Modification of small- and middle-scale solar wind structures by the bow shock and magnetosheath: Correlation analysis

• Cross-correlation analysis of solar wind-magnetosheath fluctuations.
• Correlation between solar wind and magnetosheath densities increases with the upstream density and magnetic field.
• A clear cutoff of solar wind fluctuations to be transmitted through the bow shock at 0.01 Hz.
• An increase of the correlation with the amplitude of fluctuations.

The paper is devoted to a modification of small- and middle-scale solar wind plasma and magnetic structures during their propagation through the bow shock and magnetosheath. We use data from two closely spaced THEMIS spacecraft in the solar wind and magnetosheath and we concentrate on the slow solar wind and quasiperpendicular bow shocks. We determine a cross-correlation function between time series of the solar wind and magnetosheath ion densities and magnetic field magnitudes. The correlation coefficient is calculated for data smoothed on different subintervals from 10 to 170 s and we show that the correlation coefficient between both quantities strongly depends on the duration of smoothing subintervals in the range of 10–50 s, whereas it is not significantly affected if the duration of the subintervals exceeds ≈50–100 s. It means that the fluctuations with frequencies below 0.01–0.02 Hz are of the solar wind origin, whereas bow shock and magnetosheath processes contribute to the enhanced level of density and magnetic field magnitude variations at higher frequencies. We also investigate an influence of different parameters on the modification of structures in the magnetosheath. We suggest that solar wind structures of larger amplitudes are less modified, especially if these structures are embedded in a dense solar wind and/or in the solar wind with a large interplanetary magnetic field magnitude. Under selected conditions, the modification of the structures does not depend on the interplanetary magnetic field direction with respect to the bow shock normal.