  In the Orion Nebula Cluster (ONC), protoplanetary disks exhibit ionized gas
clouds in the form of a striking teardrop shape as massive stars irradiate the
disk material. We present the first spatially and spectrally resolved
observations of 12 proplyds, using Integral Field Spectroscopy observations
performed with the MUSE instrument in Narrow Field Mode (NFM) on the VLT. We
present the morphology of the proplyds in seven emission lines and measure the
radius of the ionization front (I-front) of the targets in four tracers,
covering transitions of different ionization states for the same element. We
also derive stellar masses for the targets. The measurements follow a
consistent trend of increasing I-front radius for a decreasing strength of the
far-UV radiation as expected from photoevaporation models. By analyzing the
ratios of the I-front radii as measured in the emission lines of Ha, [OI] 6300,
[OII] 7330, and [OIII] 5007, we observe the ionization stratification, that is,
the most ionized part of the flow being the furthest from the disk (and closest
to the UV source). The ratios of I-front radii scale in the same way for all
proplyds in our sample regardless of the incident radiation. We show that the
stratification can help constrain the densities near the I-front by using a 1D
photoionization model. We derive the upper limits of photoevaporative mass-loss
rates by assuming ionization equilibrium, and estimate values decreasing
towards lower impinging radiation. We do not find a correlation between Mloss
and stellar mass. The highest mass-loss rate is for the proplyd 244-440. These
values of Mloss, combined with estimates of the disk mass with ALMA, confirm
previous estimates of the short lifetime of these proplyds. This work
demonstrates the potential of this dataset and offers a new set of observables
to be used to test current and future models of external photoevaporation.
