  We consider the potential magnetic field associated with a helical electric
line current flow, idealizing the near-potential coronal field within which a
highly localized twisted current structure is embedded. It is found that this
field has a significant axial component off the helical magnetic axis where
there is no current flow, such that the flux winds around the axis. The helical
line current field, in including the effects of flux rope writhe, is therefore
more topologically complex than straight line and ring current fields sometimes
used in solar flux rope models. The axial flux in magnetic fields around
confined current structures may be affected by the writhe of these current
structures such that the field twists preferentially with the same handedness
as the writhe. This property of fields around confined current structures with
writhe may be relevant to classes of coronal magnetic flux rope, including
structures observed to have sigmoidal forms in soft X-rays and prominence
magnetic fields. For example, ``bald patches'' and the associated heating by
Parker current sheet dissipation seem likely. Thus some measurements of flux
rope magnetic helicities may derive from external, near-potential fields. The
predicted hemispheric preference for positive and negative magnetic helicities
is consistent with observational results for prominences and sigmoids and past
theoretical results for flux rope internal fields.
