Cross-field transport and pitch-angle anisotropy of solar energetic particles in MHD turbulence

Recent modelling of solar energetic particles (SEPs) propagation through the heliospheric turbulence, also discussed in this workshop, has investigated the role of the pitch-angle scattering and the perpendicular transport in spreading particles in heliolongitude, as shown by multi-spacecraft measurements (STEREO A/B, ACE, SOHO, etc.) at 1 AU in various energy ranges. In some events the first-order pitch-angle anisotropy of the particles distribution is not-negligible. We calculate the average perpendicular displacement due to the gradient/curvature drift in an inhomogeneous turbulence accounting for pitch-angle dependence for two MHD turbulence models: (a) 3-D isotropic, (b) anisotropic as conjectured by Goldreich-Sridhar. We find in both cases that the drift scales as (1 − μ2)2 with the cosine of pitch-angle μ, in contrast with previous models for transport of SEPs. This result can impact the models of propagation of SEPs through the heliosphere.

Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays

It is generally believed that the cosmic ray spectrum below the knee is of Galactic origin, although the exact sources making up the entire cosmic ray energy budget are still unknown. Including effects of magnetic amplification, Supernova Remnants (SNR) could be capable of accelerating cosmic rays up to a few PeV and they represent the only source class with a sufficient non-thermal energy budget to explain the cosmic ray spectrum up to the knee. Now, gamma-ray measurements of SNRs for the first time allow to derive the cosmic ray spectrum at the source, giving us a first idea of the concrete, possible individual contributions to the total cosmic ray spectrum. In this contribution, we use these features as input parameters for propagating cosmic rays from its origin to Earth using GALPROP in order to investigate if these supernova remnants reproduce the cosmic ray spectrum and if supernova remnants in general can be responsible for the observed energy budget.

A solution to the cosmic ray anisotropy problem

Observations of the cosmic ray (CR) anisotropy are widely advertised as a means of finding nearby sources. This idea has recently gained currency after the discovery of a rise in the positron fraction and is the goal of current experimental efforts, e.g., with AMS-02 on the International Space Station. Yet, even the anisotropy observed for hadronic CRs is not understood, in the sense that isotropic diffusion models overpredict the dipole anisotropy in the TeV–PeV range by almost two orders of magnitude. Here, we consider two additional effects normally not considered in isotropic diffusion models: anisotropic diffusion due to the presence of a background magnetic field and intermittency effects of the turbulent magnetic fields. We numerically explore these effect by tracking test-particles through individual realisations of the turbulent field. We conclude that a large misalignment between the CR gradient and the background field can explain the observed low level of anisotropy.

The power spectrum of cosmic ray arrival directions

Various experiments show that the arrival directions of multi-TeV cosmic rays show significant anisotropies at small angular scales. It was recently argued that this small scale structure may arise naturally by cosmic ray diffusion in a large-scale cosmic ray gradient in combination with deflections in local turbulent magnetic fields. We show via analytical and numerical methods that the non-trivial power spectrum in this setup is a direct consequence of Liouville's theorem and can be related to properties of relative diffusion.

Cosmic ray transport and anisotropies to high energies

A model is introduced, in which the irregularity spectrum of the Galactic magnetic field beyond the dissipation length scale is first a Kolmogorov spectrum k-5/3 at small scales λ = 2 π/k with k the wave-number, then a saturation spectrum k-1, and finally a shock-dominated spectrum k-2 mostly in the halo/wind outside the Cosmic Ray disk. In an isotropic approximation such a model is consistent with the Interstellar Medium (ISM) data. With this model we discuss the Galactic Cosmic Ray (GCR) spectrum, as well as the extragalactic Ultra High Energy Cosmic Rays (UHECRs), their chemical abundances and anisotropies. UHECRs may include a proton component from many radio galaxies integrated over vast distances, visible already below 3 EeV.

The large-scale anisotropy with the PAMELA calorimeter

The large-scale anisotropy (or the so-called star-diurnal wave) has been studied using the calorimeter of the space-born experiment PAMELA. The cosmic ray anisotropy has been obtained for the Southern and Northern hemispheres simultaneously in the equatorial coordinate system for the time period 2006–2014. The dipole amplitude and phase have been measured for energies 1–20 TeV n-1.

The cosmic ray anisotropy below 10¹⁵ eV

The measurement of the anisotropy in the cosmic ray (CR) arrival direction distribution provides important informations on the propagation mechanisms and on the identification of their sources. In the last decade the anisotropy came back to the attention of the scientific community, thanks to several new two-dimensional representations of the CR arrival direction distribution which clearly showed the existence of anisotropies at different angular scales in both hemispheres. The origin of the observed anisotropies is still unknown. So far, no theory of CRs in the Galaxy exists yet to explain the observations leaving the standard model of CRs and that of the local magnetic field unchanged at the same time. In this paper the observations of Galactic CR anisotropy will be briefly summarized, with particular attention to the results obtained by the ARGO-YBJ experiment in the Northern Hemisphere.

Diffuse synchrotron emission from galactic cosmic ray electrons

Synchrotron diffuse radiation (SDR) emission is one of the major Galactic components, in the 100 MHz up to 100 GHz frequency range. Its spectrum and sky map provide valuable measure of the galactic cosmic ray electrons (GCRE) in the relevant energy range, as well as of the strength and structure of the Galactic magnetic fields (GMF), both regular and random ones. This emission is an astrophysical sky foreground for the study of the Cosmic Microwave Background (CMB), and the extragalactic microwave measurements, and it needs to be modelled as better as possible. In this regard, in order to get an accurate description of the SDR in the Galaxy, we use – for the first time in this context – 3-dimensional GCRE models obtained by running the DRAGON code. This allows us to account for a realistic spiral arm pattern of the source distribution, demanded to get a self-consistent treatment of all relevant energy losses influencing the final synchrotron spectrum.

Search for a positron anisotropy with PAMELA experiment

The PAMELA experiment has been collecting data since 2006; its results indicate a rise in the positron fraction with respect to the sum of electrons and positrons in the cosmic-ray (CR) spectrum above 10 GeV. This excess can be due to additional sources, as SNRs or pulsars, which can lead to an anisotropy in the local CR positron, detectable from current experiments. We report on the analysis on spatial distributions of positron events collected by PAMELA, taking into account also the geomagnetic field effects. No significant deviation from the isotropy has been observed.

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

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 direction 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 observations. 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.