Complexity Variations in the Interplanetary Magnetic Field between 0.4 and 5.3 AU

Abstract:

We have investigated how the character of magnetic fluctuations of solar wind plasma depends on radial distance from the Sun. We use measurements of the magnetic field taken at different distances from the Sun by different spacecraft: Helios between 0.4 and 1 AU, ACE and Wind at about 1 AU, and Ulysses at about 5.3 AU. Data intervals are selected to contain only what appear to be random fluctuations and to exclude solar wind structures such as coronal mass ejections, co-rotating interaction regions, heliospheric current sheets, shocks, etc. With these data we calculate the Jensen–Shannon complexity as a function of permutation entropy. Jensen-Shannon complexity maps indicate if the fluctuations in the magnetic fields are stochastic (low complexity and high entropy), or if they exhibit minimal or maximal complexity and lower entropy. The Jensen–Shannon complexity values determined from the spacecraft measurements evolve from moderate complexity and high entropy at 0.4 AU to lower complexity and higher entropy farther from the Sun. We interpret these data to mean that as the solar wind plasma expands outward, the magnetic field fluctuations evolve from chaotic (i.e., low dimensionality, deterministic fluctuations) to turbulent (i.e., low dimensionality, non-deterministic fluctuations). By separating the magnetic fluctuations into slow solar wind (<450 km/s) and fast solar wind (>550 km/s), we find that the younger solar wind (transported outward rapidly) has higher complexity than the older solar wind (transported outward slowly). These results can be tested by Solar Probe Plus to be launched in 2018.