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

Direct Plasmon-Driven Photoelectrocatalysis

NEWS COVERAGE: A news release based on this journal publication is available online: Rice researchers demo solar water-splitting technology [http://news.rice.edu/2015/09/04/rice-researchers-demo-solar-water-splitting-technology/]Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies. Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts, (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot electron injection for different particle diameters to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. Interestingly, our studies did not reveal differences in the hot-electron generation and injection efficiencies for the investigated particle dimensions and plasmon resonances

Broadband Absorption Engineering to Enhance Light Absorption in Monolayer MoS₂

Here we take a first step toward tackling the challenge of incomplete optical absorption in monolayers of transition metal dichalcogenides for conversion of photon energy, including solar, into other forms of energy. We present a monolayer MoS₂-based photoelectrode architecture that exploits nanophotonic light management strategies to enhance absorption within the monolayer of MoS₂, while simultaneously integrating an efficient charge carrier separation mechanism facilitated by a MoS₂/NiO_x heterojunction. Specifically, we demonstrate two extremely thin photoelectrode architectures for solar-fuel generation: (i) a planar optical cavity architecture, MoS₂/NiO_x/Al, that improves optical impedance matching and (ii) an architecture employing plasmonic silver nanoparticles (Ag NPs), MoS₂/Ag NPs/NiO_x/Al, that further improves light absorption within the monolayer. We used a combination of numerical simulations, analytical models, and experimental optical characterizations to gain insights into the contributions of optical impedance matching versus plasmonic near-field enhancement effects in our plasmonic photoelectrode structures. By performing three-dimensional electromagnetic simulations, we predict structures that can absorb 37% of the incident light integrated from 400 to 700 nm within a monolayer of MoS₂, a 5.9× enhanced absorption compared to that of MoS₂ on a sapphire (Al₂O₃) substrate. Experimentally, a 3.9× absorption enhancement is observed <i>in the total structure</i> compared to that of MoS₂/Al₂O₃, and photoluminescence measurements suggest this enhancement largely arises from absorption enhancements within the MoS₂ layer alone. The results of these measurements also confirm that our MoS₂/NiO_x/Al structures do indeed facilitate efficient charge separation, as required for a photoelectrode. To rapidly explore the parameter space of plasmonic photoelectrode architectures, we also developed an analytical model based on an effective medium model that is in excellent agreement with results from numerical FDTD simulations

Searching for a Solar Source of Magnetic-Field Switchbacks in Parker Solar Probe's First Encounter

Parker Solar Probe observations show ubiquitous magnetic-field reversals closer to the Sun, often referred to as "switchbacks". The switchbacks have been observed before in the solar wind near 1 AU and beyond, but their occurrence was historically rare. PSP measurements below similar to 0.2 AU show that switchbacks are, however, the most prominent structures in the "young" solar wind. In this work, we analyze remote-sensing observations of a small equatorial coronal hole to which PSP was connected during the perihelion of Encounter 1. We investigate whether some of the switchbacks captured during the encounter were of coronal origin by correlating common switchback in situ signatures with remote observations of their expected coronal footpoint. We find strong evidence that timescales present in the corona are relevant to the outflowing, switchback-filled solar wind, as illustrated by strong linear correlation. We also determine that spatial analysis of the observed region is optimal, as the implied average solar-wind speed more closely matches that observed by PSP at the time. We observe that hemispherical structures are strongly correlated with the radial proton velocity and the mass flux in the solar wind. The above findings suggest that a subpopulation of the switchbacks are seeded at the corona and travel into interplanetary space.ISSN:0038-0938ISSN:1573-093

Firefly: The Case for a Holistic Understanding of the Global Structure and Dynamics of the Sun and the Heliosphere

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere