Colloquium Schedule

Fall 2017

Magnetothermodynamics: Measuring the Equations of State of a Magnetized Plasma

Manjit Kaur, Swarthmore College
Fri., Sep. 22, 2017, 12:30 PM in Science Center 199

We have explored the thermodynamics of compressed magnetized plasmas in laboratory experiments and we call these studies "magnetothermodynamics". The experiments are carried out in the linear Swarthmore Spheromak eXperiment (SSX) device. In this device, a magnetized plasma source is located at one end of the device and at the other end, a closed conducting can is installed. We generate parcels of magnetized, relaxed plasma and observe their compression against the end wall of the conducting can. The plasma parameters such as plasma density, temperature, and magnetic field are measured during compression, using HeNe laser interferometry, ion Doppler spectroscopy and a linear B-dot probe array, respectively. To identify the instances of ion heating during compression, a PV diagram is constructed using measured density, temperature, and volume of the magnetized plasma. Various equations of state of the magnetized plasma are analyzed to estimate the adiabatic nature of the compressed plasma.

What Can We Still Learn About Climate from Radiative-Convective Equilibrium?

Tim Cronin, Massachusetts Institute of Technology
Fri., Oct. 6, 2017, 12:30 PM in Science Center 199

Understanding Earth’s climate, and how it may change due to the significant human impact on atmospheric composition, is a key scientific challenge of the 21st century. A large fraction of the uncertainty in predictions of climate change is related to how clouds and their impact on global energy balance may change with warming. Much of this problem in turn owes to our inability to resolve cloud-scale motions (~1km in size) in global climate models, which have grid boxes ~100 km in horizontal size. In this talk, I will focus on the idealized model configuration of radiative-convective equilibrium, and how it is helping us to understand cloud feedbacks on climate change.  Radiative-convective equilibrium (RCE) is the statistical state of the atmosphere and surface that is set by the overall climatic energy balance between absorbed sunlight and emitted infrared radiation, assuming a horizontally uniform surface and horizontally uniform incident sunlight. Although RCE has been used as an idealization of the climate system for over 50 years, increasing computational power in the last two decades has allowed for simulation of RCE that explicitly represents the atmospheric motions that form clouds. One intriguing finding of these simulations is that under some conditions, clouds can undergo a transition from randomly dispersed to highly aggregated. Aggregation of clouds alters the atmospheric energy balance and dries the atmosphere overall, possibly affecting both cloud and water vapor feedbacks on warming. I will talk about simulations and analysis that assess how aggregation of clouds in RCE may affect climate sensitivity, as well as bottom-up theoretical work to understand aggregation of clouds as a linear instability of RCE.  

Einstein's Last Legacy:  Ripples in Spacetime

Andrea Lommen, Haverford College
Fri., Oct. 27, 2017, 12:30 PM in Science Center 199

100 years ago Einstein made a prediction. He knew a couple things you already know, and I'll show you that you would've made the same prediction. He predicted that Gravitational Waves, ripples in spacetime, exist, but are far too small to ever be detected. He was right about the first part and wrong about the second. Thanks to LIGO we are in the midst of the era of gravitational waves, a rapidly expanding field that will tell us enormous amounts about the universe. Pulsar timing will soon make an analogous detection, but in a complementary part of the gravitational wave spectrum. I'll show you how our experiment is analogous to LIGO's experiment, and convince you that we should actually a expect an overall crinkling of spacetime in the universe from the cumulative effect of tens of thousands of super massive black holes distributed across the universe. Finally, I'll tell you about an x-ray telescope I launched to the International Space Station in June that is going to help us understand the noise in the pulsar timing data.

Nonlinear Dynamics in Nature: Mathematics for an Equation-Free W​orld​

Ethan Deyle, University of California San Diego
Fri., Nov. 17, 2017, 12:30 PM in Science Center 199

Equations have a long, proud history in the sciences. However, they are not without limits! In Earth and environmental sciences, the same fundamental laws are at play as are in the physics or chemistry lab, but the behavior we seek to explain is happening at much larger scales. It’s emergent behavior. While a great deal of effort is put into finding equations that describe these phenomena, the various mathematics of non-parametric modeling can be much better suited to the practical reality. In the first half of the talk, I’ll discuss these general ideas and give an introduction to my favored approach, “empirical dynamic modeling”. In the second half, I will describe a specific ongoing study of deoxygenation in Lake Geneva. Despite the physics of the lake being relatively simple, the dynamics of oxygen are also deeply convolved with the lake ecology. Consequently, study of the system with equation-free empirical dynamic modeling finds substantially greater traction than previous equation-based attempts.