Interstellar Field Research Articles

Astrochemistry of the Molecular Gas in Dusty Star-Forming Galaxies at the Cosmic Noon

Far-infrared and submillimeter observations have established the fundamental role of dust-obscured star formation in the assembly of stellar mass over the past ∼12 billion years. At z = 2–4, the so-called “cosmic noon”, the bulk of star formation is enshrouded in dust, and dusty star-forming galaxies (DSFGs) contain ∼50% of the total stellar mass density. Star formation occurs in dense molecular clouds, and is regulated by a complex interplay between all the ISM components that contribute to the energy budget of a galaxy: gas, dust, cosmic rays, interstellar electromagnetic fields, gravitational field, and dark matter. Molecular gas is the actual link between star-forming gas and its complex environment: much of what we know about star formation comes from observations of molecular line emissions. They provide by far the richest information about the star formation process. However, their interpretation requires complex modeling of the astrochemical networks which regulate molecular formation and establish molecular abundances in a cloud, and a modeling of the physical conditions of the gas in which molecular energy levels become populated. This paper critically reviews the main astrochemical parameters needed to obtain predictions about molecular signals in DSFGs. Molecular lines can be very bright compared to the continuum emission, but radiative transfer models are required to properly interpret the observed brightness. We review the current knowledge and the open questions about the interstellar medium of DSFGs, outlining the key role of molecular gas as a tracer and shaper of the star formation process.

Unified Picture of the Local Interstellar Magnetic Field from Voyager and IBEX

Prior to the Voyagers’ heliopause crossings, models and the community expected the magnetic field to show major rotations across the boundary. Surprisingly, the field showed no significant change in direction from the heliospheric Parker Spiral at either Voyager location. Meanwhile, a major result from the IBEX mission is the derived magnitude and direction of the interstellar field far from the Sun (∼1000 au) beyond the influence of the heliosphere. Using a self-consistent model fit to IBEX ribbon data, Zirnstein et al. reported that this “pristine” local interstellar magnetic field has a magnitude of 0.293 nT and direction of 227° in ecliptic longitude and 34.°6 in ecliptic latitude. These values differ by 27% (51%) and 44° (12°) from what Voyager 1 (2) currently observes (as of ∼2022.75). While differences are to be expected as the field undrapes away from the heliosphere, the global structure of the draping across hundreds of astronimcal units has not been reconciled. This leads to several questions: How are these distinct sets of observations reconcilable? What is the interstellar magnetic field’s large-scale structure? How far out would a future mission need to go to sample the unperturbed field? Here, we show that if realistic errors are included for the difficult-to-calibrate radial field component, the measured transverse field is consistent with that predicted by IBEX, allowing us to answer these questions through a unified picture of the behavior of the local interstellar magnetic field from its draping around the heliopause to its unfolding into the pristine interstellar medium.

Synchrotron Polarization of Gamma-Ray Burst Afterglow Shocks with Hydrodynamic-scale Turbulent Magnetic Field

Afterglows of gamma-ray bursts (GRBs) are emitted from expanding forward shocks, which are expected to have magnetic fields much stronger than the interstellar field, although the origin of the field is a long-standing problem. Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. The coherence-length scales of the fields amplified by these two processes are different by 7–10 orders of magnitude, and the polarimetric observations may distinguish them. We construct a semi-analytic model of the forward-shock afterglow polarization under the assumption of hydrodynamic-scale turbulent magnetic field. We perform numerical calculations of synchrotron polarization for the isotropic turbulence and the zero viewing angle. We find that the polarization degrees are ∼1% when the field coherence-length scale in the fluid co-moving frame is of the order of the thickness of the shocked regions. This range of polarization degree is comparable to that of the observed late-phase optical afterglows. Our model also shows that the radio polarization degrees are comparable to the optical ones on average but can be higher than the optical ones at some time intervals. The polarization angles are shown to vary randomly and continuously. These polarimetric properties are clearly different from the case of plasma kinetic instability. Simultaneous polarimetric observations of GRB afterglows at the radio and optical bands have recently started, which will help us constrain the magnetic field amplification mechanism.

Cyanopolyyne Chemistry in the L1544 Prestellar Core: New Insights from GBT Observations

We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100 m Robert C. Byrd Green Bank Telescope, we observe three emission lines of HC3N, nine lines of HC5N, five lines of HC7N, and nine lines of HC9N. HC9N is detected for the first time toward the source. The high spectral resolution (∼0.05 km s−1) reveals double-peak spectral line profiles with the redshifted peak a factor 3–5 brighter. Resolved maps of the core in other molecular tracers indicate that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modeling of the HC5N, HC7N, and HC9N lines to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5–12 K, a source size of 80″, and a gas density larger than 100 cm−3. The HC5N:HC7N:HC9N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those toward the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in light of the new observations.

An early transition to magnetic supercriticality in star formation

Magnetic fields have an important role in the evolution of interstellar medium and star formation1,2. As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas3. Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H I narrow self-absorption (HINSA)4,5 towards L15446,7—a well-studied prototypical prestellar core in an early transition between starless and protostellar phases8–10 characterized by a high central number density11 and a low central temperature12. A combined analysis of the Zeeman measurements of quasar H I absorption, H I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes13,14.

H i-to-H2 Transitions in Dust-free Interstellar Gas

Abstract We present numerical computations and analysis of atomic-to-molecular (H i-to-H2) transitions in cool (∼100 K), low-metallicity, dust-free (primordial) gas in which molecule formation occurs via cosmic-ray-driven negative ion chemistry and removal is by a combination of far-UV photodissociation and cosmic-ray ionization and dissociation. For any gas temperature, the behavior depends on the ratio of the Lyman–Werner (LW) band FUV intensity to gas density, I LW/n, and the ratio of the cosmic-ray ionization rate to the gas density, ζ/n. We present sets of H i-to-H2 abundance profiles for a wide range of ζ/n and I LW/n for dust-free gas. We determine the conditions for which H2 absorption-line self-shielding in optically thick clouds enables a transition from atomic to molecular form for ionization-driven chemistry. We also examine the effects of cosmic-ray energy losses on the atomic and molecular density profiles and transition points. For a unit Galactic interstellar FUV field intensity (I LW = 1) with LW flux 2.07 × 107 photons cm−2 s−1 and a uniform cosmic-ray ionization rate ζ = 10−16 s−1, an H i-to-H2 transition occurs at a total hydrogen gas column density of 4 × 1021 cm−2, within 3 × 107 yr, for a gas volume density of n = 106 cm−3 at 100 K. For these parameters, the dust-free limit is reached for a dust-to-gas ratio Z d ′ ≲ 10 − 5 , which may be reached for overall metallicities Z ′ ≲ 0.01 relative to Galactic solar values.

Census of High- and Medium-mass Protostars. V. CO Abundance and the Galactic X CO Factor

Abstract We present the second dust continuum data release in the Census of High- and Medium-mass Protostars (CHaMP), expanding the methodology trialed in Pitts et al. to the entire CHaMP survey area (280° < ℓ < 300°, − 4° < b < + 2°). This release includes maps of dust temperature (T d), H2 column density ( N H 2 ), gas-phase CO abundance, and temperature–density plots for every prestellar clump with Herschel coverage, showing no evidence of internal heating for most clumps in our sample. We show that CO abundance is a strong function of T d and can be fit with a second-order polynomial in log-space, with a typical dispersion of a factor of 2–3. The CO abundance peaks at 20.0 − 1.0 + 0.4 K with a value of 7.4 − 0.3 + 0.2 × 10 − 5 per H2; the low T d at which this maximal abundance occurs relative to laboratory results is likely due to interstellar UV bombardment in the largest survey fields. Finally, we show that, as predicted by theoretical literature and hinted at in previous studies of individual clouds, the conversion factor from integrated 12CO line intensity ( I 12 CO ) to N H 2 , the X CO factor, varies as a broken power law in I 12 CO with a transition zone between 70 and 90 K km s−1. The X CO function we propose has N H 2 ∝ I 12 CO 0.51 for I 12 CO ≲ 70 K km s−1 and N H 2 ∝ I 12 CO 2.3 for I 12 CO ≳ 90 K km s−1. The high- I 12 CO side should be generalizable with known adjustments for metallicity, but the influence of interstellar UV fields on the low- I 12 CO side may be sample specific. We discuss how these results expand on previous works in the CHaMP series and help tie together observational, theoretical, and laboratory studies on CO over the past decade.

Physical and numerical instabilities of radiatively cooling shocks in turbulent magnetized media

ABSTRACT We consider the influence of a fluctuating magnetic field on to the structure formation and instabilities of radiatively cooling blast waves. The study is based on an example of optically thin post-adiabatic supernova remnants (SNRs) in the homogeneous interstellar medium. By means of analytic estimations and full-scale multidimensional simulations, we investigate the roles of thermal, hydrodynamic (corrugation, pulsational, convective, Rayleigh–Taylor, linear and non-linear Vishniac) and numerical instabilities (‘carbuncle’ and grid-forced effects). It is found that of primary importance is the interplay of the thermal instability with quasi-regular and random components of the interstellar field. Bending fluctuations caused by the latter can be strongly amplified by non-linear Vishniac instability in the SNR regions where the regular component is almost normal to the shock. The instabilities driven by counter-directional pressure and density gradients are limited mostly to very narrow post-shock cooling layers, transient perturbations of the same short scales, and rather weakly magnetized environments. Some of these results can also be applied to radiative shocks separating optically thick media from thin or semitransparent ones. Several recommendations and requirements on numerical simulation techniques are formulated.

Warm dust surface chemistry

Context.Molecular hydrogen (H2) is the main constituent of the gas in the planet-forming disks that surround many pre-main-sequence stars. H2can be incorporated in the atmosphere of the nascent giant planets in disks. Deuterium hydride (HD) has been detected in a few disks and can be considered the most reliable tracer of H2, provided that its abundance throughout the disks with respect to H2is well understood.Aims.We wish to form H2and HD efficiently for the varied conditions encountered in protoplanetary disks: the densities vary from 104to 1016cm−3; the dust temperatures range from 5 to 1500 K, the gas temperatures go from 5 to a few 1000 Kelvin, and the ultraviolet radiation field can be 107stronger than the standard interstellar field.Methods.We implemented a comprehensive model of H2and HD formation on cold and warm grain surfaces and via hydrogenated polycyclic aromatic hydrocarbons in the physico-chemical code PROtoplanetary DIsk MOdel. The H2and HD formation on dust grains can proceed via the Langmuir-Hinshelwood and Eley-Ridel mechanisms for physisorbed or chemisorbed H (D) atoms. H2and HD also form by H (D) abstraction from hydrogenated neutral and ionised PAHs and via gas phase reactions.Results.H2and HD are formed efficiently on dust grain surfaces from 10 to ~700 K. All the deuterium is converted into HD in UV shielded regions as soon as H2is formed by gas-phase D abstraction reactions. The detailed model compares well with standard analytical prescriptions for H2(HD) formation. At low temperature, H2is formed from the encounter of two physisorbed atoms. HD molecules form on the grain surfaces and in the gas-phase. At temperatures greater than 20 K, the encounter between a weakly bound H- (or D-) atom or a gas-phase H (D) atom and a chemisorbed atom is the most efficient H2formation route. H2formation through hydrogenated PAHs alone is efficient above 80 K. However, the contribution of hydrogenated PAHs to the overall H2and HD formation is relatively low if chemisorption on silicate is taken into account and if a small hydrogen abstraction cross-section is used. The H2and HD warm grain surface network is a first step in the construction of a network of high-temperature surface reactions.

Precision Determination of Corotation Radii in Galaxy Disks: Tremaine–Weinberg versus Font–Beckman for NGC 3433

Abstract Density waves in galaxy disks have been proposed over the years, in a variety of specific models, to explain spiral arm structure and its relation to the mass distribution, notably in barred galaxies. An important parameter in dynamical density wave theories is the corotation radius, the galactocentric distance at which the stars and gas rotate at the same speed as the quasi-static propagating density wave. Determining corotation, and the pattern speed of a bar have become relevant to tests of cosmologically based theories of galaxy evolution involving the dynamical braking of bars by interaction with dark matter halos. Here, comparing two methods, one of that measures the pattern speed and another that measures the radius of corotation, using two instruments (an integral field spectrometer and a Fabry–Perot interferometer), and using both the stellar and interstellar velocity fields, we have determined the bar corotation radius, and three further radii of corotation for the SAB(s)b galaxy NGC 3433. The results of both methods, with both instruments, and with both disk components, give excellent agreement. This strengthens our confidence in the value of the two methods, and offers good perspectives for quantitative tests of different theoretical models.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis.

Pure methane (CH4/CD4) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C3H8(C3D8)], ethane [C2H6(C2D6)], the ethyl radical [C2H5(C2D5)], ethylene [C2H4(C2D4)], acetylene [C2H2(C2D2)], and the methyl radical [CH3(CD3)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7) from electron irradiation and CnH2n+2 (n = 4-8), CnH2n (n = 3-10), CnH2n-2 (n = 3-11), CnH2n-4 (n = 4-11), CnH2n-6 (n = 5-11), and CnH2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C3H4 and C4H6 isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C9H8) and naphthalene (C10H8) under interstellar conditions.

Heliosphere in a strong interstellar magnetic field

We present results of MHD simulations of the global structure of the heliosphere for a wide range of the interstellar magnetic field (ISMF) strengths (2-20 µG). For a strong interstellar field, the plasma flow in and around the heliosphere as well as the magnetic field draped over the heliopause have a simple structure, reminiscent of (though not identical to) the analytical model by Parker. We show how this structure evolves as the interstellar field strength is reduced down to the values in the observed range (few µG). We also present an example of using this evolution to understand the present observations of the heliosphere.

ENERGETIC NEUTRAL HYDROGEN OBSERVATIONS DEMONSTRATE THAT VOYAGER 1 IS NOT OBSERVING THE EXTRAORDINARILY STRONG INTERSTELLAR MAGNETIC FIELD

ABSTRACT It is generally believed that Voyager 1 (V1) is now in interstellar space, having crossed the heliopause at a heliocentric distance of 121.58 au in late August of 2012. Here we use recently published spectra of energetic neutral hydrogen, and the magnetic field and energetic particles directly measured by V1 to find the average pressure in the inner heliosheath (termination shock to 122 au). This pressure turns out to be surprisingly large, (3.57 ± 0.71) × 10−12 dyn cm−2, and is completely dominated by pressures of pickup ions (PUIs), created in the inner heliosheath, and their suprathermal tails (43%), and PUIs and their tails that are produced upstream of the termination shock and enter the heliosheath (46%). We compute the total particle pressure in the outer heliosheath near the heliopause from distribution functions of the interstellar plasma and locally created PUIs using profiles of proton density, proton temperature, and neutral hydrogen density from model 2 in Zank et al., and find it to be at most 7.7 × 10−13 dyn cm−2. Balancing pressure across the heliopause, thus requires an unusually large magnetic pressure (2.8 × 10−12 dyn cm−2). The resulting strength and 1σ uncertainty of the draped magnetic field in the outer heliosheath near the heliopause is 0.839 ± 0.106 nT. The 3σ lower limit field strength (0.52 nT) is greater than the field of ∼0.43 ± 0.02 nT measured by V1, implying that there is less than 1% probability that V1 is measuring the interstellar draped field.

Limits on the interstellar magnetic field imposed by observational constraints indicate that Voyager 1 remains in the inner heliosheath

The unmeasured interstellar magnetic field strength is an essential parameter for models of the heliosphere moving relative to the Local Interstellar Cloud (LIC) that surrounds it. Here we use recent measurements of the differential energy spectra of energetic neutral hydrogen (ENH) combined with Voyager 1 (V1) measurements of energetic particles to calculate the total pressure and its 1-0 limit in the inner heliosheath, the region between the termination shock at ∼90 AU and the heliopause that V1 presumably crossed at 122 AU. Balancing pressure across the heliopause, we find the magnetic pressure in the outer heliosheath at ∼ 135 AU where V1 is currently located. From this magnetic pressure we find an extraordinarily large magnetic field strength of 0.84 ± 0.08 nT compared to the 0.464 ± 0.009 nT field strength currently measured by V1. We conclude that V1 is currently not measuring the interstellar field, but rather the solar magnetic field. Voyager 1 is thus not in interstellar space but still inside a new, unanticipated, and most unusual region of heliosphere that it entered in late August 2012. We recalculate total pressures in the ordinary heliosheath (∼90 - 122 AU), in the new, co-called cold inner heliosheath (122 - 145 AU), in the outer heliosheath and in the unperturbed LIC. Pressure balance again requires that all these pressures must be equal. Constrained by the requirement of pressure equality between all regions and minimum reduced χ2 fits to the ENH spectrum and the differential intensities of particles measured by V1 in the ordinary, co-called hot heliosheath, we find the common pressure to be (3.45 ± 0.39)x10-12 dyne-cm-2. The magnetic field strength in the outer heliosheath is calculated to be 0.90 ± 0.089 nT, and in the unperturbed LIC it is 0.86 ± 0.11 nT. To achieve common pressure for all regions imposes firm limits on the LIC neutral hydrogen density, n(HI) = 0.10 ± 0.02 cm-3. Pressure balance between the LIC and the hot cloud surrounding it places firm limit on the proton density in the unperturbed LIC, n(HIl) = 0.036 ± 0.012 cm-3.

Chemistry in AGB stars: successes and challenges

Emission and absorption line observations of molecules in late-type stars are a vital component in our understanding of stellar evolution, dust formation and mass loss in these objects. The molecular composition of the gas in the circumstellar envelopes of AGB stars reflects chemical processes in gas whose properties are strong functions of radius with density and temperature varying by more than ten and two orders of magnitude, respectively. In addition, the interstellar UV field plays a critical role in determining not only molecular abundances but also their radial distributions. In this article, I shall briefly review some recent successful approaches to describing chemistry in both the inner and outer envelopes and outline areas of challenge for the future.

CHARTING THE INTERSTELLAR MAGNETIC FIELD CAUSING THEINTERSTELLAR BOUNDARY EXPLORER(IBEX) RIBBON OF ENERGETIC NEUTRAL ATOMS

The interstellar magnetic field (ISMF) near the heliosphere is a basic part of the solar neighborhood that can only be studied using polarized starlight. Results of an ongoing survey of polarized starlight are analyzed with the goal of linking the interstellar magnetic field that shapes the heliosphere to the nearby field in interstellar space. New results for the direction of the nearby ISMF, based on a merit function that utilizes polarization position angles, identify several magnetic components. The dominant interstellar field, B_pol, is aligned with the direction L,B= 36.2,49.0 (+/-16.0) degrees and is within 8 degrees of the IBEX Ribbon ISMF direction. Stars tracing B_pol have the same mean distance as stars that do not trace B_pol, but show weaker polarizations consistent with lower column densities of polarizing grains. The variations in the polarization position angle directions indicate a low level of magnetic turbulence. B_pol is found after excluding polarizations that trace a separate magnetic structure that apparently is due to interstellar dust deflected around the heliosphere. Local interstellar cloud velocities relative to the LSR increase with the angles between the LSR velocities and ISMF, indicating that the kinematics of local interstellar material is ordered by the ISMF. Polarization and color excess data are consistent with an extension of Loop I to the solar vicinity. Polarizations are consistent with previous findings of more efficient grain alignment in low column density sightlines. Optical polarization and color excess data indicate the presence of nearby interstellar dust in the BICEP2 field. Color excess E(B-V) indicates an optical extinction of A_V about 0.59 mag in the BICEP2 field, while the polarization data indicate that A_V is larger than 0.09 mag. The IBEX Ribbon ISMF extends to the boundaries of the BICEP2 region.

Shape and evolution of wind-blown bubbles of massive stars: on the effect of the interstellar magnetic field

The winds of massive stars create large (>10 pc) bubbles around their progenitors. As these bubbles expand they encounter the interstellar coherent magnetic field which, depending on its strength, can influence the shape of the bubble. We wish to investigate if, and how much, the interstellar magnetic field can contribute to the shape of an expanding circumstellar bubble around a massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical simulations of bubbles, using a single star model, combined with several different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic field. This covers the typical field strengths of the interstellar magnetic fields found in the galactic disk and bulge. Furthermore, we present two simulations that include both a 5 muG interstellar magnetic field and a 10,000 K interstellar medium and two different ISM densities to demonstrate how the magnetic field can combine with other external factors to influence the morphology of the circumstellar bubbles. Our results show that low magnetic fields, as found in the galactic disk, inhibit the growth of the circumstellar bubbles in the direction perpendicular to the field. As a result, the bubbles become ovoid, rather than spherical. Strong interstellar fields, such as observed for the galactic bulge, can completely stop the expansion of the bubble in the direction perpendicular to the field, leading to the formation of a tube-like bubble. When combined with a warm, high-density ISM the bubble is greatly reduced in size, causing a dramatic change in the evolution of temporary features inside the bubble. The magnetic field of the interstellar medium can affect the shape of circumstellar bubbles. This effect may have consequences for the shape and evolution of circumstellar nebulae and supernova remnants, which are formed within the main wind-blown bubble.

MAGNETIC FLUX TUBE INTERCHANGE AT THE HELIOPAUSE

The magnetic field measured by Voyager 1 prior to its heliocliff encounter on 2012.65 showed an unexpectedly complex transition from the primarily azimuthal inner-heliosheath field to the draped interstellar field tilted by some 20° to the nominal azimuthal direction. Most prominent were two regions of enhanced magnetic field strength depleted in energetic charged particles of heliospheric origin. These regions were interpreted as magnetic flux tubes connected to the outer heliosheath that provided a path for the particles to escape. Despite large increases in strength, the field's direction did not change appreciably at the boundaries of these flux tubes. Rather, the field's direction changed gradually over several months prior to the heliocliff crossing. It is shown theoretically that the heliopause, as a pressure equilibrium layer, can become unstable to interchange of magnetic fields between the inner and the outer heliosheaths. The curvature of magnetic field lines and the anti-sunward gradient in plasma kinetic pressure provide conditions favorable for an interchange. Magnetic shear between the heliosheath and the interstellar fields reduces the growth rates, but does not fully stabilize the heliopause against perturbations propagating in the latitudinal direction. The instability could create a transition layer permeated by magnetic flux tubes, oriented parallel to each other and alternately connected to the heliosheath or the interstellar regions.

DRAPING OF THE INTERSTELLAR MAGNETIC FIELD OVER THE HELIOPAUSE: A PASSIVE FIELD MODEL

As the local interstellar plasma flows past our heliosphere, it is slowed and deflected around the magnetic obstacle of the heliopause. The interstellar magnetic field, frozen into this plasma, then becomes draped around the heliopause in a characteristic manner. We derive the analytical solution for this draped magnetic field in the limit of weak field intensity, assuming an ideal potential flow around the heliopause, which we model as a Rankine half-body. We compare the structure of the model magnetic field with observed properties of the Interstellar Boundary Explorer (IBEX) ribbon and with in situ observations at the Voyager 1 spacecraft. We find reasonable qualitative agreement, given the idealizations of the model. This agreement lends support to the secondary ENA model of the IBEX ribbon and to the interpretation that Voyager 1 has crossed the heliopause. We also predict that the magnetic field measured by Voyager 2 after it crosses the heliopause will not be significantly rotated away from the direction of the undisturbed interstellar field.

Anisotropies in TeV Cosmic Rays Related to the Local Interstellar Magnetic Field from the IBEX Ribbon

The Interstellar Boundary Explorer (IBEX) observes enhanced Energetic Neutral Atoms (ENAs) emission in the keV energy range from a narrow (~20° wide) "ribbon" in the sky that appears to be centered on the direction of the local interstellar (LIS) magnetic field. The Milagro collaboration, the Asγ collaboration and the IceCube observatory have recently made global maps of cosmic ray fluxes in the TeV energy range, revealing anisotropic structures ordered in part by the local interstellar magnetic field and the interstellar flow. This paper following from a recent publication in Science makes the link between these disparate observations by developing a simple model of the magnetic structure surrounding the heliosphere in the Local Interstellar Medium (LISM) that is consistent with both IBEX ENA fluxes and TeV cosmic ray anisotropies. The model also employs the revised velocity direction of the LIC derived from neutral He observations by IBEX. By modeling the propagation of cosmic rays through this magnetic field structure, we specifically show that (1) the large-scale TeV anisotropy provides a roughly consistent orientation for the local interstellar magnetic field at the center of the IBEX Ribbon and corroborates the ~ 3 μG magnitude of the local interstellar magnetic field derived from IBEX observations of the global heliosphere; (2) and small-scale structures in cosmic rays (over < 30° angular scales) are influenced by the interstellar field interaction with the heliosphere at energies < 10 TeV. Thus, we provide a link between IBEX ENA observations, IBEX neutral observations of interstellar He, and TeV cosmic ray anisotropies, which are strongly influenced by the interactions between the local interstellar magnetic field, the flow of the local interstellar plasma, and the global heliosphere.