Mark Peter Rast

Interactive desktop analysis of high resolution simulations: application to turbulent plume dynamics and current sheet formation

The ever increasing processing capabilities of the supercomputers available to computational scientists today, combined with the need for higher and higher resolution computational grids, has resulted in deluges of simulation data. Yet the computational resources and tools required to make sense of these vast numerical outputs through subsequent analysis are often far from adequate, makingsuchanalysisofthedataapainstaking,ifnotahopeless,task.Inthispaper, we describe a new tool for the scientific investigation of massive computational datasets. This tool (VAPOR) employs data reduction, advanced visualization, and quantitative analysis operations to permit the interactive exploration of vast datasets using only a desktop PC equipped with a commodity graphics card. We describe VAPORs use in the study of two problems. The first, motivated by stellar envelope convection, investigates the hydrodynamic stability of compressible thermal starting plumes as they descend through a stratified layer of increasing density with depth. The second looks at current sheet formation in an incompressible helical magnetohydrodynamic flow to understand the early spontaneous development of quasi two-dimensional (2D) structures embedded within the 3D solution. Both of the problems were studied at sufficiently high spatial resolution, a grid of 504 2 by 2048 points for the first and 1536 3 points for the second, to overwhelm the interactive capabilities of typically available analysis resources.

THE SCALES OF GRANULATION, MESOGRANULATION, AND SUPERGRANULATION

Solar granulation is described as an advection-fragmentation process in the upper layers of the convection zone. The fundamental hydrodynamic unit is the downflow plume, and from its structure the granular scale follows. Moreover, through the collective advective interaction of many small-scale and short-lived granular plumes, large spatial and long temporal mesogranular and supergranular scales naturally arise. We illustrate and examine this process of scale selection using a simplified n-body advective-interaction model. For parameters set by granulation observations and numerical plume simulations, clustering scales remarkably close to observed mesogranulation and supergranulation result.

A prototype discovery environment for analyzing and visualizing terascale turbulent fluid flow simulations

Scientific visualization is routinely promoted as an indispensable component of the knowledge discovery process in a variety of scientific and engineering disciplines. However, our experiences with visualization at the National Center for Atmospheric Research (NCAR) differ somewhat from those described by many in the visualization community. Visualization at NCAR is used with great success to convey highly complex results to a wide variety of audiences, but the technology only rarely plays an active role in the day-to-day scientific discovery process. We believe that one reason for this is the mismatch between the size of the primary simulation data sets produced and the capabilities of the software and visual computing facilities generally available for their analysis. Here we describe preliminary results of our efforts to facilitate visual as well as non-visual analysis of terascale scientific data sets with the aim of realizing greater scientific return from such large scale computation efforts.

The Thermal Starting Plume as an Acoustic Source

We propose that solar acoustic oscillations are excited by localized cooling events and new downflow-plume formation at the solar surface. The excitation process involves, in successive stages, radiative cooling, buoyant acceleration, and advective inflow. Pressure fluctuations induced at each stage result in monopolar, dipolar, and quadrupolar acoustic emission. We examine this excitation mechanism in detail, measure the acoustic energy output by such events, and discuss possible observational implications for helioseismic spectra.

Latitudinal Variation of the Solar Photospheric Intensity

We have examined images from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO) in search of latitudinal variation in the solar photospheric intensity. Along with the expected brightening of the solar activity belts, we have found a weak enhancement of the mean continuum intensity at polar latitudes (continuum intensity enhancement ~0.1%-0.2%, corresponding to a brightness temperature enhancement of ~2.5 K). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and Ca II K intensity distributions shifted toward higher values with little change in shape from their midlatitude distributions. Since the enhancement is of low spatial frequency and of very small amplitude, it is difficult to separate from systematic instrumental and processing errors. We provide a thorough discussion of these and conclude that the measurement captures real solar latitudinal intensity variations.

On the asymmetry of solar acoustic line profiles

We study a simplified model of solar acoustic oscillations and show how asymmetries in spectral lines depend both on the acoustic source depth, as previously recognized, and on the acoustic source type. We provide a unified description of modal line asymmetries and high-frequency pseudomode locations, suggesting an inversion on power spectra minima to determine source properties and a correction to Lorentzian line shapes based upon the relative locations of spectral peaks and valleys. We also consider nonadiabatic effects due to Newtonian cooling and demonstrate that these do not lead to notable differences between velocity and intensity power spectral line shapes. We argue more generally that it is unlikely that any nonadiabatic effect can be responsible for the observed differences. Finally, we discuss the importance of both multiplicative and additive background power to the spectra and show how additive noise can reduce the apparent line asymmetry of a mode. We note that information on solar convective motions can be potentially extracted from three components of the acoustic power spectra: the additive background yielding information on the spectrum of nonoscillatory motions at the height of observation, the multiplicative background reflecting the source spectrum, and the power minima providing the source depth and physical nature. For stochastically excited linear waves only the first of these contributes significantly to spectral differences between observed variables.

Persistent North-South Alignment of the Solar Supergranulation

We have found evidence of an alignment of the solar supergranulation in the direction parallel to the Suns rotation axis. Signatures of the alignment are apparent in both time-averaged images and in three-dimensional power spectra. The north-south organization is persistent in time, extending over many supergranular lifetimes. It occurs over a wide latitudinal extent, to ±60°, and shows variation on a 10°-30° scale. These properties, as well as the rotation rate of the pattern, suggest a underlying larger scale dynamical cause. We examine a mechanism by which giant cell motions may contribute to such alignment.

Ionization effects in three-dimensional solar granulation simulations

These numerical studies show that ionization influences both the transport and dynamical properties of compressible convection near the surface of the Sun. About two-thirds of the enthalpy transported by convective motions in the region of partial hydrogen ionization is carried as latent heat. The role of fast downflow plumes in total convective transport is substantially elevated by this contribution. Instability of the thermal boundary layer is strongly enhanced by temperature sensitive variations in the radiative properties of the fluid, and this provides a mechanism for plume initiation and cell fragmentation in the surface layers. As the plumes descend, temperature fluctuations and associated buoyancy forces are maintained because of the increased specific heat of the partially ionized material. This can result is supersonic vertical flows. At greater depths, ionization effects diminish, and the plumes are decelerated by significant entrainment of surrounding fluid.

Bright rings around sunspots

There are two possible explanations for why sunspots are dark: the partial suppression by the sunspot magnetic fields of convective energy transport from the underlying layers, or the removal of energy from the sunspot by enhanced hydromagnetic wave radiation. Both processes would reduce the energy emitted radiatively. The first explanation is currently favoured, and predicts that the blocked energy should show up as a bright ring around the spot, with the actual brightness of the ring sensitive to details of solar convective transport and sunspot structure. Previous searches for these bright rings were inconclusive because of the presence of bright, vertical magnetic flux tubes near the spots, and a lack of sufficient precision in the observations. Here we report high-photometric-precision observations of bright rings around eight sunspots. The rings are about 10 K warmer than the surrounding photosphere and extend at least one sunspot radius out from the penumbra. About 10% of the radiative energy missing from the sunspots is emitted through the bright rings. We also report observations of a second set of sunspots, for which simultaneous magnetic field measurements demonstrate that the rings are not associated with vertical flux tubes.

The Role of Subsurface Flows in Solar Surface Convection: Modeling the Spectrum of Supergranular and Larger Scale Flows

We model the solar horizontal velocity power spectrum at scales larger than granulation using a two-component approximation to the mass continuity equation. The model takes four times the density scale height as the integral (driving) scale of the vertical motions at each depth. Scales larger than this decay with height from the deeper layers. Those smaller are assumed to follow a Kolomogorov turbulent cascade, with the total power in the vertical convective motions matching that required to transport the solar luminosity in a mixing length formulation. These model components are validated using large scale radiative hydrodynamic simulations. We reach two primary conclusions: 1. The model predicts significantly more power at low wavenumbers than is observed in the solar photospheric horizontal velocity spectrum. 2. Ionization plays a minor role in shaping the observed solar velocity spectrum by reducing convective amplitudes in the regions of partial helium ionization. The excess low wavenumber power is also seen in the fully nonlinear three-dimensional radiative hydrodynamic simulations employing a realistic equation of state. This adds to other recent evidence suggesting that the amplitudes of large scale convective motions in the Sun are significantly lower than expected. Employing the same feature tracking algorithm used with observational data on the simulation output, we show that the observed low wavenumber power can be reproduced in hydrodynamic models if the amplitudes of large scale modes in the deep layers are artificially reduced. Since the large scale modes have reduced amplitudes, modes on the scale of supergranulation and smaller remain important to convective heat flux even in the deep layers, suggesting that small scale convective correlations are maintained through the bulk of the solar convection zone.