A generalized two-component model of solar wind turbulence and ab initio diffusion mean-free paths and drift lengthscales of cosmic rays

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 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

The radial variation of the solar wind turbulence spectra near the kinetic break scale from Parker Solar Probe measurements

In this study we examine the radial dependence of the inertial and dissipation range indices, as well as the spectral break separating the inertial and dissipation range in power density spectra of interplanetary magnetic field fluctuations using Parker Solar Probe data from the fifth solar encounter between ∼0.1 and ∼0.7 au. The derived break wavenumber compares reasonably well with previous estimates at larger radial distances and is consistent with gyro-resonant damping of Alfvénic fluctuations by thermal protons. We find that the inertial scale power-law index varies between approximately −1.65 and −1.45. This is consistent with either the Kolmogorov (−5/3) or Iroshnikov–Kraichnan (−3/2) values, and has a very weak radial dependence with a possible hint that the spectrum becomes steeper closer to the Sun. The dissipation range power-law index, however, has a clear dependence on radial distance (and turbulence age), decreasing from −3 near 0.7 au (4 days) to −4 [±0.3] at 0.1 au (0.75 days) closer to the Sun

Spectroscopic imaging of the sun with MeerKAT: opening a new frontier in solar physics

Solar radio emissions provide several unique diagnostics to estimate different physical parameters of the solar corona, which are otherwise simply inaccessible. However, imaging the highly dynamic solar coronal emissions spanning a large range of angular scales at radio wavelengths is extremely challenging. At gigahertz frequencies, MeerKAT radio telescope is possibly globally the best-suited instrument at present for providing high-fidelity spectroscopic snapshot solar images. Here, we present the first published spectroscopic images of the Sun made using the observations with MeerKAT in the 880–1670 MHz band. This work demonstrates the high fidelity of spectroscopic snapshot MeerKAT solar images through a comparison with simulated radio images at MeerKAT frequencies. The observed images show extremely good morphological similarities with the simulated images. Our analysis shows that below ∼900 MHz MeerKAT images can recover essentially the entire flux density from the large angular-scale solar disk. Not surprisingly, at higher frequencies, the missing flux density can be as large as ∼50%. However, it can potentially be estimated and corrected for. We believe once solar observation with MeerKAT is commissioned, it will enable a host of novel studies, open the door to a large unexplored phase space with significant discovery potential, and also pave the way for solar science with the upcoming Square Kilometre Array-Mid telescope, of which MeerKAT is a precursor

A comparison of turbulence-reduced drift coefficients of importance for the modulation of galactic cosmic-ray protons in the supersonic solar wind

The 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

An AB initio model for the modulation of galactic cosmic-ray electrons

The modulation of galactic cosmic-ray electrons is studied using an ab initio three-dimensional steady state cosmicraymodulation code in which the effects of turbulence on both the diffusion and drift of these cosmic-rays are treated as self-consistently as possible. A significant refinement is that a recent two-component turbulence transport model is used. This model yields results in reasonable agreement with observations of turbulence quantities throughout the heliosphere. The sensitivity of computed galactic electron intensities to choices of various turbulence parameters pertaining to the dissipation range of the slab turbulence spectrum, and to the choice of model of dynamical turbulence, is demonstrated using diffusion coefficients derived from the quasi-linear and extended nonlinear guiding center theories. Computed electron intensities and latitude gradients are also compared with spacecraft observations

Sensitivity of cosmic-ray proton spectra to the low-wavenumber behavior of the 2D turbulence power spectrum

In 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

An AB initio model for cosmic-ray modulation

A proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays (CRs) is of vital importance for a better understanding of CR modulation in the heliosphere. This study presents an ab initio model for CR modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for solar minimum heliospheric conditions, utilizing boundary values chosen so that model results are in reasonable agreement with spacecraft observations of turbulence quantities in the solar ecliptic plane and along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modeled slab and two-dimensional (2D) turbulence energy spectra. The modeled 2D spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers. There currently exist no models or observations for the wavenumber where this drop-off occurs, and it is considered to be the only free parameter in this study. The modeled 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 CR drifts are modeled in a self-consistent way, also employing a recently developed model for wavy current sheet drift. The resulting diffusion and drift coefficients are applied to the study of galactic CR protons and antiprotons using a 3D, steady-state CR modulation code, and sample solutions in fair to good agreement with multiple spacecraft observations are presented

Permutation entropy analysis of magnetic field turbulence at 1AU revisited

Permutation entropy analysis is a relatively recent addition to the palette of techniques used to study spacecraft observations of heliospheric magnetic field fluctuations at Earth, allowing one to characterize the underlying processes driving the observed fluctuations. This study investigates the effects of data averaging, data gaps, and underlying periodicities on the results of such an analysis, utilizing synthetic data sets and thereby developing efficient treatments for each of these effects. Furthermore, WIND spacecraft observations are employed to validate the results of an earlier permutation entropy analysis by Weck et al. (2015, https://doi.org/10.1103/PhysRevE.91.023101), confirming the results of that study and showing how dat averaging effects can significantly affect the results so acquired. Lastly, as a novel application of this technique, Advanced Composition Explorer spacecraft data taken from 1998 to 2008 are analyzed to investigate whether the permutation entropy so calculated displays a solar cycle dependence. It is shown that although solar cycle dependencies have been reported for observed turbulence quantities such as the magnetic variance, there is no significant dependence discernible in the permutation entropy, and therefore in the underlying processes driving the turbulenc

Time-dependent cosmic ray modulation

Time-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