The files pfss_????????_??.gif show extrapolated field lines plotted
on an EIT image. When EIT data are not avilable, an SXT image is used.
The field lines come from the PFSS model as implemented by Schrijver
and DeRosa (2003, Solar Physics, 212, 165).  The model is based on MDI
magnetograms assimilated into a flux dispersal model to reflect
constantly evolving solar maagnetic field.  Here, pink and green lines
represent open field lines with negative and positive footpoint
polarities, respectively, white lines a subset of open field lines
that reach the ecliptic at the source surface (2.5 Rsun), therefore
intersecting Earth.  Blue lines stand for closed field lines.  Field
tracing is done from both the photosphere and the source surface.  The
files wso_pfss_????????.gif show the same thing on the basis of daily
updated magnetograms from Wilcox Solar Observatory.  These are
provided to fill the period of SOHO data gap, but a few dates are
overlapping with MDI (e.g., 29-Apr-1998) for comparison.  Because of
reduced spatial resolution, more field lines are on larger scales.