Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind

TL;DR

This study uses STEREO/HI1 images to observe the transition in the solar wind from structured, low-beta flow to turbulent, high-beta flow, revealing the onset of turbulence and instabilities in the young solar wind.

Contribution

The paper introduces novel image analysis techniques to quantify the transition in solar wind structure and demonstrates the onset of turbulence and instabilities in the young solar wind.

Findings

Fading of radial striae in the corona observed.

Brightening of dense solar wind puffs ('flocculae') detected.

Transition consistent with hydrodynamic or MHD instabilities leading to turbulence.

Abstract

Above the top of the solar corona, the young slow solar wind transitions from low-beta, magnetically structured flow dominated by radial structures, to high-beta, less structured flow dominated by hydrodynamics. This transition, long inferred via theory, is readily apparent in the sky region close to 10 degrees from the Sun, in processed, background-subtracted solar wind images. We present image sequences collected by the STEREO/HI1 instrument in 2008 Dec, covering apparent distances from approximately 4 to 24 degrees from the center of the Sun and spanning this transition in large-scale morphology of the wind. We describe the observation and novel techniques to extract evolving image structure from the images, and we use those data and techniques to present and quantify the clear textural shift in the apparent structure of the corona and solar wind in this altitude range. We demonstrate that the change in apparent texture is due both to anomalous fading of the radial striae that characterize the corona, and to anomalous relative brightening of locally dense puffs of solar wind that we term "flocculae;" and show that these phenomena are inconsistent with smooth radial flow, but consistent with onset of hydrodynamic or MHD instabilities leading to a turbulent cascade in the young solar wind.