Cross-scale energy transport in space plasmas


The solar wind is a supersonic magnetized plasma streaming far into the heliosphere. Although cooling as it flows, it is rapidly heated upon encountering planetary obstacles. At Earth, this interaction forms the magnetosphere and its sub-regions. The present paper focuses on particle heating across the boundary separating the shocked solar wind and magnetospheric plasma, which is driven by mechanisms operating on fluid, ion and electron scales. The cross-scale energy transport between these scales is a compelling and fundamental problem of plasma physics. Here, we present evidence of the energy transport between fluid and ion scales: free energy is provided in terms of a velocity shear generating fluid-scale Kelvin–Helmholtz instability. We show the unambiguous observation of an ion-scale magnetosonic wave packet, inside a Kelvin–Helmholtz vortex, with sufficient energy to account for observed ion heating. The present finding has universal consequences in understanding cross-scale energy transport, applicable to environments experiencing velocity shears during comparable plasma regimes.

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Figure 1: Pictogram depicting the Cluster spacecraft configuration, wave-packet observations and the KH simulated data.
Figure 2: Overview plot showing time series data collected by the Cluster spacecraft for the boundary crossing on 6 June 2002.
Figure 3: Experimental and theoretical dispersion relations used for wave mode identification of RH, LH and FMW intervals.
Figure 4: Growth rates calculated for the FMW interval.
Figure 5: Cluster data showing the mixing region where cross-scale energy transport takes place.


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The work of T.W.M. and K.N. was supported by National Science Foundation Grants 0847120 and 1502774. The work of A.P.D. was supported by the Academy of Finland Grants 288472 and 267073/2013. The authors would like to thank M. A. Balikhin for valuable discussion. The authors would like to acknowledge the work performed by the Cluster FGM, EFW, CIS and PEACE instrument teams as well as the Cluster Science Archive and the Cluster Active Archive for the use of their data.

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K.N. provided the idea, initiated the study and guided the work of graduate student T.W.M., who identified the KH event from Cluster data, screened the data for plasma mixing regions and higher frequency plasma waves, analysed wave properties, created 2.5D MHD simulations of the event and prepared figures for the manuscript. A.P.D. computed the experimental dispersion relation using the two-spacecraft method. All authors contributed to the writing and editing of the manuscript and discussed the methods, results and scientific implications at all stages. All authors discussed the text and commented on the manuscript.

Correspondence to T. W. Moore.

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Moore, T., Nykyri, K. & Dimmock, A. Cross-scale energy transport in space plasmas. Nature Phys 12, 1164–1169 (2016).

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