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Events on Thursday, February 29th, 2024

Plasma Seminar
Investigations of fundamental Alfvénic Wave Physics
Time: 1:00 pm - 2:00 pm
Place: B343 Sterling Hall
Speaker: Seth Dorfman, Space Science Institute
Abstract: nvestigations of fundamental Alfvénic wave physics in the laboratory and in space Low frequency Alfvénic waves and fluctuations are ubiquitous in laboratory and space plasmas, and these fundamental modes of a magnetized plasma often serve as building blocks for more complicated structures and dynamics. The linear and non-linear properties of these waves may play key roles in the turbulent solar wind, heating of the solar corona, and the environment near the Earth's bow shock. In this seminar, I will present a vision for how laboratory and spacecraft studies focused on these fundamental building blocks can help us develop a more complete picture of important space physics phenomena. Our recent work on the Large Plasma Device at UCLA successfully isolated important non-linear Alfvénic phenomena that may be building blocks of the turbulent solar wind [e.g., Dorfman and Carter, PRL 2016]. Recent results include a proof-of-principle measurement of the Parametric Decay Instability (PDI) growth rate [Dorfman, et. al, in prep]; PDI has been previously shown to bound the solar wind parameter space. We also recorded the first observation of residual energy in a non-linear Alfvén wave interaction [Abler, et. al, in prep]; this is important because residual energy is observed in the inertial range of the turbulent cascade (i.e. there is more energy in the magnetic than the velocity fluctuations), but an MHD Alfvén wave has equal amounts of energy in fluctuations of each type. On the spacecraft study side, I will introduce the Earth's ion foreshock as a natural laboratory for wave studies and show a new method to detect the foreshock edge that also has wide implications for the interpretation of minimum variance techniques commonly used to determine wave properties [Dorfman, et. al, 2023]. After examining these various examples, I will discuss the prospects of a new Solar Wind Machine aimed at producing magnetized plasma turbulence in the laboratory for detailed study to complement and extend spacecraft observations [Dorfman, et. al, Heliophysics Decadal White Paper 2022]. You are invited to join us on April 18-20 for a workshop to refine physics targets and develop candidate machine designs:
Host: Jan Egedal
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
New probes of ultrahigh energy cosmic ray source evolution
Time: 2:30 pm - 3:30 pm
Place: Supernova @WIPAC
Speaker: Marco Muzio, Penn State University
Abstract: Despite first observing cosmic rays with energies above an EeV (10^18 eV) in the 1960s, the source of these particles remains an open question. Modern observatories, in particular the Pierre Auger Observatory and Telescope Array, have firmly established that the cosmic ray spectrum continues up to ~10^20.3 eV and have significantly advanced our understanding of these particles. However, limited statistics, uncertainties in particle physics, and significant deflections in the Galactic magnetic field have made progress towards discovering their astrophysical source extremely challenging. One key astrophysical input needed to understand ultrahigh energy cosmic ray data is the distribution of their sources, or the source evolution. In this talk, I will focus on multimessenger observations which have the potential to pin down the source evolution for the very first time.
Host: Lu Lu
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Astronomy Colloquium
Exploring the diversity of H2-H2O subNeptunes
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Raymond T. Pierrehumbert, University of Oxford
Abstract: Astronomical observations directly probe the properties of only the outer portions of a planet's atmosphere. When both mass and radius observations are available, the resulting mean density provides further, though highly degenerate, constraints on the composition of the interior. In this talk, I will discuss the kinds of inferences that can be drawn when the two kinds of information are put together. The emphasis will be on planets whose fluid layer is composed of H2 and H2O with various proportions, potentially interacting with a silicate core. An important physical consideration constraining plausible interior structures is that for liquid water interiors, the solubility of H2 is constrained by Henry's Law solubility, whereas for supercritical water interiors H2 (and other gases) are completely miscible with the interior. We will discuss the range of possible H2:H2O ratios in the outer atmosphere that can be compatible with a supercritical water atmosphere. Although an H2 layer is miscible with a supercritical water interior, there is a stable density jump at the interface, which inhibits mixing between the two layers; an essential missing piece of the puzzle is the quantification of the rate of such mixing. Once mixing begins, the moistening of the H2 layer leads to additional phenomena, including both water vapour feedback and generation of steep radiative layers near the interface through compositional stabilization of the lower atmosphere. I will also discuss thermal evolution models and implications of interaction of the H2:H2O fluid layer with a basal magma ocean. K2-18b and GJ1214b will be used as the archetypes of two very different types of subNeptunes, but I will also discuss results from a recent JWST survey of subNeptunes selected to have densities compatible with a potentially H2O-rich composition.
Host: Ke Zhang
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