arXiv subjects

Mark Peter Rast

Publications and source records attributed to Mark Peter Rast.

4 recordsLinked to original sources

arXiv2023

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.

astro-ph.SR↗

arXiv2021

Contact statistics in populations of noninteracting random walkers in two dimensions

The interaction between individuals in biological populations, dilute components of chemical systems, or particles transported by turbulent flows depends critically on their contact statistics. This work clarifies those statistics under the simplifying assumptions that the underlying motions approximate a Brownian random walk and that the particles can be treated as noninteracting. We measure the contact-interval (also called the waiting-time or inter-arrival-time), contact-count, and contact-duration distributions in populations of individuals undergoing noninteracting continuous-space-time random walks on a periodic two-dimensional plane (a torus), as functions of the population number density, walker radius, and random-walk step size. The contact-interval is exponentially distributed for times longer than the mean-free-collision time but not for times shorter than that, and the contact duration distribution is strongly peak at the ballistic-crossing time for head-on collisions, when the ballistic-crossing time is short compared to the mean step duration. While successive contacts between individuals are independent, the probability of repeat contact decreases with time after a previous contact. This leads to a negative duration dependence of the waiting-time interval and over dispersion of the contact-count probability density function for all time intervals. The paper demonstrates that for populations of small particles (walker radius small compared to the mean-separation or random-walk step size) the mean-free-collision interval, the ballistic-crossing time, and the random-walk-step duration can be used to construct temporal scalings which allow for common waiting-time, contact-count, and contact-duration distributions across different populations. Semi-analytic approximations for both the waiting-time and contact-duration distributions are also presented.

math-ph↗

arXiv2021

Identifying acoustic wave sources on the Sun I. Two-dimensional waves in a simulated photosphere

The solar acoustic oscillations are likely stochastically excited by convective dynamics in the solar photosphere, though few direct observations of individual source events have been made and their detailed characteristics are still unknown. Wave source identification requires measurements that can reliably discriminate the local wave signal from the background convective motions and resonant modal power. This is quite challenging as these 'noise' contributions have amplitudes several orders of magnitude greater than the sources and the propagating wave fields they induce. In this paper, we employ a high-temporal-frequency filter to identify sites of acoustic emission in a radiative magnetohydrodynamic simulation. The properties of the filter were determined from a convolutional neural network trained to identify the two-dimensional acoustic Green's function response of the atmosphere, but once defined, it can be directly applied to an image time series to extract the signal of local wave excitation, bypassing the need for the original neural network. Using the filter developed, we have uncovered previously unknown properties of the acoustic emission process. In the simulation, acoustic events are found to be clustered at mesogranular scales, with peak emission quite deep, about 500 km below the photosphere, and sites of very strong emission can result from the interaction of two supersonic downflows that merge at that depth. We suggest that the method developed, when applied to high-resolution high-cadence observations, such as those forthcoming with Daniel K. Inouye Solar Telescope (DKIST), will have important applications in chromospheric wave-studies and may lead to new investigations in high-resolution local-helioseismology.

astro-ph.SR↗

arXiv2016

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.

astro-ph.SR↗