[Paper Review] [Plasma 2020 Decadal] Multipoint Measurements of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence

K. G. Klein, Olga Alexandrova+22 more|arXiv (Cornell University)|Mar 13, 2019

Solar and Space Plasma Dynamics10 citations

TL;DR

This paper proposes the HelioSwarm mission—a multi-spacecraft swarm designed to measure solar wind turbulence across magnetohydrodynamic (MHD), transition, and kinetic scales simultaneously. By deploying nine spacecraft in a dynamically evolving formation, HelioSwarm enables three-dimensional, multi-scale correlation and derivative calculations, resolving energy transfer and dissipation in magnetized plasma turbulence with unprecedented spatial and spectral resolution.

ABSTRACT

A multi-institutional, multi-national science team will soon submit a NASA proposal to build a constellation of spacecraft to fly into the near-Earth solar wind in a swarm spanning a multitude of scales in order to obtain critically needed measurements that will reveal the underlying dynamics of magnetized turbulence. This white paper, submitted to the Plasma 2020 Decadal Survey Committee, provides a brief overview of turbulent systems that constitute an area of compelling plasma physics research, including why this mission is needed, and how this mission will achieve the goal of revealing how energy is transferred across scales and boundaries in plasmas throughout the universe.

Motivation & Objective

• To resolve the unresolved mechanisms of energy transfer across scales in magnetized plasma turbulence, a fundamental problem in astrophysical and space plasma physics.
• To address the lack of in situ multi-scale measurements in the solar wind, which currently limits understanding of turbulence dynamics at kinetic scales.
• To enable direct measurement of spatial gradients and wavevector-frequency power distributions using multi-spacecraft formations.
• To close the gap between theoretical models and observational data by providing simultaneous coverage of inertial and dissipation ranges.
• To advance understanding of plasma heating, particle acceleration, and dynamo processes across cosmic environments through in situ solar wind observations.

Proposed method

• Deploy a swarm of nine small satellites in a dynamically evolving formation to span spatial scales from 50 km (kinetic scale) to 3000 km (inertial range).
• Use three-dimensional baseline vectors (λ) between spacecraft to ensure full coverage of MHD, transition, and kinetic scales in all orthogonal directions (RTN coordinates).
• Construct 382 polyhedra from at least four spacecraft to analyze 3D turbulence structure and quantify elongation (E) and planarity (P) of spatial configurations.
• Apply multi-point correlation techniques to calculate two-point correlations with both time and space as independent variables, enabling scale-resolved cascade rate analysis.
• Utilize advanced formation flying techniques to compute spatial derivatives and map power distribution in frequency–wavevector space across scales.
• Leverage low-resource, small-satellite technology to achieve high-resolution, multi-scale observations previously unattainable with single-spacecraft or limited-formation missions.

Experimental results

Research questions

• RQ1 How is energy transferred across scales in magnetized plasma turbulence, particularly from MHD to kinetic scales?
• RQ2 What is the role of magnetic fields in organizing the turbulent cascade and influencing energy dissipation?
• RQ3 How do the spatial structures of turbulent fluctuations evolve across the inertial and dissipation ranges?
• RQ4 What mechanisms dissipate turbulent energy into ion and electron thermal energy at kinetic scales?
• RQ5 How do intermittency and anisotropy affect the distribution of turbulent power in wavevector space?

Key findings

• The proposed HelioSwarm configuration achieves full three-dimensional coverage of MHD, transition, and kinetic scales simultaneously, with baseline vectors spanning 50 km to 3000 km.
• The mission design ensures optimal spatial distribution with 382 distinct polyhedra formed from at least four spacecraft, enabling robust 3D turbulence structure analysis.
• Near-Earth regions with high-quality 3D coverage—defined by √(E² + P²) ≤ 0.6 and L₁ ≥ 3L₂—are identified and highlighted, indicating optimal measurement conditions.
• The mission enables the first multi-scale, multi-point calculation of two-point correlations with both time and space as independent variables.
• The formation allows direct measurement of spatial derivatives and wavevector-frequency power distributions, crucial for resolving cascade dynamics.
• The mission’s multi-spacecraft design overcomes limitations of single-point missions (e.g., ACE, Wind) and enables studies of intermittency and anisotropy at unprecedented resolution.