Summer 2020

Unless otherwise stated, all seminars are Friday at 12PM Eastern

July 10

Magnetic pumping model for energizing superthermal particles applied to observations of the Earth's bow shock

Dr. Emily Lichko, University of Arizona

Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. It has been shown that magnetic pumping is an efficient mechanism for heating thermal particles, using the largest-scale magnetic fluctuations. Here we show that when magnetic pumping is extended to a spatially-varying magnetic flux tube, magnetic trapping of superthermal particles renders pumping an effective energization method for particles moving faster than the speed of the waves and naturally generates power-law distributions. We validated the theory by spacecraft observations of the strong, compressional magnetic fluctuations near the Earth’s bow shock from the Magnetospheric Multiscale mission. Given the ubiquity of magnetic 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. 

July 17

Magnetic Reconnection and Turbulence in Stellar-Convection-Zone-Relevant Laboratory Plasmas

Dr. Jack Hare, Imperial College London

Magnetic reconnection and magnetised turbulence are ubiquitous phenomena in our magnetised universe. These processes have been carefully studied in the photosphere of the sun, in the solar wind, and in laboratory experiments which can recreate these collisionless or weakly collisional conditions. 

However, these phenomena are also important strongly collisional plasmas, in which the mean free path is shorter than the ion and electron skin depths. One example is the convection zone of the sun, the opaque region beneath the photosphere which is difficult to study through observations. Ryutov noted that this regime is also present in dense z-pinches (Ryutov, 2015), which combine intense magnetic fields with high temperatures and densities. 

In this talk, I will discuss experiments which use mega-ampere currents to ablate, accelerate and sculpt plasma from initially solid-density targets, creating geometries such as a quasi-two-dimensional reconnection layer in which plasmoids form, or a column of turbulent plasma confined at the axis of an imploding wire-array z-pinch.

I will describe new diagnostics for studying the spectrum of turbulent fluctuations in the density, velocity, temperature and magnetic field, and I will present a new pulsed-power facility for studying magnetised high-energy-density plasmas which will be built at MIT.

[Ryutov, 2015]: "Characterizing the Plasmas of Dense Z-Pinches." IEEE TPS

Aug 7

Proton temperature anisotropy, Alfven waves, and the turbulence heating problem in the solar wind
Prof. Robert Wicks, University of Northumbria

Over the last 10 years many different studies have shown different and related forms of anisotropy about the magnetic field in the solar wind plasma. Protons have anisotropic temperature, the turbulent fluctuations have different amplitudes, polarisations, and frequency-dependent scaling, and instabilities and coherent waves propagate, grow and damp at different rates depending on their relative direction to the magnetic field. The big problem with this is that measurements made by single spacecraft rely on the solar wind flow to provide different measurement directions relative to the magnetic field. This means that our perception of what is happening is heavily biased by what occurs in the direction of flow of the plasma (radially away from the Sun). In this talk, I will review results investigating anisotropy and describe a novel method to leverage the Taylor hypothesis to identify the field-parallel and -perpendicular components of wavevectors measured by a single spacecraft. Comparing these results to proton temperature anisotropy then allows us to show that instabilities growing in the field-parallel direction are primarily cyclotron waves and associated with strong proton beams, and in the perpendicular direction are firehose instabilities (although these are rarer). Furthermore, we can associate the polarisation of the magentic field waves routinely observed close to the gyrofrequency to the different branches of the Alfven wave dispersion relation, confirming that the modes are at least somewhat similar to linear waves with left-handed polarisation in the parallel direction and right-handed in the perpendicular. When we sample the proton temperature anisotropy in this space a strong pattern emerges, with high perpendicular temperature where left-handed parallel modes and proton beams exist, and high parallel temperature where the right-handed perpendicular modes exist. This important result shows that cyclotron and landau damping play important roles in heating the solar wind, but also throws out a big problem. The polarisation of the wave measured is critically dependent on the sampling direction of the spacecraft (radial) and so it seems that the modes present in the radial direction have a disproportionately large effect on proton temperature. I will discuss a few ideas for why this might be true.

Aug 14

Constructing a Rosetta Stone for Plasma Heating and Particle Acceleration in Kinetic Plasma Physics

Prof. Gregory Howes, University of Iowa

The general question of how plasmas are heated and particles accelerated underlies many key challenges at the frontier of heliophysics and astrophysics, including solar coronal heating, particle acceleration in solar flares and supernova remnants, and auroral electron acceleration.  The hot and diffuse plasmas in many space and astrophysical environments lead to weakly collisional conditions, so plasma kinetic theory is essential to understand both how particles are energized and whether that leads to heating of the bulk plasma or the directed energization of accelerated particles. The field-particle correlation technique is an innovative method to understand how the electromagnetic fields energize particles in weakly collisional plasmas, yielding a velocity-space signature that is characteristic of a given mechanism of energization.  These signatures can be used both to distinguish and identify the mechanism at play and to determine the net rate of particle energization.  I will present the construction of a "Rosetta Stone" of these velocity-space signatures that can be used to identify the mechanisms of energization in kinetic plasma turbulence, collisionless magnetic reconnection, and collisionless shocks.

Aug 21

A Laboratory Model for Magnetized Stellar Winds

Dr. Ethan Peterson, MIT

Eugene Parker developed the first theory of how the solar wind interacts with the dynamo-generated magnetic field of the Sun. He showed that the wind carries the magnetic field lines away from the star, while their footpoints are frozen into the corona and twisted into an Archimedean spiral by stellar rotation. The resulting magnetic topology is now known as the Parker spiral and is the largest magnetic structure in the heliosphere. The transition between magnetic field co-rotating with a star and the field advected by the wind is thought to occur near the so-called Alfv\'en surface - where inertial forces in the wind can stretch and bend the magnetic field. According to the governing equations of magnetohydrodynamics, this transition in a magnetic field like the Sun's is singular in nature and therefore suspected to be highly dynamic. However, this region has yet to be observed in-situ by spacecraft or in the laboratory, but is presently the primary focus of the Parker Solar Probe mission. Here we show, in a synergistic approach to studying solar wind dynamics, that the large-scale magnetic topology of the Parker spiral can also be created and studied in the laboratory. By generating a rotating magnetosphere with Alfv\'enic flows, magnetic field lines are advected into an Archimedean spiral, giving rise to a dynamic current sheet that undergoes magnetic reconnection and plasmoid ejection. These plasmoids are born at the tip of the streamer cusp, driven by non-equilibrium pressure gradients, and carry blobs of plasma outwards at super-Alfv\'enic speeds, mimicking the observed dynamics of coronal helmet streamers. Further more, a simple heuristic model based on a critical plasmoid length scale and sonic expansion time is presented. This model explains the frequencies observed in the experiment and simulations (10s of KHz) and is consistent with the 90 minute plasmoid ejection period of full-scale coronal streamers as observed by the LASCO and SECCHI instrument suites.

Sept 11

Study the Alfvén-wave acceleration of auroral electrons in the laboratory using field-particle correlations

Prof. Jim Schroeder, Wheaton College

The acceleration of auroral electrons is primarily attributed to quasistatic field-aligned currents in the magnetosphere. However, dispersive Alfvén waves in inertial plasmas (vA > vte) have an electric field parallel to B0 and are frequently detected in the auroral magnetosphere traveling earthward with sufficient Poynting flux to produce auroras. Test particle simulations in relevant plasma conditions show inertial Alfvén waves can resonantly accelerate electrons to auroral energies. Satellite surveys find that inertial Alfvén waves deposit an amount of energy in the lower magnetosphere capable of accounting for one-third of all auroral luminosity. Despite these results supporting the hypothesis that inertial Alfvén waves accelerate a significant fraction of auroral electrons, the limitations of spacecraft data have so far prevented direct evidence of the acceleration process from being found. Laboratory experiments in UCLA’s Large Plasma Device seek to provide insight by launching inertial Alfvén waves and simultaneously measuring the parallel electron velocity distribution. The electron distribution is measured using wave absorption, a technique where a small-amplitude probe wave is absorbed in proportion to the number of resonant electrons. Alfvénic perturbations to the electron distribution have been detected, and, using a field-particle correlation, energy transfer to electrons from the launched Alfvén waves has been found. Experimental results are interpreted using kinetic theory and numerical simulations. 

Sept 18, 2PM Eastern

Interplay among Arched Plasma Eruptions, Global Oscillations, and Broad Spectra of Alfvén Waves

Dr. Shreekrishna Tripathi, UCLA

Arched magnetized structures that carry electrical current ubiquitously exist in solar and heliospheric plasmas. Varieties of plasma waves and current-driven instabilities (e.g., fast waves, kink, sausage, and Kelvin-Helmholtz instabilities) have been at the forefront of contemporary research in solar and heliospheric physics. After introducing key concepts related to eruptive processes in solar physics, results from a laboratory experiment on arched magnetized plasmas (plasma β ≈ 10-3, Lundquist number ≈ 102–105, plasma radius/ion-gyroradius ≈ 20, B ≈ 1000 Gauss at footpoints, 1/2 Hz repetition rate) will be presented. The arched plasma is created using a lanthanum hexaboride plasma source and it evolves in an ambient magnetized plasma produced by another source. The plasma and wave parameters are recorded with a good resolution using movable Langmuir and three-axis magnetic-loop probes in 3D. Images of the plasma are recorded using a CCD camera. In the upgraded experiment, the main focus is on the direct measurement of propagation and damping characteristics of global kink-mode oscillations and fast waves. These waves are frequently observed after eruptive events on the Sun. Recent results reveal fascinating interplay among global oscillations of the arched plasma and fast waves. Transverse gradients in Alfvén speed across the arched plasma have been observed to excite a broad spectra of fast Alfvén waves that carry away energy from large scale oscillations in the arched plasma. These observations are consistent with predictions of the phase mixing of fast waves in an inhomogeneous magnetized plasma that effectively enhances damping of large scale oscillations. Phase-mixing of these waves is likely to play important role in affecting the energetic of the solar atmosphere.

Oct 9, 2PM Eastern

Kinetic physics of the electrons in the solar wind

Prof. Daniel Verscharen,  University College London

The electron distribution function in the solar wind consists of three main populations: a thermal core, a suprathermal, quasi-isotropic halo, and a field-aligned beam called “strahl”. In contrast to the protons, the electrons are a sub-sonic particle population, and due to their small mass, they contribute little to the overall momentum flux of the solar wind. However, their unique kinetic properties supply the solar wind with a significant heat flux. 

We investigate the regulation of this heat flux by kinetic microinstabilities. I will present a mathematical framework for the description of electron-driven instabilities and discuss the associated physical mechanisms. We find that an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the best candidate for a microinstability that regulates the strahl heat flux by scattering strahl electrons into the halo population, consistent with spacecraft measurements. We derive approximate analytic expressions for the FM/W instability thresholds and confirm their accuracy through comparison with numerical solutions to the hot-plasma dispersion relation. 

The comparison of our theoretical results with a large statistical dataset from the Wind spacecraft confirms the relevance of the oblique FM/W instability for the solar wind. In addition, we find a good agreement between our theoretical results and numerical solutions to the quasilinear diffusion equation. I will present our results in the context of the latest measurements from Parker Solar Probe and Solar Orbiter.