The Large Scale Structure of the Solar Minimum Corona
SOHO JOP Proposal 44
Douglas Biesecker and Sarah Gibson
Scientific Objective:
Our goal is to understand the large-scale, stable, coronal structure of
equatorial helmet streamers and polar coronal holes that can persist
for several solar rotations at solar minimum. Although the corona
does change dramatically, even during solar minimum, when a Coronal
Mass Ejection blows a huge quantity of mass outwards, it is to this
stable background structure that the corona returns. This
observed behavior suggests that it may prove useful to consider solar
minimum CME's as perturbations of the background corona. Understanding
the large-scale corona is also fundamental to understanding how and
where the solar wind is accelerated. The magnetic field structure in
particular is fundamental to solar wind studies: knowledge of where
the coronal field is open and where it is closed, as well as an
understanding of how the expansion of the field varies from a purely
radial expansion, are essential to studies of how the coronal field is
related to fast and slow solar wind passing the earth. We also would
like to clarify where the wind becomes important to coronal force
balance, i.e. where the sonic surface lies.
We therefore would like to quantify the density, temperature, and
magnetic field distribution throughout the large-scale, stable, mostly
static solar corona (i.e. 1 Rsun - 3 Rsun). We would approach this
problem by using coronal observations as constraints on magnetostatic
models. In previous work (Gibson, Bagenal, & Low JGR, 101, 4813, 1996;
Gibson & Bagenal, JGR, 100, 198651, 1995; Bagenal & Gibson, JGR,
96,17663, 1991), existing coronal models were used and new models were
developed to investigate the balance between magnetic forces and
gravitational and thermal forces in the presence of both bulk and sheet
currents. These models were constrained by observations of the white
light corona (Mauna Loa MkIII and SMM Coronagraphs) and, to a lesser
extent, of the photospheric magnetic field (Stanford WSO Magnetographs).
Density, temperature, and magnetic field distributions were found that
matched the data to within observational accuracy. However, the
considerable degeneracy of acceptable solutions that were found implied
that in order to specify the coronal plasma properties with confidence,
additional observational constraints were needed.
Complete text of proposal.