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This Week at Physics

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Events During the Week of February 24th through March 3rd, 2019

Monday, February 25th, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Quantifying heating by magnetic pumping through in situ spacecraft observations
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Emily Lichko , UW Madison
Abstract: Superthermal electrons and ions in power-law tails are observed throughout the universe in a variety of astrophysical systems, but how these particles are energized is an open question. It is well known that plasma can be heated by waves, but most theories of particle energization are based on wave-particle resonances which are only effective at particle velocities near the phase velocity of the wave, v ~ ω/k. Starting from the drift kinetic equation, we have derived a magnetic pumping model, similar to the magnetic pumping well-known in fusion research, where particles are heated by the largest scale turbulent fluctuations. We have shown that this is a complementary heating mechanism to the turbulent cascade in the solar wind, effective up to v ≤ ω/k, which results in power-law distributions like those observed in the solar wind [1]. However, compressional Alfvénic turbulence has the ability to magnetically trap superthermal particles. Magnetic trapping renders magnetic pumping an effective Fermi heating process for particles with v >> ω/k, and produces superthermal power-law distributions. To test this, we used satellite observations of the strong, compressional magnetic fluctuations near the Earth's bow shock from the Magnetospheric MultiScale (MMS) mission and found strong agreement with our model. Given the ubiquity of such fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated. [1] E. Lichko, J. Egedal, W. Daughton, and J. Kasper. Astrophys. J. Lett. 2, 850 (2017)
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Astronomy Colloquium
From Cores to Disks: Probing the Initial Conditions for Stars and Planets
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies are served at 3:30 PM, Talk begins at 3:45 PM
Speaker: Sarah Sadavoy, Harvard CfA
Abstract: Star formation is a multi-scale problem and there are numerous observational and theoretical challenges to connect large molecular clouds that span several parsecs to the planet-forming disks around young stars with sizes of ~ 100 au. I bridge the astrophysical processes seen across these widely different scales using observations of interstellar dust and gas. This presentation focuses on my recent observations of cloud structure from the Herschel Space Observatory and dust polarization with the ALMA interferometer. In particular, my ALMA observations represent the first unbiased survey of dust polarization in young protostellar disks and probe both magnetic fields and dust grain growth at early protostar stages. I also describe my future goals using new and existing facilities to answer fundamental questions about the formation of both stars and planets.
Host: Professor Snezana Stanimirovic
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Tuesday, February 26th, 2019

Chaos & Complex Systems Seminar
Our changing National climate: Vulnerabilities to adaptations
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: John Young, Wisconsin State Climatology Office
Abstract: The results of a national climate assessment are released at four-year intervals. They represent analyses of extensive weather and climate data. The first scientific analyses for the US and its specific climate regions was released in November 2017, and the most recent one addresses the impact of specific changes and attendant risks and adaptation challenges.

I will explain some highlights of the findings, with illustrations from trends in average seasonal climate and extremes, regional differences, and considerations of future adaptation challenges.
Host: Clint Sprott
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Council Meeting
Time: 3:30 pm
Place: 2314 Chamberlin Hall
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Wednesday, February 27th, 2019

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, February 28th, 2019

Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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Astronomy Colloquium
"Exploring Planet Habitability"
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Meredith MacGregor, Havard University
Abstract: More than 20% of nearby main sequence stars are surrounded by debris disks, where planetesimals, larger bodies similar to asteroids and comets in our own Solar System, are ground down through collisions. The resulting dusty material is directly linked to any planets in the system, providing an important probe of the processes of planet formation and subsequent dynamical evolution. I will present highlights from ongoing work that explores how planetary systems form and evolve by (1) probing the grain properties of material in debris disks, (2) connecting debris disk structure to sculpting planets, and (3) understanding the impact of stellar flares on planetary habitability. Measurements of the long wavelength spectral index determine the grain size distribution in circumstellar disks, informing constraints on composition and collision processes. Detailed modeling of ALMA millimeter observations constrains the properties of possible planets responsible for sculpting nearby debris disk systems. Resolved imaging of debris disks also detects the host stars in many cases, yielding additional insights into the radiation environment of these planetary systems. Together these results provide an exciting foundation to investigate the evolution of planetary systems through multi-wavelength observations.
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Friday, March 1st, 2019

Physics Department Colloquium
Exploring extreme states of matter at the Linac Coherent Light Source
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Frederico Fiuza, SLAC National Accelerator Laboratory
Abstract: The combination of bright coherent X-ray sources with high-power optical lasers is revolutionizing our ability to probe matter under extreme conditions. Under the action of present-day laser intensities, materials are rapidly ionized, electrons can wiggle with energies over 100 MeV, the ponderomotive pressure exceeds one billion atmospheres, and the electric and magnetic fields produced in the plasma reach 100 TV/m and 100 kiloTesla, respectively. Developments in X-ray free-electron lasers offer the possibility to probe these extreme plasma states with unprecedented temporal and spatial resolution. This combination thus opens a unique window into extreme plasma environments that cannot be produced or probed by any other means in the laboratory and that are important for applications that range from nuclear fusion and laboratory astrophysics to the development of compact radiation sources for medical imaging and therapy. I will discuss recent advances at the Linac Coherent Light Source (LCLS) at SLAC in these areas and the exciting opportunities that lie ahead.
Host: Cary Forest
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