Alfvénic turbulence in solar wind originating near coronal hole boundaries: heavy-ion effects?

B Bavassano,R Bruno,E Pietropaolo,N A Schwadron

doi:10.5194/angeo-24-785-2006

Abstract

Abstract. The mid-latitude phases of the Ulysses mission offer an excellent opportunity to investigate the solar wind originating near the coronal hole boundaries. Here we report on Alfvénic turbulence features, revealing a relevant presence of in-situ generated fluctuations, observed during the wind rarefaction phase that charaterizes the transition from fast to slow wind. Heavy-ion composition and magnetic field measurements indicate a strict time correspondence of the locally generated fluctuations with 1) the crossing of the interface between fast and slow wind and 2) the presence of strongly underwound magnetic field lines (with respect to the Parker spiral). Recent studies suggest that such underwound magnetic configurations correspond to fast wind magnetic lines that, due to footpoint motions at the Sun, have their inner leg transferred to slow wind and are stretched out by the velocity gradient. If this is a valid scenario, the existence of a magnetic connection across the fast-slow wind interface is a condition that, given the different state of the two kinds of wind, may favour the development of processes acting as local sources of turbulence. We suggest that heavy-ion effects could be responsible of the observed turbulence features.

Highlights

Ulysses observations over two out-of-ecliptic orbits at different phases of the Sun’s activity cycle have clearly established that the solar wind, though dramatically different between minimum and maximum conditions, for a major fraction of the cycle around solar minimum exhibits an almost unchanged pattern (McComas et al, 2002a,b)
The Alfvenic turbulence seen in solar wind flows originating from solar regions near the polar coronal hole boundaries is often characterized by a negative cross-helicity and a nearly zero residual energy
We have given examples showing that this unusual behaviour is seen to occur just near the crossing of the stream interface and in concurrence with periods of sub-Parker magnetic field spiral

Summary

Introduction

Ulysses observations over two out-of-ecliptic orbits at different phases of the Sun’s activity cycle have clearly established that the solar wind, though dramatically different between minimum and maximum conditions, for a major fraction of the cycle around solar minimum exhibits an almost unchanged pattern (McComas et al, 2002a,b). An ideal opportunity for a detailed study of the transition between the fast high-latitude wind and the slow low-latitude wind is offered by the mid-latitude phases of Ulysses around the aphelion, when the spacecraft, moving slowly in latitude, spends several solar rotations in the region of interest. This leads to repeated crossings of the interface between the two flows, seen as a series of compression (slow to fast transition) and rarefaction (fast to slow transition) regions. Inward fluctuations can only originate from sources in the region outside the Alfvenic critical point, since inside this point inward waves fall back to the Sun

Observations

Concluding remarks