Nicol, R. M., Chapman, Sandra C. and Dendy, R. O. (2009) Quantifying the anisotropy and solar cycle dependence of "1/f" solar wind fluctuations observed by advanced composition explorer. Astrophysical Journal, Vol.703 (No.2). pp. 2138-2151. doi:10.1088/0004-637X/703/2/2138

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Abstract

The power spectrum of the evolving solar wind shows evidence of a spectral break between an inertial range (IR) of turbulent fluctuations at higher frequencies and a "1/f" like region at lower frequencies. In the ecliptic plane at similar to 1 AU, this break occurs approximately at timescales of a few hours and is observed in the power spectra of components of velocity and magnetic field. The "1/f" energy range is of more direct coronal origin than the IR, and carries signatures of the complex magnetic field structure of the solar corona, and of footpoint stirring in the solar photosphere. To quantify the scaling properties we use generic statistical methods such as generalized structure functions and probability density functions (PDFs), focusing on solar cycle dependence and on anisotropy with respect to the background magnetic field. We present structure function analysis of magnetic and velocity field fluctuations, using a novel technique to decompose the fluctuations into directions parallel and perpendicular to the mean local background magnetic field. Whilst the magnetic field is close to "1/f," we show that the velocity field is "1/f(alpha)" with alpha not equal 1. For the velocity, the value of a varies between parallel and perpendicular fluctuations and with the solar cycle. There is also variation in a with solar wind speed. We have examined the PDFs in the fast, quiet solar wind and intriguingly, whilst parallel and perpendicular are distinct, both the B field and velocity show the same PDF of their perpendicular fluctuations, which is close to gamma or inverse Gumbel. These results point to distinct physical processes in the corona and to their mapping out into the solar wind. The scaling exponents obtained constrain the models for these processes.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science > Physics
Library of Congress Subject Headings (LCSH):	Magnetic fields, Solar wind, Turbulence, Anisotropy, Solar cycle
Journal or Publication Title:	Astrophysical Journal
Publisher:	IOP Publishing
ISSN:	0004-637X
Official Date:	1 October 2009
Dates:	Date Event 1 October 2009 Published
Volume:	Vol.703
Number:	No.2
Number of Pages:	14
Page Range:	pp. 2138-2151
DOI:	10.1088/0004-637X/703/2/2138
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	Science and Technology Facilities Council (Great Britain) (STFC), Culham Centre for Fusion Energy (CCFE)

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