Local Interstellar Magnetic Field Research Articles

Magnetic Trapping of Galactic Cosmic Rays in the Outer Heliosheath and Their Preferential Entry into the Heliosphere

This paper examines the geometry of interstellar magnetic field lines close to the boundary of the heliosphere in the direction of the unperturbed local interstellar magnetic field, where the field lines are spread apart by the heliopause (HP). Such field parting establishes a region of weaker magnetic field of about 300 au in size in the northern hemisphere that acts as a giant magnetic trap affecting the propagation of galactic cosmic rays (GCRs). The choice of an analytic model of the magnetic field in the very local interstellar medium allows us to qualitatively study the resulting magnetic field draping pattern while avoiding unphysical dissipation across the HP-impeding numerical magnetohydrodynamic (MHD) models. We investigate GCR transport in the region exterior to the heliosphere, including the magnetic trap, subject to guiding center drifts, pitch angle scattering, and perpendicular diffusion. The transport coefficients were derived from Voyager 1 observations of magnetic turbulence in the VLISM. Our results predict a ring current of energetic ions drifting around the interior of the magnetic trap. It is also demonstrated that GCRs cross the HP for the first time preferentially through a crescent-shaped region between the magnetic trap and the upwind direction. The paper includes results of MHD modeling of the heliosphere that provide the coordinates of the center of the magnetic trap in ecliptic coordinates. In addition to the heliosphere, we examine several extreme field draping configurations that could describe the astrospheres of other stars.

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.

Determining the IBEX Ribbon Transverse Profile from ENA Temporal Variations: A Proof of Concept for IMAP Observations

Since its discovery in 2009, the IBEX energetic neutral atom (ENA) Ribbon has been a subject of numerous studies. It appears at energies ∼0.5–6 keV and is most pronounced at ∼1–3 keV. It is almost circular, ∼20°–40° wide, and its center lies near the pristine local interstellar magnetic field direction, whose field lines are draped around the heliosphere. The Ribbon intensity is enhanced above the more diffuse, globally distributed flux (GDF) and varies on timescales that are delayed compared to the underlying and slowly varying GDF. We present a novel method to infer the Ribbon boundaries and transverse profile of the Ribbon using sequential time variations of ENA fluxes, with minimal modeling assumptions involved. The method utilizes the difference in temporal evolution between the total Ribbon content and GDF fluxes. We then use the inferred Ribbon transverse profile to statistically quantify the GDF contribution to the observed peak Ribbon intensity to be ∼32.23% ± 3.15% in 2009–2011. This Ribbon separation method works best during times of gradual changes in solar wind output, and with high angular resolution and ENA counting statistics; results thus provide a proof of concept for the upcoming Interstellar Mapping and Acceleration Probe ENA measurements.

Interpretation of the Spectra and Anisotropy of Galactic Cosmic Rays

Recent measurements of the spectra and anisotropy of cosmic rays (CRs) show a fine structure that reflects the spectral hardenings of CRs nuclei at the rigidity R ∼ 200 GV followed by softenings at R ∼ 10 TV, and reveal complicated energy dependence of the amplitude and phase of anisotropy from 100 GeV to PeV. Numerous studies have shown that the existence of nearby CR sources and a local interstellar magnetic field (LIMF) near the solar system are crucial for such CR spectral and anisotropic patterns. In this work, we analyze the CR spectra of different CR components and the anisotropy considering the nearby Geminga supernova remnants (SNRs) source. In the calculation process, we also introduce the anisotropic diffusion of CRs induced by the LIMF based on the spatial-dependent propagation (SDP) model. As a result, our model can simultaneously account for the CR spectra and the anisotropy from 100 GeV to PeV. Future high-precision measurements of the CR anisotropy, for example, by the LHAASO experiment, would be of the essence in the assessment of our proposed model.

PROPAGATION OF LARGE-SCALE SOLAR WIND EVENTS IN THE OUTER HELIOSPHERE FROM A NUMERICAL MHD SIMULATION

Voyager 1 occasionally detected sudden jumps of the local interstellar magnetic field strength since its heliopause crossing in August 2012. These events were believed to be associated with outward propagating solar wind shocks originating in the inner heliosphere. Here we investigate the correlation between interstellar shocks and large-scale solar wind events by means of numerical MHD simulation. The solar wind is simplified as a symmetric flow near the equatorial plane, and the interstellar neutrals are treated as a constant flow with a fixed density distribution along the upwind direction of the local interstellar medium. The charge exchanges between the solar wind plasma and the interstellar neutrals are taken into account. At a heliocentric distance of 1 AU, the solar wind data from OMNI, STEREO A and B during the period between 2010 and 2017 are used as the inner boundary conditions to drive the simulation. The simulation results showed that the solar wind gradually merges into large-scale structures as the radial distance increases, consistent with observations by New Horizons. After propagating into the inner heliosheath, the shocks are fully developed and the corresponding pressure pulses roughly agree with the observations by Voyager 2 in the inner heliosheath. The arrival of the shocks beyond the heliopause is estimated and found to be consistent with the observed signatures of interstellar shocks by Voyager 1. The possible origins of interstellar shocks in the inner heliosheath are discussed based on the simulation.

The Original Anisotropy of TeV Cosmic Rays in the Local Interstellar Medium

Abstract This paper presents the original anisotropy that TeV cosmic rays (CRs) have in the local interstellar medium. This anisotropy is obtained using a method of flux mapping with the Liouville theorem and a magnetohydrodynamic (MHD) heliosphere model of the electromagnetic field to remove the particle propagation effects hidden in the measurements made by the Tibet ASγ experiment at Earth. The original interstellar anisotropy turns out to be almost a pure dipole, which results from a diffusion flow of CRs escaping along the local interstellar magnetic field into the northern Galactic halo. The observed anisotropy maps at Earth appear quite complex because the heliosphere distorts the dipole anisotropy, generating a significant amount of high-order multipoles, while interstellar magnetic field fluctuations contribute to some weak anisotropy on small angular scales. It is found that the density gradient of these CRs points approximately toward Vela in the Local Bubble, providing experimental evidence to show that the local supernova is making a special contribution to the TeV CRs we see at Earth. This special contribution will keep growing in the future tens of thousand years. The original anisotropy also reveals that the CRs spread from the source primarily along the interstellar magnetic field, while experiencing a nearly isotropic pitch-angle scattering process caused by interstellar turbulence.

Magnitude and direction of the local interstellar magnetic field inferred from Voyager 1 and 2 interstellar data and global heliospheric model

In this Letter, we provide constraints on the direction and magnitude of the pristine (i.e., unperturbed by the interaction with the Sun) local interstellar magnetic field. The constraints are based on analysis of the interstellar magnetic field components at the heliopause measured by magnetometer instruments on board Voyager 1 and 2 spacecraft. The analysis was performed with the help of our kinetic-magnetohydrodynamical model of the global heliosphere. The model shows that the solar-induced disturbances of the interstellar magnetic field are extended relatively far from the Sun up to 400−500 AU. The field is draped around the heliopause and compressed. By comparison of the model results with Voyager data we found that the model provides results comparable with the data for the interstellar magnetic field of BLISM = 3.7−3.8 μG in magnitude and directed towards ≈125° in longitude, and ≈37° in latitude in the heliographic inertial coordinate system.

Parallax of the IBEX Ribbon Indicates a Spatially-Retained Source.

In 2009, the Interstellar Boundary Explorer (IBEX) discovered the existence of a narrow "ribbon" of intense energetic neutral atom (ENA) emission projecting approximately a circle in the sky. It is believed that the ribbon originates from outside of the heliopause in radial directions ( r ) perpendicular to the local interstellar magnetic field (ISMF), B , i.e., B∙ r = 0. Swaczyna et al. (2016a) estimated the distance to the IBEX ribbon via the parallax method comparing the ribbon position observed from the opposite sides of the Sun. They found a parallax angle of 0.41° ± 0.15°, yielding a distance of au to a portion of the ribbon at high ecliptic latitudes. In this study, we demonstrate how the apparent shift of the ribbon in the sky, and thus the apparent distance to the ribbon's source found via the parallax, depends on the transport effects of energetic ions outside the heliopause. We find that the apparent shift of the ribbon based on the "spatial retention" model with ion enhancement near B∙ r = 0, as proposed by Schwadron & McComas (2013), agrees with the parallax of the source region. Parallax is also accurate for a homogeneously-distributed emission source. However, if there is weak pitch angle scattering and ions propagate freely along the ISMF, the apparent shift is significantly smaller than the expected parallax because of the highly anisotropic source. In light of the results from Swaczyna et al. (2016a), our results indicate that the IBEX ribbon source is spatially confined.

Strong Scattering of ~keV Pickup Ions in the Local Interstellar Magnetic Field Draped Around Our Heliosphere: Implications for the IBEX Ribbon's Source and IMAP.

The leading hypothesis for the origin of the Interstellar Boundary Explorer (IBEX) "ribbon" of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the secondary ENA mechanism, whereby neutralized solar wind ions escape the heliosphere and, after several charge-exchange processes, may propagate back toward Earth primarily in directions perpendicular to the local interstellar magnetic field (ISMF). However, the physical processes governing the parent protons outside of the heliopause are still unconstrained. In this study, we compute the "spatial retention" model proposed by Schwadron & McComas (2013) in a 3D simulated heliosphere. In their model, pickup ions outside the heliopause that originate from the neutral solar wind are spatially-retained in a region of space via strong pitch angle scattering before becoming ENAs. We find that the ribbon's intensity and shape can vary greatly depending on the pitch angle scattering rate both inside and outside the spatial retention region, potentially contributing to the globally distributed flux. The draping of the ISMF around the heliopause creates an asymmetry in the average distance to the ribbon's source as well as an asymmetry in the ribbon's shape, i.e., radial cross section of ENA flux through the circular ribbon. The spatial retention model adds an additional asymmetry to the ribbon's shape due to the enhancement of ions in the retention region close to the heliopause. Finally, we demonstrate how the ribbon's structure observed at 1 au is affected by different instrument capabilities, and how the Interstellar Mapping and Acceleration Probe (IMAP) may observe the ribbon.

All-sky Measurement of the Anisotropy of Cosmic Rays at 10 TeV and Mapping of the Local Interstellar Magnetic Field

Abstract We present the first full-sky analysis of the cosmic ray arrival direction distribution with data collected by the High-Altitude Water Cherenkov and IceCube observatories in the northern and southern hemispheres at the same median primary particle energy of 10 TeV. The combined sky map and angular power spectrum largely eliminate biases that result from partial sky coverage and present a key to probe into the propagation properties of TeV cosmic rays through our local interstellar medium and the interaction between the interstellar and heliospheric magnetic fields. From the map, we determine the horizontal dipole components of the anisotropy δ 0h = 9.16 × 10−4 and δ 6h = 7.25 × 10−4 (±0.04 × 10−4). In addition, we infer the direction (229.°2 ± 3.°5 R.A., 11.°4 ± 3.°0 decl.) of the interstellar magnetic field from the boundary between large-scale excess and deficit regions from which we estimate the missing corresponding vertical dipole component of the large-scale anisotropy to be .

Time Dependence of the IBEX Ribbon and the Globally Distributed Energetic Neutral Atom Flux Using the First 9 Years of Observations

Abstract Observations from the Interstellar Boundary Explorer (IBEX) of energetic neutral atoms (ENAs) reveal two populations, those emitted from a narrow (∼20°–40°) ribbon that is centered on the local interstellar magnetic field, and a globally distributed flux (GDF) that is controlled by processes in the heliosheath. This is a third study utilizing a previously developed technique to separate ENA emissions in the ribbon from the GDF. In the first ribbon separation study, we analyzed the first year of IBEX data at the energies of 0.7 keV and above; the second study analyzed data down to 0.2 keV using the first five years of IBEX data. Here, we utilize the separation analysis from 0.7 keV and above to study time evolution in 3 year intervals over the first nine years of IBEX data. This study is the first to reveal the global time evolution of the GDF distinct from that of the IBEX ribbon. We show that the time evolution of the GDF within 40° of the upwind pressure maximum is driven by changes in the solar wind ram pressure through compression and rarefaction in the heliosheath. In contrast, the GDF is relatively stable in the region centered on the heliotail downwind with respect to the interstellar flow. The evolution of the IBEX ribbon is observed to have a time lag with respect to the upwind GDF evolution, likely due to the secondary (neutral) solar wind source. The time lag observed in the ribbon evolution is consistent with the generation of ions retained for several years beyond the heliopause. These observations lend further support to secondary solar wind models of the IBEX ribbon, but also require that there is a significant several year time lag for reneutralization of ions that form the IBEX ribbon. We use this study of the 9 year separation of the IBEX ribbon from the globally distributed flux to prepare for a formal IBEX data release of ribbon and globally distributed flux maps to the heliophysics community.

The Local Interstellar Magnetic Field Observed by Voyager 1 and IBEX

Observations by the Interstellar Boundary Explorer (IBEX) have revealed the presence of the IBEX Ribbon, an almost circular band of increased ENA emissions that is believed to be centered on the direction of the interstellar magnetic field. IBEX measures a range of interstellar species including H, He, and O. IBEX has been able to distinguish primary He coming directly from the interstellar medium from secondary He that is created through neutralization of He+ atoms in the heliosheath. Together with the observations of Lyman-α radiation from SOHO/SWAN, which is resonantly absorbed and re-emitted from interstellar H atoms that move into the heliosphere, the directions of interstellar flow for each of the observed interstellar species (H, primary He, secondary He, and primary O) line-up along a symmetry plane — the interstellar B–V plane — that should contain the interstellar magnetic field direction (B) and the interstellar flow direction (V). The B–V plane also contains the center of the IBEX Ribbon, which supports the concept that the Ribbon center is close to the direction of the interstellar magnetic field. Further, the deflection of the various interstellar species along the B–V plane are consistent with the compression of the heliosheath by the interstellar magnetic field pressure. Lastly, the interstellar magnetic field in the outer heliosheath is directly observed by Voyager 1 (V1). The direction of the interstellar magnetic field observed by V1 is offset from the Ribbon center, consistent with draping around the heliopause, and time variations in the interstellar magnetic field are consistent with the effects of large-scale compression and rarefaction in the outer heliosheath. Thus, we summarize the array of observations that suggest a consistent direction of the interstellar magnetic field, and the effects of the interstellar magnetic field on the structure and time variations in the heliosheath.

The Role of Pickup Ion Dynamics Outside of the Heliopause in the Limit of Weak Pitch Angle Scattering: Implications for the Source of the IBEX Ribbon.

We present a new model of the Interstellar Boundary Explorer (IBEX) ribbon based on the secondary energetic neutral atom (ENA) mechanism, under the assumption that there is negligible pitch angle scattering of pickup ions (PUIs) outside the heliopause. Using the results of an MHD-plasma/kinetic-neutral simulation of the heliosphere, we generate PUIs in the outer heliosheath, solve their transport using guiding center theory, and compute ribbon ENA fluxes at 1 AU. We implement several aspects of the PUI dynamics, including (1) parallel motion along the local interstellar magnetic field (ISMF), (2) advective transport with the interstellar plasma, (3) the mirror force acting on PUIs propagating along the ISMF, and (4) betatron acceleration of PUIs as they are advected within an increasing magnetic field towards the heliopause. We find that ENA fluxes at 1 AU are reduced when PUIs are allowed to move along the ISMF, and ENA fluxes are reduced even more by the inclusion of the mirror force, which pushes particles away from IBEX lines-of-sight. Inclusion of advection and betatron acceleration do not result in any significant change in the ribbon. Interestingly, the mirror force reduces the ENA fluxes from the inner edge of the ribbon more than its outer edge, effectively reducing the ribbon's width by ∼6° and increasing its radius projected on the sky. This is caused by the asymmetric draping of the ISMF around the heliopause, such that ENAs from the ribbon's inner edge originate closer to the heliopause, where the mirror force is strongest.

Effects of Solar Activity on the Local Interstellar Magnetic Field Observed by Voyager 1 and IBEX

Abstract Interstellar Boundary Explorer (IBEX) observations have shown enhanced energetic neutral atom emission from a narrow, circular ribbon likely centered on the direction of the magnetic field in the local interstellar medium (LISM). IBEX observations are compared to Voyager 1's direct observations of the magnetic field in the LISM, which reveals structured changes that are shown here to be driven by pressure fluctuations transmitted across the heliopause boundary separating the LISM from the shocked solar wind beyond the termination shock. The changes in the local interstellar magnetic field observed by Voyager 1 occurred in 2015 and 2016, when the most powerful coronal mass ejections released from the Sun over the last 11-year solar cycle arrived in the outer heliosphere. These events propagated over more than two years through the solar system, ultimately pounding the heliopause and causing transient deviations of the magnetic fields detected by Voyager 1 in the LISM. Thus, we identify the response in pressure variations at Voyager 1 in the LISM with the activity of major solar events several years earlier, during the most active period of the last solar maximum.

An Improved Analytical Model of the Local Interstellar Magnetic Field: The Extension to Compressibility

Abstract A previously published analytical magnetohydrodynamic model for the local interstellar magnetic field in the vicinity of the heliopause (Röken et al. 2015) is extended from incompressible to compressible, yet predominantly subsonic flow, considering both isothermal and adiabatic equations of state. Exact expressions and suitable approximations for the density and the flow velocity are derived and discussed. In addition to the stationary induction equation, these expressions also satisfy the momentum balance equation along stream lines. The practical usefulness of the corresponding, still exact, analytical magnetic field solution is assessed by comparing it quantitatively to results from a fully self-consistent magnetohydrodynamic simulation of the interstellar magnetic field draping around the heliopause.

Following the interstellar magnetic field from the heliosphere into space with polarized starlight

Starlight linearly polarized by aligned interstellar dust grains provides the necessary data for tracing the structure of the very local interstellar magnetic field (ISMF). Two methods have been developed to recover the ISMF direction from polarized starlight, using data from an ongoing polarization survey. Both methods rely on the probability distribution function for polarized light. Method 1 calculates the ISMF direction from polarization position angles regardless of the data accuracy, while Method 2 relies on high-probability data points. The ISMF direction Bibex recovered by Method 1 corresponds to the closest ISMF to the heliosphere, traced by the center of the IBEX Ribbon arc. Method 2 reveals a new direction for the more distant ISMF, Bnew, toward l=41.1° ± 4.1° and b= 25.8° ± 3.0°, which differs by 30.4° ± 5.6° from the IBEX ISMF direction. Polarizations of filament stars that are located within 25° of a pole of Bnew, where background polarizations would be minimal, show the highest statistical probabilities of tracing the filament ISMF. The IBEX ISMF direction orders the kinematics of interstellar clouds within 15 pc, and Bnew must therefore dominate beyond 15 pc. These new data are consistent with the location of the Sun in the rim of an expanding superbubble shell associated with the evolved Loop I superbubble.

The Local Interstellar Magnetic Field and the IBEX Ribbon

It is well known that the Interstellar Boundary Explorer (IBEX) discovered a region of enhanced energetic neutral atom (ENA) emission seen in all-sky maps as a ribbon. The enhanced fluxes of ENAs were between 2 and 3 times greater than adjacent regions of the sky. The ribbon itself was not predicted by any models of the heliosphere interface. In the paper by Ratkiewicz, Strumik and Grygorczuk published in the Astrophysical Journal, 756:3, 2012 the authors presented some arguments to show that it was possible to predict the IBEX ribbon. In the current paper the numerical results obtained by three-dimensional MHD simulations of the interaction between the solar wind and interstellar medium by Ratkiewicz et al. [2012] are summarized. Some new aspects of this issue are discussed.

The Heliosphere as Seen in TeV Cosmic Rays

Measurements from several cosmic-ray air shower experiments reveal that the anisotropy of TeV cosmic-ray flux does not agree with a dipole pattern commonly expected from the Compton-Getting effect or from the diffusion of cosmic rays in Galactic magnetic fields. TeV cosmic-ray anisotropy maps often show fine features, some of which are slightly time-dependent. Because the size of the heliosphere is larger than the gyroradius of TeV cosmic rays in the interstellar magnetic field, the electric and magnetic fields of the heliosphere may distort the pattern of cosmic-ray anisotropy that one would see in the local interstellar medium without the presence of the heliosphere. We have developed a method of mapping cosmic-ray anisotropy using Liouville's theorem. In this paper, we show how to use cosmic-ray anisotropy features to determine the direction of the local interstellar magnetic field, the hydrogen deflection plane, the size and shape of the heliotail, and the geometry of the heliosphere bow wave.

THE ENERGY-DEPENDENT POSITION OF THE IBEX RIBBON DUE TO THE SOLAR WIND STRUCTURE

ABSTRACT Observations of energetic neutral atoms (ENAs) allow for remote studies of the condition of plasma in the heliosphere and the neighboring local interstellar medium. The first results from the Interstellar Boundary Explorer (IBEX) revealed an arc-like enhancement of the ENA intensity in the sky, known as the ribbon. The ribbon was not expected from the heliospheric models prior to the launch of IBEX. One proposed explanation for the ribbon is the mechanism of secondary ENA emission. The ribbon reveals energy-dependent structure in the relative intensity along its circumference and in its position. That is, the geometric center of the ribbon varies systematically by about 10° in the energy range 0.7–4.3 keV. Here, we show by analytical modeling that this effect is a consequence of the helio-latitudinal structure of the solar wind reflected in the secondary ENAs. Along with a recently measured distance to the ribbon’s source just beyond the heliopause, our findings support the connection of the ribbon with the local interstellar magnetic field by the mechanism of secondary ENA emission. However, the magnitude of the center shift in the highest IBEX energy channel is much larger in the observations than expected from the modeling. This may be due to another, not currently recognized, process of ENA generation.

INTERSTELLAR NEUTRAL HELIUM IN THE HELIOSPHERE FROM IBEX OBSERVATIONS. IV. FLOW VECTOR, MACH NUMBER, AND ABUNDANCE OF THE WARM BREEZE

ABSTRACT Following the high-precision determination of the velocity vector and temperature of the pristine interstellar neutral (ISN) He via a coordinated analysis summarized by McComas et al., we analyzed the Interstellar Boundary Explorer (IBEX) observations of neutral He left out from this analysis. These observations were collected during the ISN observation seasons 2010–2014 and cover the region in the Earth's orbit where the Warm Breeze (WB) persists. We used the same simulation model and a parameter fitting method very similar to that used for the analysis of ISN He. We approximated the parent population of the WB in front of the heliosphere with a homogeneous Maxwell–Boltzmann distribution function and found a temperature of ∼9500 K, an inflow speed of 11.3 km s−1, and an inflow longitude and latitude in the J2000 ecliptic coordinates 251.°6, 12.°0. The abundance of the WB relative to ISN He is 5.7% and the Mach number is 1.97. The newly determined inflow direction of the WB, the inflow directions of ISN H and ISN He, and the direction to the center of the IBEX Ribbon are almost perfectly co-planar, and this plane coincides within relatively narrow statistical uncertainties with the plane fitted only to the inflow directions of ISN He, ISN H, and the WB. This co-planarity lends support to the hypothesis that the WB is the secondary population of ISN He and that the center of the Ribbon coincides with the direction of the local interstellar magnetic field (ISMF). The common plane for the direction of the inflow of ISN gas, ISN H, the WB, and the local ISMF is given by the normal direction: ecliptic longitude 349.°7 ± 0.°6 and latitude 35.°7 ± 0.6 in the J2000 coordinates, with a correlation coefficient of 0.85.