Bogdan Hnat

Self-organization of internal pedestals in a sandpile

The temperature profiles of magnetically confined plasmas can display distinctive longlived internal pedestals, caused by self-created transport barriers. This raises theoretical physics issues, distinct from the plasma physics modelling challenge, which concern the class of transport processes, physical principles, and control parameters that can generate such phenomenology. Here, we show that such structures can arise naturally through avalanching transport in a sandpile model. A single control parameter, that governs the spatial range of each rapid critical-gradient-triggered redistribution event, determines the occurrence and regularity of these effects.

Scaling and a Fokker-Planck model for fluctuations in geomagnetic indices and comparison with solar wind as seen by Wind and ACE

The evolution of magnetospheric indices on temporal scales shorter than that of substorms is characterized by bursty, intermittent events that may arise from turbulence intrinsic to the magnetosphere or that may reflect solar wind-magnetosphere coupling. This leads to a generic problem of distinguishing between the features of the system and those of the driver. We quantify scaling properties of short-term (up to few hours) fluctuations in the geomagnetic indices AL and AU during solar minimum and maximum, along with the parameter that is a measure of the solar wind driver. We find that self-similar statistics provide a good approximation for the observed scaling properties of fluctuations in the geomagnetic indices, regardless of the solar activity level, and in the parameter at solar maximum. This self-similarity persists for fluctuations on timescales at least up to about 1–2 hours. The scaling exponent of AU index fluctuations show dependence on the solar cycle, and the trend follows that found in the scaling of fluctuations in . The values of their corresponding scaling exponents, however, are always distinct. Fluctuations in the AL index are insensitive to the solar cycle, as well as being distinct from those in the parameter. This approximate self-similar scaling leads to a Fokker-Planck model which, we show, captures the probability density function of fluctuations and provides a stochastic dynamical equation (Langevin equation) for time series of the geomagnetic indices.

Scaling in long term data sets of geomagnetic indices and solar wind ϵ as seen by WIND spacecraft

[4] In this paper we use a larger 10-year data set for the AE indices to obtain a more accurate statistical determination of the functional form of the PDF of fluctuations over a more extensive dynamic range, including characterization of extremal events up to 10 standard deviations for the first time. We apply structure functions to characterize and compare both the low and higher order moments for all quantities. A 4-year subset of the index data, corresponding to the same period in the solar cycle as that used to produce � , is used to facilitate this comparison. We then verify these results by direct examination of the fluctuation PDF using the full 10-year AE indices dataset.

Sandpile Model with Tokamaklike Enhanced Confinement Phenomenology

Confinement phenomenology characteristic of magnetically confined plasmas emerges naturally from a simple sandpile algorithm when the parameter controlling redistribution scale length is varied. Close analogs are found for enhanced confinement, edge pedestals, and edge localized modes (ELMs), and for the qualitative correlations between them. These results suggest that tokamak observations of avalanching transport are deeply linked to the existence of enhanced confinement and ELMs.

Scaling of solar wind ϵ and the AU, AL and AE indices as seen by WIND

We apply the finite size scaling technique to quantify the statistical properties of fluctuations in AU, AL and AE indices and in the epsilon parameter that represents energy input from the solar wind into the magnetosphere. We find that the exponents needed to rescale the probability density functions (PDF) of the fluctuations are the same to within experimental error for all four quantities. This self-similarity persists for time scales up to similar to4 hours for AU, AL and epsilon and up to similar to2 hours for AE. Fluctuations on shorter time scales than these are found to have similar long-tailed (leptokurtic) PDF, consistent with an underlying turbulent process. These quantitative and model-independent results place important constraints on models for the coupled solar wind-magnetosphere system.

Anisotropic Intermittency of Magnetohydrodynamic Turbulence

A higher-order multiscale analysis of spatial anisotropy in inertial range magnetohydrodynamic turbulence is presented using measurements from the STEREO spacecraft in fast ambient solar wind. We show for the first time that, when measuring parallel to the local magnetic field direction, the full statistical signature of the magnetic and Elsasser field fluctuations is that of a non-Gaussian globally scale-invariant process. This is distinct from the classic multiexponent statistics observed when the local magnetic field is perpendicular to the flow direction. These observations are interpreted as evidence for the weakness, or absence, of a parallel magnetofluid turbulence energy cascade. As such, these results present strong observational constraints on the statistical nature of intermittency in turbulent plasmas.

TEMPERATURE ANISOTROPY IN THE PRESENCE OF ULTRA LOW FREQUENCY WAVES IN THE TERRESTRIAL FORESHOCK

We report the first study of the correlation between elevated solar wind core plasma temperatures and temperature anisotropy in the terrestrial foreshock. Plasma temperature is enhanced near the fire hose marginal stability threshold in the presence of ultra low frequency (ULF) large amplitude magnetic perturbations, which are intrinsically right-hand circularly polarized. Direct comparison of contemporaneous anisotropic temperatures in the upstream solar wind and the foreshock suggests that the net heating of plasma is mediated via increase of the parallel temperature in the foreshock region where the ULF waves are present. We consider the possibility that a mechanism based on Landau damping, where solar wind plasma temperature parallel to the background magnetic field is increased by interaction with oblique compressible fast magneto-acoustic ULF waves, influences temperature anisotropy.

INTERMITTENCY AND SELF-SIMILARITY IN ‘NATURAL PARAMETERS’ IN SOLAR WIND TURBULENCE

Abstract. The solar wind provides a natural laboratory for observations of MHD turbulence over extended temporal scales. We identify self-similarity in the probability density functions of fluctuations in certain solar wind bulk plasma parameters as seen by the WIND spacecraft. Whereas the fluctuations of speed ν and IMF magnitude B are multi-fractal, we find that the fluctuations in the ion density ρ , energy densities B 2 and ρv 2 as well as MHD-approximated Poynting flux vB 2 are self-similar on the timescales up to ~26 hours. We argue that these are ‘natural parameters’ in which to cast the solar wind measurements which are taken in an Eulerian frame of reference. The intermittency of the system is then expressed in these parameters through the non-Gaussian nature of the single curve that describes the fluctuations PDF up to this timescale. Self-similarity implies that a Fokker-Planck model exists for the timeseries and we derive this here along with the associated Langevin equation.

Nonlinear Waves in the Terrestrial Quasiparallel Foreshock

We provide strongly conclusive evidence that the cubic nonlinearity plays an important part in the evolution of the large amplitude magnetic structures in the terrestrial foreshock. Large amplitude nonlinear wave trains at frequencies above the proton cyclotron frequency are identified after nonharmonic slow variations are filtered out by applying the empirical mode decomposition. Numerical solutions of the derivative nonlinear Schrödinger equation, predicted analytically by the use of a pseudopotential approach, are found to be consistent with the observed wave forms. The approximate phase speed of these nonlinear waves, indicated by the parameters of numerical solutions, is of the order of the local Alfvén speed. We suggest that the feedback of the large amplitude fluctuations on background plasma is reflected in the evolution of the pseudopotential.