  This white paper submitted for 2020 Decadal Assessment of Plasma Science
concerns the importance of multi-spacecraft missions to address fundamental
questions concerning plasma turbulence. Plasma turbulence is ubiquitous in the
universe, and it is responsible for the transport of mass, momentum, and energy
in such diverse systems as the solar corona and wind, accretion discs, planet
formation, and laboratory fusion devices. Turbulence is an inherently
multi-scale and multi-process phenomenon, coupling the largest scales of a
system to sub-electron scales via a cascade of energy, while simultaneously
generating reconnecting current layers, shocks, and a myriad of instabilities
and waves. The solar wind is humankind's best resource for studying the
naturally occurring turbulent plasmas that permeate the universe. Since
launching our first major scientific spacecraft mission, Explorer 1, in 1958,
we have made significant progress characterizing solar wind turbulence. Yet,
due to the severe limitations imposed by single point measurements, we are
unable to characterize sufficiently the spatial and temporal properties of the
solar wind, leaving many fundamental questions about plasma turbulence
unanswered. Therefore, the time has now come wherein making significant
additional progress to determine the dynamical nature of solar wind turbulence
requires multi-spacecraft missions spanning a wide range of scales
simultaneously. A dedicated multi-spacecraft mission concurrently covering a
wide range of scales in the solar wind would not only allow us to directly
determine the spatial and temporal structure of plasma turbulence, but it would
also mitigate the limitations that current multi-spacecraft missions face, such
as non-ideal orbits for observing solar wind turbulence. Some of the
fundamentally important questions that can only be addressed by in situ
multipoint measurements are discussed.
