  We analyze 5 epochs of NICER data of the black hole X-ray binary MAXI
J1820+070 during the bright hard-to-soft state transition in its 2018 outburst
with both reflection spectroscopy and Fourier-resolved timing analysis. We
confirm the previous discovery of reverberation lags in the hard state, and
find that the frequency range where the (soft) reverberation lag dominates
decreases with the reverberation lag amplitude increasing during the
transition, suggesting an increasing X-ray emitting region, possibly due to an
expanding corona. By jointly fitting the lag-energy spectra in a number of
broad frequency ranges with the reverberation model reltrans, we find the
increase in reverberation lag is best described by an increase in the X-ray
coronal height. This result, along with the finding that the corona contracts
in the hard state, suggests a close relationship between spatial extent of the
X-ray corona and the radio jet. We find the corona expansion (as probed by
reverberation) precedes a radio flare by ~5 days, which may suggest that the
hard-to-soft transition is marked by the corona expanding vertically and
launching a jet knot that propagates along the jet stream at relativistic
velocities.

High Energy Astrophysical Phenomena