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This Week at Physics

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Events on 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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