# 2021

## Dec 10: Modeling dissipative effects in general-relativistic plasmas and beyond

### Elias Most (Institute of Advanced Studies and Princeton University)

**Abstract:** Relativistic plasmas are central to the study of black hole accretion, jet physics, neutron star mergers, and compact object magnetospheres. Going beyond common approaches used in the literature, I will describe a fully relativistic covariant 14-moment based two-fluid system appropriate for the study of electron-ion or electron-positron plasmas. This generalized Israel-Stewart-like system of equations of motion is obtained directly from the relativistic Boltzmann-Vlasov equation. Crucially, this new formulation can account for non-ideal effects, such as anisotropic pressures and heat fluxes. Specializing to the case of non-resistive plasmas, I will present a novel numerical scheme capable of solving these equations in the strongly and weakly collisional limits.

Bridging the gap between formulations of general-relativistic plasmas and dynamical gravity, I will then present a novel reformulation of the Einstein field equations. By exploiting the Palatini formalism, I will show how these equations can be recast into a form resembling non-linear electrodynamics in a medium. Such a formulation might permit the use of advanced numerical methods, such as constraint transport, in simulations of vacuum spacetimes.

## Dec 03: Time-dependent screening of the electric field in pulsar discharges and its implications for coherent radio emission

### Elizabeth Tolman (Institute of Advanced Studies)

**Abstract:** Pulsars produce coherent radio emission, the source of which has remained enigmatic. Recent computational work suggests this emission may be produced in nonstationary pair plasma discharges in the pulsar polar cap, where the pulsar electric field is screened by newly-produced electron-positron pairs, leading to the production of plasma waves which escape from the magnetosphere as radio emission. In this talk, we present the physical principles governing the evolution of wave energy in the polar cap's time-dependent, relativistic, collisionless pair plasma, for both linear waves and nonlinear waves. These principles are then used to analytically determine the change in the pulsar polar cap electric field amplitude as it is screened. Finally, we discuss the implications of this model for coherent radio emission and the interpretation of pulsar observations.

## Nov 19: Simulations of the collisionless magnetorotational instability with Particle-in-Cell: Revisited

### Fabio Bacchini (University of Colorado)

**Abstract:** The magnetorotational instability (MRI) is a fundamental process occurring in astrophysical accretion disks. The MRI promotes accretion by driving turbulence on macroscopic scales. In the process, magnetic reconnection and other collective plasma phenomena can accelerate particles to high (possibly nonthermal) energies. The resulting plasma emission may be measurable with observational campaigns.

In radiatively inefficient accretion flows, plasmas are expected to be collisionless. This invokes the use of Particle-in-Cell (PiC) methods to study the dynamics of the MRI in these environments. However, the scale separation between macroscopic and kinetic scales is extremely large for realistic systems. For this reason, only a small number of works have been produced where the MRI is studied with PiC simulations, often with a very limited exploration of the available parameter space and with limited dimensionality.

We are revisiting PiC simulations of the MRI by exploring the parameter space (temporal and spatial scale separation) in detail, and considering different field geometries in the 2D and 3D cases. We find that even in the 2D pair-plasma case, very large simulations are needed to minimize the effect of the limited scale separation; this carries over to the 3D case. Moreover, the field geometry and the dimensionality of the simulation heavily influence the results. Our findings are important in the context of ultimately perform ion-electron 3D simulations of the MRI with PiC, in order to gain insight into the microphysics of observationally targeted RIAFs.

## Nov 05: The Magneto-thermal Instability in Galaxy Clusters: Theory and Simulations

### Lorenzo Perrone (Cambridge University)

**Abstract:** In the intracluster medium (ICM) of galaxies, exchanges of heat across magnetic field lines are strongly suppressed. This anisotropic heat conduction, in the presence of a large-scale temperature gradient, destabilizes the outskirts of galaxy clusters via the magneto-thermal instability (MTI), and thus supplies a source of observed ICM turbulence. Our aim is to take a fresh look at the problem and construct a general theory that (a) explains the MTI saturation mechanism and (b) provides scalings and estimates for the turbulent levels. We simulate MTI turbulence with a Boussinesq code, SNOOPY, which allows us to carry out an extensive sampling of the parameter space. In two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; at a characteristic ‘buoyancy scale’, the energy is dumped into large-scale g-modes, which subsequently dissipate. In three dimensions, on the other hand, most energy is dissipated at the same scale as its injection, and turbulent eddies are vertically elongated at or below the thermal conduction length, but relatively isotropic on larger scales. Similar to 2D, however, the saturated turbulent energy levels and the integral scale follow clear power-laws that depend on the thermal diffusivity, temperature gradient, and buoyancy frequency. We also show that the MTI amplifies magnetic fields, through a fluctuation dynamo, to equipartition strengths provided that the integral scale of MTI turbulence is larger than the viscous dissipation scale. Finally, we show that our scaling laws are consistent with extant observations of ICM turbulence if the thermal conductivity is reduced by a factor of ~10 from its Spitzer value, and that on global cluster scales the stable stratification significantly reduces the vertical elongation of MTI motions.

## Oct 29: The Early Light from Weak and Aspherical Stellar Explosions

### Itai Linial (Hebrew University of Jerusalem)

**Abstract:** The first photons from a supernova explosion emerge when the supernova’s radiation-mediated shock breaks out of the stellar surface, heralding an energetic transient event. These early shock-breakout photons carry invaluable information about the progenitor’s properties, its immediate environment, and the explosion mechanism. I will discuss this phenomenon in the context of weak stellar explosions (with explosion energy much smaller than the star's binding energy), and explain how the radiative dissipation of a blast wave sets a minimal energy scale below which shock acceleration cannot unbind any mass from the stellar envelope, and comment on implications regarding pre-supernova eruptions and early supernovae light curves. I will then present recent results regarding the bolometric light curve expected from the breakout of aspherical stellar explosions, and discuss the prospects of their discovery and characterization.

## Oct 29: Electron Heating in 2D PIC Simulations of Quasi-Perpendicular Low-Beta Shocks

### Aaron Tran (Columbia University)

**Abstract:** The electron heating physics in collisionless shocks imprints upon micro-scale dissipation and waves seen in situ by heliospheric spacecraft, as well as X-ray emission from supernova remnants and galaxy clusters. How much do electrons heat, and how do they heat? We model low-beta heliospheric shocks with 1D and 2D fully-kinetic particle-in-cell simulations, taking fast Mach number 1–4 and total plasma beta 0.25. In the parameter space of quasi-perpendicular shocks (pre-shock magnetic field angles ~55–90 deg. from shock normal), we explore two regimes. For field angles near 90 deg., electrons are advected downstream by the magnetic field. Here, the parallel electric field of ion-scale oblique whistler waves—only captured in 2D—accelerates electrons into streams along the field, which then relax via two-stream-like instability. At more oblique field angles, subsonic electrons stream easily in and out of the shock. Here, electrons can gain energy from a cross-shock parallel electric potential jump and a secular potential rise along an oblique whistler wave train traveling just ahead of the shock. Post-shock electrons are colder than ions (Te/Ti ~ 0.1–1) in both regimes.

## Oct 22: Accretion onto spinning supermassive binary black holes approaching merger

### Luciano Combi (Instituto Argentino de Radioastronomía)

**Abstract:** A supermassive binary black hole (SMBBH) might form after two galaxies merge. If the binary reaches subparsec separations, gravitational waves (GW) can efficiently extract energy from the system and the black holes might eventually merge. Contrary to most stellar-mass black holes, SMBBHs might also be powerful sources of electromagnetic radiation accreting the gas of the galaxy. In order to distinguish the electromagnetic signatures of these systems from normal AGNs, we need to analyze the complex non-linear dynamics that occur between the gas and the dynamical spacetime. In this talk, I will present our latest results on GRMHD simulations of accretion disks around spinning SMBBHs. We model the spacetime of a spinning BBH in the inspiral regime using a semi-analytical approximation that we use as a background to evolve the plasma GRMHD equations. We analyze the accretion mechanisms, variability, and electromagnetic luminosities of the circumbinary disk and mini-disks that form around each black hole, focusing on the role of the spin. Finally, we use ray-tracing calculations to obtain light-curves, images, and spectra based on the simulation data.

## Oct 22: Faster Accretion Disk Simulations for Analysis of Black Hole Images

### Tamar Faran (Hebrew University of Jerusalem)

**Abstract: **Observations show evidence for the existence of relativistic shock waves in various extreme astrophysical environments, such as gamma-ray bursts and the outflows of binary neutron star mergers. Detailed theoretical models for shock wave hydrodynamics and the radiation that accompanies shock passage are crucial for a correct interpretation of observations, which can help uncover the properties of the systems. In this talk I will discuss our hydrodynamic solution for the interiors of an ultra-relativistic blast wave (Faran & Sari 2021). We obtain a self-similar solution that follows Blandford & McKee (1976) close to the shock wave, at ultra-relativistic velocities, but also describes the flow in the interiors of the blast wave, where the flow reaches mildly-relativistic to Newtonian velocities. I will then discuss our ongoing work on the breakout of a relativistic shock wave from a stellar edge, and highlight applications of this work to low luminosity gamma ray bursts and to the gamma-ray flare that followed GW170817.

## Oct 15: Faster Accretion Disk Simulations for Analysis of Black Hole Images

### Ben Prather (University of Illinois)

**Abstract: **The EHT Collaboration (EHTC) has now published high resolution images of total and polarized emission from the black hole M87. The interpretation of these and forthcoming images by the EHTC makes heavy use of libraries of simulations modeling the observed systems, conducted using general relativistic magnetohydrodynamics (GRMHD) and imaged via general relativistic radiative transfer (GRRT). While the imaging process is well-understood, there is a clear need for a broader array of longer GRMHD simulations as observational data accumulates. I will present a new performance-portable GRMHD code designed to address this need, KHARMA (Kokkos-based High-Accuracy Relativistic Magnetohydrodynamics, soon to have Adaptive mesh refinement). KHARMA leverages the Kokkos performance portability library to generate efficient code for different computer architectures, and uses the new Parthenon Eulerian simulation framework for many other code functions, keeping KHARMA simple while also fast, featureful, and extensible. I will end by presenting some future projects making use of this new capability.

## Oct 08: Decay of primordial magnetic fields via reconnection can explain cosmic-void observations

### David Hosking (Oxford University)

**Abstract: **It has been suggested that the weak magnetic field hosted by the intergalactic medium (IGM) in voids is a relic from the early Universe. If so, the modern-day strength and coherence length of such fields could be “predicted” from reasonable assumptions about the properties that the primordial field had at its genesis, provided the evolution of the field in the intervening time were understood. Previously held theories based on magnetogenesis at the electroweak phase transition (EWPT) indicated that the primordial field would have decayed too quickly to be consistent with the observational constraints on modern fields. Thus, the “relic-field” hypothesis appeared unlikely. However, recent numerical developments have shown that these decay theories are flawed: they do not predict the “inverse transfer” of magnetic energy to larger scales that has been observed in simulations. In my talk, I present a new theory of the decay based on the conservation of the “fluctuation level” of magnetic helicity, with dynamics controlled by the reconnection of magnetic-field lines. I show that this theory explains the “inverse-transfer” phenomenon, and predicts a slower decay of primordial fields, thus restoring consistency between the relic-field hypothesis and the observational constraints. Finally, I show that the theory I present is robust, in that, unlike previous models, it does not depend on the large-scale asymptotic of the magnetic-energy spectrum at magnetogenesis.

## Oct 01: Black hole flares: ejection of accreted magnetic flux through 3D plasmoid-mediated reconnection

### Bart Ripperda (Princeton University and Center for Computational Astrophysics)

**Abstract:** Magnetic reconnection can power bright and rapid flares originating from the inner magnetosphere of accreting black holes. We conduct extremely high resolution general-relativistic magnetohydrodynamics simulations, capturing plasmoid-mediated reconnection in a 3D magnetically arrested disk for the first time. We will show that an equatorial, plasmoid-unstable current sheet forms in a transient, non-axisymmetric, low-density magnetosphere within the inner ten Schwarzschild radii. Magnetic flux is expelled from the event horizon through reconnection at the converged universal plasmoid-mediated rate in this current sheet. The reconnection is fed by the highly-magnetized plasma in the jet, heating the plasma trapped in flux tubes to temperatures proportional to the jet's magnetization. Expelled flux tubes can orbit for an orbital period as low-density hot spots, in accordance with Sgr A* observations by the GRAVITY interferometer. Reconnection near the horizon produces sufficiently energetic plasma to explain flares from accreting black holes, such as the TeV emission observed from M87. The drop in mass accretion rate during the flare, and the resulting low-density magnetosphere make it easier for very high energy photons produced by reconnection-accelerated particles to escape. The extreme resolution results in a converged plasmoid-mediated reconnection rate that directly determines the timescales and properties of the flare.

## Sep 24: Electromagnetic fireworks: Fast radio bursts from rapid reconnection in the compressed magnetar wind

### Jens Mahlmann (Princeton University)

**Abstract:** It was recently proposed that extragalactic fast radio bursts can be produced during magnetic reconnection in a compressed current sheet of the magnetar wind, triggered by the strong fast magnetosonic pulse produced by a magnetar flare. The current sheet is fragmented into a self-similar chain of magnetic islands and the coherent radiation is produced by time-dependent plasma currents at their interfaces during coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations. We compute the efficiency of the coherent emission and obtain scalings for its peak frequency. Consistent with expectations, a fraction of the incident magnetic flux, scaling with the reconnection rate, is reconnected and converted into fireworks of high-frequency fast waves with an efficiency factor f~0.002. In analogy to previous analytical estimates, we find that fast magnetosonic pulses can trigger relatively narrow-band GHz emission with radio luminosities sufficient to explain extragalactic fast radio bursts. The mechanism also provides a natural explanation for the nanosecond sub-structure recently detected in FRB 20200120E.

## Jun 4: *Shock physics in the merging galaxy cluster A2146*

*Shock physics in the merging galaxy cluster A2146*

### Prof. Helen Russell (University of Nottingham)

**Abstract:** The galaxy cluster A2146 is undergoing a major merger and hosts two huge, bright Mach 2 shock fronts, which provide a unique opportunity to measure the electron-ion equilibration timescale along with other key transport phenomena. Collisionless shocks occur over a wide range of scales from accretion shocks to supernova remnants and heliospheric shocks. However, only clusters, particularly A2146 allow us to map the large-scale equilibration with a single observation, which is unaffected by cross-calibration uncertainties. I will present preliminary results from a new 2.4Ms Chandra observation that provides a measure of this timescale, reveals the detailed shock structure and traces the disruption of both cool cores, including heating and mixing of cool gas blobs in ram pressure stripped tails.

## May 14: *Alfvénic fluctuations and switchbacks in the solar wind*

*Alfvénic fluctuations and switchbacks in the solar wind*

### Prof. Anna Tenerani (UT Austin)

**Abstract:** Large amplitude, turbulent Alfvénic fluctuations have been commonly observed in the solar wind since the first in-situ measurements, and they are thought to provide a possible mechanism to heat the solar corona and accelerate the solar wind. Despite Alfvénic fluctuations have been studied for many decades now, a complete understanding of their origin and nonlinear evolution still remains elusive. Observations show that such fluctuations are characterized by a nearly constant magnetic field amplitude, a condition which remains largely to be understood and that points to an intrinsic degree of coherence of s such fluctuations. The Parker Solar Probe mission, by probing regions of space never explored before closer to the sun, has provided a wealth of data showing the ubiquitous and persistent occurrence of the so-called switchbacks. Switchbacks are magnetic field lines which are strongly perturbed to the point that they lead to local inversions of the radial magnetic field. The corresponding signature in the velocity field is that of a local radial speed jet displaying the well-known velocity/magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. Switchbacks are thus an extreme case of those Alfvénic fluctuations that permeate the solar wind further away. While there is not yet a general consensus on what is the origin of switchbacks and their connection to coronal activity, a first necessary step is to understand how they evolve and how long they can persist in the solar wind. In this talk, we will discuss the evolution of Alfvénic fluctuations and switchbacks in the solar wind. We will focus on how their evolution is affected by parametric instabilities and the possible role of expansion and kinetic effects by comparing theory with observations in the inner heliosphere. We will finally discuss what are the implications of our results for models of switchback generation and related open questions.

## Apr 30: *A systematic exploration of magnetized winds solutions in protoplanetary discs*

*A systematic exploration of magnetized winds solutions in protoplanetary discs*

### Dr. Geoffroy Lesur (CNRS Grenoble)

**Abstract:** The recent developments in our understanding of the chemical composition, the ionisation equilibrium and the dynamics of protoplanetary discs has led to the conclusion that magnetised disc wind (MDW) are probably playing an important role in shaping the long term evolution of these objects. Most of our understanding of these winds comes from complex global direct numerical simulations which include detailed microphysics and which explore only a very limited subspace of parameters. Hence, it is very difficult to draw firm conclusions about the nature of MDWs, their property, and the long-term evolution of discs subject to MDWs. In this talk, I will present a systematic exploration of MDW solutions, using a simplified self-similar approach, which can then be used in secular models to predict the evolution of a disc, in a way similar to the "alpha disc" model. I will also discuss the solutions topology (top/down symmetry, midplane/surface accretion layers, laminar stress, etc...) which have often been mis-interpreted in the literature.

## Apr 23: *The dynamics of pair and electron-ion relativistic collisionless shock waves*

*The dynamics of pair and electron-ion relativistic collisionless shock waves*

### Dr. Arno Vanthieghem (Stanford)

**Abstract:** Among other powerful relativistic astrophysical objects, gamma-ray bursts and blazars provide ideal environments to understand the acceleration mechanisms of high-energy charged particles. The radiative spectra observed in such objects are generally attributed to particles energized in relativistic collisionless shock waves. We present the main aspects of an analytical model that describe the dynamics of the precursor of relativistic unmagnetized collisionless shock waves in electron-positron plasmas. This model is then extended to electron-ion plasmas by accounting for the longitudinal electrostatic field playing an important role in the strong non-adiabatic heating and slowdown of the electrons in the shock front frame. This model is validated by large-scale ab initio Particle-In-Cell simulations of relativistic unmagnetized pair and electron-ion shocks with various mass ratios and Lorentz factors of the shock wave. We then introduce a hydrodynamical model allowing to treat the dynamics of a loaded plasma in the context of neutron shell propagating upstream of a decelerating ion blast wave. We show how the neutron shell affects the dynamics of the shock up to a critical regime presenting particle acceleration.

## Apr 16: *Phenomenology and theory of galactic cosmic-ray propagation*

*Phenomenology and theory of galactic cosmic-ray propagation*

**Abstract:** Cosmic rays are the most energetic particles in the local Universe as they are known to reach energies above few Joules. How and where they are produced have been a science puzzle for several decades now, whose solution has been driving the rising of multi-messenger astrophysics as well as novel theoretical approaches. Of particular interest is the energy range below ~PeV as we expect that these particles have been all accelerated in the most extreme objects in our own Galaxy. Additionally, the past decade has seen an unprecedented improvement in the quality and quantity of data about their energy spectrum and chemical composition, allowing us to infer global properties as the galactic grammage and the average residence time. These quantities are crucial to test any more fundamental description of the transport of charged particles in the interstellar plasma. Even more thrilling, these new measurements, together with a deeper scrutiny of the diffuse gamma-ray emission from the Galactic plane, have revealed unexpected new features in the cosmic-ray elemental spectra that are challenging the commonly accepted scenario of how these particles are energized and propagate through interstellar space.

In my talk I will provide an overview of these recent ﬁndings, and discuss some of the new ideas proposed to explain these anomalies.

## Mar 26: *Extreme particle acceleration in pulsars: from the magnetosphere to the nebula*

*Extreme particle acceleration in pulsars: from the magnetosphere to the nebula*

**Abstract: **Pulsars blow ultra-magnetized relativistic winds loaded with electron-positron pairs created and launched in the magnetospheric regions. A generic feature of pulsar winds is a large-scale oscillating current sheet, or striped wind, forming where the magnetic field polarity reverses, thus providing an ideal environment for studying magnetic reconnection and particle acceleration under extreme physical conditions (relativistic, collisionless and radiative regime), and yet perhaps applicable to other high-energy astrophysical objects such as AGN jets and gamma-ray bursts. Reconnection in the innermost parts of the wind is thought to power the observed high-energy pulsed emission from pulsars. Recent global particle-in-cell simulations suggest that reconnection proceeds in the plasmoid-dominated regime and consumes the field until the complete dissipation of the striped wind. At the wind termination shock, the remaining magnetic energy may be dissipated via turbulent reconnection and shear-flow acceleration, which may power the bright synchrotron nebula surrounding pulsars and possibly the mysterious Crab gamma-ray flares.

## Mar 19: *MHD turbulence: simultion, observation, impact on particle transport*

*MHD turbulence: simultion, observation, impact on particle transport*

**Abstract: **The multiphase nature of ISM and diversity of driving mechanisms give rise to spatial variation of turbulence properties. In particular, we find that the proportion of magnetosonic modes increases with increasing compressive forcing from turbulence simulations. Nevertheless, the employed model of turbulence is often oversimplified being assumed to be only Alfvenic or even hydrodynamic due to a lack of observational evidence. Employing our novel method, the signature from polarization analysis (SPA), we unveiled the dominant plasma modes in interstellar turbulence. The method is based on the statistical properties of polarized synchrotron radiation. We find that Alfven and magnetosonic modes are distinguishable. Its application leads to the first discovery of magnetosonic modes in the Cygnus X region, overlapping to a high degree with Fermi cocoon, completely in line with our theoretical expectations on cosmic ray transport. A highly promising research field is foreseen to unroll with ample results anticipated from the advanced analysis of high resolution synchrotron polarization data and multiple-messenger comparison, that will shed light on the role of turbulence in various processes, particularly cosmic ray transport.

## Feb 26: *Non ideal effects in protoplanetary disks: transition from turbulent to Dead Zone in local and global stratified simulations*

*Non ideal effects in protoplanetary disks: transition from turbulent to Dead Zone in local and global stratified simulations*

**Abstract: **The dynamical evolution of protoplanetary disks (PPDs) is of key interest for building a comprehensive theory of planet formation and to explain the observational properties of these objects. Much of what we have learned about PPDs has come from MHD simulations (Balbus and Hawley (2003)) carried out both by means of local shearing box simulations (LSBS) and global simulations (GS). Placing these models in the context of global disk structure and its interaction with the central star makes the problem very challenging, involving a wide range of temporal and spatial scales, coupled via nonlinear dynamical processes, including radiation, disk chemistry and dust.

In this talk I will address the physics of the transition between the active turbulent layer of the PPD and the non-ideal layers, the so called “dead zone”. I will present my recently published result with an isothermal shearing box setup, using the magnetohydrodynamics code Athena++, in which we quantify how the turbulence level is affected by the presence of a non-uniform ohmic resistivity in the radial - x direction. Comparing the turbulent activity to that of ideal simulations, the turbulence inhibited area shows density fluctuations and magnetic activity at its boundaries, driven by energy injection from the active (ideal) zone boundaries. We find magnetic dissipation to be significantly stronger in the ideal regions, and the turbulence penetration through the boundary of the dead zone is determined by the value of the resistivity itself, through the ohmic dissipation process, though the thickness of the transition does not play a significant role in changing the dissipation.

I will also introduce my preliminary work with Athena++ on global simulations of a vertically stratified disk, in which we will take into account the chemistry and thermodynamics of the disk to determine the non-ideal coefficient and the location and effectiveness of the wind lunching. This process is particularly important for planet formation as it gives us information on the area/mass ratio of the grains that form at the disk midplane, which are the fundamental bricks for planet formation.

## Feb 19: *Primordial vs astrophysical models for the emergence of cosmic magnetism: an observational and theoretical challange*

*Primordial vs astrophysical models for the emergence of cosmic magnetism: an observational and theoretical challange*

**Abstract: **Galaxy clusters and filaments of the cosmic web are giant and perfect plasma laboratories in the Universe, which can tell us about the growth of cosmic magnetism from either primordial or astrophysical seed field, as well as about the acceleration of cosmic rays by violent accretion processes.

I will review recent results in the numerical modelling of such objects, with the final goal of understanding the mysterious origin of cosmic magnetism, and use existing or future radio observations to test and kill competing theoretical models, spanning from the range of radio jets from AGN to that of giant cosmic filaments.

## Jan 22: *Violation of the zeroth law of turbulence in space plasmas*

*Violation of the zeroth law of turbulence in space plasmas*

**Abstract: **The movement and thermalization of turbulent energy between scales in plasmas is both fundamentally interesting and important for a wide range of modelling applications. In strongly magnetized space plasmas, energy in electromagnetic fluctuations is usually assumed to flow to the smallest scales, eventually heating electrons. During this talk I will show that when turbulence is dominated by outward-propagating perturbations, as occurs in the solar wind, the energy hits a “barrier” near the ion-gyroradius scale. The effect violates the “zeroth law of turbulence”, which states that large-scale turbulent statistics are independent of microphysical dissipation, suggesting interesting implications for plasma thermodynamics. It also provides a testable explana- tion for the long-standing puzzle of what determines the scale and slope of the “ion-kinetic transition” in observed magnetic- energy spectra.