  It is well established that solar eruptions are powered by free magnetic
energy stored in current-carrying magnetic field in the corona. It has also
been generally accepted that magnetic flux ropes (MFRs) are a critical
component of many coronal mass ejections (CMEs). What remains controversial is
whether MFRs are present well before the eruption. Our aim is to identify
progenitors of MFRs, and investigate pre-eruptive magnetic properties
associated with these progenitors. Here we analyze 28 MFRs erupting within 45
deg from the disk center from 2010 to 2015. All MFRs'feet are well identified
by conjugate coronal dimmings. We then calculate magnetic properties at the
feet of the MFRs, prior to their eruptions, using Helioseismic and Magnetic
Imager (HMI) vector magnetograms. Our results show that only 8 erupting MFRs
are associated with significant non-neutralized electric currents, 4 of which
also exhibit pre-eruptive dimmings at the foot-prints. Twist and current
distributions are asymmetric at the two feet of these MFRs. The presence of
pre-eruption dimmings associated with non-neutralized currents suggests the
pre-existing MFRs. Furthermore, evolution of conjugate dimmings and electric
currents within the foot-prints can provide clues about the internal structure
of MFRs and their formation mechanism.
