Cosmic ray small-scale anisotropies in quasi-linear theory

Mertsch, Philipp; Ahlers, M.

doi:10.1088/1475-7516/2019/11/048

Cited by 11 publications

(21 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We follow the steps outlined in Ref. [6]. For further details we refer to this paper and references therein.…”

Section: Analytical Calculationmentioning

confidence: 99%

See 1 more Smart Citation

Cosmic Ray Small-Scale Anisotropies in Slab Turbulence

Kuhlen¹,

Mertsch²

2021

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

View full text Add to dashboard Cite

In the standard picture of cosmic ray transport the propagation of charged cosmic rays through turbulent magnetic fields is described as a random walk with cosmic rays scattering on magnetic field turbulence. This is in good agreement with the highly isotropic cosmic ray arrival directions as this diffusion process effectively isotropizes the cosmic ray distribution. High-statistics observatories like IceCube and HAWC have however observed significant deviations from isotropy down to very small angular scales. This is in strong tension with this standard picture of cosmic ray propagation. While large scale multipoles arise naturally, for example due to the earth's motion relative to the isotropic cosmic ray distribution, there is no intuitive mechanism to account for the observed anisotropies at smaller angular scales. By relaxing one of the standard assumptions of quasi linear theory and treating correlations between fluxes of cosmic rays from different directions explicitly we show that higher multipoles also are to be expected from particle propagation through turbulent magnetic fields. We present a first analytical calculation of the angular power spectrum assuming a physically motivated model of the magnetic field turbulence and find good agreement with numerical simulations.

show abstract

“…We follow the steps outlined in Ref. [6]. For further details we refer to this paper and references therein.…”

Section: Analytical Calculationmentioning

confidence: 99%

“…The authors of Ref. [6] take into account these correlations and put forward a model to predict the APS based on a perturbative expansion of the time-evolution operator. However, a rather unrealistic white-noise power spectrum of turbulence was adopted to allow for some explicit analytical results.…”

Section: Introductionmentioning

confidence: 99%

Cosmic Ray Small-Scale Anisotropies in Slab Turbulence

Kuhlen¹,

Mertsch²

2021

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

View full text Add to dashboard Cite

show abstract

“…Instead, one can only hope to predict statistical moments of the phase space density for a statistical ensemble of turbulent magnetic fields. Traditionally, one is mostly interested in the first moment, the ensemble average, though see Mertsch and Ahlers (2019) for the computation of a second-order moment.…”

Section: Derivation Of the Fokker-planck Equationmentioning

confidence: 99%

“…There are however situations where even Galactic transport is not diffusive or where the diffusive picture is questionable. These include the escape of Galactic CRs from the CR halo around the knee (DeMarco et al 2007;Giacinti et al 2015), near source transport Kachelrieß et al 2015), stochastic acceleration (O'Sullivan et al 2009;Winchen and Buitink 2018), also in relativistic turbulence (Demidem et al 2019), and the study of CR anisotropies Schwadron et al 2014;Mertsch and Funk 2015;Ahlers and Mertsch 2015;López-Barquero et al 2016;Pohl and Rettig 2016;Kumar et al 2019;Mertsch and Ahlers 2019). In the following, we will briefly review the use of test particle simulations and discuss the results for a few physics cases.…”

Section: Applicationsmentioning

confidence: 99%

Test particle simulations of cosmic rays

Mertsch

2020

Astrophys Space Sci

View full text Add to dashboard Cite

Modelling of cosmic ray transport and interpretation of cosmic ray data ultimately rely on a solid understanding of the interactions of charged particles with turbulent magnetic fields. The paradigm over the last 50 years has been the so-called quasi-linear theory, despite some wellknown issues. In the absence of a widely accepted extension of quasi-linear theory, wave-particle interactions must also be studied in numerical simulations where the equations of motion are directly solved in a realisation of the turbulent magnetic field. The applications of such test particle simulations of cosmic rays are manifold: testing transport theories, computing parameters like diffusion coefficients or making predictions for phenomena beyond standard diffusion theories, e.g. for cosmic ray small-scale anisotropies. In this review, we seek to give a low-level introduction to test particle simulations of cosmic rays, enabling readers to perform their own test particle simulations. We start with a review of quasi-linear theory, highlighting some of its issues and suggested extensions. Next, we summarise the state-of-theart in test particle simulations and give concrete recipes for generating synthetic turbulence. We present a couple of examples for applications of such simulations and comment on an important conceptual detail in the backtracking of particles.

show abstract

“…Interestingly, it has been suggested that the turbulent Galactic magnetic field could be the source of small-scale anisotropies Ahlers 2014;Ahlers & Mertsch 2015;Battaner et al 2015;López-Barquero et al 2016;Mertsch & Ahlers 2019). This idea has been investigated using test particle simulations of CRs in synthetic turbulence where sky maps of the CR arrival directions can be obtained from the phase-space density back-tracked along the CR trajectories to an earlier time (see Ahlers & Mertsch 2017 for a review).…”

Section: Introductionmentioning

confidence: 99%

No longer ballistic, not yet diffusive--the formation of cosmic ray small-scale anisotropies

Kuhlen,

Phan,

Mertsch

2021

Preprint

View full text Add to dashboard Cite

The arrival directions of TeV-PeV cosmic rays are remarkably uniform due to the isotropization of their directions by scattering on turbulent magnetic fields. Small anisotropies can exist in standard diffusion models, however, only on the largest angular scales. Yet, high-statistics observatories like IceCube and HAWC have found significant deviations from isotropy down to small angular scales. Here, we explain the formation of small-scale anisotropies by considering pairs of cosmic rays that get correlated by their transport through the same realisation of the turbulent magnetic field. We argue that the formation of small-scale anisotropies is the reflection of the particular realisation of the turbulent magnetic field experienced by cosmic rays on time scales intermediate between the early, ballistic regime and the late, diffusive regime. We approach this problem in two different ways: First, we run test particle simulations in synthetic turbulence, covering for the first time the TV rigidities of observations with realistic turbulence parameters. Second, we extend the recently introduced mixing matrix approach and determine the steady-state angular power spectrum. Throughout, we adopt magneto-static, slab-like turbulence. We find excellent agreement between the predicted angular power spectra in both approaches over a large range of rigidities. In the future, measurements of small-scale anisotropies will be valuable in constraining the nature of the turbulent magnetic field in our Galactic neighborhood.

show abstract

Cosmic ray small-scale anisotropies in quasi-linear theory

Abstract: The distribution of arrival directions of cosmic rays is remarkably isotropic,

Cited by 11 publications

References 16 publications

Cosmic Ray Small-Scale Anisotropies in Slab Turbulence

Cosmic Ray Small-Scale Anisotropies in Slab Turbulence

Test particle simulations of cosmic rays

No longer ballistic, not yet diffusive--the formation of cosmic ray small-scale anisotropies

Contact Info

Product

Resources

About