How flux ropes evolve, how the core field develops, and on what time scale, are all unknown.
Observations seem to indicate that the core field grows as a function of downtail distance.
Hesse et al. In addition to addressing the core-field question, we look at scale sizes and geometry, and analyze an event the strongly suggests multiple island formation. ISEE1 and 2 magnetic field data in GSM coordinates from Mar 5, 1979. By, centered on a bipolar Bz, and an increase in Btotal to well above the background - all typical signatures of a flux rope. The bipolar signature is negative then positive - opposite the usual sense.