A Permutation Entropy Analysis of Voyager Interplanetary Magnetic Field Observations

The permutation entropy analysis technique is here employed to study Voyager 2 observations of heliospheric field fluctuations from ∼6 AU to ∼34 AU for the first time. The properties of the technique, especially regarding the classification of a given process as either chaotic or stochastic, are illustrated in some detail and it is indicated how the technique is best applied and interpreted to the data set in question. Proceeding from this, conclusions are made regarding the stochasticity of the processes driving turbulence as a function of radial distance from the Sun, which is here found to increase with distance toward a value theoretically associated with Brownian motion, exceeding that value depending on the data bin size considered. At larger radial distances, however, it is argued that this trend may be influenced by a strongly declining signal‐to‐noise ratio. Intriguingly, this technique also serves to identify intervals of anomalously moderate to low stochasticity, which are briefly investigated here. Plain Language Summary Analyzing spacecraft data measuring the Sun's magnetic field using the permutation entropy analysis technique allows on to ascertain whether the processes generating fluctuations in the magnetic field are chaotic (more deterministic) or stochastic (more random). This study presents for the first time the results of such an analysis of Voyager spacecraft data, and shows that fluctuations in the Sun's magnetic field, out to a distance of approximately 34 AU (near Pluto's orbit), appear to be primarily stochastic in nature. However, we identify a fair number of intervals with low permutation entropy, that could correspond to interesting phenomena that require further analysis. This raises the intriguing possibility that the permutation entropy analysis technique may serve as a useful tool in identifying intervals of interest to scientists in large data sets. Key Points Permutation entropy analysis is here, for the first time, applied to Voyager data Permutation entropy is found to imply that the driving processes for heliospheric magnetic field (HMF) turbulence are predominantly stochastic Permutation entropy is found to increase with heliocentric radial distance, although this effect may be influenced by instrumental uncertainties at the largest radial distances considered here