Fall 2020

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


Oct 23

Understanding Our Heliospheric Shield: Laying the Groundwork to Predict Habitable Astrospheres

Prof. Merav Opher, Boston University

The heliosphere is an immense shield that protects the solar system from harsh, galactic radiation. This radiation affects not only life on Earth, but human space exploration as well. In order to understand the evolution of the heliosphere’s shielding properties, we need to understand its structure and large-scale dynamics. The heliosphere is a template for all other astrospheres, enabling predictions about the conditions necessary to create habitable planets. Space science is at a pivotal point in generating new understandings of the heliosphere due to the flood of new in situ data from the Voyager 1 (V1), Voyager 2 (V2), and New Horizon spacecraft, combined with the energetic neutral atom (ENA) maps generated by IBEX and Cassini.

I this talk I will review some of the most pressing aspects that need understanding in the heliosphere. Among them, the shape of the heliosphere. The canonical view of the structure of the heliosphere is that it has a long comet-like tail. This view is not universally accepted and there is vigorous debate as to whether it possesses a long comet-like structure, is bubble shaped, or is “croissant”-like, a debate that is driven by observations and modeling.

Opher et al. (2015) suggest a heliosphere with two lobes, described as “croissant”-like. An extension of the single ion global 3D MHD model that treats PUIs created in the supersonic solar wind as a fluid separate and distinct from the thermal solar wind plasma yields a heliosphere that is  reduced in size and rounder in shape (Opher et al. 2020). In contrast, Izmodenov et al. 2020 argue that a long/extended tail confines the plasma.

One direct way to probe the structure of the tail is through energetic neutral atom (ENA) maps. ENA images of the tail by Interstellar Boundary Explorer (IBEX) at energies of 0.5-6keV exhibit a multi-lobe structure. These lobes are attributed to signatures of slow and fast wind within the extended heliospheric tail as part of the 11-year solar cycle (McComas et al. 2013; Zirnstein et al. 2017). Higher energy ENA observations (>5.2 keV) from the Cassini spacecraft, in conjunction with >28 keV in-situ ions from V1&2/LECP (Dialynas et al. 2017), in contrast, support the interpretation of bubble-like heliosphere.

Regardless of the shape of the heliotail, there is an agreement between models that the solar magnetic field in the inner heliosheath (IHS) possesses a “slinky-like” structure (Opher et al. 2015; Pogorelov et al. 2015; Izmodenov et al. 2015) that helps confine the plasma in the IHS.

I will review some of the recent discoveries and challenges as part of the recently funded NASA Science Center SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics).


Oct 30

The Earth's Ion Foreshock: A natural laboratory for ion beam-generated  waves and non-linear wave processes

Dr. Seth Dorfman, SRI


Waves generated by accelerated particles are important throughout our  heliosphere.  These particles often gain their energy at shocks via  Fermi acceleration.  At the Earth's bow shock, this mechanism  accelerates ion beams back into the solar wind; the beams can then  generate ultra low frequency (ULF) waves below the ion cyclotron 
frequency via an ion-ion right hand resonant instability. These ULF waves influence the shock structure and particle acceleration, lead to coherent structures in the magnetosheath, and are ideal for non-linear interaction studies relevant to turbulence.

We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock [1]. This is made possible by employing the two ARTEMIS spacecraft orbiting the moon at ∼60 Earth radii from Earth to characterize crescent-shaped reflected ion beams and relatively monochromatic ULF waves.  Using ARTEMIS data, the ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from 
resonant beam instability theory and prior satellite measurements.

Building on this result, new work is underway to determine the statistical properties of the ULF waves at the location of ARTEMIS and make comparisons with the global hybrid-Vlasov code Vlasiator.  For example, an analysis of all foreshock events observed by ARTEMIS from 2011-2019 shows a clear preference for the left-hand polarized waves (in the spacecraft frame) expected from the ion-ion right hand resonant instability.  However, unlike a 2.5-D Vlasiator simulation in which the waves are entirety left-hand polarized, ARTEMIS data shows a significant right-hand component that is unlikely to be directly generated by the ion beam.  This component may therefore result from non-linear processes that are 3-D in nature with potential applications to turbulence and dissipation in the heliosphere.

[1] S. Dorfman, H. Hietala, P. Astfalk, and V. Angelopoulos, Geophys. Res. Let. 44 (2017).


Nov 6

Experimental evidence of detached bow shock formation in the interaction of a laser-produced plasma with a magnetized obstacle

Dr. Joseph Levesque, University of Michigan

The magnetic field produced by planets with active dynamos, like the Earth, can exert sufficient pressure to oppose supersonic stellar wind plasmas, leading to a standing bow shock upstream of the pressure-balance surface, known as the magnetopause. Scaled laboratory experiments studying the interaction of an inflowing solar wind analog with a strong, external magnetic field can provide another way to study magnetospheric physics and complement existing models. In this talk I present experimental evidence of the formation of a magnetized bow shock in the interaction of a supersonic, super-Alfvenic plasma with a strongly magnetized obstacle at the OMEGA laser facility.

 

The plasma source for these experiments is generated by the simultaneous laser-irradiation of two thin carbon discs, the resulting counter-propagating plasma plumes collide and subsequently expand outward toward the magnetized obstacle, which is a thin, current-carrying wire. We measure the plasma number density in the interaction region using Spatially resolved, optical Thomson scattering, from which we infer the presence of what appears to be a fast magnetosonic shock far upstream of the obstacle. Proton images additionally provide a measurement of large-scale features of the magnetic field topology based on proton deflections, and further suggest the formation of a bow shock by an inferred compression of the magnetic field in our system. From these images we determine the shock standoff distance and analyze the evolution of the bow shock for two applied field strengths.


Nov 20

Relativistic electron-ion shocks in gamma-ray bursts: What about pair loading?
Dr. Daniel Groselj, Columbia

In the external gamma-ray burst (GRB) collisionless shocks, intense radiation gives rise to abundant production of electron-positron pairs in two-photon collisions. The upstream medium is thus transformed into a mixed composition electron-ion-positron plasma. How exactly the pair loading affects the structure, and the resulting particle acceleration, of external GRB shocks is largely unknown. Here, I present a set of first-principles kinetic simulations to elucidate the microscale physics of relativistic pair-plasma-loaded, weakly magnetized shocks. I will show that even moderate changes in the plasma composition can significantly impact the shock dynamics. More specifically, I will demonstrate that (i) the transition from a Weibel-mediated to a magnetized type shock becomes a function of the pair loading factor Z, (ii) the energy fraction transferred from ions to pairs is only weakly dependent on Z, and (iii) pair-loaded shocks are efficient particle accelerators in the limit of vanishing magnetization. These findings have important implications for the modeling of the early afterglow emission of GRBs.


Dec 4, 10AM

Solar Wind Turbulence: in-situ observations from magneto-fluid to kinetic plasma scales
Prof. Olga Alexandrova, Observatoire de Paris, LESIA

This seminar is devoted to solar wind turbulence from MHD to kinetic plasma scales. Solar wind turbulence was mostly studied at MHD scales: there, magnetic fluctuations follow the Kolmogorov spectrum. The fluctuations are mostly incompressible and they have non-Gaussian statistics (intermittency), due to the presence of coherent structures in the form of current sheets, as it is widely accepted. Kinetic range of scales is less known and the subject of debates.

We study the transition from Kolmogorov inertial range to small kinetic scales with a number of space missions. It becomes evident that if at ion scales (100-1000 km) turbulent spectra are variable, at smaller scales they follow a general shape. Thanks to Cluster/STAFF, the most sensitive instrument to measure magnetic fluctuations by today, we could resolve electron scales (1 km, at 1 AU) and smaller (up to 300 m) and show that the end of the electromagnetic turbulent cascade happens at electron Larmor radius scale, i.e., we could establish the dissipation scale in collisionless plasma. Recently, we have confirmed these results closer to the Sun (at 0.3 AU).

Furthermore, we show that intermittency is not only related to current sheets, but also to cylindrical magnetic vortices, which are present within the inertial range as well as in the kinetic range. This result is in conflict with the classical picture of turbulence at kinetic scales, consisting of a mixture of kinetic Alfven waves. The dissipation of these waves via Landau damping may explain the turbulent dissipation. How does this picture change if turbulence is not only a mixture of waves but also filled with coherent structures such as magnetic vortices?

These vortices seem to be an important ingredient in other instances, such as astrophysical shocks: for example, they are observed downstream of Earth's and Saturn's bow-shocks. With the new data of Parker Solar Probe and Solar Orbiter we hope to study these vortices closer to the Sun to better understand their origin, stability and interaction with charged particles.


Dec 18

Using magnetic fields and microgravity to explore the physics of dusty plasmas

Prof. Edward Thomas, Jr, Auburn

Over the last three decades plasma scientists have learned how to control a new type of plasma system known as a “complex” or “dusty” plasma.  These are four-component plasma systems that consist of electrons, ions, neutral atoms, and charged, solid, nanometer- to micrometer-sized particles.  The presence of these microparticles allow us to “tune” the plasma to have solid-like, fluid-like, or gas-like properties.  This means that dusty plasmas are not just a fourth state of matter – they can take on the properties of all four states of matter.  

From star-forming regions to planetary rings to fusion experiments, charged microparticles can be found in many naturally occurring and man-made plasma systems.  Therefore, understanding the physics of dusty plasmas can provide new insights into a broad range of astrophysical and technological problems.  This presentation introduces the physical properties of dusty plasmas – focusing on how the small charge-to-mass ratio of the charged microparticles gives rise to many of the characteristics of the system.  In particular, dusty plasmas can be used to study a variety of processes in non-equilibrium or dissipative systems such as self-organization and energy cascade as well as a variety of transport and instability mechanisms.  This presentation will discuss results from our studies of dusty plasmas in high (B ≥ 1 T) magnetic fields using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station.