Estimation of the entropy of the solar wind flow

We analyze a time series of velocities of the low-speed stream measured by the Hellos spacecraft in the inner heliosphere, which is the region of spac e dominated by the solar wind flow. We use a nonlinear filter to give a fai thful representation of the solar wind nonlinear behavior. We have demonstr ated that the influence of noise in the data can be much mon efficiently re duced by a nonlinear filter than with the conventional (linear) filters, an d this allows a more realistic calculation of the solar wind entropy. The r esulting Kolmogorov entropy is positive, and possibly the largest Lyapunov exponent is also positive locally, which would exhibit exponential sensitiv ity to initial conditions. Thus, these results show that the solar wind in the inner heliosphere is most Likely a deterministic chaotic system.