# Heliophysics Seminars

The Heliophysics seminars are intended to:

• allow guests and local members of the plasma physics community to present heliophysics related research and foster collaborations
• facilitate development of theoretical tools for understanding fundamental physical processes such as reconnection, turbulence, waves, and transport that control the dynamics in the context of the heliosphere
• provide a forum to facilitate cross-fertilization between laboratory plasma physics, astrophysics, and heliospheric science
Heliophysics seminars are usually held Thursdays, @3 PM, in the Theory Conference Room, T169.

### Past

• Mercury‘s Dynamic Magnetosphere
Prof. James Slavin, University of Michigan, abstract, slides
[#s743, 06 Dec 2018]
MESSENGER’s exploration of Mercury has led to many important discoveries and a global perspective on its magnetosphere, exosphere, and interior as a coupled system. Mercury’s proximity to the Sun, weak planetary magnetic field, electrically conducting core, and sodium-dominated exosphere give rise to a highly dynamic magnetosphere unlike that of any other planet. The strong interplanetary magnetic fields so close to the Sun result in a high rate of energy transfer from the solar wind into Mercury’s magnetosphere. Surprisingly, direct solar wind impact on the surface during coronal mass ejection impact has been found to be infrequent. Electric currents induced in Mercury’s highly conducting interior buttress the weak planetary magnetic field against direct impact for all but the strongest solar events. Kinetic effects associated with the large orbits of planetary ions about the magnetic field and the small dimensions of the magnetosphere are observed to significantly affect some fluid instabilities such as Kelvin-Helmholtz waves along the magnetopause. As at Earth, magnetic reconnection, dipolarization fronts, and plasmoid ejection are closely associated with substorms in Mercury’s magnetosphere, and MESSENGER frequently observed energetic electrons with energies of tens to several hundred thousand electron volts. However, no “Van Allen” radiation belts with durable trapping are present.
• The lunar plasma wake and electron phase-space holes
Prof. Ian Hutchinson, MIT, abstract, slides
[#s926, 30 Nov 2018]
Wakes of plasma flowing past unmagnetized bodies like probes, moons, or large particles are usually unsteady. Detailed theory and simulations show instabilities excited by the velocity distribution distortions give rise to electron holes (soliton-like BGK modes). We have recently discovered from spacecraft observations that the solar wind wake of the moon is full of electron holes, in agreement with predictions. Transverse instability of these holes determines their evolution and persistence and how they eventually merge into the background plasma.

2 related papers:
"Prediction and Observation of Electron Instabilities and Phase Space Holes Concentrated in the Lunar Plasma Wake", Ian H. Hutchinson, David M. Malaspina, Geophysical Res. Lett. 2018

"Transverse instability of electron phase-space holes in multi-dimensional Maxwellian plasmas", I. H. Hutchinson J. Plasma Phys. 2018
• On the role of magnetic reconnection in kinetic-range turbulence and the existence of cascades in the entire phase space from hybrid-Vlasov-Maxwell simulations
Silvio Cerri, Princeton University , abstract, slides
[#s735, 30 May 2018]
Understanding the properties of turbulent fluctuations and how turbulent energy is dissipated in weakly collisional plasmas is a fundamental step towards understanding how turbulence feeds back on the evolution of several astrophysical systems. In this context, space plasmas are probably the best laboratory for the study of plasma turbulence in a weakly collisional regime, as the Earth’s environment has become accessible to increasingly accurate direct measurements. In situ observations of the solar wind and the terrestrial magnetosheath have indeed provided relevant constraints on the turbulent energy spectra, determining the typical values of their slopes and revealing the presence of breaks in the electromagnetic fluctuation cascade at kinetic scales. A first break in the turbulent spectrum is indeed encountered at the proton kinetic scales and separates the so-called “MHD inertial range” spectrum from the kinetic spectrum that arises at scales smaller than the proton gyroradius (also referred to as the “dissipation” or “dispersion” range). Such transition is a clear evidence of a change in the physics underlying the cascade process, and its understanding is today a matter of a strong debate. Very high resolution measurements by MMS have also recently pointed out the presence of structures in the particle (electron) distribution function that can be interpreted as a cascade in velocity space.
In this talk I will present some recent developments in the investigation of the properties of kinetic-range turbulence via high-resolution hybrid-kinetic (fully-kinetic ions and fluid electrons) simulations both in 2D and 3D. In particular, I will show the first numerical evidence that has led to the suggestion of a link between magnetic reconnection, ion break and turbulent energy transfer in the sub-ion-gyroradius cascade[1,2] (also known as “reconnection-mediated scenario” for plasma turbulence). Finally, I will show the first evidence for a six-dimensional (“dual”) phase-space cascade of ion-entropy fluctuations in a 3D3V simulation of electromagnetic turbulence: such phase-space cascade is shown to be anisotropic with respect to the background magnetic fleld in both real and velocity space and suggests that both linear and non-linear phase mixing are simultanously at work[3].
[1] S. S. Cerri & F. Califano, New J. Phys. 19, 025007 (2017)
[2] Luca Franci, Silvio Sergio Cerri et al., Astrophys. J. Lett. 850, L16 (2017)
[3] S. S. Cerri, M. W. Kunz & F. Califano, Astrophys. J. Lett. 856, L13 (2018)
• Magnetic Reconnection during Turbulence and the Role it Plays in Dissipation and Heating
Mike Shay, U. Delaware , abstract, slides
[#s707, 09 May 2018]
Turbulence plays an important role in many plasmas, including those in accretion disks, in the heliosphere, and in the laboratory. In plasmas with low collisionality, such as those in the heliosphere, exactly how this turbulent energy damps away is an open question, with ramifications for the heating of the solar corona and the solar wind. Magnetic reconnection, where magnetic field lines break and reform in a plasma, is one possible mechanism for damping this turbulent energy and heating the plasma, but the role it may play is uncertain. Recently, however, significant progress has been made in understanding plasma heating in isolated reconnection sites. Can this new knowledge shed light on the properties of plasma heating during turbulence?
In this talk, after reviewing our understanding of heating due to reconnection, I will lay out a framework for applying reconnection heating predictions to turbulent systems, and show initial results for testing this framework using fully kinetic PIC simulations. In addition, I will discuss recent MMS observations of reconnection in Earth's turbulent magnetosheath. I will then explore the statistics of magnetic reconnection in kinetic simulations of turbulence. By statistics, I mean the number of x-lines, the spread of reconnection rates, and how these quantities vary in time. How these statistics vary in different turbulence regimes and its impact on reconnection heating will be discussed.
• Collisionless damping of slow magnetosonic waves (and related compressional fluctuations)
Bill Dorland, University of Maryland , abstract
[#s663, 30 Mar 2018]
Compressional perturbations are observed in the solar wind even when the collision time is much longer than an inferred wave period. This is puzzling. Lithwick & Goldreich argued that the parallel wavenumbers of the slow modes would be inherited from the Alfvén cascade, which would itself be well-described as being in critical balance. For most parameters, this argument favors rapid damping of compressional fluctuations, $\gamma \sim k_\parallel v_A \sim k_\perp v_\perp$. Schekochihin et al. argued instead that the compressional perturbations would evolve in Lagrangian fashion, maintaining their original (possibly very long) wavelengths along the magnetic field, even as the field itself developed ever-shorter parallel wavelengths. Although compressional waves would still experience Landau and/or Barnes damping in this picture, the rate could be very small. Kanekar et al. observed that stochastic echoes could “fluidize” the compressional fluctuations, allowing them to evade collisionless damping altogether. It remains unclear which mechanism is dominant, if any. I will present recent work on this problem by R. Meyrand, A. Kanekar, A. Schekochihin, and myself.
• Magnetic Reconnection in MHD and Kinetic Turbulence
Nuno Loureiro, MIT , abstract
[#s631, 21 Feb 2018]
Recent works have revisited the current understanding of Alfvénic turbulence to account for the role of magnetic reconnection [1-3]. Theoretical arguments suggest that reconnection inevitably becomes important in the inertial range, at the scale where it becomes faster than the eddy turn over time. This leads to a transition to a new sub-inertial interval, suggesting a route to energy dissipation that is fundamentally different from that envisioned in the usual Kolmogorov-like phenomenology.
These concepts can be extended to weakly collisional plasmas, where reconnection is enabled by electron inertia rather than resistivity [4,5]. Although several different cases must then be considered (whether the eddies themselves are on MHD or kinetic scales, whether the plasma beta is large or small, etc.), a common result to all of them is that the energy spectrum exhibits a scaling with the perpendicular wave number that scales between $k_\perp^{−8/3}$ and $k_\perp^{−3}$, in favourable agreement with many numerical results and observations.
This talk aims to review these results, and discuss their implications.
[1] Nuno F. Loureiro & Stanislav Boldyrev, Phys. Rev. Lett. 118, 245101 (2017)
[2] A. Mallet, A. A. Schekochihin & B.D.G. Chandran, Mon. Not. R. Astron. Soc. 468, 4862 (2017)
[3] Stanislav Boldyrev & Nuno F. Loureiro, Astrophys. J. 844, 125 (2017)
[4] Nuno F. Loureiro & Stanislav Boldyrev, Astrophys. J. 850, 182 (2017)
[5] Alexander A. Schekochihin & Benjamin D. G. Chandran, J. Plasma Phys. 83, 905830609 (2017)