  We address the problem of calculating the transverse magnetic field in the
solar wind outside of the hypothetical sphere called the source surface where
the solar wind originates. This calculation must overcome a widely used
fundamental assumption about the source surface -- the field is normally
required to purely radial at the source surface. Our model rests on the fact
that a change in the radial field strength at the source surface is a change in
the field line density. Surrounding field lines must move laterally in order to
accommodate this field line density change. As the outward wind velocity drags
field lines past the source surface this lateral component of motion produces a
tilt implying there is a transverse component to the field.
  An analytic method of calculating the lateral translation speed of the field
lines is developed. We apply the technique to an interval of approximately two
Carrington rotations at the beginning of 2011 using 2-h averages of data from
the Helioseismic Magnetic Imager instrument on the Solar Dynamics Observatory
spacecraft. We find that the value of the transverse magnetic field is
dominated on a global scale by the effects of high latitude concentrations of
field lines being buffetted by supergranular motions.
