  Observations show that the accretion flows in low-luminosity active galactic
nuclei (LLAGNs) probably have a two-component structure with an inner ADAF and
an outer truncated accretion disk. As shown by Taam et al. (2012), the
truncation radius as a function of mass accretion rate is strongly affected by
including the magnetic field within the framework of disk evaporation model,
i.e., an increase of the magnetic field results in a smaller truncation radius
of the accretion disk. In this work, we calculate the emergent spectrum of an
inner ADAF + an outer truncated accretion disk around a supermassive black hole
based on the prediction by Taam et al. (2012). It is found that an increase of
the magnetic field from $\beta=0.8$ to $\beta=0.5$ (with magnetic pressure
$p_{\rm m}=B^2/{8\pi}=(1-\beta)p_{\rm tot}$, $p_{\rm tot}=p_{\rm gas}+p_{\rm
m}$) results in an increase of $\sim 8.7$ times of the luminosity from the
truncated accretion disk. We found that the equipartition of gas pressure to
magnetic pressure, i.e., $\beta=0.5$, failed to explain the observed
anti-correlation between $L_{\rm 2-10 keV}/L_{\rm Edd}$ and the bolometric
correction $\kappa_{\rm 2-10 keV}$ (with $\kappa_{\rm 2-10 keV} = L_{\rm
bol}/L_{\rm 2-10 keV}$). The emergent spectra for larger value $\beta=0.8$ or
$\beta=0.95$ can well explain the observed $L_{\rm 2-10 keV}/L_{\rm
Edd}$-$\kappa_{\rm 2-10 keV}$ correlation. We argue that in the disk
evaporation model, the electrons in the corona are assumed to be heated only by
a transfer of energy from the ions to electrons via Coulomb collisions, which
is reasonable for the accretion with a lower mass accretion rate. Coulomb
heating is the dominated heating mechanism for the electrons only if the
magnetic field is strongly sub-equipartition, which is roughly consistent with
observations.
