  A very slight rotation-induced latitudinal temperature variation (presumably
on the order of several Kelvin) on the solar surface is theoretically expected.
While recent high-precision solar brightness observations reported its
detection, confirmation by an alternative approach using the strengths of
spectral lines is desirable, for which reducing the noise due to random
fluctuation caused by atmospheric inhomogeneity is critical. Towards this
difficult task, we carried out a pilot study of spectroscopically investigating
the relative variation of temperature (T) at a number of points in the solar
circumference region near to the limb (where latitude dependence should show up
if any exists) based on the equivalent widths (W) of 28 selected lines in the
5367--5393A and 6075--6100A regions. Our special attention was paid to i)
clarifying which kind of lines should be employed and ii) how much precision
would be accomplished in practice. We found that lines with strong
T-sensitivity (|log W/log T|) should be used and that very weak lines should be
avoided because they inevitably suffer large relative fluctuations (Delta_W/W).
Our analysis revealed that a precision of Delta_T/T ~ 0.003 (corresponding to
~15K) can be achieved at best by a spectral line with comparatively large |log
W/log T|, though this can possibly be further improved if other more suitable
lines are used. Accordingly, if many such favorable lines could be measured
with sub-% precision of Delta_W/W and by averaging the resulting Delta_T/T from
each line, the random noise would eventually be reduced down to <~1K and
detection of very subtle amount of global T-gradient might be possible.
