  Corona structures and processes during the pre-impulsive stage of solar
eruption are crucial to understanding the physics leading to the subsequent
explosive energy release. Here we present the first microwave imaging study of
a hot flux rope structure during the pre-impulsive stage of an eruptive M7.7
solar flare, with the Nobeyama Radioheliograph (NoRH) at 17 GHz. The flux rope
is also observed by the SDO/AIA in its hot passbands of 94 and 131 \AA\. In the
microwave data, it is revealed as an overall arcade-like structure consisting
of several intensity enhancements bridged by generally weak emissions, with
brightness temperatures ($T_B$) varying from ~10,000~K to ~20,000 K. Locations
of microwave intensity enhancements along the structure remain relatively fixed
at certain specific parts of the flux rope, indicating that the distribution of
emitting electrons is affected by the large scale magnetic configuration of the
twisted flux rope. Wavelet analysis shows a pronounced 2-min period of the
microwave $T_B$ variation during the pre-impulsive stage of interest. The
period agrees well with that reported for AIA sunward-contracting loops and
upward ejective plasmoids (suggested to be reconnection outflows). This
suggests that both periodicities are controlled by the same reconnection
process that takes place intermittently at a 2-min time scale. We infer that at
least a part of the emission is excited by non-thermal energetic electrons via
the gyro-synchrotron mechanism. The study demonstrates the potential of
microwave imaging in exploring the flux rope magnetic geometry and relevant
reconnection process during the onset of solar eruption.
