arXiv subjects

G. P. Zank

Publications and source records attributed to G. P. Zank.

At least 55 records · Page 3Linked to original sources

arXiv2013

Characteristic Length of Energy-Containing Structures at the Base of a Coronal Hole

An essential parameter for models of coronal heating and fast solar wind acceleration that rely on the dissipation of MHD turbulence is the characteristic energy-containing length $λ_{\bot}$ of the squared velocity and magnetic field fluctuations ($u^2$ and $b^2$) transverse to the mean magnetic field inside a coronal hole (CH) at the base of the corona. The characteristic length scale defines directly the heating rate. We use a time series analysis of solar granulation and magnetic field measurements inside two CHs obtained with the New Solar Telescope (NST) at Big Bear Solar Observatory. A data set for transverse magnetic fields obtained with the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard {\it Hinode} spacecraft was utilized to analyze the squared transverse magnetic field fluctuations $b_t^2$. Local correlation tracking (LCT) was applied to derive the squared transverse velocity fluctuations $u^2$. We find that for $u^2$-structures, Batchelor integral scale $λ$ varies in a range of 1800 - 2100 km, whereas the correlation length $ς$ and the $e$-folding length $L$ vary between 660 and 1460 km. Structures for $b_t^2$ yield $λ\approx 1600$ km, $ς\approx 640$ km, and $L \approx 620$ km. An averaged (over $λ, ς$, and $L$) value of the characteristic length of $u^2$-fluctuations is 1260$\pm$500 km, and that of $b_t^2$ is 950$\pm$560 km. The characteristic length scale in the photosphere is approximately 1.5-50 times smaller than that adopted in previous models (3-30$\times10^3$ km). Our results provide a critical input parameter for current models of coronal heating and should yield an improved understanding of fast solar wind acceleration.

astro-ph.SR↗

arXiv2010

Can IBEX Identify Variations in the Galactic Environment of the Sun using Energetic Neutral Atom (ENAs)?

The Interstellar Boundary Explorer (IBEX) spacecraft is providing the first all-sky maps of the energetic neutral atoms (ENAs) produced by charge-exchange between interstellar neutral \HI\ atoms and heliospheric solar wind and pickup ions in the heliosphere boundary regions. The 'edge' of the interstellar cloud presently surrounding the heliosphere extends less than 0.1 pc in the upwind direction, terminating at an unknown distance, indicating that the outer boundary conditions of the heliosphere could change during the lifetime of the IBEX satellite. Using reasonable values for future outer heliosphere boundary conditions, ENA fluxes are predicted for one possible source of ENAs coming from outside of the heliopause. The ENA production simulations use three-dimensional MHD plasma models of the heliosphere that include a kinetic description of neutrals and a Lorentzian distribution for ions. Based on this ENA production model, it is then shown that the sensitivities of the IBEX 1.1 keV skymaps are sufficient to detect the variations in ENA fluxes that are expected to accompany the solar transition into the next upwind cloud. Approximately 20% of the IBEX 1.1 keV pixels appear capable of detecting the predicted model differences at the $ 3 σ$ level, with these pixels concentrated in the Ribbon region. Regardless of the detailed ENA production model, the success of the modeled \BdotR\ directions in reproducing the Ribbon locus, together with our results, indicate that the Ribbon phenomenon traces the variations in the heliosphere distortion caused by the relative pressures of the interstellar magnetic and gaseous components.

astro-ph.SR↗

arXiv2010

Modulation of waves due to charge-exchange collisions in magnetized partially ionized space plasma

A nonlinear time dependent fluid simulation model is developed that describes the evolution of magnetohydrodynamic waves in the presence of collisional and charge exchange interactions of a partially ionized plasma. The partially ionized plasma consists of electrons, ions and a significant number of neutral atoms. In our model, the electrons and ions are described by a single fluid compressible magnetohydrodynamic (MHD) model and are coupled self-consistently to the neutral gas, described by the compressible hydrodynamic equations. Both the plasma and neutral fluids are treated with different energy equations that describe thermal energy exchange processes between them. Based on our self-consistent model, we find that propagating Alfvénic and fast/slow modes grow and damp alternately through a nonlinear modulation process. The modulation appears to be robust and survives strong damping by the neutral component.

physics.space-ph↗

arXiv2010

From Micro- to Macro-scales in the Heliosphere and Magnetospheres

From a broader perspective, the heliosphere and planetary magnetospheres provide a test bed to explore the plasma physics of the Universe. In particular, the underlying nonlinear coupling of different spatial and temporal scales plays a key role in determining the structure and dynamics of space plasmas and electromagnetic fields. Plasmas and fields exhibit both laminar and turbulent properties, corresponding to either well organized or disordered states, and the development of quantitative theoretical and analytical descriptions from physics based first principles is a profound challenge. Limited observations and complications introduced by geometry and physical parameters conspire to complicate the problem. Dimensionless scaling analysis and statistical methods are universally applied common approaches that allow for the application of related ideas to multiple physical problems. We discuss several examples of the interplay between the scales in a variety of space plasma environments, as exemplified in the presentations of the session \textit{From Micro- to Macro-scales in the Heliosphere and Magnetospheres.}

physics.space-ph↗

arXiv2009

Self-consistent Simulations of Plasma-Neutral in a Partially Ionized Astrophysical Turbulent Plasma

A local turbulence model is developed to study energy cascades in the heliosheath and outer heliosphere (OH) based on self-consistent two-dimensional fluid simulations. The model describes a partially ionized magnetofluid OH that couples a neutral hydrogen fluid with a plasma primarily through charge-exchange interactions. Charge-exchange interactions are ubiquitous in warm heliospheric plasma, and the strength of the interaction depends largely on the relative speed between the plasma and the neutral fluid. Unlike small-length scale linear collisional dissipation in a single fluid, charge-exchange processes introduce channels that can be effective on a variety of length scales that depend on the neutral and plasma densities, temperature, relative velocities, charge-exchange cross section, and the characteristic length scales. We find, from scaling arguments and nonlinear coupled fluid simulations, that charge-exchange interactions modify spectral transfer associated with large-scale energy-containing eddies. Consequently, the turbulent cascade rate prolongs spectral transfer among inertial range turbulent modes. Turbulent spectra associated with the neutral and plasma fluids are therefore steeper than those predicted by Kolmogorov's phenomenology. Our work is important in the context of the global heliospheric interaction, the energization and transport of cosmic rays, gamma-ray bursts, interstellar density spectra, etc. Furthermore, the plasma-neutral coupling is crucial in understanding the energy dissipation mechanism in molecular clouds and star formation processes.

physics.space-ph↗

arXiv2009

3D Hall MHD Modeling of Solar Wind Plasma Spectra

We present fully self consistent 3D simulations of compressible Hall MHD plasma that describe spectral features relevant to the solar wind plasma. We find that a $k^{-7/3}$ spectrum sets in for the fluctuations that are smaller than ion gyro radius. We further investigate scale dependent anisotropy led by nonlinear processes relevant to the solar wind plasma. Our work is important particularly in understanding the role of wave and nonlinear cascades in the evolution of the solar wind, structure formation at the largest scales.

physics.space-ph↗

arXiv2009

Whistler Wave Turbulence in Solar Wind Plasma

Whistler waves are present in solar wind plasma. These waves possess characteristic turbulent fluctuations that are characterized typically by the frequency and length scales that are respectively bigger than ion gyro frequency and smaller than ion gyro radius. The electron inertial length is an intrinsic length scale in whistler wave turbulence that distinguishably divides the high frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. We present nonlinear three dimensional, time dependent, fluid simulations of whistler wave turbulence to investigate their role in solar wind plasma. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like $k^{-7/3}$ spectrum. By contrast, the small scale turbulent fluctuations exhibit a Navier-Stokes like evolution where characteristic turbulent eddies exhibit a typical $k^{-5/3}$ hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.

physics.space-ph↗

arXiv2009

Exclusion of Tiny Interstellar Dust Grains from the Heliosphere

The distribution of interstellar dust grains (ISDG) observed in the Solar System depends on the nature of the interstellar medium-solar wind interaction. The charge of the grains couples them to the interstellar magnetic field (ISMF) resulting in some fraction of grains being excluded from the heliosphere while grains on the larger end of the size distribution, with gyroradii comparable to the size of the heliosphere, penetrate the termination shock. This results in a skewing the size distribution detected in the Solar System. We present new calculations of grain trajectories and the resultant grain density distribution for small ISDGs propagating through the heliosphere. We make use of detailed heliosphere model results, using three-dimensional (3-D) magnetohydrodynamic/kinetic models designed to match data on the shape of the termination shock and the relative deflection of interstellar neutral H and He flowing into the heliosphere. We find that the necessary inclination of the ISMF relative to the inflow direction results in an asymmetry in the distribution of the larger grains (0.1 micron) that penetrate the heliopause. Smaller grains (0.01 micron) are completely excluded from the Solar System at the heliopause.

astro-ph.EP↗

arXiv2009

Density Spectrum in the Solar Wind Plasma

The density fluctuation spectrum in the solar wind reveals a Kolmogorov-like scaling with a spectral slope of -5/3 in wavenumber space. The energy transfer process in the magnetized solar wind, characterized typically by MHD turbulence, over extended length-scales remains an unresolved paradox of modern turbulence theories, raising the question of how a compressible magnetofluid exhibits a turbulent spectrum that is characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfvénic, supersonic and strongly magnetized plasma fluid. It is shown that a Kolmogorov-like density spectrum can develop by plasma motions that are dominated by Alfvénic cascades whereas compressive modes are dissipated.

physics.space-ph↗

arXiv2009

Turbulent Density Spectrum in Solar Wind Plasma

physics.space-ph↗

arXiv2009

Spectral features of solar wind turbulent plasma

Spectral properties of a fully compressible solar wind Hall Magnetohydrodynamic plasma are investigated by means of time dependent three dimensional Hall MHD simulations. Our simulations, in agreement with spacecraft data, identify a spectral break in turbulence spectra at characteristic length-scales associated with electromagnetic fluctuations that are smaller than the ion gyroradius. In this regime, our 3D simulations show that turbulent spectral cascades in the presence of a mean magnetic field follow an omnidirectional anisotropic inertial range spectrum close to $k^{-7/3}$. The onset of the spectral break in our simulations can be ascribed to the presence of nonlinear Hall interactions that modify the spectral cascades. Our simulations further show that the underlying charachteristic turbulent fluctuations are spectrally anisotropic, the extent of which depends critically on the local wavenumber. The fluctuations associated with length scales smaller than the ion gyroradius are highly compressible and tend to exhibit a near equipartition in the velocity and magnetic fields. Finally, we find that the orientation of velocity and magnetic field fluctuations critically determine the character of nonlinear interactions that predominantly govern a Hall MHD plasma, like the solar wind.

physics.space-ph↗

arXiv2009

Turbulent Spectra in the Solar Wind Plasma

Observations of interstellar scintillations at radio wavelengths reveal a Kolmogorov-like scaling of the electron density spectrum with a spectral slope of -5/3 over six decades in wavenumber space. A similar turbulent density spectrum in the solar wind plasma has been reported. The energy transfer process in the magnetized solar wind plasma over such extended length-scales remains an unresolved paradox of modern turbulence theories raising the especially intriguing question of how a compressible magnetized solar wind exhibits a turbulent spectrum that is a characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfvénic, supersonic and strongly magnetized plasma. It is shown that the observed Kolmogorov-like (-5/3) spectrum can develop in the solar wind plasma by supersonic plasma motions that dissipate into highly subsonic motion that passively convect density fluctuations.

physics.space-ph↗

arXiv2009

Self-Organization in a Driven Dissipative Plasma System

We perform a fully self-consistent 3-D numerical simulation for a compressible, dissipative magneto-plasma driven by large-scale perturbations, that contain a fairly broader spectrum of characteristic modes, ranging from largest scales to intermediate scales and down to the smallest scales, where the energy of the system are dissipated by collisional (Ohmic) and viscous dissipations. Additionally, our simulation includes nonlinear interactions amongst a wide range of fluctuations that are initialized with random spectral amplitudes, leading to the cascade of spectral energy in the inertial range spectrum, and takes into account large scale as well as small scale perturbation that may have been induced by the background plasma fluctuations, also the non adiabatic exchange of energy leading to the migration of energy from the energy containing modes or randomly injected energy driven by perturbations and further dissipated by the smaller scales. Besides demonstrating the comparative decays of total energy and dissipation rate of energy, our results show the existence of a perpendicular component of current, thus clearly confirming that the self-organized state is non-force free.

physics.space-ph↗

arXiv2008

The Heliosphere in Time

Because of the dynamic nature of the interstellar medium, the Sun should have encountered a variety of different interstellar environments in its lifetime. As the solar wind interacts with the surrounding interstellar medium to form a heliosphere, different heliosphere shapes, sizes, and particle contents result from the different environments. Some of the large possible interstellar parameter space (density, velocity, temperature) is explored here with the help of global heliosphere models, and the features in the resulting heliospheres are compared and discussed. The heliospheric size, expressed as distance of the nose of the heliopause to the Sun, is set by the solar wind - interstellar pressure balance, even for extreme cases. Other heliospheric boundary locations and neutral particle results correlate with the interstellar parameters as well. If the H0 clouds identified in the Millennium Arecibo survey are typical of clouds encountered by the Sun, then the Sun spends ~99.4% of the time in warm low density ISM, where the typical upwind heliosphere radii are up to two orders of magnitude larger than at present.

astro-ph↗

arXiv2008

Minimum Dissipative Relaxed States Applied to Laboratory and Space Plasmas

The usual theory of plasma relaxation, based on the selective decay of magnetic energy over the (global) magnetic helicity, predicts a force-free state for a plasma. Such a force-free state is inadequate to describe most realistic plasma systems occurring in laboratory and space plasmas as it produces a zero pressure gradient and cannot couple magnetic fields with flow. A different theory of relaxation has been proposed by many authors, based on a well-known principle of irreversible thermodynamics, the principle of minimum entropy production rate which is equivalent to the minimum dissipation rate (MDR) of energy. We demonstrate the applicability of minimum dissipative relaxed states to various self-organized systems of magnetically confined plasma in the laboratory and in the astrophysical context. Such relaxed states are shown to produce a number of basic characteristics of laboratory plasma confinement systems and solar arcade structure.

physics.space-ph↗

arXiv2008

Self-consistent interaction of neutrals and shocks in the local interstellar medium

The problem of hydrogen neutrals interacting with the heliospheric bow shock region has received considerable attention recently, motivated primarily by the hope that the Voyager spacecraft may soon encounter the first of the heliospheric boundaries. The complexity of the charge exchange interactions has limited our analytic understanding so far. In this work we develop a semi-analytic one-dimensional model based upon a double expansion technique to investigate the self-consistent interaction of shock with neutrals. The underlying method uses the Boltzmann transport equation that describes neutral transport, and is coupled to plasma protons predominantly through charge exchange processes. The gyrotropic distribution function of the neutrals is first expanded in terms of Legendre polynomials followed by associated Laguerre harmonics. The resulting set of equations can be cast into a square matrix whose eigenvalues depend upon sources and the plasma distribution. The interstellar protons, in a high $β$ limit, are described by a hydrodynamic fluid with source terms that couple neutrals and plasma and further modify both distributions in a self-consistent manner.

physics.space-ph↗

arXiv2008

Passive Convection of Density Fluctuations in the Local Interstellar Medium

We have developed a time-dependent three-dimensional model of isotropic, adiabatic, and compressible magnetohydrodynamic plasma to understand nonlinear cascades of density fluctuations in local interstellar medium. Our simulations, describing evolution of initial supersonic, super Alfvénic plasma modes, indicate that nonlinear interactions lead to damping of plasma motion. During the process, turbulent cascades are governed predominantly by the Alfvénic modes and velocity field fluctuations evolve towards a state charachterized by near incompressibility. Consequently, density field is advected passively by the velocity field. Our findings thus demonstrate that the observed density fluctuations in the interstellar medium are the structures passively convected by the background velocity field.

physics.space-ph↗

arXiv2008

The effects of a kappa-distribution in the heliosheath on the global heliosphere and ENA flux at 1 AU

We investigate heliosheath energetic neutral atom (ENA) fluxes at keV energies, by assuming that the heliosheath proton distribution can be approximated by a kappa-distribution. The choice of the kappa parameter derives from observational data of the solar wind (SW). This has direct applications to the upcoming IBEX mission. We will look at all-sky ENA maps within the IBEX energy range (10 eV to 6 keV), as well as ENA energy spectra in several directions. We find that the use of kappa, as opposed to a Maxwellian, gives rise to greatly increased ENA fluxes above 1 keV, while medium energy fluxes are somewhat reduced. We show how IBEX data can be used to estimate the spectral slope in the heliosheath, and that the use of kappa reduces the differences between ENA maps at different energies. We also investigate the effect introducing a kappa-distribution has on the global interaction between the SW and the local interstellar medium (LISM), and find that there is generally an increase in energy transport from the heliosphere into the LISM, due to the modified profile of ENA's energies. This results in a termination shock that moves out by 4 AU, a heliopause that moves in by 9 AU and a bow shock 25 AU farther out, in the nose direction.

astro-ph↗