Credit: HINODE JAXA/NASA/PPARC

The Sun emits a hot gas of charged particles known as 'solar wind', which takes 10 days or fewer to reach the Earth. There, it can interact with the planet's magnetic field lines to produce aurorae, for instance. Solar wind comes in fast and slow varieties. Fast solar wind emerges from coronal holes — darker and colder regions of the Sun's corona with open magnetic field lines. There has been, however, no consensus on the source of the slow wind, but Louise Harra and co-workers now propose an end to the debate (Astrophys. J. 676, L147–L150; 2008).

Using the Extreme Ultraviolet Imaging Spectrometer (EIS) and X-ray Telescope (XRT) aboard the Hinode probe (which also carries an optical telescope), Harra et al. studied active regions of the Sun and measured the speed of plasma flow. The weakest regions — that is, at the edges of the bright regions — have the strongest outflow. The authors' large-scale model of the magnetic field shows that loops connecting one region of high-velocity outflow (to the right of an active region) and another region (a smaller western bipole, whose two opposite magnetic polarities move apart over time) can reach as far as 600,000 km. Despite the vast separation distance, the magnetic fields from these regions collide and create the hot gas of the solar wind.

Aside from improving our understanding of the Sun, such studies will also contribute to our ability to model the effects of solar wind on the shape and structure of the Earth's magnetic field. Particularly strong blasts have, in the past, affected the electricity grid as well as disrupted satellite-based communications systems.