Session YO8: Space and Astrophysical Plasmas

A model for how induced reversed-currents form the 3-part CME structure

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.   

Resistively untangling plasma knots

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.   

Wave generation and heat flux suppression in astrophysical plasma systems

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.   

The Electric Field of the Sun and Solar Wind

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^-24m² ≤ 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. 

  

The Occurrence Rate of Ion Driven Instabilities in the Solar Wind

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.
Instead of focusing on a single source of free energy or a single kind of unstable mode, we assess the stability of solar wind observations against ion free energy sources using an automated implementation of Nyquist's instability criterion, accounting for the destabilizing effects of proton and alpha temperature anisotropies, temperature disequilibrium, and relative drifts between components. 53.7% of a representative sample of intervals from the Wind spacecraft are unstable, though only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities have maximum growth rates slower than ion-kinetic-scale timescales or the measurement cadence. Instabilities are more likely to arise when a proton beam is resolved, as well as for cases with relatively large alpha drift speeds or a departure of the core proton temperature from isotropy. This automated analysis method promises to offer insight into the role instabilities play in a variety of non-equilibrium plasma systems.   

Stability analysis of core-strahl electron distributions in the solar wind

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.   

Generation of Plasmas Around Spinning Black Holes

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.    

Axion-Driven Cosmic Magnetogenesis during the QCD Crossover

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.