Polar Coronal Holes during the Past Solar Cycle: Ulysses Observations

Details Polar Coronal Holes during the Past Solar Cycle: Ulysses Observations Polar Coronal Holes during the Past Solar Cycle: Ulysses Observations

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh14a..02v&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SH14A-02

Astronomy and Astrophysics

Astrophysics

[2162] Interplanetary Physics / Solar Cycle Variations, [2164] Interplanetary Physics / Solar Wind Plasma, [2169] Interplanetary Physics / Solar Wind Sources, [7511] Solar Physics, Astrophysics, And Astronomy / Coronal Holes

Scientific paper

During its nearly 19 year mission, Ulysses pioneered novel measurements of the three-dimensional heliosphere and particularly provided the first in situ observations of solar wind from polar coronal holes (PCHs). These PCH-associated solar wind streams show long-term variability in both dynamic and also compositional signatures. Between the polar passages in 1994-95 and in 2007-08, the C and O freeze-in temperatures measured in high-latitude solar wind have decreased by ~15 % and are now around 0.86 MK and 1.0 MK, respectively. Si and Fe ionization states also exhibit a substantial cooling with a reduction of 0.4 and 0.5 charge states on average, respectively. On the other hand, there no significant changes of the elemental composition of the solar wind, as exhibited through the First Ionization Potential fractionation effect, which has remained at f = 1.8±0.3 during both sets of polar passages, i.e., enhanced to the photospheric composition (f = 1). Thus, it appears that the PCH of cycle 23 are cooler overall than those of cycle 22, while their elemental composition has remained unchanged, thus confirming their status as the “ground state” of the solar wind. These observations, together with the observed ~15 % reduction of the heliospheric magnetic field (Smith and Balogh, 2008), and the ~17 % and ~14 % reductions in density and temperature, respectively (McComas et al., 2008), provide a unique test for theories of the solar wind and its composition, in particular for the concept of freezing-in of charge states and of the FIP fractionation effect. We will present results from this analysis of SWICS data and also discuss the scientific implications of these novel results.

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