553 research outputs found
The implications of simple estimates of the 2D outerscale based on measurements of magnetic islands for the modulation of galactic cosmic-ray electrons
No full textThe behavior of the 2D turbulence power spectrum at the lowest wavenumbers has a significant effect on the perpendicular diffusion coefficients of charged particles in the heliosphere derived from various scattering theories, and subsequently used to model the transport of cosmic rays (CRs) and solar energetic particles. In this regard, the lengthscale at which the energy-containing range begins, as opposed to that at which the inertial range commences, is of particular interest. This 2D outerscale has, however, never before been directly observed. Recently, direct measurements of magnetic islands in the solar wind have been reported by various authors. Assuming that these may provide an estimate of the 2D ultrascale, the direct calculation of the 2D outerscale becomes possible, should an observationally motivated form for the 2D turbulence power spectrum be employed. This study presents the results of such a calculation and provides comparisons of these with previous estimates of the 2D outerscale. Furthermore, the sensitivity of galactic CR electron intensities, calculated using a 3D ab initio CR modulation model, is demonstrated, and conclusions are drawn therefro
A comparison of turbulence-reduced drift coefficients of importance for the modulation of galactic cosmic-ray protons in the supersonic solar wind
No full textThe study of the modulation of cosmic rays in the heliosphere relies heavily on a thorough understanding of the transport of these charged particles in the turbulent solar wind. Drift effects due to gradients and the curvature of the background magnetic field have long been known to be reduced in the presence of turbulence, and as such, several forms for the drift coefficient that include the effect of turbulence have been proposed. The present study aims to investigate the qualitative effects of various turbulence-reduced drift coefficients on cosmic ray intensities computed using an ab initio 3D steady-state cosmic-ray modulation code. Results from a two-component turbulence transport models are used as inputs for the basic turbulence quantities. Furthermore, an expression for the perpendicular mean free path is derived here from a modification of the non-linear guiding center theory of Matthaeus et al. (2003) assuming a 2D turbulence power spectrum with a k-1k-1 energy range wavenumber dependence, and is used in conjunction with the various proposed turbulence-reduced drift coefficients. Cosmic-ray intensities computed using different drift coefficients but assuming the same turbulence conditions are found to differ widely. This study emphasises the need to gain a better understanding of the effect of turbulence on drifts in the heliosphereNational Research Foundation (NRF), South Afric
Sensitivity of cosmic-ray proton spectra to the low-wavenumber behavior of the 2D turbulence power spectrum
No full textIn this study, a novel ab initio cosmic ray (CR) modulation code that solves a set of stochastic transport equations
equivalent to the Parker transport equation, and that uses output from a turbulence transport code as input for the
diffusion tensor, is introduced. This code is benchmarked with a previous approach to ab initio modulation. The
sensitivity of computed galactic CR proton spectra at Earth to assumptions made as to the low-wavenumber
behavior of the two-dimensional (2D) turbulence power spectrum is investigated using perpendicular mean free
path expressions derived from two different scattering theories. Constraints on the low-wavenumber behavior of
the 2D power spectrum are inferred from the qualitative comparison of computed CR spectra with spacecraft
observations at Earth. Another key difference from previous studies is that observed and inferred CR intensity
spectra at 73 AU are used as boundary spectra instead of the usual local interstellar spectrum. Furthermore, the
results presented here provide a tentative explanation as to the reason behind the unusually high galactic proton
intensity spectra observed in 2009 during the recent unusual solar minimu
Effects of various dissipation range onset models on the 26-day variations of low-energy galactic cosmic-ray electrons
No full textThe effect of various models presented by Leamon et al. (2000) for the dissipation range cutoff wavenumber on the 26-day variations of galactic cosmic-ray electrons in a Fisk-Parker hybrid field is investigated, by means of a three-dimensional steady-state numerical modulation code. Analytical expressions for the mean free paths parallel and perpendicular to the heliospheric magnetic field are adapted from the works of Teufel and Schlickeiser (2003) and Shalchi et al. (2004), respectively. Note that only solar minimum conditions are considered, and that only qualitative agreement with data is sought. Effective diffusion for galactic electrons pertaining to 26-day variations is found to be dominated by the ratio of the perpendicular to parallel mean free paths at low energies, and the relationship between changes in cosmic-ray intensities and the modulation parameter postulated by Zhang (1997) is found to no longer hold when this ratio drops below a critical value. Use of ion inertial scale dependent models for the dissipation range cutoff leads to possible second linearities in the relative amplitudes as functions of latitude gradient
Cosmic-ray modulation: an ab initio approach
No full textA better understanding of cosmic-ray modulation
in the heliosphere can only be gained through a proper understanding
of the effects of turbulence on the diffusion and drift
of cosmic rays. We present an ab initio model for cosmic-ray
modulation, incorporating for the first time the results yielded
by a two-component turbulence transport model. This model
is solved for periods of minimum solar activity, utilizing
boundary values chosen so that model results are in fair to
good agreement with spacecraft observations of turbulence
quantities, not only in the solar ecliptic plane but also along
the out-of-ecliptic trajectory of the Ulysses spacecraft.
These results are employed as inputs for modelled slab
and 2D turbulence energy spectra. The latter spectrum is
chosen based on physical considerations, with a drop-off
at the very lowest wavenumbers commencing at the 2D
outerscale. There currently exist no models or observations
for this quantity, and it is the only free parameter
in this study. The modelled turbulence spectra are used
as inputs for parallel mean free path expressions based
on those derived from quasi-linear theory and perpendicular
mean free paths from extended nonlinear guiding
center theory. Furthermore, the effects of turbulence on
cosmic-ray drifts are modelled in a self-consistent way,
employing a recently developed model for drift along
the wavy current sheet. The resulting diffusion coefficients
and drift expressions are applied to the study of
galactic cosmic-ray protons and antiprotons using a threedimensional,
steady-state cosmic-ray modulation code, and
sample solutions in fair agreement with multiple spacecraft
observations are presented.National Research
Foundation (NRF
Time-dependent cosmic ray modulation
No full textTime-dependent cosmic ray modulation is calculated over multiple solar cycles using our well established two-dimensional time-dependent modulation model. Results are compared to Voyager 1, Ulysses and IMP cosmic ray observations to establish compatibility. A time-dependence in the diffusion and drift coefficients, implicitly contained in recent expressions derived by Teufel and Schlickeiser (2002), Shalchi et al. (2004), Minnie et al. (2007), Engelbrecht (2008), is incorporated into the cosmic ray modulation model. This results in calculations which are compatible with spacecraft observations on a global scale over consecutive solar cycles. This approach compares well to the successful compound approach of Ferreira and Potgieter (2004). For both these approaches the magnetic field magnitude, variance of the field and current sheet tilt angle values observed at Earth are transported time-dependently into the outer heliosphere. However, when results are compared to observations for extreme solar maximum, the computed step-like modulation is not as pronounced as observed. This indicates that some additional merging of these structures into more pronounced modulation barriers along the way is neede
A detailed calculation of neutral hydrogen ionization frequencies used in turbulence transport models in the heliosphere
No full textIt is generally accepted that the solar wind is significantly heated beyond ~10 AU by the turbulent decay of pickup ion generated Alfvénic fluctuations. Here, we present a detailed and general calculation of the pickup ion ionization frequencies, and we evaluate these quantities within the solar wind termination shock along the stagnation line. For this supersonic solar wind region, inside of the solar wind termination shock, our results compare well with earlier estimates of these frequencies. When, in the future, turbulence transport models are extended into the heliosheath, the methodology outlined in this paper can be used to calculate ionization frequencies in this hot and dense plasma region where the resulting calculations become more comple
An ab initio approach to the historical modulation of cosmic rays
No full textMSc (Astrophysical Sciences), North-West University, Potchefstroom CampusThe relationship between observed cosmic-ray intensities and the heliospheric magnetic
field during the early space age is generally considered to be anomalous, relative to their
relationship observed at later times. Various explanations for this behaviour have been
proposed, but these do not take into account the influence of magnetic turbulence on the
transport of cosmic-rays. The aim of this study is to take this into account, by analysing
historic heliospheric magnetic observations in an appropriatemanner to gain insight as to
the behaviour of magnetic variances and correlation lengthscales during this period. These
quantities are then used as inputs for an ab initio cosmic-ray modulation model, using
theoretically-motivated expressions for cosmic ray diffusion and drift coefficients. Galactic
cosmic-ray intensities computed with this model, when compared with neutron monitor
observations, are found to be in qualitative agreement. From this it can be concluded that
cosmic-ray modulation conditions during the early space age were not as anomalous as
previously thought, and that it is essential to take turbulence into account in the study of
historic cosmic-raymodulation.Master
Diffusion in pulsar wind nebulae: an investigation using magnetohydrodynamic and particle transport models
No full textWe study the transport of high-energy particles in pulsar wind nebulae (PWN) using three-dimensional magnetohydrodynamic (MHD) and test-particle simulations, as well as a Fokker–Planck particle transport model. The latter includes radiative and adiabatic losses, diffusion, and advection on the background flow of the simulated MHD nebula. By combining the models, the spatial evolution of flux and photon index of the X-ray synchrotron emission is modelled for the three nebulae G21.5−0.9, the inner regions of Vela, and 3C 58, thereby allowing us to derive governing parameters: the magnetic field strength, average flow velocity, and spatial diffusion coefficient. For comparison, the nebulae are also modelled with the semi-analytic Kennel & Coroniti model but the Porth et al. model generally yields better fits to the observational data. We find that high velocity fluctuations in the turbulent nebula (downstream of the termination shock) give rise to efficient diffusive transport of particles, with average Péclet number close to unity, indicating that both advection and diffusion play an important role in particle transport. We find that the diffusive transport coefficient of the order of ∼ 2 × 1027(Ls/0.42 Ly) cm2 s− 1 (Ls is the size of the termination shock) is independent of energy up to extreme particle Lorentz factors of γp ∼ 101
A generalized two-component model of solar wind turbulence and ab initio diffusion mean-free paths and drift lengthscales of cosmic rays
No full textWe 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 from 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
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