Abstract
Abstract. Magnetosheath parameters are usually described by gasdynamic or magnetohydrodynamic (MHD) models but these models cannot account for one of the most important sources of magnetosheath fluctuations – the foreshock. Earlier statistical processing of a large amount of magnetosheath observations has shown that the magnetosheath magnetic field and plasma flow fluctuations downstream of the quasiparallel shock are much larger than those at the opposite flank. These studies were based on the observations of a single spacecraft and thus they could not provide full information on propagation of the fluctuations through the magnetosheath. We present the results of a statistical survey of the magnetosheath magnetic field fluctuations using two years of Cluster observations. We discuss the dependence of the cross-correlation coefficients between different spacecraft pairs on the orientation of the separation vector with respect to the average magnetic field and plasma flow vectors and other parameters. We have found that the correlation length does not exceed ~1 RE in the analyzed frequency range (0.001–0.125 Hz) and does not depend significantly on the magnetic field or plasma flow direction. A close connection of cross-correlation coefficients computed in the magnetosheath with the cross-correlation coefficients between a solar wind monitor and a magnetosheath spacecraft suggests that solar wind structures persist on the background of magnetosheath fluctuations.
Highlights
The magnetosheath is a region bounded at its outer edge by the bow shock and by the magnetopause at its inner edge
The results suggest that mirror type fluctuations originate from the compression region downstream of the quasi-perpendicular bow shock and that the growth of the fluctuations cannot be described by linear approximations
In order to check the influence of the fluctuation strength on our results, we repeated the analysis shown in Figs. 7– 11 with the half of the data exhibiting the largest standard deviations (SD>3.5 nT); the results were exactly the same
Summary
Estimates of the global plasma properties in the magnetosheath are predominantly based on the results of the gasdynamic model predictions of Spreiter et al (1966) and Spreiter and Stahara (1980). The magnetosheath fluctuations are often treated as an ensemble of plasma waves, the direction of the magnetic field with respect to the s/c separation vectors would influence the value of the correlation coefficient. Such investigation should reflect the dependence of the correlation coefficient on the separation distance shown in previous figures. The background directions of the magnetic field and plasma velocity differ at locations of different spacecraft, but we think that this fact cannot spoil our analysis because we investigate the flank magnetosheath where the draping effect is not so pronounced and reasonable correlations were obtained for separations shorter than 1 RE. A larger magnetosheath-solar wind correlation can probably be interpreted as an increasing portion of solar wind variations in the magnetosheath and the trends in Fig. 11 suggest that these variations correlate over longer distances in the magnetosheath than intrinsic magnetosheath fluctuations
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