Persistence of Ion Cyclotron Waves and Stochasticity of Kinetic Alfvén Waves in the Solar Wind

Daniele Telloni

doi:10.3390/atmos12010044

Abstract

This paper investigates the nature of the physical processes underlying the origin of the Ion Cyclotron Waves (ICWs) and Kinetic Alfvén Waves (KAWs) in the solar wind, by studying their Waiting Time Distributions (WTDs). The results show that ICWs and KAWs do not share common statistical properties: while KAWs independently occur as stochastic, uncorrelated wave packets governed by Poisson statistics, ICWs are highly correlated, thus departing from the Poisson hypothesis. The results based on the WTD analysis may cast more light on the mechanisms actively at work in the generation of the two wave modes. Specifically, while the stochastic character of KAWs may be reminiscent of the random convection-driven jostling of the flux-tube foot-points that generates the Alfvén waves in the lower solar atmosphere, the correlations among the ICW events can be effectively explained on the basis of the persistent nature of the mechanism underlying the local origin of ICWs, namely the proton cyclotron instability. Alternative explanations for the observed distribution of ICW waiting times, based on a piecewise-constant Poisson process involving time-varying rates, are also reported.

Highlights

The power-law Waiting Time Distributions (WTDs) observed for Ion Cyclotron Waves (ICWs) clearly indicates that Poisson statistics cannot account for the temporal distribution of ICW packets: nonstochastic, correlated clusters are present in the ICW packet time sequence
Since the statistical properties of WTDs are related to the physical processes underlying the origin of the corresponding events, it appears evident that ICWs and Kinetic Alfvén Waves (KAWs) are generated by two different mechanisms
Despite more refined statistical analyses, performed on the latest generation data provided by the novel solar missions Parker Solar Probe and Solar Orbiter, are required to definitevely solve the problem of the origin of ICWs and KAWs, some clues may be advanced even on the basis of the results of the present work, which represents a first step for further investigations

Summary

Introduction

Over the last 60 years or so a fleet of space missions dedicated to the exploration of the. Since the first ever evidence for highly correlated plasma velocity and magnetic field fluctuations in the solar wind by Belcher and Davis [12] in 1971 using Mariner 5 data, which marks de facto the first observation of Alfvén waves in the inner heliosphere, the Alfvénic and turbulent properties of the solar wind have been extensively studied in-situ by a series of interplanetary probes orbiting at different distances from the Sun (as the pioneering Helios missions, which in the 1970s approached the Sun at 0.3 AU, and the Voyager missions, the most distant deep-space probes, which both reach the final frontier of the heliosphere, entering the interstellar medium, more than 30 years after their launch) as well as in and out of the ecliptic (as Ulysses which, from its polar orbit, provided the first in-situ measurement of the high-latitude heliosphere).

Data Analysis

Discussion and Conclusions