Plasma Physics Seminar
 
   
  Spring 2021 Schedule  
 
January 27th, Wed. 3:30PM
Virtual Presentation
Kevin Genestreti, SwRI
The onset of reconnection in Earth's magnetotail

The elongated tail of our nightside magnetosphere stores energy from the solar wind. The oppositely-directed magnetic fields in the northern and southern tail are separated by a current sheet, which periodically becomes very thin then "short circuits". Understanding the causes of magnetic reconnection - the "short circuit" mechanism - is an important yet elusive problem in space physics. We have learned a tremendous amount about reconnection by observing it while it is occurring in space, yet it is still debated how reconnection starts in the first place. Ambiguity stems from the difficulty of determining the accurate time history of a reconnection event over the vast range of relevant spatial scales. The standard picture is that reconnection of the solar wind and Earth's dayside magnetic fields drives global (~1015 km3) magnetospheric convection, which can thin the current sheet by compressing it or depleting its internal pressure. However, tail reconnection does not necessarily follow dayside reconnection and some thin current sheets remain stable. Microscopic (~107 km3) magnetic reconnection sites are formed after one of several proposed instabilities is triggered. While several instabilities accompany reconnection, causal relationships have been difficult to verify.

We start the seminar by summarizing the basic concepts of magnetospheric reconnection and the wide-ranging impacts of reconnection on near-Earth space. We then report a case study where many space and ground-based observatories witnessed magnetotail reconnection being initiated. We find that the tail current sheet became thin by evacuating its internal thermal pressure without significant dayside reconnection. The solar wind prompted the pressure evacuation and, eventually, initiated reconnection by momentarily compressing the tail. Reconnection was initiated in multiple locations once the current sheet surpassed the threshold for the electron-tearing instability, which requires a sufficiently thin current sheet with a weak magnetic field and a low ion-to-electron temperature ratio. One reconnection site quickly engulfed the others, becoming the dominant region that shredded the tail's magnetic field, fitting with simple models.
February 3rd, Wed. 3:30PM
Virtual Presentation
Gareth Roberg-Clark, Max-Planck-Institut fur Plasmaphysik
Calculating the linear critical gradient for the ion-temperature-gradient mode in magnetically confined plasmas

A first-principles method to calculate the critical temperature gradient for the onsetof the ion-temperature-gradient mode (ITG) in linear gyrokinetics is presented. We find that conventional notions of the connection length previously invoked in tokamak research should be revised and replaced by a generalized correlation length to explain this onset in stellarators. Simple numerical experiments and gyrokinetic theory show that localized "spikes" in shear, a hallmark of stellarator geometry, are generally insufficient to constrain the parallel correlation length of the mode. ITG modes that localize within bad drift curvature wells that have a critical gradient set by peak drift curvature are also observed. A case study of nearly helical stellarators of increasing field period demonstrates that the critical gradient can indeed be controlled by manipulating magnetic geometry, but underscores the need for a general framework to evaluate the critical gradient. We conclude that average curvature and global shear set the correlation length of resonant ITG modes near the absolute critical gradient, the physics of which is included through direct solution of the gyrokinetic equation. Our method, which handles general geometry and is more efficient than conventional gyrokinetic solvers, could be applied to future studies of stellarator ITG turbulence optimization.
February 10th, Wed. 3:30PM
Virtual Presentation
Open


February 17th, Wed. 3:30PM
Virtual Presentation
Open


February 24th, Wed. 3:30PM
Virtual Presentation
Open


March 3rd, Wed. 3:30PM
Virtual Presentation
Open


March 10th, Wed. 3:30PM
Virtual Presentation
Open


March 17th, Wed. 3:30PM
Virtual Presentation
Spring Break


March 24th, Wed. 3:30PM
Virtual Presentation
Marina Battaglia, FHNW, Switzerland
X-ray diagnostics of accelerated particles in solar flares

March 31st, Wed. 3:30PM
Virtual Presentation
Joe Penano, NRL


April 7th, Wed. 3:30PM
Virtual Presentation
Open


April 14th, Wed. 3:30PM
Virtual Presentation
Katherine Goodrich, UC-Berkeley


April 21st, Wed. 3:30PM
Virtual Presentation
Open


April 28th, Wed. 3:30PM
Virtual Presentation
Open


May 5th, Wed. 3:30PM
Virtual Presentation
Open