We ascertain the influence of the By component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF Bz, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF By. It is important to note that since transpolar arcs can persist for 20--30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to snapshots of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF By is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in Bz is needed for theta aurora formation.