skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: REDUCTION OF DRIFT EFFECTS DUE TO SOLAR WIND TURBULENCE

Abstract

Gradient and curvature drift play a key role in the modulation of cosmic rays. Reduction in the drift coefficient due to turbulence has been demonstrated unambiguously through direct numerical simulations, but a theory that can explain these results is still lacking. We introduce a parameterized form of the drift coefficient based on direct numerical simulations and show that good agreement with observed proton energy spectra at Earth can be found when it is used in a numerical modulation model. We show that the turbulence ultrascale, for which no observations currently exist, plays an important role in drift reduction. The magnitude at Earth and spatial dependence of this quantity required to fit cosmic-ray observations at Earth are argued to be plausible based on the required properties of the two-dimensional turbulence spectrum at large scales.

Authors:
;  [1]
  1. Unit for Space Physics, North-West University, 2520 Potchefstroom (South Africa)
Publication Date:
OSTI Identifier:
21474359
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 725; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/725/1/1366
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPUTERIZED SIMULATION; COSMIC RADIATION; ENERGY SPECTRA; HELIOSPHERE; PROTONS; SOLAR WIND; SPACE DEPENDENCE; SUN; TURBULENCE; ATMOSPHERES; BARYONS; ELEMENTARY PARTICLES; FERMIONS; HADRONS; IONIZING RADIATIONS; MAIN SEQUENCE STARS; NUCLEONS; RADIATIONS; SIMULATION; SOLAR ACTIVITY; SOLAR ATMOSPHERE; SPECTRA; STARS; STELLAR ACTIVITY; STELLAR ATMOSPHERES; STELLAR WINDS

Citation Formats

Burger, R. A., and Visser, D. J. REDUCTION OF DRIFT EFFECTS DUE TO SOLAR WIND TURBULENCE. United States: N. p., 2010. Web. doi:10.1088/0004-637X/725/1/1366.
Burger, R. A., & Visser, D. J. REDUCTION OF DRIFT EFFECTS DUE TO SOLAR WIND TURBULENCE. United States. doi:10.1088/0004-637X/725/1/1366.
Burger, R. A., and Visser, D. J. 2010. "REDUCTION OF DRIFT EFFECTS DUE TO SOLAR WIND TURBULENCE". United States. doi:10.1088/0004-637X/725/1/1366.
@article{osti_21474359,
title = {REDUCTION OF DRIFT EFFECTS DUE TO SOLAR WIND TURBULENCE},
author = {Burger, R. A. and Visser, D. J.},
abstractNote = {Gradient and curvature drift play a key role in the modulation of cosmic rays. Reduction in the drift coefficient due to turbulence has been demonstrated unambiguously through direct numerical simulations, but a theory that can explain these results is still lacking. We introduce a parameterized form of the drift coefficient based on direct numerical simulations and show that good agreement with observed proton energy spectra at Earth can be found when it is used in a numerical modulation model. We show that the turbulence ultrascale, for which no observations currently exist, plays an important role in drift reduction. The magnitude at Earth and spatial dependence of this quantity required to fit cosmic-ray observations at Earth are argued to be plausible based on the required properties of the two-dimensional turbulence spectrum at large scales.},
doi = {10.1088/0004-637X/725/1/1366},
journal = {Astrophysical Journal},
number = 1,
volume = 725,
place = {United States},
year = 2010,
month =
}
  • We extend a two-component model for the evolution of fluctuations in the solar wind plasma so that it is fully three-dimensional (3D) and also coupled self-consistently to the large-scale magnetohydrodynamic equations describing the background solar wind. The two classes of fluctuations considered are a high-frequency parallel-propagating wave-like piece and a low-frequency quasi-two-dimensional component. For both components, the nonlinear dynamics is dominanted by quasi-perpendicular spectral cascades of energy. Driving of the fluctuations by, for example, velocity shear and pickup ions is included. Numerical solutions to the new model are obtained using the Cronos framework, and validated against previous simpler models. Comparing results frommore » the new model with spacecraft measurements, we find improved agreement relative to earlier models that employ prescribed background solar wind fields. Finally, the new results for the wave-like and quasi-two-dimensional fluctuations are used to calculate ab initio diffusion mean-free paths and drift lengthscales for the transport of cosmic rays in the turbulent solar wind.« less
  • The understanding of the small-scale termination of the turbulent energy cascade in collisionless plasmas is nowadays one of the outstanding problems in space physics. In the absence of collisional viscosity, the dynamics at small scales is presumably kinetic in nature; the identification of the physical mechanism which replaces energy dissipation and establishes the link between macroscopic and microscopic scales would open a new scenario in the study of turbulent heating in space plasmas. We present a numerical analysis of kinetic effects along the turbulent energy cascade in solar-wind plasmas which provides an effective unified interpretation of a wide set ofmore » spacecraft observations and shows that, simultaneously with an increase in the ion perpendicular temperature, strong bursts of electrostatic activity in the form of ion-acoustic turbulence are produced together with accelerated beams in the ion distribution function.« less
  • Nonlinear processes in the solar wind drive a turbulent cascade of energy from large to small scales. Many aspects of this turbulent evolution can be described using fluid models, but near the wavelength of ion cyclotron waves, the spectrum steepens; evidence that the fluctuations are damped, heating the ambient plasma. Fluid models can be modified to approximate the physics of these kinetic processes by modifying the dissipation terms and by including extra terms which approximate some aspects of kinetic interactions.
  • MHD turbulence features in the solar wind are briefly reviewed. Focus is first on the evolution of fluctuations of Alfvenic type. The role of interplanetary sources and the influence of interactions with structures convected by the solar wind are discussed. Compressible fluctuations are then examined, with special attention to their nature and origin.
  • We study the wave-particle interaction and the evolution of electromagnetic waves propagating through a solar-wind-like plasma composed of cold electrons, isotropic protons, and a small portion of drifting anisotropic He{sup +2} (T {sub α} = 6 T {sub ∥α}) and O{sup +6} (T {sub O} = 11 T {sub ∥O}) ions as suggested in Gomberoff and Valdivia and Gomberoff et al., using two approaches. First, we use quasilinear kinetic theory to study the energy transfer between waves and particles, with the subsequent acceleration and heating of ions. Second, 1.5 D (one spatial dimension and three dimensions in velocity space) hybridmore » numerical simulations are performed to investigate the fully nonlinear evolution of this wave-particle interaction. Numerical results of both approaches show that the temperatures of all species evolve anisotropically, consistent with the time-dependent wave-spectrum energy. In a cascade effect, we observe the emergence of modes at frequencies higher than those initially considered, peaking at values close to the resonance frequencies of O{sup +6} ions (ω ∼ Ω {sub cO}) and He{sup +2} ions (ω ∼ Ω {sub cα}), being the peak due to O{sup +6} ions about three times bigger than the peak associated with He{sup +2} ions. Both the heating of the plasma and the energy cascade were more efficient in the nonlinear analysis than in the quasilinear one. These results suggest that this energy cascade mechanism may participate in the acceleration and heating of the solar wind plasma close to the Sun during fast streams associated with coronal holes.« less