Events at Physics
Events on Friday, February 25th, 2022
- Theory Seminar (High Energy/Cosmology)
- Underdamped Axionic Blue Isocurvature Perturbations
- Time: 1:00 pm
- Place: Chamberlin 5280
- Speaker: Sai Chaitanya Tadepalli, UW Madison
- Abstract: Previous computations of strongly blue tilted axionic isocurvature spectra were computed in the parametric region in which the lightest time-dependent mass is smaller than the Hubble expansion rate during inflation, leading to an overdamped time evolution. In this talk, we present the strongly blue tilted axionic isocurvature spectrum in an underdamped time evolution parametric regime. Somewhat surprisingly, there exist parametric regions with a strong resonant spectral behavior that leads to rich isocurvature spectral shapes and large amplitude enhancements. We focus on computing this resonant spectrum analytically in parametric regions amenable to such computations. Because the spectrum is sensitive to nonperturbative classical field dynamics, we will discuss a wide variety of analytic techniques that are used like decoupling, nonlinear field redefinition, a time-space effective potential obtained by integrating out high-frequency fluctuations, and a piecewise mass-model.
- Physics Department Colloquium
- Magnetic Reconnection, a Celestial Phenomenon in the Laboratory
- Time: 3:30 pm
- Place: 2241 Chamberlin Hall
- Speaker: Jan Egedal and the WiPPL team , UW Madison
- Abstract: The Earth is embedded in the Sun's extended atmosphere, where the Earth's magnetic field acts as a shield against the incoming solar wind plasma. However, this shielding is not perfect. Through the process of magnetic reconnection solar particles can penetrate the magnetosphere and greatly influence the conditions in space that affect the Earth and its technological systems---magnetic reconnection controls space weather. Notable aspects of reconnection include its ability to convert magnetic energy into particle energy while changing the topology of the magnetic field lines. Although reconnection occurs in microscopic diffusion regions, it often governs the macroscopic properties and behavior of the systems. It controls the evolution of explosive events such as solar flares, coronal mass ejections and magnetic storms in the Earth's magnetotail. The latter drives the auroral phenomena.
The phenomenon is explored on the Terrestrial Reconnection Experiment (TREX) at UW-Madison, where the absolute rate of reconnection is set by an external drive. A shock interface between the supersonically driven plasma inflow and a region of magnetic flux pileup permits the normalized reconnection rate to self-regulate to a fixed value. In agreement with numerical and theoretical results, the width of the electron diffusion regions is characterized by the kinetic length scale of the electrons. While the reconnection layers are modulated by a current-driven instability, their characteristics remain consistent with a 3D simulation for which off-diagonal stress in the electron pressure tensor is responsible for fast reconnection.
- Host: Cary Forest