Scale-Dependent Kurtosis of Magnetic Field Fluctuations in the Solar Wind: A Multi-Scale Study With Cluster 2003–2015

O. W. Roberts*, O. Alexandrova, L. Sorriso-Valvo, Z. Vörös, R. Nakamura, D. Fischer, A. Varsani, C. Philippe Escoubet, M. Volwerk, P. Canu, S. Lion, K. Yearby

*Corresponding author for this work

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Abstract

During the lifetime of the Cluster mission, the inter-spacecraft distances in the solar wind have changed from the large, fluid, scales (∼104 km), down to the scales of protons (∼102 km). As part of the guest investigator campaign, the mission achieved a formation where a pair of spacecraft were separated by ∼7 km. The small distances and the exceptional sensitivity of the search coil magnetometer provide an excellent data set for studying solar wind turbulence at electron scales. In this study, we investigate the intermittency of the magnetic field fluctuations in the slow solar wind. Using 20 time intervals with different constellation orientations of Cluster we cover spatial scales between 7 and 104 km. We compare time-lagged increments from a single spacecraft with spatially lagged increments using multiple spacecraft. As the turbulent cascade proceeds to smaller scales in the inertial range, the deviation from Gaussian statistics is observed to increase in both temporal and spatial increments in the components transverse to the mean field direction. At ion scales, there is a maximum of kurtosis, and at sub-ion scales, the fluctuations are only weakly non-Gaussian. In the compressive component the deviation from Gaussian statistics is variable: it may increase throughout the inertial and sub-ion ranges, but also, it may have a maximum at magnetohydrodynamic scales associated with large scale magnetic holes. The observations show differences in kurtosis of time and space increments when the spacecraft pairs are transverse to the flow, indicating its spatial anisotropy.

Key Points
Multipoint Cluster magnetic field data (from 2003 to 2015) are used to investigate intermittency in solar wind plasma
Two different methods are used to calculate fluctuations: temporal increments and spatial increments
Spatial scales from ∼101 to ∼104 km are covered

Plain Language Summary
Turbulence in the slow solar wind is investigated using multi-spacecraft measurements for different satellites configurations. Twenty time intervals of more than 1 hr are analyzed. We compare differences in two time-delayed magnetic field measurements (time-lags) and of magnetic field measurements between spacecraft pairs (space-lags). Space-lags give the fluctuations along different satellite baseline directions and scales (from 7 to 9,000 km) while time-lags give the fluctuations along the flow direction. The magnetic field fluctuations' intermittency, which can be thought of as the “patchiness” or “roughness” is investigated. Differences are observed between the time-lagged measurements and the spatially lagged measurements when the spacecraft pairs are transverse to the flow.
Original languageEnglish
Article numbere2021JA029483
Number of pages25
JournalJournal of Geophysical Research: Space Physics
Volume127
Issue number9
DOIs
Publication statusPublished - 19 Sept 2022
Externally publishedYes

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