A technique has been developed recently to determine the polarities of
interplanetary magnetic fields relative to their origins at the Sun b
y comparing energetic electron flow directions with local magnetic fie
ld directions. Here we use heat flux electrons from the Los Alamos Nat
ional Laboratory (LANL) plasma detector on the ISEE 3 spacecraft to de
termine the field polarities. We examine periods within well-defined m
agnetic sectors when the field directions appear to be reversed from t
he normal spiral direction of the sector. About half of these intrasec
tor field reversals (IFRs) are cases in which the polarities match tho
se of the surrounding sectors, indicating that those fields have been
folded back toward the Sun. The more interesting cases are those with
polarity reversals. We find no clear cases of isolated reverse polarit
y fields, which suggests that islands of reverse polarity in the solar
source dipole field probably do not exist. The IFRs with polarity rev
ersals are strongly associated with periods of bidirectional electron
flows, suggesting that those fields occur only in conjunction with clo
sed fields. We propose that both those IFRs and the bidirectional flow
s are signatures of coronal mass ejections (CMEs). In that case, many
interplanetary CMEs are larger and more complex than previously though
t, consisting of both open and closed field components.