The Dusty Heart of NGC 4151 Revealed by $\lambda\sim$1-40 $\mu$m
  Reverberation Mapping and Variability: A Challenge to Current Clumpy Torus
  Models
  We probe the dusty environment of the archetypical type-1 AGN in NGC 4151
with comprehensive IR reverberation mapping over several decades, in J
($\sim1.22\mu$m), H ($\sim1.63\mu$m), K ($\sim2.19\mu$m), L ($\sim3.45\mu$m)
and N-band ($\sim10.6\mu m$), plus multiple measurements at 20--40 $\mu$m. At
1--4$\mu$m, the hot dust reverberation signals come from two distinct dust
populations at separate radii ($\sim$0.033 pc and $\sim$0.076 pc), with
temperatures of $\sim$1500--2500 K and $\sim$900--1000 K, consistent with the
expected properties of sublimating graphite and silicate dust grains. The
domination of the torus infrared output by carbon and silicate grains near
their sublimation temperatures and radii may account for the general similarity
of AGN near-IR spectral energy distributions. The torus inner edge defined by
the hottest dust remains at roughly the same radius independent of the AGN
optical luminosity over $\sim$25 years. The emission by hot dust warmed
directly by the optical/UV AGN output has increased gradually by
$\sim$4\%/year, indicating a possibly growing torus. A third dust component at
$\sim$700 K does not seem to participate directly in the IR reverberation
behavior and its emission may originate deep in the circumnuclear torus. We
find a reverberation signal at $\sim10\mu$m with an inferred radius for the
warm dust of $\sim$2.2--3.1 pc. The lack of variability at 20--40$\mu$m
indicates the far-IR emission comes from even more extended regions. The torus
properties revealed by dust reverberation analysis are inconsistent with
predictions from pure clumpy torus models. Instead, the longer wavelength
emission possibly originates in a flared torus or the polar wind.
