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A Consistent Scenario for the IBEX Ribbon, Anisotropies in Tev Cosmic Rays, and the Local Interstellar Medium

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A Consistent Scenario for the IBEX Ribbon, Anisotropies in Tev Cosmic Rays, and the Local Interstellar Medium ASTRA Proc., 2, 9–16, 2015 www.astra-proceedings.net/2/9/2015/ doi:10.5194/ap-2-9-2015 © Author(s) 2015. CC Attribution 3.0 License. A Consistent Scenario for the IBEX Ribbon, Anisotropies in TeV Cosmic Rays, and the Local Interstellar Medium N. A. Schwadron1,2, P. Frisch3, F. C. Adams4, E. R. Christian5, P. Desiati6, H. O. Funsten7, J. R. Jokipii8, D. J. McComas2,10, E. Moebius1, and G. Zank9 1University of New Hampshire, Durham, NH, 03824, UK 2Southwest Research Institute, San Antonio, TX 78228, USA 3University of Chicago, Department of Astronomy and Astrophysics, Chicago, IL 60637, USA 4University of Michigan, Ann Arbor, MI, 48109, USA 5Goddard Space Flight Center, Greenbelt, MD, 20771 6IceCube Research Center and Astronomy Department, University of Wisconsin, Madison, WI 53706, USA 7Los Alamos National Laboratory, Los Alamos, NM, 87545, USA 8University of Arizona, Tucson, AX 85721, USA 9University of Alabama, Huntsville, AL, Huntsville, AL 35899, USA 10University of Texas at San Antonio, San Antonio, TX, USA Correspondence to: N. A. Schwadron ([email protected]) Received: 2 April 2015 – Revised: 28 July 2015 – Accepted: 24 August 2015 – Published: 8 September 2015 Abstract. The Interstellar Boundary Explorer (IBEX) observes enhanced ∼ keV energy Energetic Neutral Atoms (ENAs) from a narrow “ribbon” that stretches across the sky and appears to be centered on the direc- tion of the local interstellar magnetic field. The Milagro collaboration, the Asγ collaboration and the IceCube observatory have made global maps of TeV cosmic rays. This paper provides links between these disparate ob- servations. We develop a simple diffusive model of the propagation of cosmic rays and the associated cosmic ray anisotropy due to cosmic ray streaming against the local interstellar flow. We show that the local plasma and field conditions sampled by IBEX provide characteristics that consistently explain TeV cosmic ray anisotropies. These results support models that place the interstellar magnetic field direction near the center of the IBEX ribbon. 1 Introduction ergy atoms from the interstellar medium including oxygen, helium and hydrogen atoms (Möbius et al., 2009). The IBEX maps revealed the existence of a narrow ribbon The Interstellar Boundary Explorer Mission (IBEX), of higher flux ENA emissions (McComas et al., 2009a; Fun- launched October 2008, has the objective to discover the sten et al., 2009; Fuselier et al., 2009). The ribbon forms a global interaction between the solar wind and the local in- roughly circular structure centered on (gal. long `, gal. lat. b) ∼ ◦ ± ◦ − ◦± terstellar medium (McComas et al., 2009b). IBEX measures (210:5 2:6 , 57.1 1.0), with the center of the rib- ∼ ◦ from ∼ 0.01 to ∼ 6 keV neutral atoms. In the ∼ keV en- bon arc varying by 11 over the measured energy range ergy range these particles, dubbed energetic neutral atoms (Funsten et al., 2013). The center of the IBEX ribbon likely (ENAs), are created predominantly from charge-exchange defines the direction of the local interstellar (LISM) mag- between neutral interstellar hydrogen and plasma protons netic field (Schwadron et al., 2009). However, until the phys- in the solar wind and interstellar medium (McComas et al., ical mechanism for the ribbon is identified, it will be diffi- 2009b, 2011). IBEX also makes measurements of lower en- cult to know how the interstellar field direction is related in Published by Copernicus Publications. 10 N. A. Schwadron et al.: A Scenario for TeV Cosmic Ray Anisotropies, the IBEX Ribbon, and the LISM detail to the IBEX ribbon. A number of heliospheric mod- imately degenerate. However, analysis resulting in a spe- els have been devised with the property that the ribbon may cific solution was associated with large uncertainties along be highly sensitive to the interstellar magnetic field direc- the parameter tube. The more recent IBEX analyses have tion (Heerikhuisen et al., 2010, 2014; Frisch et al., 2010; taken into account a much larger observational baseline, al- Ratkiewicz et al., 2012; Schwadron and McComas, 2013). lowing derivation of specific solutions. In the Heliocentric For the purposes of this paper, we take the association of the coordinates, Schwadron et al.(2015) found an ISN flow IBEX ribbon center with the interstellar magnetic field direc- gal. long. ` D 183:7◦ ±0:8◦, gal. lat. b D −15:0◦ ±1:5◦, and tion as a partially-supported conjecture, and attempt to build speed 25:4 ± 1:1 kms−1. This yields an angle of 80:4◦ ± 5◦ a consistent scenario for the TeV cosmic ray anisotropies. between the magnetic field and the velocity expressed with Further, this study supplies a test for the association be- respect to the LSR velocity frame that describes our galactic tween the IBEX ribbon and the interstellar magnetic field. neighborhood. This angle is 46:6◦ ±2:5◦ for velocities given This paper also discusses another observational signature of with respect to the Sun. this LISM magnetic field in TeV cosmic ray anisotropies This yields an angle between that magnetic field and the (Schwadron et al., 2014). flow velocity in the LSR frame of 80:4◦ ± 5◦. This angle is There are numerous studies that complement the analysis 46:6◦ ± 2:5◦ in the Heliocentric reference frame. presented here. While Schwadron et al.(2014) studied the global TeV anisotropy, there are also features in TeV cosmic ray maps that may be directly related to heliospheric struc- 3 Local Interstellar Structure ture. For example, Desiati and Lazarian(2013) considered the scenario where heliospheric magnetic instabilities (see The heliosphere interacts with a clumpy decelerating inter- Pogorelov et al., 2013) cause resonant scattering which may stellar flow that moves away from the Lower Centaurus-Crux re-distribute the anisotropic cosmic rays in the 10’s TeV en- (LCC) association near the origin of the Loop I superbub- ergy scale. Such scattering processes may produce a complex ble (Frisch et al., 2011). The Sun is likely located within angular structure in the arrival direction distribution of cos- or near the Local Interstellar Cloud (LIC), less than 0.1 pc mic rays, that trace the heliospheric structure. Another com- from the LIC edge (Wood et al., 2000; Frisch and Mueller, plementary analysis is presented by Effenberger et al.(2012), 2011). In the direction facing away from the galactic center, which examines Galactic transport models for cosmic rays the closest cloud to the LIC is the “Blue Cloud” (BC, Lalle- involving the diffusive motion of these particles in the inter- ment et al., 1994; Hébrard et al., 1999). The conditions for stellar medium. Because of the large-scale structure of the colliding clouds are observed within ∼ 10 pc (Linsky et al., Galactic magnetic field, cosmic ray diffusion is found to be 2008), and relative intercloud velocities are observed up to highly anisotropic. As a result, significant variability is found 50 kms−1. Gry and Jenkins(2015) model the LIC kinematics in the energy spectra at different positions along the Sun’s by omitting outlying velocity components, and conclude that Galactic orbit. Locally, polarized starlight and the center of a shock front is driving into the cloud from the rough location the IBEX ribbon arc trace the same interstellar field direc- of ` D 124◦, b D −67◦ (or RA, DEC D 13◦, −4◦). Figure 1 tions to within uncertainties, suggesting that the ordering of provides a rough picture of local interstellar structure near the interstellar field persists over much larger spatial scales the edge of LIC and at the front of the Loop I superbubble. than that of the heliosphere (Frisch et al., 2015). These results An interstellar magnetic field entrained in the inhomo- highlight the need to account for anisotropic diffusion and the geneous interstellar material associated with the expanding need for accurate Galactic magnetic field models. The work Loop I shell provides an explanation for inhomogeneities presented here offers new insights into local structure of the in the local interstellar magnetic field. Measurements by magnetic field in the interstellar medium. IBEX and Ulysses of interstellar He inside of the heliosphere provide the LIC velocity (e.g., McComas et al., 2015b,a; Schwadron et al., 2015; Bzowski et al., 2015; Leonard et al., 2 Interstellar flow and field characteristics 2015; Möbius et al., 2015) and velocities for 15 local clouds have been determined with the triangulation of the radial ve- Our understanding of the characteristics of the local inter- locity component towards different stars behind the clouds stellar medium (LISM) depend critically on a limited set of (Redfield and Linsky, 2008). The Loop I superbubble shell observations. Very recent analysis of Interstellar Boundary is centered ≈ 78 pc at galactic coordinates of `, b ≈ 346◦, Explorer observations has allowed us to update estimates of 3◦ (Wolleben, 2007). Pressure equilibrium between the gas LISM flow using observations of interstellar neutral (ISN) and magnetic field in the LIC yield magnetic field strength He (e.g., McComas et al., 2015b,a; Schwadron et al., 2015; of ∼ 2:7 µG (Slavin and Frisch, 2008). This value is similar Bzowski et al., 2015; Leonard et al., 2015; Möbius et al., to the ∼ 3 µG field strength derived from the globally dis- 2015). A feature of the IBEX data is that the LIC physi- tributed ENA fluxes (Schwadron et al., 2011). cal characteristics are determined from a narrow tube in pa- The IBEX magnetic field direction should be apparent in rameter space where the interstellar parameters are approx- the light of nearby stars that is linearly polarized in the inter- ASTRA Proc., 2, 9–16, 2015 www.astra-proceedings.net/2/9/2015/ N. A. Schwadron et al.: A Scenario for TeV Cosmic Ray Anisotropies, the IBEX Ribbon, and the LISM 11 4 Source of Large-scale anisotropy in TeV cosmic rays We consider a TeV cosmic ray model for the large-scale anisotropy associated with streaming of cosmic rays against the LIC plasma flow.
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