Session CI02: Invited: Astro/Space

Earth’s Alfvén wings driven by the April 2023 Coronal Mass Ejection

Like supersonically fast fighter jets creating sonic shocks in the air, planet Earth typically moves in the magnetized solar wind at super-Alfvénic speeds and generates a bow shock. Here we report a rare regime of Earth's magnetosphere interaction with a sub-Alfvénic solar wind brought by an erupted magnetic flux rope from the Sun, called a coronal mass ejection (CME). The terrestrial bow shock disappears, leaving the magnetosphere exposed directly to the cold CME plasma and the strong magnetic field from the Sun's corona. Our global magnetohydrodynamic (MHD) simulations show that the magnetosphere transforms from its typical windsock-like configuration to having wings that magnetically connect our planet to the Sun. NASA's Magnetospheric Multiscale (MMS) mission further observes cold CME ions simultaneously with energized counter-streaming electrons, indicating dual-wing reconnection between the CME and Alfvén-wing field lines. The observations show dynamic generation of new wing filaments, and dual-wing reconnection converting the open flux in the wings to closed flux, forming dayside magnetic flux circulation. The picture revealed by the MMS measurements differs from the typical cartoons of Alfvén wings, and goes beyond the global MHD simulation which does not produce dual-wing reconnection. How much the dayside flux circulation modifies the typical Dungey cycle is among the open questions brought by the unusual cosmic event. Our work creates new possibilities to understand how sub-Alfvénic plasma wind may impact astrophysical bodies in our solar and other stellar systems.   

Electron-only magnetic reconnection in lunar-relevant laser-driven mini-magnetospheres

Mini-magnetospheres are ion-scale structures that are ideal for studying the kinetic-scale physics of collisionless space plasmas. Such ion-scale magnetospheres can be found on local regions of the Moon, associated with the interaction of the solar wind with the lunar crustal magnetic field.

  We present laboratory observation of magnetic reconnection in laser-driven lunar-like ion-scale magnetospheres on the Large Plasma Device (LAPD) at UCLA. In our experiment, we use a high-repetition rate (1 Hz), nanosecond laser to drive a fast moving plasma that expands into the field generated by a pulsed magnetic dipole embedded into a background plasma and magnetic field [1]. The dipole and background fields are oriented to be anti-parallel, allowing a magnetic reconnection geometry. Taking advantage of the high-repetition rate, we acquire time resolved, volumetric data of the magnetic and electric fields, measured with magnetic flux and emissive probes around the reconnection point. We notably find that Hall physics plays an important role in mini-magnetospheres, and that the observed reconnection is dominated by electron dynamics [2]. We use spatially resolved Thomson scattering to further investigate the plasma density evolution, as well as heating associated with the mini-magnetosphere and reconnection process. Using particle-in-cell simulations reproducing the experiment, and comparing the terms of the generalized Ohm’s law,  we uncover the microphysics driving this reconnection and show that the reconnection electric field is mostly sustained by the electron pressure anisotropy on the electron scale.

By comparing several dimensionless parameters, we show that our experiment operates in a physical regime relevant to lunar mini-magnetospheres and therefore can contribute to understanding the physics driving reconnection on the Moon in a context where in-situ data is scarce and limited.

[1] D. B. Schaeffer et al. Physics of Plasmas 29, 042901 (2022)

[2] L. Rovige et al. The Astrophysical Journal, In press (2024)

    

Laboratory Modelling of Accretion Disks and Jets on Pulsed-Power Generators and Intense Lasers

Rotating plasma disks orbiting a central object, such as a black hole, are ubiquitous in the universe. However, questions regarding their dynamical evolution, such as the mechanisms of angular momentum transport and the role of magnetic fields in seeding instabilities, turbulence, and launching jets, remain outstanding. In this talk, I will give an overview of a new generation of laboratory experiments conducted at high-energy-density facilities (the MAGPIE pulsed-power generator and the OMEGA laser), designed to probe plasma physics relevant to accretion disks and jet-launching regions in astrophysics [1-5]. 

 

A differentially rotating plasma column is driven and sustained by the collision of multiple inflowing plasma jets. The free-boundary design allows the plasma to expand axially, forming supersonic rotating jets that remain collimated as they propagate through the vacuum chamber. Both laser and pulsed-power experiments drive high magnetic Reynolds numbers, transonic plasma flows with a quasi-Keplerian rotation curve.

 

The experiments are supported by 3-D MHD simulations performed using the code Gorgon and 2-D collisional-kinetic particle-in-cell simulations using the code OSIRIS, which model the formation, evolution, and structure of differentially rotating plasmas. I will discuss the potential of these experiments to study the magneto-rotational instability, the Omega-effect, and the overall effect of magnetic fields in high-Rm rotating plasmas on laboratory scales.

[1] Ryutov, Astrophys. Space Sci (2011)

[2] Bocchi et al., The Astrophys. J. (2013)

[3] Valenzuela-Villaseca et al., Phys. Rev. Lett. (2023)

[4] Valenzuela-Villaseca et al., IEEE Trans. Plasma Sci. (2024)

[5] Valenzuela-Villaseca et al., Journal of Plasma Phys. (2024, in press)   

Temporal properties of magnetohydrodynamic turbulence and Implications for Energetic Particle Transport

The temporal properties of compressible magneto-hydrodynamic (MHD) turbulence is a fundamental problem which has important implications for particle acceleration and transport in astrophysical plasmas. Here, by analyzing the spatio-emporal properties of compressible MHD turbulence, we derive a new spectral power density function verified by simulations. This new function reveals that the low frequency fluctuations are dominated by modes with small parallel wavenumbers with respect to the mean background magnetic field. Furthermore, for fluctuations with dynamically significant parallel wavenumbers, broadening around their eigenfrequencies is described by this function in close agreement with simulations. We use this formalism to present the scaling properties of individual MHD modes. The broadening around eigenfrequencies is a direct consequence of nonlinear processes and is different for the three fundamental MHD modes. Our results provide a new window to investigate the temporal properties of turbulence and will enable further studies on the interaction between compressible MHD turbulence and energetic plasmas.   

Magnetogenesis in collisionless plasma: from Weibel instability to turbulent dynamo

Astronomical observations suggest pervasive, dynamically important magnetic fields in our Galaxy and the intracluster medium. Their origin remains a long-standing question in astrophysics and cosmology. It is widely believed that such fields first arose as weak 'seeds' generated by cosmic batteries, and were subsequently amplified by the turbulent plasma flows to current levels via the 'dynamo' process. However, a complete understanding of these processes in a weakly collisional plasma is still lacking. Our first-principles numerical and theoretical study provides a unified paradigm for understanding the origin and evolution of cosmic magnetism by taking into account the effects of nonequilibrium micro-physics of collisionless plasmas on macroscopic astrophysical processes. We apply an external mechanical force to a weakly collisional, initially unmagnetized plasma. The driven large-scale motions are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary 'seed' magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until energy equipartition with the turbulent flow is reached. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through generic large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.   

Magnetogenesis via the canonical battery effect

Magnetic fields are ubiquitous in the Universe, underpinning the dynamics of stars, planets, galaxies, accretion disks, neutron stars, and the intergalactic medium. These structures, composed of magnetized plasmas capable of supporting convective flows, typically exhibit large-scale magnetic fields attributed to turbulent dynamo effects. However, dynamo processes require an initial seed magnetic field, and the mechanisms generating these weak fields remain poorly understood.

 

In this talk, we demonstrate that the "canonical battery effect" is responsible for seed magnetogenesis. By taking the curl of the first moment of the Vlasov equation for any species, we derive an equation describing the time evolution of canonical vorticity. This equation reveals that, given an initially zero magnetic field and fluid vorticity, the only term capable of spontaneously generating magnetic fields is the canonical battery term, related to the species' pressure tensor.

 

We show that the canonical battery term generalizes popular magnetogenesis mechanisms. Firstly, it is shown trivially that the well-known Biermann battery is a specific case of the canonical battery for isotropic, scalar pressure. Secondly, particle-in-cell simulations demonstrate that the canonical battery also underlies the Weibel instability.

 

The canonical battery term enables the prediction of new pressure tensor configurations that induce spontaneous magnetogenesis. One such configuration, a 2D-localized pressure anisotropy, is shown to generate quadrupole magnetic fields which are important for various processes such as magnetic reconnection and flux rope generation. This mechanism is verified by particle-in-cell simulations. Various potential studies and extensions arising from this framework are discussed.