Using data from ACE/SWICS, we observe significant changes in the average solar
wind composition and plasma properties on the time-scale of a solar cycle. We
now know that the interplanetary magnetic field decreased by 30% between the
last two solar minima, and that ionic charge states of O have been reported to
change towards lower values in the fast wind. In a recently published analysis
(Lepri et al., ApJ 768:94, 2013), we find a systematic change of C, O, Si and
Fe ionic charge states towards lower ionization distributions. Most
importantly, charge-state ratios in both the fast and slow wind decrease from
solar maximum to solar minimum indicating a global decrease of electron
temperatures in the corona. It is also likely that the density in the low
corona decreased towards solar minimum.
The most surprising aspect of this analysis concerns the heavy ion content of
the solar wind. We show that there was a ~50% decrease of heavy ion abundances
(He, C, O, Si, and Fe) relative to H as the Sun went from solar maximum to
solar minimum. The Figure shows Carrington-rotation-averaged elemental
abundances relative to O and H. Slow solar wind data is shown in magenta and
fast wind data is shown in blue in each of the panels. Panel (a) shows C/O,
and (b) and (c) show Si/O and Fe/O, respectively. Panels (d)-(h) show the
abundance of a specific heavy ion as compared to H and for increasing mass
from He, C, O, Si, to Fe. All quantities are shown as log values. Both the
fast and slow wind become more depleted in heavy ions at solar minimum with
slow wind showing enhanced depletion as compared to fast wind. During this
entire time, the relative abundances in the slow wind remain organized by
their First Ionization Potential (FIP).
The fact that heavy ion abundances drop at solar minimum for both fast and
slow wind suggests global changes in the processes that accelerate the wind or
in the properties of loops that release plasma in the wind.
This item was contributed by
by Micah J. Weberg, Sue T. Lepri and Thomas H. Zurbuchen of the University of Michigan.
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Last modified 28 Apr 2013.