Spring 2021

Unless otherwise stated, all seminars are Friday at 2PM Eastern


February 5th

Dynamic, Astrophysically- and Solar-Relevant MHD Plasma Experiments

Prof. Paul Bellan, Caltech
 
Dynamics relevant to solar and astrophysical plasmas is being investigated using MHD-regime lab experiments.  High-speed imaging resolves sub-Alfven time scales and reveals unexpected phenomena.  Images show that highly collimated MHD-driven, finite-beta plasma flows occur and can be considered a lab ‘replica’ of an astrophysical jet. Having both axial and azimuthal magnetic fields, the jet can be considered to be a flux rope, i.e., a plasma-confining flux tube with embedded helical magnetic field. The jet velocity is proportional to axial electric current and the jet flows axially from where the flux tube radius is small to where it is large.  Jet stagnation compresses embedded azimuthal magnetic flux resulting in self-collimation. An expanding solar coronal loop is effectively two jets pointing at each other carrying a common current and axial flux.  Jets kink when they breach the Kruskal-Shafranov stability limit. Lateral acceleration of a sufficiently strong kink produces a substantial effective gravity that provides the environment for a spontaneously developing fine-scale, fast Rayleigh-Taylor instability. This instability involves ‘heavy’ plasma interchanging with ‘light’ plasma in a series of ripples that choke the current channel to less than the ion skin depth. This cascade from the MHD scale to the ion skin depth scale can result in a fast magnetic reconnection whereby the jet breaks off.  Several distinct abrupt phenomena are observed when this happens. These include: radiation of a hard X-ray burst, emission of a whistler wave burst, localized EUV emission, and dimming of visible light.  
 

February 12th

Flux Ropes, Turbulence, and Collisionless Perpendicular Shock Waves: High Plasma Beta Case

G.P. Zank1, Masaru Nakanotoni1, Lingling Zhao1, Laxman Adhikari1, and Senbei Du2

1Department of Space Science and Center for Space Plasma and Aeronomic Research

The University of Alabama in Huntsville

2Los Alamos National Lab

With the onset of solar maximum and the likely increased prevalence of interplanetary shock waves, Parker Solar Probe is likely to observe numerous shocks in the next few years. An outstanding question that has received surprisingly little attention has been how turbulence interacts with collisionless shock waves. Turbulence in the supersonic solar wind is described frequently as a superposition of a majority 2D and a minority slab component. We formulate a collisional perpendicular shock-turbulence transmission problem in a way that enables investigation of the interaction and transmission of quasi-perpendicular fluctuations such as magnetic flux ropes/islands and vortices as well as entropy and acoustic modes in the large plasma beta regime. We focus on the transmission of an upstream spectrum of these modes, finding that the downstream spectral amplitude is typically increased significantly (a factor of 10 or more), and that the upstream spectral index of the inertial range, and indeed the general spectral shape, is unchanged for the downstream magnetic variance, kinetic energy, and density variance. A comparison of the theoretically predicted downstream magnetic variance, kinetic energy, and destiny variance spectra with those observed at 1 au, 5 au, and 84 au by Wind, Ulysses, and Voyager 2 shows excellent agreement. The overall theoretically predicted characteristics of the transmission of turbulence across shocks observed in the solar wind appear to be largely consistent with recent observational studies by Pitna et al. 2016, Pitna et al. 2020, and Borovsky 2020.


February 26th

Alfven Wave Damping and Heating in the Solar Corona

Dr. Michael Hahn, Columbia University

Understanding the mechanism by which the solar corona is heated to over a million Kelvin has been an unresolved problem in astrophysics for over 80 years. One theory is that energy is carried by Alfven waves into the corona where the waves are damped, thereby converting their energy into plasma heating. Using spectroscopic observations, we have found evidence that Alfven waves do carry enough energy for the heating and are indeed damped at low heights in the corona, as required by wave heating models. However, the physical processes that cause the wave damping are unknown. We are now investigating the cause of this damping through both observations and laboratory experiments. Recently, we studied intensity fluctuations in EUV images obtained by the Sun Watcher with Active Pixels (SWAP) instrument on the Proba2 satellite. These intensity fluctuations are proportional to density fluctuations, and show that density fluctuations grow in amplitude at heights similar to where the Alfven waves are damped. The density fluctuations change the local Alfven speed and are expected to cause reflection of the Alfven waves. Thus, the density fluctuations may help trap Alfven wave energy and promote dissipation through turbulence between the outward and reflected waves. We have also been carrying out laboratory experiments using the Large Plasma Device at the University of California Los Angeles. There, we have studied the propagation of Alfven waves through Alfven-speed gradients similar to those in the corona. Our results confirm that the transmission of Alfven wave energy is significantly reduced by the gradient. Surprisingly, though, we have not observed any reflection of the Alfven waves, which is the mechanism predicted by theory to be responsible for the reduced transmittance.


March 5th

Ion Wakes: Modeling Dust Plasma Interactions

Prof. Lorin Matthews, Baylor University

The interaction of an object within a streaming fluid is a phenomenon widely encountered in physics, spanning a range of length scales, from the familiar meter- to cm-sized wakes observed behind rocks in flowing streams (Fig. 1a), the kilometers-long wakes observed in cloud patterns as air flows past ocean islands (Fig. 1b), to the wakes produced in the bow shock of a speeding neutron star covering distances on the order of a parsecs (Fig. 1c).  Plasma, a gas consisting of electrons, ions, and neutral molecules, can also be considered as a fluid.  When plasma is moving with respect to an immersed object (such as a micron-sized dust grain), the object becomes charged and the trajectories of the ions in the plasma are altered as they flow past the charged body. Depending on flow velocity and the magnitude of the perturbing potential, ions can be focused into a region downstream of the object, creating an ion wakefield (Fig. 1d). Here we report results of coupled numerical models of the plasma discharge, ion wakefield and particle interactions in ground-based lab experiments and in microgravity experiments onboard the ISS.

Figure 1


March 12th

Parallel shock experiments on the Big Red Ball: Understanding the role of non-linear whistler waves

Doug Endrizzi and Cary Forest, University of Wisconsin

Whistler waves are a common feature of space plasmas, notably present upstream of shocklets, SLAMS, and switchbacks in the solar wind. In the laboratory, they have been studied for at least 60 years, being easily generated from RF antennas, electron beams, and in pulsed power experiments. On the Big Red Ball vacuum vessel, strong whistler waves appeared in recent parallel shock experiments, where a supersonic high-density plasma piston was launched parallel to the magnetic field. These experiments explored a large range in plasma beta (from 0.04 up to above 1.0) and had diagnostic coverage allowing for 2D views of the interaction. Results will be presented showing the generation of dispersive whistler waves from an abrupt nonlinear ramp at the leading edge of the piston. Analysis of the whistler electric and magnetic fields will show how they mediate shock-like behavior in the cylindrical geometry.

 


March 26th

Magnetic Reconnection Rate in Collisionless Plasmas

Prof. Yi-Hsin Liu, Dartmouth University

Magnetic reconnection is the process whereby the change in the magnetic field lines' connectivity allows for a rapid release of magnetic energy into the thermal and kinetic energy of the surrounding plasma. The magnitude of the reconnection electric field parallel to the reconnection x-line (where magnetic field lines break and rejoin) not only determines how fast reconnection processes magnetic flux, but can also be crucial for generating super-thermal particles. Observations and numerical simulations have revealed that collisionless magnetic reconnection in the steady-state tends to proceed with a normalized reconnection rate of an order of 0.1 in disparate systems. However, the explanation of fast reconnection remains an open question. In this talk, I will present a series of theory, modeling, and MMS (Magnetospheric Multiscale mission) observational studies on this issue.

We propose that this value 0.1 is essentially an upper bound value constrained by the force-balance at the upstream and downstream regions, independent of the dissipation-scale physics, independent of the mechanism that localizes the x-line. The prediction from this model compares favorably to particle-in-cell simulations of magnetic reconnection in both the non-relativistic and extremely relativistic limits, from symmetric to asymmetric reconnection.  Lately, we have included thermal pressure effects in our model to predict the rate in the high-beta limit.  We also extend our study from 2D to the 3D system, studying the impact of a short x-line extent in the out-of-plane direction. Finally, we show that the maximum plausible reconnection rate could determine some of the 3D nature of magnetic reconnection, particularly the orientation of the x-line. These results could be interesting to researchers who study solar, magnetospheric, astrophysical, and laboratory plasmas.


April 2nd

Prof. Craig Kletzing, University of Iowa


April 9th

Prof. Walter Gekelman, UCLA


April 23rd

Dr. Emily Mason, NASA Goddard


May 14th

Dr. Shiva Kavosi, University of New Hampshire