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

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° north observed with Helios 1 and -1° south observed onboard of Helios 2. The longitudinal flow angle migrates about +1° 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⊙ over the poles and 30 R⊙ 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 Alfv́n surface are responsible for systematic drifts in the solar wind flow direction.