Where the wind blows from
Astronomers have pinpointed the origin of the solar wind with the help of some space probes. The particle stream consists of two components: a slow component, originating from long, narrow structures at the equator, and a fast component, which originates from many areas distributed across the Sun. The solar wind is a stream of particles - mainly electrons, protons and helium nuclei - emitted by our host star. Normally invisible to the eye, it sometimes causes stunning natural phenomena. This is how auroras are formed when it hits Earth's atmosphere, and comets form a tail as the solar wind pushes the evaporating particles outward.
The speed of the particle stream near the Earth's orbit is between 200 and 900 km/s, divided into an uneven slow component and a faster component, which was previously assumed to originate from the region around the sun's poles. Guided by the star's magnetic lines, the particles travel through our solar system and finally disappear into the depths of space.
In previous work, Jet Propulsion Laboratory's Richard Woo and colleagues sought the origin of the slow solar wind by analyzing communications signals from spacecraft that disappeared behind the Sun. They looked for scattered signals when the probes were close to the visible edge of the sun's disk. A kind of twinkle or brief flash should then indicate irregular gas movement near the surface. Woo's working group found the strongest effects where the radio waves had to pass through so-called stalks. They therefore suspected that the uneven slow winds originated in these areas.
To test this assumption, Shadia Habbal, Silvano Fineschi of the Harvard-Smithsonian Center for Astrophysics, and Woo, along with others, measured wind speeds using an instrument on board the Solar and Heliospheric Observatory (SOHO). They used it to capture light that was first emitted by oxygen ions deep in the Sun's atmosphere and then scattered in all directions by other oxygen ions, finally traveling outwards with the solar wind. The faster the solar wind is, the smaller the scattering effect. They were able to confirm that the slow solar wind actually comes from the stalks (Astrophysical Journal Letters 10/97).
It was also shown that the fast part of the wind leaves the sun over a large range of longitudes - not only near the poles, as previously assumed. But this observation is still controversial, because the sun's magnetic field should actually confine the particles and only release them at the poles.
In the case of the slow solar wind, however, most researchers believe that the question of its origin is all but resolved. Alan Title of the Lockheed Palo Alto Research Laboratory acknowledged the results as a "huge step forward".
The Heidelberger Verlag Spektrum der Wissenschaft is the operator of this portal. Its online and print magazines, including "Spektrum der Wissenschaft", "Gehirn&Geist" and "Spektrum – Die Woche", report on current research findings.
