Astronomy & Astrophysics

Free access article

Issue		A&A Volume 366, Number 1, January IV 2001
Page(s)		331 - 338
Section		The Sun
DOI		10.1051/0004-6361:20000225

A&A 366, 331-338 (2001)
DOI: 10.1051/0004-6361:20000225

Long-term variations of the flow direction and angular momentum of the solar wind observed by Helios

K. Scherer¹, E. Marsch², R. Schwenn² and H. Rosenbauer²

¹ dat-hex, Obere Straße 11, 37191 Katlenburg-Lindau, Germany
² Max-Planck Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany

(Received 18 May 1999 / Accepted 8 November 2000 )

Abstract
The flow directions of solar wind protons were measured in situ by the Helios spacecraft. A long-term average of the velocity shows a systematic drift in the latitudinal flow angle of about $+1^\circ$ north observed with Helios 1 and $-1^\circ$ south observed onboard of Helios 2. The longitudinal flow angle migrates about $+1^\circ$ west over a period of almost 10 years for Helios 1 and 6 years for Helios 2. This systematic change with time of the plasma flow direction may be caused by solar-cycle variations of the orientation of the Sun's magnetic field which partially corotates with the Sun inside the Alfvén surface (varying in distance between $10 R_{\odot}$ over the poles and $30 R_{\odot}$ near the equator). These variations must have been imprinted on the solar wind flow when it detached from corotation with the Sun near the Alfvén point. The angular momentum of the wind is intimately connected with the flow and field directions. The gain of total angular momentum of the wind equals the loss of angular momentum of the Sun, which is caused by the torque exerted on the rotating Sun through the magnetic field of the expanding corona. Implications of the Helios observations for models of the magnetic fields of the Sun as well as the solar wind are discussed. We show evidence, that changes of the solar magnetic field inside the Alfvén surface are responsible for systematic drifts in the solar wind flow direction.

Key words: Sun: solar wind -- Sun: magnetic fields -- Sun: activity

Offprint request: K. Scherer, dat-hex@t-online.de

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