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Register a new userThe sidereal anisotropy of multi-TeV cosmic rays in an expanding Local Interstellar Cloud
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The sidereal anisotropy of galactic cosmic ray (GCR) intensity observed with the Tibet Air Shower (AS) experiment still awaits theoretical interpretation. The observed global feature of the anisotropy is well reproduced by a superposition of the bi-directional and uni-directional flows (BDF and UDF, respectively) of GCRs. If the orientation of the deduced BDF represents the orientation of the local interstellar magnetic field (LISMF), as indicated by best-fitting a model to the data, the UDF deviating from the BDF orientation implies a significant contribution from the streaming perpendicular to the LISMF. This perpendicular streaming is probably due to the drift anisotropy, because the contribution from the perpendicular diffusion is expected to be much smaller than the drift effect. The large amplitude deduced for the UDF indicates a large spatial gradient of the GCR density. We suggest that such a density gradient can be expected at the heliosphere sitting close to the boundary of the Local Interstellar Cloud (LIC), if the LIC is expanding. The spatial distribution of GCR density in the LIC reaches a stationary state because of the balance between the inward cross-field diffusion and the adiabatic cooling due to the expansion. We derive the steady-state distribution of GCR density in the LIC based on radial transport of GCRs in a spherical LIC expanding at a constant rate. By comparing the expected gradient with the observation by Tibet experiment, we estimate the perpendicular diffusion coefficient of multi-TeV GCRs in the local interstellar space.
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We develop a model anisotropy best-fitting to the two-dimensional sky-map of multi-TeV galactic cosmic ray (GCR) intensity observed with the Tibet III air shower (AS) array. By incorporating a pair of intensity excesses in the hydrogen deflection plane (HDP) suggested by Gurnett et al., together with the uni-directional and bi-directional flows for reproducing the observed global feature, this model successfully reproduces the observed sky-map including the skewed feature of the excess intensity from the heliotail direction, whose physical origin has long remained unknown. These additional excesses are modeled by a pair of the northern and southern Gaussian distributions, each placed ~50 degree away from the heliotail direction. The amplitude of the southern excess is as large as ~0.2 %, more than twice the amplitude of the northern excess. This implies that the Tibet AS experiment discovered for the first time a clear evidence of the significant modulation of GCR intensity in the heliotail and the asymmetric heliosphere.
We present the first full-sky analysis of the cosmic ray arrival direction distribution with data collected by the HAWC 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 holds 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 $delta_{0h} = 9.16 times 10^{-4}$ and $delta_{6h} = 7.25 times 10^{-4}~(pm0.04 times 10^{-4})$. In addition, we infer the direction ($229.2pm 3.5^circ$ RA , $11.4pm 3.0^circ$ Dec.) 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 $delta_N sim -3.97 ^{+1.0}_{-2.0} times 10^{-4}$.
In our recent work, we build a propagation scenario to simultaneously explain the spectra and anisotropy of cosmic rays (CRs) by considering spatially dependent propagation (SDP) model and nearby Geminga supernova remnant (SNR) source. But the phase of anisotropy is still inconsistent with the experimental data. Recent observations of CR anisotropy show that the phase is consistent with local regular magnetic field (LRMF) observed by Interstellar Boundary Explorer (IBEX) below 100 TeV, which indicates that diffusion along LRMF is important. In this work, we further introduce the LRMF and take into account the effect of corresponding anisotropic diffusion to explain the anisotropy of CRs. We find that when the diffusion coefficient perpendicular to the LRMF is much smaller than the parallel one, the phase of anisotropy points to $sim R.A.= 3^h$, which accords with experimental observation below 100 TeV. We also analyze the influence of the ratio of perpendicular and parallel diffusion coefficient on the anisotropy and the energy dependence of the ratio. The results illustrate that with the decrease of perpendicular diffusion, the anisotropic phase changes from the direction of nearby source to the LRMF below 100 TeV, meanwhile it changes from the galactic center (GC) to opposite direction of LRMF above 100 TeV. When the perpendicular diffusion coefficient grows faster than the parallel one with energy, the diffusion approaches to the isotropic at high energy, the phase of anisotropy shifts from the LRMF to the GC above 100 TeV. This could be helpful to ascertain the energy dependence of diffusion coefficients.
In this paper we report the first observation in the Southern hemisphere of an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This measurement was performed using cosmic ray induced muons recorded by the partially deployed IceCube observatory between May 2009 and May 2010. The data include a total of 33$times 10^{9}$ muon events with a median angular resolution of $sim3^{circ}$ degrees. A sky map of the relative intensity in arrival direction over the Southern celestial sky is presented for cosmic ray median energies of 20 and 400 TeV. The same large-scale anisotropy observed at median energies around 20 TeV is not present at 400 TeV. Instead, the high energy skymap shows a different anisotropy structure including a deficit with a post-trial significance of -6.3$sigma$. This anisotropy reveals a new feature of the Galactic cosmic ray distribution, which must be incorporated into theories of the origin and propagation of cosmic rays.
After two years of operation, the High-Altitude Water Cherenkov (HAWC) Observatory has analyzed the TeV cosmic-ray sky over an energy range between $2.0$ and $72.8$ TeV. The HAWC detector is a ground-based air-shower array located at high altitude in the state of Puebla, Mexico. Using 300 light-tight water tanks, it collects the Cherenkov light from the particles of extensive air showers from primary gamma rays and cosmic rays. This detection method allows for uninterrupted observation of the entire overhead sky (2~sr instantaneous, 8.5~sr integrated) in the energy range from a few TeV to hundreds of TeV. Like other detectors in the northern and southern hemisphere, HAWC observes an energy-dependent anisotropy in the arrival direction distribution of cosmic rays. The observed cosmic-ray anisotropy is dominated by a dipole moment with phase $alphaapprox40^{circ}$ and amplitude that slowly rises in relative intensity from $8times10^{-4}$ at 2 TeV to $14times10^{-4}$ around 30.3 TeV, above which the dipole decreases in strength. A significant large-scale ($>60^{circ}$ in angular extent) signal is also observed in the quadrupole and octupole moments, and significant small-scale features are also present, with locations and shapes consistent with previous observations. Compared to previous measurements in this energy range, the HAWC cosmic-ray sky maps improve on the energy resolution and fit precision of the anisotropy. These data can be used in an effort to better constrain local cosmic-ray accelerators and the intervening magnetic fields.
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