June 6, 2011 | The solar minimum that bottomed out from 2006 to 2010 was the longest and deepest since modern space observations began. Among other effects, it reorganized the areas of flux from open magnetic field lines that produce solar wind. NCAR postdoctoral researcher Liang Zhao is using data from the last two minima to revise a model of how open magnetic flux is transported through the solar atmosphere.
The Sun’s magnetic polarity shifts about every 11 years. This cycle reorients magnetic fields and generates a cyclic peak in solar activity, including coronal mass ejections (CMEs) that can produce damaging space weather on Earth. In previous work, Zhao identified three types of solar wind:
• slow, hot wind originating from the streamer-stalk region, a zone underneath the sheet of electromagnetic current at low solar latitudes
• cooler wind from higher solar latitudes
• very hot wind produced by CMEs at various points over brief periods
Using data from recent observing missions, Zhao found that the streamer stalk region was only about half as wide in the most recent solar minimum (spanning about 20° in latitude) as in the previous minimum (40–50°). The area beyond the streamer stalks was thus larger, but its open magnetic field was about 30% weaker. As a result, similar amounts of open magnetic flux were produced in both solar minima outside the streamer stalk region.
The findings prompted Zhao and Lennard Fisk (University of Michigan) to revise a model developed by Fisk and colleagues. In the model, open magnetic field flux moves slowly toward the equator, twisted by the Sun’s differential rotation. However, since the new data suggest that open magnetic flux is conserved outside the streamer stalk region, the revised model keeps the two areas more distinct, with the pole-to-equator flow of open magnetic flux deflected before it reaches the streamer stalk region.