Multipoint observations of plasma phenomena made in space by Cluster

Abstract

Plasmas are ubiquitous in nature, surround our local geospace environment, and permeate the universe. Plasma phenomena in space give rise to energetic particles, the aurora, solar flares and coronal mass ejections, as well as many energetic phenomena in interstellar space. Although plasmas can be studied in laboratory settings, it is often difficult, if not impossible, to replicate the conditions (density, temperature, magnetic and electric fields, etc.) of space. Single-point space missions too numerous to list have described many properties of near-Earth and heliospheric plasmas as measured both in situ and remotely (see http://www.nasa.gov/missions/#.U1mcVmeweRY for a list of NASA-related missions). However, a full description of our plasma environment requires three-dimensional spatial measurements. Cluster is the first, and until data begin flowing from the Magnetospheric Multiscale Mission (MMS), the only mission designed to describe the three-dimensional spatial structure of plasma phenomena in geospace. In this paper, we concentrate on some of the many plasma phenomena that have been studied using data from Cluster. To date, there have been more than 2000 refereed papers published using Cluster data but in this paper we will, of necessity, refer to only a small fraction of the published work. We have focused on a few basic plasma phenomena, but, for example, have not dealt with most of the vast body of work describing dynamical phenomena in Earth's magnetosphere, including the dynamics of current sheets in Earth's magnetotail and the morphology of the dayside high latitude cusp. Several review articles and special publications are available that describe aspects of that research in detail and interested readers are referred to them (see for example, Escoubet et al.

On the Fourier Contribution of Strong Current Sheets to the High‐Frequency Magnetic Power SpectralDensity of the Solar Wind

“…The standard method for describing the parts of the magnetic power spectrum is via an energy‐cascade picture. Sketches of the cascade concept of the solar wind magnetic spectrum can be found in Figure 19 of Goldstein et al (), Figure 2 of Howes (), and Figure 14of Matthaeus et al (). The first column of Figure lists the main concepts used in that method of describing the solar wind's magnetic power spectrum.…”

The Complex Structure of Magnetic Field Discontinuities in the Turbulent Solar Wind

“…The characteristic scales at which this break-down occurs are given by the proton gyro-radius ρ p = v th,p /Ω p (being v th,p the proton thermal speed and Ω p the proton gyrofrequency) and/or the proton skin depth d p = c/ω cp (being c the speed of light and ω cp the proton plasma frequency) [2,[4][5][6]. At these scales the dynamics could be mediated by kinetic-Alfvén fluctuations, whistler-like perturbations and coherent structures such as vortexes and current sheets [7].The most narrow current sheets and filaments are present at electron scales, where turbulent energy eventually dissipates [8,9], even if the energy-dissipation mechanisms in a weakly-collisional plasma such as the turbulent solar wind are far from being understood. Recent kinetic simulations…”

“…The most narrow current sheets and filaments are present at electron scales, where turbulent energy eventually dissipates [8,9], even if the energy-dissipation mechanisms in a weakly-collisional plasma such as the turbulent solar wind are far from being understood. Recent kinetic simulations…”

“…Magnetic reconnection is the change of topology of the magnetic field, with subsequent conversion of energy into flows, heat, and non-thermal effects. Usually, reconnection and turbulence have been studied as separate topics, but more recently it has been suggested that these effects might coexist [7,9,14]. Namely, reconnection of thin current sheets can take place in magnetohydrodynamics (MHD), as well as in plasma-kinetic models [10].…”

The calculation of moment uncertainties from velocity distribution functions with random errors

“…The spin‐integrated differential particle flux in cm −2 s −1 sr −1 keV −1 as a function of time is shown in Figure a. Density ( n ) and heat flux parallel to the magnetic field ( H || ) were calculated using a trapezoidal integration scheme that was validated against existing Cluster moments code [ Goldstein et al , ] and are shown in Figures b and c, respectively. From reported N(v) and f ( v ) values, the covariance matrix was calculated following section 2.…”