Turbulence in the solar wind and planetary magnetospheres
The solar wind streaming off the Sun in all directions at a speed that often exceeds 400 km/s is described by magnetohydrodynamics at large scales. This complex description should include collective and dissipative processes, where energy is transferred from larger to smaller scales. How turbulence develops and the energy cascade terminates and how the energy is partitioned between larger and smaller scales were among the still-open questions that the EU-funded project STORM (Solar system plasma turbulence: Observations, intermittency and multifractals) sought to address. The answers are fundamental to understanding processes of particle acceleration and plasma heating in solar corona and solar wind as well as other astrophysical plasmas. Observations by European spacecraft like Ulysses in the solar wind and Cluster and Venus Express in planetary plasma environments brought valuable insights into fluctuations of plasma and fields that describe the turbulent state of ionized matter in the solar system. STORM scientists used different approaches to analyse the structure and topology of these fluctuations in various regions of the solar system. Besides the solar wind, they looked into turbulence developed at the interface of the magnetospheres of Venus, Earth and Saturn and the solar wind. High time resolution measurements of plasma and magnetic field from Cluster, Venus Express, Giotto and Cassini satellites offered the chance to probe the smallest scales ever explored in the solar wind. Moreover, advanced models based on a statistical approach, like the (multi)fractal geometry, could be tested in a creative way to bring new insight into universal processes like turbulence. One of the main outcomes of STORM is a software library for non-linear data analysis that gathers methods able to reveal the structure of turbulence. It includes methods from lower-order analysis, like the power spectral density analysis, to higher-order analyses, like the probability distribution functions and multifractals. It is a versatile tool that can ingest a broad spectrum of spacecraft and synthetic data. The data analysis tools can be freely downloaded from the project website and used to effectively analyse time series, either provided by STORM databases or from other past, ongoing and even future space missions. Scientific results obtained during STORM contributed to drawing a more complete picture of turbulence in solar system plasmas. All new information provided about turbulence in solar wind and planetary magnetospheres will open up novel ways of looking at societal challenges like the space weather and/or the unstable behaviour of plasmas in fusion power plants.
Keywords
Turbulence, solar wind, plasma turbulence, magnetohydrodynamics, STORM, multifractals
