Isidoros Doxas
University of Colorado Boulder
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Featured researches published by Isidoros Doxas.
Physics of Plasmas | 1999
W. Horton; J.P. Smith; R. Weigel; C. Crabtree; Isidoros Doxas; Brent Goode; John R. Cary
The solar-wind driven magnetosphere–ionosphere system is a classic example of a complex dynamical system (CDS). The defining properties of a CDS are (1) sensitivity to initial conditions; (2) multiple space-time scales; (3) bifurcation sequences with hysteresis in transitions between attractors; and (4) noncompositionality. Noncompositionality means that the behavior of the system as a whole is different from the dynamics of its subcomponents taken with passive or no couplings. In particular the dynamics of the geomagnetic tail plasma depends on its coupling to the dissipative ionospheric plasma and on the nature of the solar-wind driving electric field over a suitably long (many hours) previous time interval. These complex dynamical system features are shown here in detail using the known WINDMI model for the solar-wind driven magnetosphere–ionosphere (MI) system. Numerous features in the bifurcation sequence are identified with known substorm and storm characteristics.
Physics of fluids. B, Plasma physics | 1992
John R. Cary; Isidoros Doxas; D. F. Escande; A. D. Verga
Velocity diffusion has been observed to be significantly above that given by quasilinear theory in both test‐particle and self‐consistent simulations of the dynamics of turbulent electrostatic fields. Test‐particle simulations and analytic calculations for motion in a set of randomly phased waves find the diffusion coefficient to be enhanced by a factor of 2.5 at intermediate values of the overlap parameter. Self‐consistent simulations find that the turbulent spectrum can evolve naturally into a form such that the diffusion enhancement is present when the linear growth time of the modes is long compared with the effective discretization time, which is nearly an order of magnitude greater than the resonant broadening time.
IEEE Transactions on Plasma Science | 2004
Isidoros Doxas; Wendell Horton; Wei-Tai Lin; Stanley Seibert; Manish Jay Mithaiwala
WINDMI-RC is a family of physics-based models that range in dimensionality from low-order models of dimension d = 8, which model the flow of energy between the eight highest global energy components of the tail, to high-order models with dimension d /spl sim/ 100 or more, for models that resolve the nonlinear dynamics of the system into different latitude zones. The models are intrinsically three-dimensional in configuration space and use the basic geometry of the Tsyganenko magnetic field model to define the geometrical quantities. The models satisfy the constraints of the conservations laws of energy and electrical charge in their network of nodes and branches that follow from the structure of the system. These models describe the injection of plasma from the plasma sheet and across the Alfve/spl acute/n layer into the inner ring current. The transport uses the storm-time solar wind dynamo electric field as the driver for the network model coupled to the inner magnetospheric corotation electric field.
Physics of Plasmas | 1997
Isidoros Doxas; John R. Cary
An enhancement of the velocity diffusion over the quasilinear value is observed in the regime where the autocorrelation time is much smaller than the linear growth time or resonance broadening time. The diffusion enhancement occurs when the resonance broadening time is small compared with the linear growth time. These simulations are self consistent and have enough modes to be in the continuous spectrum limit. That is, even at the initial amplitudes the intermode spacing is sufficiently small that the resonance overlap parameter is large. A possible mechanism for the enhanced diffusion (spontaneous spectrum discretization) is discussed.
Journal of Geophysical Research | 1994
Isidoros Doxas; T. W. Speiser; P. B. Dusenbery; W. Horton
An identifying signature is proposed for the existence and location of the neutral line in the magnetotail. The signature, abrupt density and temperature changes in the Earth-tail direction, was first discovered in test particle simulations. Such temperature variations have been observed is ISEE data (Huang et al., 1992), but their connection to the possible existence of a neutral line in the tail has not yet been established. The proposed signature develops earlier than the ion velocity space ridge of Martin and Speiser (1988) but can only be seen by spacecraft in the vicinity of the neutral line, while the latter can locate a neutral line remotely.
PLOS ONE | 2014
Vishnu Sreekumar; Simon Dennis; Isidoros Doxas; Yuwen Zhuang; Mikhail Belkin
A correlation dimension analysis of people’s visual experiential streams captured by a smartphone shows that visual experience is two-scaled with a smaller dimension at shorter length scales than at longer length scales. The bend between the two scales is a phase transition point where the lower scale primarily captures relationships within the same context and the higher dimensional scale captures relationships between different contexts. The dimensionality estimates are confirmed using Takens’ delay embedding procedure on the image stream, while the randomly permuted stream is shown to be space-filling thereby establishing that the two-scaled structure is a consequence of the dynamics. We note that the structure of visual experience closely resembles the structure of another domain of experience: natural language discourse. The emergence of an identical structure across different domains of human experience suggests that the two-scaled geometry reflects a general organizational principle.
Proceedings of the National Academy of Sciences of the United States of America | 2010
Isidoros Doxas; Simon Dennis; William Oliver
The paragraph spaces of five text corpora, of different genres and intended audiences, in four different languages, all show the same two-scale structure, with the dimension at short distances being lower than at long distances. In all five cases the short-distance dimension is approximately eight. Control simulations with randomly permuted word instances do not exhibit a low dimensional structure. The observed topology places important constraints on the way in which authors construct prose, which may be universal.
Multiscale Coupling of Sun-Earth Processes | 2005
W. Horton; M.J. Mithaiwala; E. Spencer; Isidoros Doxas
Abstract An important problem in magnetospheric physics is to develop integrated dynamical systems capable of modeling storm and substorm databases with the long term aim of developing space weather forecasting tools. WINDMI is a family of physics-based models that range in dimensionality, d, from low-order models which model the flow of energy between the dominant global energy components to high-order models with d ~ 102 – 103, for models that resolve the nonlinear dynamics of the system into different latitudes. The models are intrinsically three-dimensional in configuration space, and use the basic geometry of the Tsyganenko magnetic field model to define the geometrical quantities. Optimal values of the model parameters are found using the genetic algorithm on given storm and substorm databases. The model satisfy the constraints of the conservations laws of energy and electrical charge in their network of nodes and branches that follow the topology of the ambient magnetic field. The new WINDMI-RC model, which builds on the WINDMI model, includes the energy coupling of injected plasma from the plasma sheet across the Alfven layer into the ring current.
Journal of Geophysical Research | 2001
Brent Goode; John R. Cary; Isidoros Doxas; W. Horton
A method for distinguishing between data from a strange attractor and data from colored random noise is presented. For both types of data the apparent dimension, as measured by the scaling of the correlation function with length, is finite and noninteger for certain length scales. This would seem to indicate that such measurements by themselves are insufficient for concluding that the dynamics of the underlying system are low-dimensional. To distinguish these two types of data, we have developed the variance growth test. The test looks for the increase in the variance, as measured by the average squared deviation, of a subset of data as the length of that subset is increased. For strange attractor data the variance saturates once the length of the subset exceeds the characteristic first return time. In contrast, the variance of colored random noise continues to increase with increasing subset length indefinitely, with a scaling law that is related to the apparent correlation dimension. Application of the method to the Bargatze data set shows that the AL index behaves like a deterministic dynamical system.
Physics of Plasmas | 1999
Isidoros Doxas; W. Horton
A physics based model for the coupled solar WIND–Magnetosphere–Ionosphere system (WINDMI) is described. The model is based on truncated descriptions of the collisionless microscopic energy transfer processes occurring in the quasineutral layer, and includes a thermal flux limit neglected in the Magnetohydrodynamic (MHD) closure of the moment equations. All dynamically relevant parameters of the model can be computed analytically. The system is both Kirchhoffian and Hamiltonian, ensuring that the power input from the solar wind is divided into physically realizable energy sub-components, a property not shared by data-based filters. The model provides a consistent mathematical formalism in which different models of the solar wind driver, ionospheric dissipation, global field configuration, and substorm trigger mechanism can be inserted, and the coupling between the different parts of the system investigated.