Physics – Plasma Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh11a1496r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SH11A-1496
Physics
Plasma Physics
[7511] Solar Physics, Astrophysics, And Astronomy / Coronal Holes, [7536] Solar Physics, Astrophysics, And Astronomy / Solar Activity Cycle, [7827] Space Plasma Physics / Kinetic And Mhd Theory, [7846] Space Plasma Physics / Plasma Energization
Scientific paper
The stark differences between the current solar minimum and the previous one offer a unique opportunity to develop new constraints on mechanisms for heating and acceleration of the solar wind. Following on from studies by McComas et al. (2008) and Schwadron and McComas (2008), we have used Ulysses measurements made during Ulysses' first and third orbit southern and northern polar passes (i.e., during near-solar minimum conditions) to deduce a linear relationship between proton number density (Np) and radial magnetic field strength (Br) in the high-speed quiescent solar wind. We have explored ideas that this could have resulted from interplanetary processes, or the super-radial expansion of the coronal magnetic field close to the Sun, but find that neither possibility can produce the observed relationship. Instead, we suggest that it is a natural consequence of a coronal heating law of the form Q ~ B. This interpretation is supported by the results of one-dimensional simulations, which show that Q ~ Np(1 AU) for a range of heating scale-lengths and field-line geometries. Additionally, our results suggest a systematic temporal decrease in the polar magnetic field strength during the 4 solar minimum polar passes: As the polar magnetic fields have weakened both from the spacecraft's traversal from southern to northern hemisphere, and from the previous minimum to the current one, commensurate drops in coronal heating have resulted in decreases in the density of the polar coronal-hole plasma. We have also computed the X-ray spectral radiance, which serves as an independent proxy for the power dissipated through coronal heating, to assess whether it also scales linearly with B.
Linker Jon A.
McComas David John
Mikic Zoran
Riley Pete
Schwadron Nathan A.
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