Photometric Trends in the Visible Solar Continuum and Their Sensitivity to the Center-to-Limb Profile

Solar irradiance variations over solar rotational time-scales are largely determined by the passage of magnetic structures across the visible solar disk. Variations on solar cycle time scales are thought to be similarly due to changes in surface magnetism with activity. Understanding the contribution of magnetic structures to total solar irradiance and solar spectral irradiance requires assessing their contributions as a function of disk position. Since only relative photometry is possible from the ground, the contrasts of image pixels are measured with respect to a center-to-limb intensity profile. Using nine years of full-disk red and blue continuum images from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we examine the sensitivity of continuum contrast measurements to the center-to-limb profile definition. Profiles which differ only by the amount of magnetic activity allowed in the pixels used to determine them yield oppositely signed solar cycle length continuum contrast trends; either agreeing with the result of Preminger et al. (2011) showing negative correlation with solar cycle or disagreeing and showing positive correlation with solar cycle. Changes in the center-to-limb profile shape over the solar cycle are responsible for the contradictory contrast results, and we demonstrate that the lowest contrast structures, internetwork and network, are most sensitive to these. Thus the strengths of the full-disk, internetwork, and network photometric trends depend critically on the magnetic flux density used in the quiet-sun definition. We conclude that the contributions of low contrast magnetic structures to variations in the solar continuum output, particularly to long-term variations, are difficult, if not impossible, to determine without the use of radiometric imaging.Comment: Accepted to ApJ. 11 pages, 5 figure

Supergranulation as the largest buoyantly driven convective scale of the Sun

Supergranulation is characterized by horizontally divergent flows with typical length scales of 32 Mm in the solar photosphere. Unlike granulation, the size of which is comparable to both the thickness of the radiative boundary layer and local scale height of the plasma in the photosphere, supergranulation does not reflect any obvious length scale of the solar convection zone. Early suggestions that the depth of second helium ionization is important are not supported by numerical simulations. Thus the origin of the solar supergranulation remains largely a mystery. Moreover, observations of flows in the photosphere using either Doppler imaging or correlation or feature tracking show a monotonic decrease in power at scales larger than supergranulation. Both local area and global spherical shell simulations of solar convection by contrast show the opposite, a power law increase in horizontal flow amplitudes to low wavenumber. Here we examine this disparity, and investigate how the solar supergranulation may arise as a consequence of strong photospheric driving and non-local heat transport by cool diving plumes. Using three dimensional anelastic simulations with surface driving, we show that the kinetic energy of largest convective scales in the upper layers of a stratified domain reflects the depth of transition from strong buoyant driving to adiabatic stratification below. We interpret the observed monotonic decrease in solar convective power at scales larger than supergranulation to be a consequence of this rapid transition, and show how the supergranular scale can be understood as the largest buoyantly driven mode of convection in the Sun

Spring 2010: Research Opportunity for Undergraduates

The goal of this effort was to engage a broader range of undergraduates (beyond those with the highest grades) in research, and to document what strategies work and where the difficulties lie in providing that broader population a meaningful undergraduate research experience. This then may lead to a more formalized undergrad research program in the department (if we decide we want to go that way), or more informally to some shared insights. Here I document as completely as possible what I have learned. Many of these things are well known to those with more experience, but I made no attempt to separate my naiveté from the presentation of the results. Those with limited time may wish to initially focus only on those passages marked in cyan italic to determine whether there is anything worthy of their closer attention

Is There Such a Thing as Quiet Sun?

The Cycle 23–Cycle 24 minimum was deep and prolonged, similar to minima of the late 19th and early 20th centuries but quite different from those between the overlapping cycles of the early space age. This provides a unique opportunity to study the Sun at very low levels of magnetic activity. Here we examine the quiet Sun, defining it to be those portions of the Sun for which continuum intensity variations are dominated by thermal perturbations as op- posed to opacity fluctuations due to the presence of magnetic fields. We briefly present evidence that: (1) The expected thermal signature of the solar super- granulation can not be separated from magnetic contributions without masking the contribution of at least 95% of the pixels. By this measure, at most 5% of the Sun is truly quiet. (2) There was a rapid decay of active network magnetic fields entering this solar minimum, a consequent increase in the internetwork area, but a nearly constant fractional area covered by network fields. This sug- gests the continuous fragmentation and decay of active region fields into weaker field components, but also, possibly, an underlying continuous flux concentration mechanism maintaining the network field. (3) One of the first flux emergence episodes of Cycle 24 did not occur as a coherent active region, but instead in the form of disorganized spatially-dispersed small-scale magnetic elements. Under the paradigm of a deep-rooted dynamo, this suggests an episode of incoherent field loss fromthe generation region or a failed/shredded omega loop rise through the convection zone.

Single-particle dispersion in stably stratified turbulence

We present models for single-particle dispersion in vertical and horizontal directions of stably stratified flows. The model in the vertical direction is based on the observed Lagrangian spectrum of the vertical velocity, while the model in the horizontal direction is a combination of a continuous-time eddy-constrained random walk process with a contribution to transport from horizontal winds. Transport at times larger than the Lagrangian turnover time is not universal and dependent on these winds. The models yield results in good agreement with direct numerical simulations of stratified turbulence, for which single-particle dispersion differs from the well studied case of homogeneous and isotropic turbulence

Coupled Analysis and Visualization of High Resolution Astrophysical Simulations

Computational physics has benefited from on-going microprocessor innovations, which have enabled larger and larger numerical simulations. One consequence of these technological advancements has been an explosion in the amount of data generated. For many modelers, available software tools and computing resources are proving inadequate for investigation of high-resolution numerical outputs. In this paper we discuss the general problems associated with very large data visualization and analysis and our work on a particular solution to those through the development of VAPOR (open source, available at http://www.vapor.ucar.edu): a desktop application that leverages today\u27s powerful CPUs and GPUs to enable visualization and analysis of terascale data sets using only a commodity PC or laptop. We briefly illustrate VAPOR\u27s utility through the exploration of a high-resolution simulation aimed at understanding the effects of hydrogen ionization on convective dynamics in stellar envelopes

VAPOR: Visual, Statistical, and Structural Analysis of Astrophysical Flows

In this paper we discuss recent developments in the capabilities of VAPOR: a desktop application that leverages today’s powerful CPUs and GPUs to enable visualization and analysis of terascale data sets using only a commodity PC or laptop. We review VAPOR’s current capabilities, highlighting support for Adaptive Mesh Refinement (AMR) grids, and present new developments in interactive feature-based visualization and statistical analysis

Identifying Acoustic Wave Sources on the Sun. II. Improved Filter Techniques for Source Wavefield Seismology

In this paper we refine a previously developed acoustic-source filter (Bahauddin & Rast 2021), improving its reliability and extending its capabilities. We demonstrate how to fine-tune the filter to meet observational constraints and to focus on specific wavefront speeds. This refinement enables discrimination of acoustic-source depths and tracking of local-source wavefronts, thereby facilitating ultra-local helioseismology on very small scales. By utilizing the photospheric Doppler signal from a subsurface source in a MURaM simulation, we demonstrate that robust ultra-local three-dimensional helioseismic inversions for the granular flows and sound speed to depths of at least 80 km below the photosphere are possible. The capabilities of the National Science Foundation's new Daniel K. Inouye Solar Telescope (DKIST) will enable such measurements of the real Sun.Comment: One mp4 video file include