Michael Hahn, Enrico Landi, Daniel Wolf Savin
We have measured the widths of spectral lines from a polar coronal hole using
the Extreme Ultraviolet Imaging Spectrometer onboard Hinode. Polar coronal
holes are regions of open magnetic field and the source of the fast solar wind.
We find that the line widths decrease at relatively low heights. Previous
observations have attributed such decreases to systematic effects, but we find
that such effects are too small to explain our results. We conclude that the
line narrowing is real. The non-thermal line widths are believed to be
proportional to the amplitude of Alfven waves propagating along these open
field lines. Our results suggest that Alfven waves are damped at unexpectedly
low heights in a polar coronal hole. We derive an estimate on the upper limit
for the energy dissipated between 1.1 and 1.3 solar radii and find that it is
enough to account for up to 70% of that required to heat the polar coronal hole
and accelerate the solar wind.
View original:
http://arxiv.org/abs/1202.1743
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