Cross-Scale

Science: Overview

The goal of Cross-Scale is to study fundamental processes that transport, convert, or release energy in collisionless plasmas. Despite the apparently bewildering array of plasma environments within the Solar System (the solar corona; the solar wind; planetary magnetospheres; cometary comas) and throughout the Universe (accretion disks; supernovae; galactic discs; planetary nebulae; and many more), there are just a small number of processes, one of which is dynamically and energetically dominant in most regions of interest. Crucially, different length and time scales are affected by different physical processes. It is the interplay of these processes which results in the complexity, and consequently the large scale effects, of plasma processes such as shocks and reconnection.

For the first time, Cross-Scale will be able to quantify the coupling dynamics and efficiencies from the largest (fluid-like) to the smallest (electron) scales and reveal how, in turn, these smallest scales control or mediate the large-scale effects.

Collisionless Shocks

Collisionless plasma shocks are some of the most spectacular, visually striking and energetic events in the Universe. Generated by supernovae, stellar winds, or the rapid motion of objects such as neutron stars, they can have a number of important effects. Supernova shock waves can trigger the collapse of galactic nebulae and hence the formation of planetary systems. They are also responsible for heating and deflecting the surrounding plasma, and can blow bubbles out of galactic disks, changing the large scale magnetic configuration of entire galaxies. Collisionless shocks can also accelerate particles to extraordinarily high energies.

Even modest Mach number shocks are fundamentally variable in time and space. They exhibit reformation, a quasi-periodic variation in the shock profile on scales comparable to the proton gyroradius. This results in a non-planar, and varying, shock profile, with important consequences for how particles are deflected, heated and accelerated. In particular, it is clear that the average effects of a shock are not the effects of a shock with the average shock profile – that is, the spatial and temporal variations are an intrinsic and fundamental part of the action and consequences of a collisionless shock. It is essential to study these variations simultaneously on electron, ion and fluid scales to measure the interactions between physical processes which occur within shocks, and how these produce their large scale effects.

Key collisionless shock questions that Cross-Scale will answer include:

• How is incident energy partitioned by the shock?
  • How are ions reflected and decelerated by the structured shock electric field?
  • How are electron dynamics affected by shock reformation and ion dynamics?
  • How do ions thermalise?
• How do shocks respond to changes in the upstream plasma?
• How do shocks accelerate particles?
  • How do ions generate and scatter from waves in the foreshock?
  • How does rippling and reformation affect reflection and acceleration?
  • What do electron foreshock processes tell us about basic beam-plasma interactions?

Magnetic Reconnection

Magnetic reconnection is a fundamental plasma process in the Universe. It plays a key role in the interaction of the Earth’s magnetospheric environment with the solar wind, in the onset of solar and stellar flares and CME formation, in the interaction of our solar system with its interstellar neighbourhood and in astrophysical contexts such as pulsar magnetospheres and active galactic nuclei. Indeed, we might expect the process to occur in any system in which magnetic fields and plasmas of different origin may interact.

Although the sizes of such systems are widely disparate, theoretically we expect that reconnection is initiated in each case on relatively small scales, where the kinetic effects of the typical particle populations in each system become important, rather than the large, flow (MHD) scales. Specifically, these processes occur on the distance and time scales of the relevant electron and ion gyromotions. When gradients in the magnetic field become sharp, the ions in a plasma become demagnetized (i.e. they no longer obey the frozen-in field (MHD) approximation) and may diffuse across the field. However, the electrons remain magnetized until the gradient becomes significant on the electron gyroscale, and an electron diffusion region forms. A complex system of electron currents is then thought to arise within and around these regions. These currents control the change of topology of the magnetic field which is seen through observations of the larger scales, and which propagates the effects to global scales.

We know that reconnection occurs when magnetic fields are sheared across relatively thin current layers. However, we have little understanding of exactly where and when a reconnection site will form, for example during a substorm, because we have a poor understanding of the microphysics of the reconnection process. To date we have been unable to make the coordinated measurements, at high enough time resolution, to be able to reach an understanding of the processes which occur on the electron scales, and their relation to the larger scale boundary conditions, such as the degree of shear, whether the process is operating in a steady state or transitory fashion, etc. Cross-Scale will be the first opportunity to make the required coordinated measurements across all the relevant scales for reconnection, covering the electron dynamics at high time and spatial resolution, the ion scale to examine the field and plasma structures surrounding the diffusion regions and the MHD scale to examine both the external drivers and the large scale consequences, such as the level of particle acceleration, the formation of flux ropes or the properties of the plasma outflows.

Key reconnection questions that will be answered by Cross-Scale include:

• How is reconnection initiated?
  • What triggers reconnection within a current sheet?
  • How are thin current sheets formed?
• What is the role of external driving in reconnection onset?
• What parameters control the structure of the reconnection site?
  • What is the scale size of the reconnection neutral line?
  • How significant are the effects of a guide field, velocity shears and density gradients?
  • What are the effects of different particle populations on the reconnection process?
• What are the consequences of reconnection?
• How does magnetic topology change during reconnection?
  • How are ions and electrons energized as a consequence of reconnection?
  • Formation of multiple vs single reconnection sites and how X or O lines move?
  • How are Alfvén waves and other plasma waves generated near the reconnection site and how these waves interact with ambient plasma?

Turbulence

Turbulence is pervasive in many astrophysical and solar system plasmas, over a vast range of scales from the inter-galactic medium to sub-electron gyroscales. Plasma turbulence is present, and dynamically important, in many interesting plasma environments, is responsible for accelerating particles and also greatly affects their propagation: without an understanding of turbulence, we cannot predict many of the most important effects of plasmas within the Solar System, far beyond, and within technologically important devices such as tokamaks. Nevertheless, the time-varying, structured, bursty and highly nonlinear nature of turbulence make analysis very complex and many aspects are poorly understood. Indeed, some aspects which appear universal in both neutral fluid and plasma turbulence, such as intermittency, remain enigmatic after decades of study. A fundamental limitation is the availability of multi-point measurements of turbulent fluids, to allow detailed analysis of the interactions between scales which is the key property of turbulence. Cross-Scale offers the prospect of the most comprehensive plasma turbulence measurements to date and will shed light on both plasma turbulence, its consequences in Solar System, astrophysical and engineering plasmas, as well as on turbulence as a universal process.

The main outcome of turbulence theories is to obtain the stationary, i.e. spatial, spectra of the turbulent fluctuations of fields and particles. These spectra are important because they contain most of the physics of the system. From these spectra we can deduce the scales of the processes by which the energy, or other invariants, like for instance the magnetic helicity, are injected, transferred, and dissipated in the system. In classical fluids as well as in dissipative magnetofluids the existence of a non-linear cascade is well documented; in particular the last part of the diagram - the dissipation – is relatively clear. In the case of dissipative MHD, it is described by the viscosity term in the dynamical equation and the diffusivity term in the induction equation. The dissipation then appears to be a simple function of the constraints exerted on the fluid; it is assimilated to ‘friction’ and ‘resistivity’, and results into heating. The main reason for this simplicity is the efficiency of the interaction between the particles, in a regime where the mean free path between collisions is very small. By contrast, in collisionless plasmas the mean free path between two binary collisions is larger than the scale of the gradients, and thus collisions cannot provide the dissipation.
Collisionless dissipation processes, such as Landau and cyclotron damping, or bounce resonances, control the shape of the turbulence spectrum. These processes intervene at different scales. The Cross Scale mission will provide k-spectra at the various relevant scales and will therefore offer unique information about what controls the turbulent cascade in collisionless plasmas.

Key turbulence questions that will be answered by Cross-Scale include:

• What is the relation between dissipation and cascades in turbulent collisionless plasmas?
  • What is the nature (and direction) of the energy cascade in homogeneous turbulent collision free plasmas?
  • What is the relation between large and small scales in the case of inhomogeneous plasmas; is there an energy cascade in magnetospheric boundaries?
  • Does intermittency exist? What role does it play?
  • What is the role of turbulence at triggering explosive phenomena, such as magnetic reconnection?
• How does turbulence transport plasmas?
• What is the role of turbulence in accelerating particles?
  • What is the efficiency of acceleration through particle interaction with stochastic waves?
  • What is the role of ULF turbulence to accelerate relativistic electrons?
• What role do mode coupling, wave-particle coupling and other isolated phenomena play in the overall turbulent nature of collisionless plasmas?

Required Observations

The ambitious science goals for Cross-Scale require temporally-resolved in situ measurements on at least three length scales: fluid, ion, and electron within regions in which the relevant collisionless processes are active. There is no better, nor any practical alternative, environment than the Earth's outer magnetosphere and nearby interplanetary medium. The Earth's bow shock and travelling interplanetary shocks provide excellent laboratories for collisionless shocks that have been well-used in the past to study the isolated elements of the shock process. The sub-solar magnetopause and mid-distant geomagnetic tail are known sites for transient and energetic magnetic reconnection. And many regions of geospace, including the solar wind, foreshock, magnetosheath, and plasma-sheet, are home to turbulent plasmas.

Uniquely, Cross-Scale will address not the details of the individual physical phenomena (e.g., electron dynamics or heating deep within shock or reconnection layers) but the manner in which the different phenomena and scales couple to one another in a feedback loop. Existing studies based on single, dual, and more recently 4-spacecraft formations, show that collisionless plasmas are highly structured and variable. This demands a multi-point apprach to each of the 3 physical scales.

At the fluid level, characterisation of the bulk plasma moments (density, velocity, temperature, ...) or equivalently low-resolution particle distributions together with the dc electromagnetic fields is essential. Further characterisation of energetic particles, composition, and high-resolution fields are not mandatory to assess the overall energy input to, and back reaction on, the physical coupling. At ion kinetic scales some information on the particle distributions up to at least suprathermal energies is required. The electron scale places considerable demands on the electron phase-space resolution, and both the particle and fields temporal resolution. Additionally, at the smallest scales electric fields play a pivotal role and should be captured in full 3D if at all feasible.

This provides the initial concept for the Cross-Scale mission.