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Key summary
Quantifying cross-scale energy transfer in space plasmas is a long-standing problem. The goal of our team is to review open questions involving multi-scale energy transport, dissipation, and particle energization in turbulent plasmas, with an emphasis on the synergy between state-of-art kinetic simulations and in-situ multi-spacecraft observations with current missions. As the community is now progressing towards cross-scale spacecraft constellations, e.g., HelioSwarm, we will identify appropriate multi-spacecraft analysis techniques that are critical to bring closure to these problems and enhance the science return of such missions.
Abstract
How plasmas process and transport energy between large and small scales, eventually dissipating it to heat surrounding environments, is one of the most compelling problems in space and astrophysical plasmas. The underlying processes driving this transport and dissipation are fundamentally multi-scale in nature. At large scales, plasmas behave as conducting fluids in which they are described using magnetohydrodynamics (MHD). At small scales, they behave kinetically where the dynamics of ions and electrons become essential. Near-Earth space plasmas are the most accessible natural laboratories where the physics at work can be probed in situ. Multi-spacecraft missions with four identical satellites, forming a tetrahedral configuration in space, namely ESA’s Cluster (launched in 2000) and NASA’s Magnetospheric Multiscale (MMS, launched in 2015), have been valuable tools to investigate the spatial structure of plasma processes.
Cluster and MMS allow us to probe plasmas at a single scale; however, energy transfer is inherently a cross-scale process where the electromagnetic and flow energy, contained at large scales, cascades to smaller scales before eventually dissipating into heat. To address such a problem, simultaneous observations of plasmas across multiple scales are critically needed. The community is now progressing towards a larger constellation of satellites to probe plasmas at multiple scales simultaneously. HelioSwarm (HS), consisting of 9 spacecraft, is such a mission recently selected by NASA to be launched in 2028. Due to a large number of works that consider turbulent energy cascade and various analysis techniques for single- and four-spacecraft measurements, the proposed ISSI team will focus on two objectives.
- We will synthesize and compare different measures of energy transfer rates in various frameworks. The focus will be placed on measures for energy transfer across MHD and kinetic scales and between different forms (electromagnetic, flow, and thermal) for each plasma species. We will also address the associated mechanisms. The aim is to consolidate our current understanding and identify key knowledge gaps.
- We will review techniques for single and multi-spacecraft measurements and test them for the calculations of energy dissipation measures in numerical simulations (MHD, Hall-MHD, kinetic) using virtual probes. The purpose is to understand expected measures, discuss the advantages and disadvantages of different techniques, and identify the development needs of tools for future missions.
The knowledge of energy transfer in turbulent plasmas is now rapidly evolving as driven by new high-resolution observations by Solar Orbiter (launched in 2020), Parker Solar Probe (launched in 2018), and MMS (launched in 2015). Meanwhile, all the formulations have not been carefully applied and systematically compared, leading to misleading or incomplete conclusions. There is an urgent need in the community to step back and review formulations in order to facilitate the discussion of the energy transfer problems and the comparison of the relative importance of various processes. This project will help identify gaps and development needs in order to maximize the scientific returns of future cross-scale spacecraft constellations such as HelioSwarm (to be launched in 2028).