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A very slight rotation-induced latitudinal temperature variation (presumablyon the order of several Kelvin) on the solar surface is theoretically expected.While recent high-precision solar brightness observations reported itsdetection, confirmation by an alternative approach using the strengths ofspectral lines is desirable, for which reducing the noise due to randomfluctuation caused by atmospheric inhomogeneity is critical. Towards thisdifficult task, we carried out a pilot study of spectroscopically investigatingthe relative variation of temperature (T) at a number of points in the solarcircumference region near to the limb (where latitude dependence should show upif any exists) based on the equivalent widths (W) of 28 selected lines in the5367--5393A and 6075--6100A regions. Our special attention was paid to i)clarifying which kind of lines should be employed and ii) how much precisionwould be accomplished in practice. We found that lines with strongT-sensitivity (|log W/log T|) should be used and that very weak lines should beavoided 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 |logW/log T|, though this can possibly be further improved if other more suitablelines are used. Accordingly, if many such favorable lines could be measuredwith sub-% precision of Delta_W/W and by averaging the resulting Delta_T/T fromeach line, the random noise would eventually be reduced down to <~1K anddetection of very subtle amount of global T-gradient might be possible.

Towards Spectroscopically Detecting the Global Latitudinal Temperature Variation on the Solar Surface