Recent advances in wavelet analyses: Part 1. A review of concepts

Cluster fluxgate magnetometer (FGM) and ion spectrometer (CIS) data are employed to analyze magnetic field fluctuations within the plasma sheet during passages through the magnetotail region in the summers of 2001 and 2002 and, in particular, to look for characteristics of magnetohydrodynamic (MHD) turbulence. Power spectral indices determined from power spectral density functions are on average larger than Kolmogorov's theoretical value for fluid turbulence as well as Kraichnan's theoretical value for MHD plasma turbulence. Probability distribution functions of the magnetic fluctuations show a scaling law over a large range of temporal scales with non-Gaussian distributions at small dissipative scales and inertial scales and more Gaussian distribution at large driving scales. Furthermore, a multifractal analysis of the magnetic field components shows scaling behavior in the inertial range of the fluctuations from about 20 s to 13 min for moments through the fifth order. Both the scaling behavior of the probability distribution functions and the multifractal structure function suggest that intermittent turbulence is present within the plasma sheet. The unique multispacecraft aspect and fortuitous spacecraft spacing allow us to examine the turbulent eddy scale sizes. Dynamic autocorrelation and cross correlation analysis of the magnetic field components allow us to determine that eddy scale sizes fit within the plasma sheet. These results suggest that magnetic field turbulence is occurring within the plasma sheet resulting in turbulent energy dissipation.

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Plasma sheet turbulence observed by Cluster II

Many phenomena in the Earth's magnetotail have characteristic temporal scales of several minutes and spatial scales of a few Earth radii ( R E ). Examples of such transient and localized mesoscale phenomena are bursty bulk flows, beamlets, energy dispersed ion beams, flux ropes, traveling compression regions, night-side flux transfer events, and rapid flappings of the current sheet. Although most of these observations are linked to specific interpretations or theoretical models they are inter-related and can be the different aspects of a physical process or origin. Recognizing the inter-connected nature of the different transient and localized phenomena in the magnetotail, this paper reviews their observations by highlighting their important characteristics, with emphasis on the new results from Cluster multipoint observations. The multi-point Cluster measurements have provided, for the first time, the ability to distinguish between temporal and spatial variations, and to resolve spatial structures. Some examples of the new results are: flux ropes with widths of 0.3 R E , transient field aligned currents associated with bursty bulk flows and connected to the Hall current at the magnetic reconnection, flappings of the magnetotail current sheet with time scales of 100 s–10 min and thickness of few thousand km, and particle energization including velocity and time dispersed ion structures with the latter having durations of 1–3 min. The current theories of these transient and localized processes are based largely on magnetic reconnection, although the important role of the interchange and other plasma modes are now well recognized. On the kinetic scale, the energization of particles takes place near the magnetic X-point by non-adiabatic processes and wave-particle interactions. The theory, modeling and simulations of the plasma and field signatures are reviewed and the links among the different observational concepts and the theoretical frameworks are discussed. The mesoscale processes in the magnetotail and the strong coupling among them are crucial in developing a comprehensive understanding of the multiscale phenomena of the magnetosphere.

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Transient and localized processes in the magnetotail: a review

http://www.lac.inpe.br/~margarete/JASRWavelet.pdf

On wavelet techniques in atmospheric sciences

Multiscale phenomena in the near-Earth magnetosphere

Multifractal and intermittent nature of substorm-associated magnetic turbulence in the magnetotail

Abstrsct-One of the central issues in substorm research is what determines the substorm intensity. Through an introduction on what constitutes a ma~etosphe~c substorm, we discuss several parameters which are available to measure the substorm intensity. In terms of ionospheric quantities, we have the aurora1 electroject indices, the total current in the westward aurora1 electrojet, the area of bright aurora, the maximum poleward advance of the aurora1 bulge, and the duration of aurora1 substorm activities. In terms of magnetospheric quantities, we have the innermost location of the substorm injection boundary and the amount ofcurrent reduction in the cross-tail current within the substorm current wedge. A measure reflecting substorm activities in both the ionosphere and the magnetosphere is the total substorm energy dissipation but its drawback lies in the difficulty of assessing it accurately if the energy loss due to plasmoids is to be included. We also discuss the predictability of substorm intensity, which leads us to the issue of whether a substorm is a directly-driven or an unloading process. The recent success in predicting the aurora1 electrojet index from solar wind parameters with a cross-correlation of -0.9 suggests that substorm activities over a long time scale are primarily directly-driven while those over a short time scale are governed by impulsive unloading processes. This understanding allows us to reconcile the apparently conflicting dual nature of magnetospheric substorms.

A.T.Y. Lui

Papers

Multiscale phenomena in the near-Earth magnetosphere

Multifractal and intermittent nature of substorm-associated magnetic turbulence in the magnetotail

What determines the intensity of magnetospheric substorms

What determines the intensity of magnetospheric substorms