Effect of Intermittent Structures on the Spectral Index of Magnetic field in the Slow Solar Wind

Abstract

Abstract. Intermittent structures are ubiquitous in the solar wind turbulence, and they can significantly affect the power spectral index of magnetic field fluctuations which reflects the cascading process of the turbulence. However, the relationship between intermittency magnitude and the spectral index has not been shown yet. Here we present the continuous variation of the magnetic spectral index in the inertial range as a function of the intermittency magnitude. By using the measurements from the WIND spacecraft, we find 42,272 intervals with different levels of intermittency magnitude and with duration of 5–6 minutes from 46 slow-wind streams between 2005 and 2013. Among them, each of the intermittent intervals is composed of one dominant intermittent structure and background turbulent fluctuations. For each interval, a spectral index αB is determined for the Fourier spectrum of magnetic field fluctuations in the inertial range between 0.01 Hz and 0.3 Hz. A parameter Imax, which corresponds to the maximum of the trace of partial variance increments of the intermittent structure, is introduced as an indicator of the intermittency magnitude. Our statistical result shows that as Imax increases from 0 to 20, the magnetic spectrum becomes steeper gradually and the spectral index αB decreases from -1.63 to -2.01. Accordingly, an empirical relation is established between αB and Imax. The result will help us to know more details about the contributions of the intermittent structures on the power spectra, and further about the physical nature of the energy cascade taking place in the solar wind. It will also help to improve the turbulence theories that contains intermittent structures.