  The solar magnetic fields emerging from the photosphere into the chromosphere
and corona are comprised of a combination of "closed" and "open" fields. The
closed magnetic field lines are defined as those having both ends rooted in the
solar surface, while the open field lines are those having one end extending
out into interplanetary space and the other rooted at the Sun's surface. Since
the early 2000's, the amount of total unsigned open magnetic flux estimated by
coronal models have been in significant disagreement with in situ spacecraft
observations, especially during solar maximum. Estimates of total open unsigned
magnetic flux using coronal hole observations (e.g., using extreme ultraviolet
(EUV) or Helium (He) I) are in general agreement with the coronal model results
and thus show similar disagreements with in situ observations. While several
possible sources producing these discrepancies have been postulated over the
years, there is still no clear resolution to the problem. This paper provides a
brief overview of the problem and summarizes some proposed explanations for the
discrepancies. In addition, two different ways of estimating the total unsigned
open magnetic flux are presented, utilizing the Wang-Sheeley-Arge (WSA) model,
and one of the methods produce surprisingly good agreement with in situ
observations. The findings presented here suggest that active regions residing
near the boundaries of mid-latitude coronal holes are the probable source of
the missing open flux. This explanation also brings in line many of the seemly
contradictory facts that have made resolving this problem so difficult.
