle of the ISMF and the Parker spiral angle at the latitude 34.6° of V1 was (22 ± 3)° and the corresponding difference of the elevation angle was (0 ± 8)°. During 2012 the deviation from the Parker spiral angle was somewhat smaller. The Interstellar Magnetic field has a West to East polarity, opposite to the direction of planetary motions. The magnitude of the ISMF varied smoothly in the range 0.38 nT to 0.59 nT with an average strength 0.49nT. The strongest Interstellar Fields were observed behind a shock at 2012/297 that was preceded by 2.2 KHz plasma oscillations, which implies an Interstellar electron density ne = 0.05/cm−3. The ISMF was observed after V1 crossed a current sheet CS0 having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an Interstellar Magnetic field draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed earlier in the heliosheath at 2012/167 and 2011/276 with high inclinations (99 +/−10)° and (89 ± 10)°, respectively). The transition from heliosheath to Interstellar Magnetic Fields is related to a two-step increase in the cosmic ray intensity observed by V1 from 2012.30 to 2012.65. The first step-increase began near the end of an unusual away-polarity sector, and it reached a plateau when V1 moved into a toward-polarity sector that ended at CS0. The second step-increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.
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Interstellar Magnetic Fields OBSERVED BY VOYAGER 1 BEYOND THE HELIOPAUSE
The Astrophysical Journal, 2014Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) was beyond the heliopause between 2013.00 and 2014.41, where it was making in situ observations of the Interstellar Magnetic field (ISMF). The average azimuthal angle and elevation angle of the Magnetic field B were (λ) = 292.°5 ± 1.°4 and (δ) = 22.°1 ± 1.°2, respectively. The angles λ and δ varied linearly at (1.°4 ± 0.°1) yr{sup –1} and (–1.°1 ± 0.°1) yr{sup –1}, respectively, suggesting that V1 was measuring the draped ISMF around the heliopause. The distributions of hourly averages of λ and δ were Gaussian distributions, with most probable values 292.°5 and 22.°1, and standard deviations (SDs) 1.°3 and 1.°1, respectively. The small SD indicates little or no turbulence transverse to B . An abrupt decrease in B from 0.50 nT on 2013/129.9 to 0.46 nT on 2013/130.6 was observed, possibly associated with a weak reverse shock or magnetoacoustic pressure wave following a burst of electron plasma oscillations. Between 2013/130.6 and 2013/365.3, (B) = 0.464 ± 0.009 nT, (λ) = 292.°6 ± 0.°8, and (δ) = 22.°1 ± 1.°1. The corresponding distribution of hourly averages of B was Gaussian with the most probable value 0.464 nT and σ = 0.009 nT. Since the uncertaintymore » σ corresponds to the instrument and digitization noise, these observations provided an upper limit to the turbulence in the ISMF. The distributions of the hourly increments of B were Gaussian distributions with σ = 0.05 nT, 0.°4, and 0.°4, respectively, indicating that the V1 did not detect evidence of ''intermittent bursts'' of Interstellar turbulence.« less
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voyager 1 observations of the Interstellar Magnetic field and the transition from the heliosheath
The Astrophysical Journal, 2014Co-Authors: L F Burlaga, N F NessAbstract:Voyager 1 (V1) has been observing Interstellar Magnetic Fields for more than one year beginning ≈2012/209, when V1 crossed a current sheet, a "CS0" having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an Interstellar Magnetic field B draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed at ≈2012/167 and ≈2011/276 with high inclinations (99° ± 10° and 89° ± 10°, respectively). From 2013.0 to ≈2013.6, the difference between the azimuthal angle λ of B from the Parker spiral angle at the latitude 346 of V1 was λ – λP = 22° ± 3° and the corresponding difference of the elevation angle δ was δ – δP = 23° ± 8°. During 2012, the deviation from the Parker spiral angle was somewhat smaller. The Interstellar Magnetic field has a "west to east polarity," opposite to the direction of planetary motions. The magnitude of B varied smoothly in the range 0.38-0.59 nT with an average B = 0.486 ± 0.045 after 2012/237.7. The transition from heliosheath to Interstellar Magnetic Fields is related to a "two-step" increase in the cosmic ray intensity observed by V1 from ≈2012.30 to ≈2012.65. The first step increase began near the end of an unusual "away-polarity" sector, and it reached a plateau when V1 moved into a "toward-polarity" sector that ended at CS0. The second step increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.
