A possible truncated-Lévy-flight statistics recovered from interplanetary solar-wind velocity and magnetic-field fluctuations

¹ Istituto Fisica Spazio Interplanetario del CNR - 00133 Roma, Italy
² Dipartimento di Fisica, Università della Calabria - 87036 Rende (Cs), and Istituto Nazionale di Fisica della Materia, Sezione di Cosenza - Cosenza, Italy

Received: 13 October 2003
Accepted: 3 February 2004

Abstract

Interplanetary solar-wind fluctuations have been studied in the inner heliosphere and within the MHD range of scales. We found that fluctuations are such that velocity and magnetic-field vectors, which initially keep their orientation around a given direction in space during a certain time interval, abruptly change direction to fluctuate around a new orientation. This behavior is then repeated several times per hour. This kind of phenomenon resembles a Lévy-flight behavior and stimulated us to compare these observations with a Lévy statistics, particularly sensitive to long-range correlations. In particular, we considered the distribution function of velocity and magnetic-field vector differences within fast and slow wind. This analysis showed that our observations can be reasonably fitted by a truncated-Lévy-flight (TLF) distribution. Moreover, we found a clear radial dependence for the PDFs of these fluctuations to evolve from Gaussian-like to possible TLF only within fast wind. We provide an explanation for what we observe in terms of a competing action between quasi-stochastic, propagating fluctuations and convected structures, both contributing to solar-wind turbulent fluctuations.