Browsing by Author "Usmanov, A. V."
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Item Alfvénic Turbulence Simulation in a Realistic Solar Wind(Astronomical Society of the Pacific, 2010) Usmanov, A. V.; Goldstein, MelvynWe present initial results from a new numerical model to simulate magnetohydrodynamic (MHD) turbulence in the solar wind above the Alfvénic critical point. Previously, we had defined a “virtual” heliosphere that contained a tilted rotating current sheet, microstreams, as well as Alfvén waves (Goldstein et al. 1999a). In this new restructured approach, we use the global, time-stationary, WKB Alfvén wave-driven solar wind model (Usmanov & Goldstein 2003a) to define the initial state of the system. Consequently, current sheets, and fast and slow streams are computed self-consistently from an inner photospheric boundary. To this steady-state configuration, we add fluctuations close to, but above, the surface where the flow becomes super-Alfvénic. The time-dependent MHD equations are then solved using a semi-discrete third-order Central Weighted Essentially Non-Oscillatory (CWENO) numerical scheme in the frame of reference corotating with the Sun. The computational domain now includes the entire sphere; the geometrical singularity at the poles is removed using the multiple grid approach described in Usmanov (1996). Wave packets are introduced at the inner boundary such as to satisfy Faraday’s Law (Yeh & Dryer 1985) and their nonlinear evolution is followed in time.Item Cosmic-Ray Diffusion Coefficients throughout the Inner Heliosphere from a Global Solar Wind Simulation(IOP, 2017-06-16) Chhiber, R.; Subedi, P.; Usmanov, A. V.; Matthaeus, W. H.; Ruffolo, D.; Goldstein, Melvyn; Parashar, T. N.We use a three-dimensional magnetohydrodynamic simulation of the solar wind to calculate cosmic-ray diffusion coefficients throughout the inner heliosphere (2R⊙ - 3au). The simulation resolves large-scale solar wind flow, which is coupled to small-scale fluctuations through a turbulence model. Simulation results specify background solar wind fields and turbulence parameters, which are used to compute diffusion coefficients and study their behavior in the inner heliosphere. The parallel mean free path (mfp) is evaluated using quasi-linear theory, while the perpendicular mfp is determined from nonlinear guiding center theory with the random ballistic interpretation. Several runs examine varying turbulent energy and different solar source dipole tilts. We find that for most of the inner heliosphere, the radial mfp is dominated by diffusion parallel to the mean magnetic field; the parallel mfp remains at least an order of magnitude larger than the perpendicular mfp, except in the heliospheric current sheet, where the perpendicular mfp may be a few times larger than the parallel mfp. In the ecliptic region, the perpendicular mfp may influence the radial mfp at heliocentric distances larger than 1.5 au; our estimations of the parallel mfp in the ecliptic region at 1 au agree well with the Palmer "consensus" range of 0.08–0.3 au. Solar activity increases perpendicular diffusion and reduces parallel diffusion. The parallel mfp mostly varies with rigidity (P) as P.³³ and the perpendicular mfp is weakly dependent on P. The mfps are weakly influenced by the choice of long-wavelength power spectra.Item Low-density anomalies and sub-Alfvénic solar wind(AGU, 2005-01-28) Usmanov, A. V.; Goldstein, Melvyn; Ogilvie, K. W.; Farrell, W. M.; Lawrence, G. R.During 10–12 May 1999, the solar wind density dropped to an anomalously low value of ∼0.1 cm⁻³. The density depletion occurred in the midst of relatively slow wind flow, in between faster flows, and was apparently associated with neither a coronal mass ejection nor a fast corotating stream. While the magnetic field intensity did not show any notable variation across the density depletion, plasma analyzers on the ACE and Wind spacecraft revealed an abnormally strong nonradial flow component with an azimuthal speed that peaked at ∼100 km s⁻¹. Usmanov et al. [2000b] suggested that the density anomaly was, in fact, a rarefaction at the trailing edge of relatively fast flow that formed as a result of suppression of coronal outflow from a region that earlier provided fast wind flow. The suppression in turn may have resulted from a rapid restructuring of solar magnetic fields during the polar field reversal. Here we show results from a two-dimensional time-dependent MHD simulation applied to the helioequatorial plane. The initially longitude-independent Parker solar wind and Archimedean spiral magnetic field are disturbed by a low-velocity/high-density jump on an inner computational boundary at 20 R⊙. We follow the development and propagation of the rarefaction to Earth orbit and compare pseudo-time series with near-Earth spacecraft measurements. We show that a strong rarefaction can develop behind the fast flow and that simulation results and spacecraft observations are generally in agreement. The simulated radial magnetic field shows a relatively small variation across the density anomaly compared with that of the density. The stream interaction generates strong azimuthal velocities in the slow flow region, as observed. The simulation shows a sub-Alfvénic flow region embedded within the low-density region that does not extend all the way back to the Sun but which has become disconnected as the depletion propagates to Earth orbit. We discuss also the correlation between low-density and sub-Alfvénic events in the solar wind as inferred from spacecraft observations using the OMNI 2 data set from 1963 to 2003.Item An MHD Solar Wind Model with Turbulence Transport(Astronomical Society of the Pacific, 2009) Usmanov, A. V.; Matthaeus, W. H.; Breech, B.; Goldstein, MelvynWe present initial results from an axisymmetric steady-state solar wind model that describes properties of the large-scale solar wind, interplanetary magnetic field, and turbulence throughout the heliosphere from 0.3 AU to 100 AU. The model is based on numerical solutions of large-scale Reynolds-averaged magnetohydrodynamic (MHD) equations coupled with a set of small-scale transport equations for the turbulence energy, normalized cross-helicity, and correlation scale. The combined set of the time-dependent equations is solved in the frame of reference corotating with the Sun by a time-relaxation method. We use the model to study the global solar wind structure and the distribution of turbulence throughout the heliosphere.Item Shear-Driven Transition to Isotropically Turbulent Solar Wind Outside the Alfv´en Critical Zone(AAS, 2020-09-15) Ruffolo, D.; Matthaeus, W. H.; Chhiber, R.; Usmanov, A. V.; Yang, Y.; Bandyopadhyay, R.; Parashar, T. N.; Goldstein, Melvyn; DeForest, C. E.; Wan, M.; Chasapis, A.; Maruca, B. A.; Velli, M.; Kasper, J. C.Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic ``flocculated'' fluctuations, we propose that the dynamics of the inner solar wind just outside the Alfvén critical zone, and in the vicinity of the first β=1 surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfvén critical zone, as suggested by global magnetohydrodynamic (MHD) simulations that include self-consistent turbulence transport. We argue that this dynamical evolution accounts for features observed by {\it Parker Solar Probe} ({\it PSP}) near initial perihelia, including magnetic ``switchbacks'', and large transverse velocities that are partially corotational and saturate near the local Alfvén speed. Large-scale magnetic increments are more longitudinal than latitudinal, a state unlikely to originate in or below the lower corona. We attribute this to preferentially longitudinal velocity shear from varying degrees of corotation. Supporting evidence includes comparison with a high Mach number three-dimensional compressible MHD simulation of nonlinear shear-driven turbulence, reproducing several observed diagnostics, including characteristic distributions of fluctuations that are qualitatively similar to {\it PSP} observations near the first perihelion. The concurrence of evidence from remote sensing observations, {\it in situ} measurements, and both global and local simulations supports the idea that the dynamics just above the Alfvén critical zone boost low-frequency plasma turbulence to the level routinely observed throughout the explored solar system.Item A three-dimensional MHD solar wind model with pickup protons(AGU, 2006-07-07) Usmanov, A. V.; Goldstein, MelvynWe have developed a three-dimensional (3-D) steady-state MHD model of the solarcorona and solar wind that covers the region from the coronal base to 100 AU and thataccounts for the effects of pickup protons in the distant heliosphere. The model expandsthe two-region model of Usmanov and Goldstein (2003) to include a region III thatextends from 1–100 AU and incorporates a population of interstellar neutral hydrogen andits interaction with solar wind protons. Following the approach of Isenberg (1986) andWhang (1998), we consider the solar wind outside 1 AU as a combination of threecomoving species, solar wind protons, electrons, and pickup protons, and solve the 3-Dsteady-state MHD equations with source terms due to photoionization and chargeexchange. Separate energy equations are included for solar wind and pickup protons. Weshow that the pickup protons cause a deceleration of the solar wind and an increase inaverage plasma temperature with heliocentric distance beyond 10 AU. We compute theglobal structure of the solar wind from the coronal base to 100 AU and compare ourresults with Voyager 1 and 2 observations.Item Three-Dimensional Solar Wind Model with Pickup Protons(European Space Agency, 2005-09) Usmanov, A. V.; Goldstein, MelvynWe have developed a three-dimensional steady-state MHD model of the solar corona and solar wind. The model covers the region from the coronal base to 100 AU and accounts for the effects of pickup protons in the distant heliosphere. To attain these ends, the two-region model of Usmanov and Goldstein (2003) was modified to include a region III that extends from 1–100 AU and incorporates a population of interstellar pickup protons and its interaction with the solar wind protons. We compute the global structure of the solar wind from the coronal base out to 100 AU and compare our results with Voyager 2 observations.Item A tilted-dipole MHD model of the solar corona and solar wind(AGU, 2003-09-30) Usmanov, A. V.; Goldstein, Melvyn[1] We simulate the three-dimensional structure of the heliosphere during solar activity minimum by specifying boundary conditions at the coronal base. We compare the output of the model with Ulysses observations obtained during the spacecraft's first fast latitude transition in 1994–1995. The polytropic MHD equations are solved for a steady coronal outflow that includes the addition of Alfvén wave momentum and energy in the WKB approximation. A solution for the outflow in a tilted dipole magnetic field in the inner computational region (1–20 R⊙) is combined with a three-dimensional solution in the outer region which extends to 10 AU. The inner region solution is essentially the same as in the work of Usmanov et al. [2000] but has been obtained for slightly different boundary conditions using a different numerical algorithm. The dipole orientation is chosen to match the one inferred from photospheric magnetic field observations at the Wilcox Solar Observatory. The steady solution in the outer region is constructed using a marching-along-radius method and models both solar rotation and interaction regions. The bimodality of solar wind with a rapid change in flow parameters with latitude and the observed extent of the slower wind belt are reproduced well. We compare our simulation also with the results of Bruno et al. [1986] and empirical models of coronal density. We show that the simulation results are in good agreement with the empirical model of Wang and Sheeley [1990].