Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session YO8: Space and Astrophysical Plasmas |
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Chair: Li-Jen Chen, NASA Goddard Room: OCC C120-122 |
Friday, November 9, 2018 9:30AM - 9:42AM |
YO8.00001: A model for how induced reversed-currents form the 3-part CME structure Magnus A Haw, Pakorn Wongwaitayakornkul, Hui Li, Paul M Bellan We report here a new model for explaining the three-part structure of CMEs consisting of a core prominence, a density cavity surrounding the prominence, and a shock-like leading edge feature. The model proposes that the cavity in a CME forms because a rising electric current in the core prominence induces an oppositely directed electric current in the background plasma; this eddy current is required to satisfy the frozen-in magnetic flux condition in the background plasma. The magnetic force between the inner core electric current and the oppositely directed induced eddy current propels the background plasma away from the core creating a density cavity and a density pileup at the cavity edge. The model is supported by laboratory experiments, 3D numerical MHD simulations, and can predict cavity widths to first order. This mechanism has broad applicability because the predicted widths are relatively independent of both the current injection mechanism and the injection timescale. |
Friday, November 9, 2018 9:42AM - 9:54AM |
YO8.00002: Resistively untangling plasma knots Christopher Berg Smiet, Tobias De Jong, David Kok, Hugo de Blank, Dirk Bouwmeester When a plasma containing highly tangled magnetic field lines is allowed to relax in a high-beta plasma environment, the helicity in the initial configuration gives rise to an ordered self-organizing magnetic configuration. This situation is reminiscent of what happens when the twisted field of a coronal loop is ejected into the high-beta plasma of the solar wind forming a magnetic cloud. We study the resistive evolution of these structures in as simple a geometry as possible; 3D MHD simulations on a fixed Eulerian grid. We briefly describe the equilibrium of the self-organized equilibrium attained: nested toroidal magnetic surfaces, with a minimum of pressure on the magnetic axis. The resistive evolution follows a universal pattern when scaled to resistive time; a Pfirsch-Schlüter like slip allows plasma to flow onto the axis, and the structure slowly expands. The rotational transform becomes nearly constant, and decays according to a power law. The magnetic energy decays faster than resistive time due to expansion perpendicular to the field direction. |
Friday, November 9, 2018 9:54AM - 10:06AM |
YO8.00003: Wave generation and heat flux suppression in astrophysical plasma systems Gareth Roberg-Clark, James Frederick Drake, Michael M Swisdak, Christopher Reynolds Thermal conduction in weakly collisional, weakly magnetized plasmas such as the intracluster medium of galaxy clusters and the solar wind is not fully understood. One possibility is that electron-scale turbulence can inhibit thermal fluxes through scattering. In our particle-in-cell (PIC) simulation model two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes oblique whistler waves. The whistlers grow to large amplitude and resonantly scatter the electrons, strongly suppressing the heat flux. Unlike classical conduction the steady state heat flux is largely insensitive to the imposed temperature gradient. The derived scaling law for thermal conduction has been confirmed in solar wind measurements by Tong et al. (arXiv 2018). We have extended our results to lower beta, which is more relevant for the solar wind and corona. In this regime whistlers gradually become subdominant and the heat flux is mostly regulated by electrostatic double layers, which modify the thermal conduction scaling. |
Friday, November 9, 2018 10:06AM - 10:18AM |
YO8.00004: Embedding an electromagnetic region in a hybrid particle simulation Mikhail Alexander Belyaev, David Jeffrey Larson, Bruce Ira Cohen We propose a new computational approach to simulating a vacuum or neutral electromagnetic (EM) region embedded or adjacent to a plasma region modeled using the hybrid equations. Our approach uses the fact that the hybrid model solves the fully EM form of the induction equation to advance the magnetic field in time. Hence, the magnetic field is solved globally on the same grid for both regions. This gives del.B = 0 to machine precision when using Yee’s algorithm for advancing the magnetic field in time. The electric fields in the hybrid and EM regions are joined together smoothly based on an electron density cutoff. The boundary between hybrid and EM regions is fully dynamic and evolves in time based on the evolution of the electron density. Because the Courant condition is more restrictive for our explicit finite difference time domain (FDTD) EM solver than for the hybrid solver, we subcycle the FDTD solver. This is not a limitation, however, as running with a ratio of 100 FDTD subcycles per hybrid timestep or more does not significantly impact the speed of the computation. |
Friday, November 9, 2018 10:18AM - 10:30AM |
YO8.00005: The Electric Field of the Sun and Solar Wind_{ } C. Fred Driscoll A simple model of solar electric fields explains the solar wind energetics and chromospheric heating, invoking only gravitational settling and photon scattering. In the (collisional) solar interior, gravity necessarily generates a radial electric field eE~ -½ m_{p} g; protons are 50% levitated, with eE(R_{s})~ 1.4eV/Mm from displaced charge Q(R_{s})~ -75.Coul. In the (weakly collisional) outer photosphere/chromosphere, electron scattering of the photon flux Γ_{E} gives eE_{ }= (Γ_{E}/c) σ_{γe}. An (averged) eE~ (4.eV/Mm) (r/R_{s})^{-2} from photon-electron cross-section σ_{γe}~ 3x10^{-24}m^{2} ≤ 10^{-3} σ(H-) can generate the observed solar wind: protons are accelerated out of the 2.keV gravity well and up to 1.3keV kinetic energy within several R_{s}, with total particle energy flux ~10^{-6} Γ_{E}. This coherent proton/electron "flow-sheath" is the K-Corona, obviating the T~100eV hydrostatic model (Van deHulst, 1950). Filamentation (~1.Mm)^{2} of the flow arises from the convection/recombination ("roiling") dynamics of surface granulations, with local electric fields generating strong currents and local magnetic fields. Statistical charge fluctuations, current filamentation, and neutral gas drag on the distant proton/electron flows produce the pervasive fluctuating magnetic fields observed by spacecraft. |
Friday, November 9, 2018 10:30AM - 10:42AM |
YO8.00006: The Occurrence Rate of Ion Driven Instabilities in the Solar Wind Kristopher G Klein, Justin C Kasper, Michael Stevens, Benjamin L Alterman, Daniel Vech Weakly collisional plasmas, including the solar wind, are frequently found in states far from thermodynamic equilibrium and are therefore susceptible to a myriad of instabilities. |
Friday, November 9, 2018 10:42AM - 10:54AM |
YO8.00007: Magneto-immutability: the resistance of weakly collisional plasmas to changes in magnetic-field strength Jonathan Squire, Matthew W. Kunz, Eliot Quataert, Alexander A. Schekochihin We propose that pressure-anisotropy causes weakly collisional plasmas to self organize so as to resist changes in magnetic field strength. We term this effect “magneto-immutability” by analogy with incompressibility (resistance to changes in density). We study magneto-immutability using simulations of magnetized (Alfvénic) turbulence in the weakly collisional Braginskii model, which show that magneto-immutable turbulence is similar, in most statistical measures, to critically balanced MHD turbulence. However, in order to minimize magnetic-field variation, the flow becomes nearly scalar, and the turbulence is modestly dominated by magnetic energy (a nonzero “residual energy”). This effect represents a key difference between magnetized kinetic and fluid turbulence at the outer scale, and should be observable in the turbulent solar wind. |
Friday, November 9, 2018 10:54AM - 11:06AM |
YO8.00008: Stability analysis of core-strahl electron distributions in the solar wind Konstantinos Horaites, Patrick Astfalk, Stanislav A Boldyrev, Frank Jenko In this work, we analyze the kinetic stability of a solar wind electron distribution composed of core and strahl subpopulations. The core is modeled by a drifting Maxwellian distribution, while the strahl is modeled by an analytic function recently derived in (Horaites et al. 2018) from the collisional kinetic equation. We perform a numerical linear stability analysis using the LEOPARD solver (Astfalk & Jenko 2017), which allows for arbitrary gyrotropic distribution functions in a magnetized plasma. We do not find evidence for a whistler instability directly associated with the electron strahl. We however find that for typical solar wind conditions, the core-strahl distribution is unstable to the kinetic Alfvén and magnetosonic modes. The maximum growth rates for these instabilities occur at wavenumbers kd_{i} ~ 1 (d_{i} is the ion inertial length), at moderately oblique angles of propagation, providing a potential source of kinetic-scale turbulence. We suggest that the whistler modes may appear as a result of nonlinear mode coupling and turbulent cascade originating at scales kd_{i} ~ 1. |
Friday, November 9, 2018 11:06AM - 11:18AM |
YO8.00009: Simulation of magnetospheric chorus wave generation with the tristan-mp pic code Ilya Kuzichev, Angel Rualdo Soto-Chavez, Jaehong Park, Andrew Gerrard, Anatoly Spitkovsky Chorus waves are one of the most important and interesting wave phenomena in the Earth’s outer radiation belt. They belong to the whistler mode and have frequencies from hundreds of Hz to several kHz. They are observed as discrete series of wave packets each having varying frequency mostly in the form of rising tones, but falling tones are also found. Chorus waves play significant role in the radiation belt dynamics: being one of the most intense wave phenomena they lead to particle acceleration and precipitation via resonant wave-particle interactions. The generation mechanisms of rising and, especially, falling tone chorus waves remain an active area of research. A common feature of most of the chorus theoretical models is that they are non-linear, which significantly reduces the opportunities to investigate corresponding processes analytically. This study shows the results of chorus wave simulation with TRISTAN-MP 2D Particle-In-Cell (PIC) code, which treats both cold and hot electrons kinetically and uses correct relativistic form of the distribution function. |
Friday, November 9, 2018 11:18AM - 11:30AM |
YO8.00010: A Parameterized Model of X-ray Solar Flare Effects on the Lower Ionosphere and HF Propagation Edlyn V. Levine, Peter J. Sultan, Lucien J. Teig We present a parameterized X-ray solar-flare effects model relating the physics of radiation transport to the observable impact of solar flares on low-altitude ionospheric absorption of High Frequency (HF) signals. Tunable parameters of time varying flare spectral energy density and characteristic flare temperature provide a novel capability to simulate HF experiments over a wide range of X-ray solar flare behavior. Results from our model are consistent with HF propagation data collected over a period of heightened solar flare activity during 5-7 September 2017, including M and X class solar flares. Our predictions and measurements are compared with results from D-RAP [Akmaev et al., 26 2010] and the Wait VLF-driven model [Wait and Spies, 1964].
Akmaev, R., A. Newman, M. Codrescu, C. Schulz, and E. Nerney (2010), Drap model validation: I. scientific report, https://www.ngdc.noaa.gov/stp/drap/DRAP- V-Report1.pdf, accessed: 2018-04-20. Wait, J., and K. Spies (1964), Characteristics of the earth-ionosphere waveguide for vlf radio waves, NBS Technical Note, U.S. 300. |
Friday, November 9, 2018 11:30AM - 11:42AM |
YO8.00011: Generation of Plasmas Around Spinning Black Holes Mikhail Medvedev, Alex Ford, Brett D Keenan Relativistic jets are ubiquitously observed in numerous systems with central black holes (BH) with a wide range of masses: from the stellar mass BH in microquasars to the supermassive BH in active galactic nuclei and quasars. These jets are thought to be powered by the Blandford-Znajek mechanism, which taps the spin energy of the BH into the EM Poynting flux. In order to operate, the entire region must be conductive, that is it must contain plasma. It the steady state regime, the plasma can only be replenished {\it in situ} via an electron-positron cascade. Here we first discuss the mechanism. Furthermore, we present the results of the numerical solutions of the appropriate equations and deduce interesting and important scaling relations between the plasma parameters, the output gamma-ray-power, mass of the central BH and the ambient radiation field from an accretion disk. Observational predictions are also made. |
Friday, November 9, 2018 11:42AM - 11:54AM |
YO8.00012: Axion-Driven Cosmic Magnetogenesis during the QCD Crossover Gianluca Gregori, Francesco Miniati, Brian Reville, Subir Sarkar We have proposed a novel mechanism for the generation of a magnetic field in the early universe during the QCD crossover assuming that dark matter is made of axions. Thermoelectric fields arise at pressure gradients in the primordial plasma due to the difference in charge, energy density and equation of state between the quark and lepton components. The axion field is coupled to the electro-magnetic field, so when its spatial gradient is misaligned with the thermoelectric field, an electric current is driven. Due to the finite resistivity of the plasma an electric field appears that is generally rotational. This seed field, while initially small, in then amplified by turbulent dynamo, driven by the same pressure gradients responsible for the thermoelectric fields, and a magnetic field is generated on sub-horizon scales. Because of the expansion of the Universe, the magnetic field unwinds and reaches a present day strength of B ~ 10^{-13} G on a characteristic scale L_{ B} ~ 20 pc. The resulting combination of B L_{B}^{1/2 }is significantly stronger than in any astrophysical scenario, providing a clear test for the cosmological origin of the field through γ-ray observations of distant blazars which probe cosmic voids. Such observations will be made with the Cherenkov telescope array. |
Friday, November 9, 2018 11:54AM - 12:06PM |
YO8.00013: Fully conservative scheme for relativistic Landau–Fokker–Planck equation Takashi Shiroto, Takashi Asahina, Yasuhiko Sentoku In our previous work, a charge-momentum-energy-conserving algorithm was developed for the relativistic Vlasov–Maxwell system by the finite-difference method [T. Shiroto et al., arXiv 1802.07238 (2018)]. The scheme is based on the central difference scheme, so numerical dissipations are not included in the algorithm. This is why we extend the conservative algorithm to the relativistic Vlasov–Fokker–Planck–Maxwell system in order to introduce the effect of dissipation by the collision terms. In the non-relativistic regime, a conservative scheme has already been developed [W.T. Taitano et al., JCP 297, 357 (2015)], and our approach is similar to theirs but based on the potential equations of Braams and Karney [B.J. Braams and C.F.F. Karney, PRL 59, 1817 (1987)]. At the moment, a mass-momentum-energy-conserving scheme for the relativistic Landau–Fokker–Planck equation is composed with explicit time integration, and one-dimensional and three-velocity-components numerical experiments are performed. However, our conservative Vlasov–Maxwell scheme is based on an implicit time integration. Therefore, we will improve the conservative scheme with the combination of the implicit scheme and the Jacobian-Free Newton–Krylov method. |
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