Variability Of The Magnetic Field Power Spectrum In The Solar Wind At Electron Scales

TL;DR

This study analyzes the variability of the magnetic field power spectrum at electron scales in the solar wind, revealing different spectral morphologies linked to wave activity and structures, using Cluster mission data.

Contribution

It identifies distinct spectral morphologies at electron scales and links them to wave activity and structures, advancing understanding of dissipation mechanisms in solar wind turbulence.

Findings

Different spectral morphologies are observed, including power laws, exponential cutoffs, and spectral knees.

Spectral knees and breaks are associated with sporadic whistler waves.

The presence of whistler waves correlates with specific spectral features.

Abstract

At the electron scales the power spectrum of solar-wind magnetic fluctuations can be highly variable and the dissipation mechanisms of the magnetic energy into the various particle species is under debate. In this paper we investigate data from the Cluster mission's STAFF Search Coil magnetometer when the level of turbulence is sufficiently high that the morphology of the power spectrum at electron scales can be investigated. The Cluster spacecraft sample a disturbed interval of plasma where two streams of solar wind interact. Meanwhile, several discontinuities (coherent structures) are seen in the large scale magnetic field, while at small scales several intermittent bursts of wave activity (whistler waves) are present. Several different morphologies of the power spectrum can be identified: (1) two power laws separated by a break (2) an exponential cutoff near the Taylor shifted electron scales and (3) strong spectral knees at the Taylor shifted electron scales. These different morphologies are investigated by using wavelet coherence, showing that in this interval a clear break and strong spectral knees are features which are associated with sporadic quasi parallel propagating whistler waves, even for short times. On the other hand, when no signatures of whistler waves at 0.1 - 0.2fce are present, a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff.