Workshop on Cross-Scale Coupling in Plasmas
Workshop on Cross-Scale Coupling in Plasmas
Most of the visible universe is in the highly ionised plasma state, and most of that plasma is collisionfree. Plasma processes are at work everywhere, from radio galaxy jets and supernova explosions to solar flares and planetary magnetospheres. Cross-Scale is an M-class mission dedicated to quantifying the coupling in plasmas between different physical scales. This cross-scale coupling, being highly variable and structured, is critical in underpinning and quantifying the physical mechanisms inferred in plasmas that are difficult to observe. As plasma regimes encounter each other, the absence of collisions raises fundamental questions about how energy is shared amongst the three main elements (electrons, ions, and overall bulk flows). These constituents, each of which operates on its own physical scale, are coupled through electromagnetic fields. Three fundamental physical processes operate to bring about the universal collisionless plasma coupling in physical environments where momentum and energy transfer is important.
Shock waves guide strong flows around obstacles or at
interfaces between two flow regimes. They are important locations for
the transfer of directed bulk flow energy into heat, with an attendant
acceleration of energetic particles.
Magnetic reconnection releases stored magnetic energy to the
plasma, and allows for exchange of material between previously
isolated regions. Moreover, the consequent change in magnetic
topologies provides a coupling between plasma regions which often
drives the global scale dynamics of the system.
Turbulence transports energy from large scales at which it is
input to small scales where it is dissipated. In the process, it
interacts strongly, and often selectively, with plasma particle
populations as either a source or sink (or both) of energy. Near-Earth
space is a unique laboratory for quantifying the physics of these
three processes. Breakthroughs have arisen due to the high quality of
data that, unlike more distant regimes, is sampled directly by plasma
and fields experiments on satellites.
Shocks, reconnection, and turbulence are controlled by dynamics which are coupled on 3 fundamental scales simultaneously: electron kinetic, ion kinetic, and fluid. It is the nonlinear interaction of 3D, time-varying structures on these 3 scales which produces the complex behaviour and consequences of these processes. Critically, most astrophysical plasmas are collisionless, which means that their constituents can be far from equilibrium with each other. The resulting nonlinear dynamics provides diverse and exotic mechanisms for momentum and energy flow and redistribution.
To date, in situ measurements have focused on terrestrial phenomena, such as the mechanisms that populate the van Allen belts. Dual spacecraft studies during the 1980's began to address the real microphysics. Present generation missions (Cluster and MMS) utilise 4 spacecraft to sample a specific volume, and hence characterise the physics operating on the single scale corresponding to the spacecraft separation. By the time MMS has flown, we shall have a catalogue of behaviour that ranges from the smallest, electron scale, to the largest fluid-like phenomena.
That knowledge is incomplete due to the ambiguity and uncertainty about the dynamics and variability of the larger contextual scales (for the electron and ion scales) and of the internal microprocesses that mediate the energy exchange (for the larger scales). The complex, dynamic nonlinear coupling of scales and physical mechanisms can not be quantified without simultaneous information on all scales.
Cross-Scale will target compelling and fundamental questions, such as:
Cross-Scale: Mission concepts and status
The complex, three dimensional nature of plasma structures has long been recognised. Previous, existing and upcoming missions have been designed to measure this 3D structure using multiple spacecraft. A minimum of four spacecraft are necessary to determine 3D structure: ESA's Cluster and NASA's upcoming MMS missions both use four spacecraft for this task. A fundamental restriction of multi-spacecraft measurements, however, is that they are sensitive to scales of the order of the spacecraft separation. With four spacecraft, multiple scales can be probed by varying this separation, but only one scale can be measured at any time.
Plasmas are not just three dimensional: they also contain time-varying structure on many scales, simultaneously. Different scales are affected by different physical processes. It is the interplay of these which results in the complexity of shocks, reconnection, and other phenomena, and consequently in their large scale effects. To understand the interplay of forces and dynamics within such regions and hence predict their effects, it is essential to measure the timedependent behaviour in 3D on the three key physical scales - electron, ion and fluid. This can only be achieved with spacecraft positioned such that some have separations comparable to each of these three physical scales, simultaneously. Thus 4 spacecraft are required at each of the three physical scales, making a complement of 12 spacecraft in total. Instrumentation on the spacecraft at each scale must be tailored to the physical processes at that scale. Near-Earth space, which is relatively accessible and contains examples of all the phenomena of interest, is the obvious target for such a mission, which we call Cross-Scale.
Cross-Scale will employ a number of ESA spacecraft which will fly with
highly complementary spacecraft from its sister mission SCOPE provided
by JAXA. Other agencies, including NASA, the Canadian Space Agency, and
Roscosmos, have expressed an interest in participation at payload or
spacecraft level. Together, the assembled fleet will separate spatial
and temporal variations simultaneously on the three key scales for the
first time. The European spacecraft, which carry a minimal payload with
strong heritage, will be launched into an elliptical Earth orbit.
Over the two year mission they will encounter various
collisionless shocks, explore regions of both spontaneous and strongly
driven reconnection, and investigate both nascent and highly evolved
plasma turbulence. There are no technologies that need to be developed
or proven for a launch in 2017, or earlier. The mission is thus low
risk for high science return. It taps directly into European
leadership in multipoint in situ space plasmas.
Cross-Scale was selected in October 2007 by ESA's Space Science Advisory
Structure to proceed to the Assessment Phase of Cosmic Vision M-class
missions. Two parallel industrial studies are now underway that will
report toward the end of 2009 on detailed mission design, programmatics,
payload accommodation, and other matters in preparation for the next ESA
Cosmic Vision selection process that will result in two missions
proceding to the Development (Phase B) Phase early in 2010. Instrument
consortia have responded to the ESA Call for Declarations of Interest
and will undertake instrument studies over the next year. Similar
studies are also underway in partner agency countries (USA and Canada).
Cross-Scale's sister mission, SCOPE, is at a mature stage within JAXA. A
key review will take place in September 2008.