Gompressive fluctuations generated by time evolution of Alfvénic perturbations in the solar wind current sheet

Abstract

The velocity‐magnetic field correlation in the solar wind low‐frequency turbulence decreases with increasing distance from the sun. A 2½‐dimensional MHD numerical simulation has been set up in order to investigate the interaction between the solar wind current sheet and Alfvén waves, as a possible decorrelation mechanism. The equilibrium structure models the magnetic field rotation in a sector boundary. On such an equilibrium we have superposed a perturbation intended to represent Alfvénic fluctuations converging with opposite correlation on the two sides of the heliospheric current sheet. In consequence of the opposite correlation a source of compressive fluctuations, related to a locally nonsolenoidal velocity field, is localized inside the current sheet. Moreover, the relation B2 = const is initially verified. During the time evolution the Alfvénic correlation is destroyed inside the current sheet arid compressive fluctuations are generated. Two kinds of such fluctuations have been identified, according to the sign of the density‐magnetic field intensity correlation. For β < 1, anticorrelated fluctuations (slow mode) remain mainly confined inside the neutral sheet, where they form pressure‐balanced structures (tangential discontinuities). Positive correlated fluctuations (fast mode), which also propagate perpendicular to the background magnetic field, also spread in the homogeneus region. The dominance of positive (negative) correlated fluctuations at frequencies lower (higher) than ∼ 0.1 hour−1 observed in the solar wind is found when β < 1. This seems to be due to the prevalence of anticorrelated fluctuations in regions where stronger nonlinear interactions transfer the fluctuation energy to small scales. Negative correlations at all scales are found for β > 1. A different dynamical mechanism is at work in the homogeneous region, producing spectra different from those found in the current sheet; this mechanism has been identified as a parametric decay. Spectra of physical quantities show features similar to those observed in the solar wind turbulence, both in the region where the Alfvénic correlation is destroyed and in the region where it is conserved.