  The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed by Suzaku for
seven times, from 2007 September 28 to October 30. The observations
successfully traced an outburst decay in which the source luminosity decreased
almost monotonically from $\sim 10^{37}$ erg s$^{-1}$ to $\sim 10^{34}$ erg
s$^{-1}$, by $\sim 3$ orders of magnitude. To investigate luminosity-dependent
changes in the accretion geometry, five of the seven data sets with a typical
exposure of $\sim 18$ ks each were analyzed; the other two were utilized in a
previous work \citep{Sakurai2012}. The source was detected up to 100 keV in the
2nd to the 4th observations, to 40 keV in the 5th, and to 10 keV on the last
two occasions. All spectra were reproduced successfully by Comptonized
blackbody model with relatively high ($\gtrsim 2.0$) optical depths, plus an
additional softer optically-thick component. The faintest three spectra were
reproduced alternatively by a single Comptonized blackbody model with a
relatively low ($\lesssim 0.8$) optical depth. The estimated radius of the
blackbody emission, including seed photons for the Comptonization, was $10 \pm
2$ km at a 0.8--100 keV luminosity of $2.4\times 10^{36}$ erg s$^{-1}$ (the 2nd
to the 4th observations). In contrast, it decreased to $7 \pm 1$ km and further
to $3 \pm 1$ km, at a luminosity of $(4.8-5.2)\times 10^{35}$ erg s$^{-1}$ (the
5th observation) and $\sim 2\times 10^{34}$ erg s$^{-1}$ (the 6th and 7th),
respectively, regardless of the above model ambiguity. This can be taken as
evidence for the emergence of a weak magnetosphere of the neutron star.
