[Paper Review] The problem of small angular scale structure in the cosmic ray anisotropy data

L. O’C. Drury|arXiv (Cornell University)|May 29, 2013

Astrophysics and Cosmic Phenomena4 references16 citations

TL;DR

This paper proposes that small-scale angular anisotropies in TeV cosmic ray data—previously unexplained—may arise from induced electric fields in the heliosphere's outer regions, particularly the heliotail, due to the solar wind's motion through the interstellar magnetic field. These fields, arising from the Lorentz transformation of the magnetic field into a frame moving with the plasma, can cause energy shifts of ~100 MV to 1 GV along particle trajectories, producing anisotropy signals at the observed 10⁻⁴ level without requiring complex particle physics beyond standard electrodynamics.

ABSTRACT

Recent observations have revealed structure on small angular scales in the anisotropy data of cosmic rays at multi TeV energies. Even though the absolute amplitudes are very small, these effects are somewhat surprising and a wide range of possible causes have been discussed. A possible origin associated with heliospheric electric fields is proposed.

Motivation & Objective

• To explain the puzzling small-scale angular structure in TeV cosmic ray anisotropy data, which cannot be accounted for by standard diffusion or pitch-angle scattering models.
• To investigate whether low-energy electric field effects in the heliosphere could produce measurable anisotropy signals at TeV energies through energy shifts along particle trajectories.
• To provide a physically plausible, frame-dependent mechanism rooted in relativistic electrodynamics that explains the low amplitude and directional correlation with the heliotail.
• To offer a testable alternative to speculative models involving strangelets or magnetic reconnection, based on well-understood plasma physics.

Proposed method

• Uses the relativistic transformation of the electromagnetic field to show that a moving plasma (solar wind) generates an effective electric field E = -V × B in the Earth's rest frame.
• Applies the line integral of the induced electric field along cosmic ray trajectories through the heliosphere: ΔE ≈ ∫(-V × B) · ds, yielding potential shifts of ~100 MV to 1 GV.
• Assumes that particles from a given direction experience nearly identical trajectories through the heliosphere, preserving the anisotropy signal across energy bands.
• Estimates the characteristic length scale (~100 AU) and field strength (1 nT) to derive order-of-magnitude potential shifts consistent with the observed anisotropy amplitude (~10⁻⁴).
• Proposes that complex, non-gyrotropic structures can emerge if the electric field structure in the heliotail is spatially inhomogeneous.
• Makes a testable prediction: electron anisotropy should be anti-correlated with proton anisotropy (hot spots in protons correspond to cold spots in electrons).

Experimental results

Research questions

• RQ1 Can heliospheric electric fields generated by the solar wind's motion through the interstellar magnetic field produce small-scale anisotropies in TeV cosmic ray data?
• RQ2 Is the observed 10⁻⁴ level anisotropy amplitude consistent with energy shifts induced by electric potentials of order 100 MV to 1 GV?
• RQ3 Why is the anisotropy structure correlated with the heliotail, and can this be explained by frame-dependent electromagnetic effects?
• RQ4 Can this mechanism naturally produce multiple, non-symmetric hot spots without requiring complex particle acceleration or scattering models?

Key findings

• The induced electric field in the heliosphere, arising from the solar wind's motion through the interstellar magnetic field, can produce energy shifts of ~100 MV to 1 GV along cosmic ray trajectories.
• These energy shifts are sufficient to generate anisotropy signals at the observed level of ~10⁻⁴ in the cosmic ray flux at TeV energies.
• The mechanism explains the directional correlation with the heliotail, as the electric field structure is expected to be concentrated in that region.
• The model predicts anti-phase anisotropy between protons and electrons: a proton hotspot should correspond to a cold spot in electron data.
• The effect is robust for TeV cosmic rays because their gyroradii are large enough to avoid cancellation of energy shifts over a gyro-orbit, allowing net energy shifts to persist.
• The model offers a simple, physically grounded explanation distinct from speculative models involving strangelets or anomalous scattering.