We analyze a time series of velocities of the low-speed stream measured by the Helios spacecraft in the inner heliosphere, which is the region of space dominated by the solar wind flow. We use a nonlinear filter to give a faithful representation of the solar wind nonlinear behavior. We have demonstrated that the influence of noise in the data can be much more efficiently reduced by a nonlinear filter than with the conventional (linear) filters, and this allows a more realistic calculation of the solar wind entropy. The resulting Kolmogorov entropy is positive, and possibly the largest Lyapunov exponent is also positive locally, which would exhibit exponential sensitivity to initial conditions. Thus, these results show that the solar wind in the inner heliosphere is most likely a deterministic chaotic system.

• Received 10 September 1999

DOI:https://doi.org/10.1103/PhysRevE.62.6496