Coronal Plumes

Steven T. Suess
NASA Marshall Space Flight Center

    Plumes are bright rays seen in coronal holes between one and
several solar radii. Being bright in white light, they are therefore
denser than interplume plasma. They have been observed for many
years, but only with difficulty since they are quite faint. Now
the Solar Heliospheric Observatory (SOHO) has provided us with
a few facts about what plumes are, why they exist, and on their
importance in the solar wind. Naturally, SOHO has also produced
a few new questions.
    One interesting fact is that plumes lie over some, but not all
magnetic flux concentrations in the photosphere. Observations
are now planned to determine what distinguishes those flux
concentrations that do have plumes and a reasonable paradigm
has been developed to guide those observations. Part of this
paradigm is that, contrary to intuition, plumes can be seen in
coronal holes because the magnetic field is strong there while
it is weak in streamers - a result which has come out of a combination
of numerical models of the global corona and SOHO empirical studies.
Perhaps a more interesting fact is that the flow speed in plumes
at, e.g., 5 solar radii, is only half that of interplume plasma. A
corollary is that flow in coronal holes is probably filamentary
down to the same scale size as that of photospheric magnetic
field. This is in marked contrast to the very smooth flow observed
from coronal holes in the solar wind at 1 AU. In fact, plumes have
proven difficult to identify at all in the solar wind and this has led
to several hypotheses for equalizing plume and interplume flow
speeds. It is notable that standard solar wind models alone are unable
to produce equal plume and interplume flow speeds at 1 AU without
some magical delayed acceleration.
    Clues to plume origin and energetics, as well as to the more
general question of coronal energetics, lie in composition differences
between plume and interplume flow. This is a new area of observation
and modeling - driven by recent advances in instrument capability.
However, some answers will only be provided later by the Solar
Probe which will dive into the solar corona and fly through the
plume/interplume forest at 4-10 solar radii.