The rate of turbulent evolution over the Sun’s poles

Ulysses magnetic field observations in the polar heliosphere have revealed the presence of a population of turbulent fluctuations on small scales with a power spectral index near 5/3 and its extension to progressively larger scales with solar distance. Using the spectral index – calculated from the second order structure function – as a tracer of the turbulent nature of the fluctuations, the rate of extension of the turbulent regime with distance is measured for both the Northern and Southern polar heliosphere. The evolution is similar in both hemispheres: the exponent of the rate of extension of the turbulent regime with solar distance is +1.1± 0.1. By comparing of the extent of the turbulent regime in the polar heliosphere with previousmeasurements of fluctuations near the ecliptic, polar fluctuations are shown to evolve more slowly than those in high speed streams at lower heliolatitudes beyond 0.3 Astronomical Units. Within this distance, however, fluctuations in high speed streams at high and low latitudes probably evolve at the same rate.