Derivation and precision of mean field electrodynamics with mesoscale fluctuations [CL]

Mean field electrodynamics (MFE) facilitates practical modeling of secular, large scale properties of magnetohydrodynamic or plasma systems with fluctuations. Practitioners commonly assume wide scale separation between mean and fluctuating quantities, to justify equality of ensemble and spatial or temporal averages. Often however, real systems do not exhibit such scale separation. This raises two questions: (1) what are the appropriate generalized equations of MFE in the presence of mesoscale fluctuations? (2) how precise are theoretical predictions from MFE? We address both by first deriving the equations of MFE for different types of averaging, along with mesoscale correction terms that depend on the ratio of averaging scale to variation scale of the mean. We then show that even if these terms are small, predictions of MFE can still have a significant precision error. This error has an intrinsic contribution from the dynamo input parameters and a contribution from the difference in the way observations and theory are projected through the measurement kernel. Minimizing the sum of these contributions can identify an optimal scale of averaging that makes the theory maximally precise. The precision error is important to quantify when comparing to observations because it quantifies the resolution of predictive power. We exemplify these principles for galactic dynamos, comment on broader implications, and identify possibilities for further work.

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H. Zhou, E. Blackman and L. Chamandy
Thu, 12 Oct 17

Comments: 27 pages, 7 figures. Submitted to Journal of Plasma Physics

Fully Kinetic Simulation of 3D Kinetic Alfven Turbulence [CL]

We present results from a fully kinetic, three-dimensional plasma turbulence simulation, resembling the typical plasma conditions found at kinetic scales of the solar wind. The spectral properties of the turbulence in the sub-ion range are consistent with theoretical expectations for kinetic Alfv\’ en waves. Furthermore, we calculate the scale-dependent anisotropy, defined by the relation $k_{\parallel}(k_{\perp})$, where $k_{\parallel}$ is a characteristic wavenumber along the local mean magnetic field at perpendicular scale $l_{\perp}\sim 1/k_{\perp}$. Over a limited range of sub-ion scales, the obtained scaling is close to $k_{\parallel}\propto k_{\perp}^{1/3}$, consistent with the standard analytical prediction for a critically balanced kinetic Alfv\’ en cascade. Our results compare favourably against a number of in-situ solar wind observations and demonstrate—from first principles—the feasibility of plasma turbulence models based on a critically balanced cascade of kinetic Alfv\’ en waves.

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D. Groselj, A. Mallet, N. Loureiro, et. al.
Wed, 11 Oct 17

Comments: submitted for publication

Extreme case of Faraday effect: magnetic splitting of ultrashort laser pulses in plasmas [CL]

The Faraday effect, caused by a magnetic-field-induced change in the optical properties, takes place in a vast variety of systems from a single atomic layer of graphenes to huge galaxies. Currently, it plays a pivot role in many applications such as the manipulation of light and the probing of magnetic fields and material’s properties. Basically, this effect causes a polarization rotation of light during its propagation along the magnetic field in a medium. Here, we report an extreme case of the Faraday effect where a linearly polarized ultrashort laser pulse splits in time into two circularly polarized pulses of opposite handedness during its propagation in a highly magnetized plasma. This offers a new degree of freedom for manipulating ultrashort and ultrahigh power laser pulses. Together with technologies of ultra-strong magnetic fields, it may pave the way for novel optical devices, such as magnetized plasma polarizers. In addition, it may offer a powerful means to measure strong magnetic fields in laser-produced plasmas.

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S. Weng, Q. Zhao, Z. Sheng, et. al.
Tue, 10 Oct 17

Comments: 18 pages, 5 figures

Insight into atmospheres of extrasolar planets through plasma processes [SSA]

Extrasolar planets appear in a chemical diversity unseen in our own solar system. Despite their atmospheres being cold, continuous and transient plasma processes do affect these atmosphere where clouds form with great efficiency. Clouds can be very dynamic due to winds for example in highly irradiated planets like HD 189733b, and lightning may emerge. Lightning, and discharge events in general, leave spectral fingerprints, for example due to the formation of HCN. During the interaction, lightning or other flash–ionisation events also change the electromagnetic field of a coherent, high energy emission which results a characteristic damping of the initial, unperturbed (e.g. cyclotron emission) radiation beam. We summarise this as ‘recipe for observers’. External ionisation by X-ray or UV e.g. from within the interstellar medium or from a white dwarf companion will introduce additional ionisation leading to the formation of a chromosphere. Signatures of plasma processes therefore allow for an alternative way to study atmospheres of extrasolar planets and brown dwarfs.

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C. Helling and I. Vorgul
Tue, 10 Oct 17

Comments: refereed proceeding (3 referees) for ‘Planetary Radio Emissions VIII’, Austrian Academy of Sciences Press

Magnetic pumping as a source of particle heating and power-law distributions in the solar wind [CL]

Based on the rate of expansion of the solar wind, the plasma should cool rapidly as a function of distance to the Sun. Observations show this is not the case. In this work, a magnetic pumping model is developed as a possible explanation for the heating and the generation of power-law distribution functions observed in the solar wind plasma. Most previous studies in this area focus on the role that the dissipation of turbulent energy on microscopic kinetic scales plays in the overall heating of the plasma. However, with magnetic pumping particles are energized by the largest scale turbulent fluctuations, thus bypassing the energy cascade. In contrast to other models, we include the pressure anisotropy term, providing a channel for the large scale fluctuations to heat the plasma directly. In this work a complete set of coupled differential equations describing the evolution, and energization, of the distribution function are derived, as well as an approximate closed form solution. Numerical simulations using the VPIC kinetic code are applied to verify the model’s analytical predictions. The results of the model for realistic solar wind scenario are computed, where thermal streaming of particles are important for generating a phase shift between the magnetic perturbations and the pressure anisotropy. In turn, averaged over a pump cycle, the phase shift permits mechanical work to be converted directly to heat in the plasma. The results of this scenario show that magnetic pumping may account for a significant portion of the solar wind energization.

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E. Lichko, J. Egedal, W. Daughton, et. al.
Fri, 6 Oct 17

Comments: 7 pages, 4 figures

Broadening of Cyclotron Resonance Conditions in the Relativistic Interaction of an Intense Laser with Overdense Plasmas [CL]

The interaction of dense plasmas with an intense laser under a strong external magnetic field has been investigated. When the cyclotron frequency for the ambient magnetic field is higher than the laser frequency, the laser’s electromagnetic field is converted to the whistler mode that propagates along the field line. Because of the nature of the whistler wave, the laser light penetrates into dense plasmas with no cutoff density, and produces superthermal electrons through cyclotron resonance. It is found that the cyclotron resonance absorption occurs effectively under the broadened conditions, or a wider range of the external field, which is caused by the presence of relativistic electrons accelerated by the laser field. The upper limit of the ambient field for the resonance increases in proportion to the square root of the relativistic laser intensity. The propagation of a large-amplitude whistler wave could raise the possibility for plasma heating and particle acceleration deep inside dense plasmas.

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T. Sano, Y. Tanaka, N. Iwata, et. al.
Tue, 3 Oct 2017

Comments: 8 pages, 8 figures, accepted for publication in PRE

Hot-cold plasma transition region: collisionless case [SSA]

We study processes at the transition region between hot (rare) and cold (dense) plasma in the collisionless regime. We use a 3-dimensional electromagnetic particle-in-cell (3-D PIC) relativistic code. Motivated by the transition region in the solar atmosphere the temperature and density ratio of the plasmas is chosen as 100 and 0.01, respectively. For better understanding of studied processes we make two types of computations: a) without any interactions among plasma particles (free expansion) and b) with the full electromagnetic interactions. In both the cases we found that the flux of cold plasma electrons and protons from colder plasma to hotter one dominates over the flux of hot plasma electrons and protons in the opposite direction. Thus, the plasma in the hotter part of the system becomes colder and denser during time evolution. In the case without any interactions among particles the cold plasma electrons and protons freely penetrate into the hot plasma. But, the cold plasma electrons are faster than cold plasma protons and therefore they penetrate deeper into the hotter part of the system than the protons. Thus, the cooling of the electron and proton components of the plasma in the hotter part of the system is different. On the other hand, in the case with the electromagnetic interactions, owing to the plasma property, which tries to keep the total electric current constant everywhere (close to zero in our case), the cold plasma electrons penetrate into the hotter part of the system together with the cold plasma protons. The plasma waves generated at the transition region during these processes reduce the number of electrons escaping from the hot plasma into the colder one. Therefore these waves support a temperature jump between hot and cold plasma.

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M. Karlicky and F. Karlicky
Mon, 25 Sep 2017

Comments: 7 pages, 9 figures