Statistical Study of Magnetic Field Spectra Around Ion Scales: STEREO Observations.

Abstract:

The solar wind magnetic turbulence in the inertial and kinetic range has been well studied using Fourier and Wavelet transform. Spectra in this range are usually described as a succession of two spectral slopes separated by a spectral break around 0.5 Hz. Our statistical study focus on this transition area. Using the twin STEREO probes, launched in October 2006, we study a large amount of magnetic data extending from 2007 to 2013. Thanks to the high resolution magnetic field data provided by the MAG instrument, the spectrum can be computed over a wide frequency range (up to 4 Hz). We conducted a spectral analysis based on the wavelet transform technique using both Daubechies and Morlet wavelets. This analysis reveals some distinct spectral shape. The major part of the spectra are like the classical description with two spectral slopes separated by a transition area more or less well defined. But for around 1% of the spectra we observe clearly defined bump shape. This bump shape is the signature of left and right handed elliptically polarized waves. We compare those different shapes and show that the wave presence is correlated with a low level of turbulence. Furthermore, by combining these results with plasma parameters obtained via the PLASTIC instrument, we try to characterize those waves and discuss their possible origins.