Photospheric Temperature Variations near the Solar Limb III

We use observations from the Solar Aspect Sensor (SAS) of RHESSI to characterize the latitude dependence of the temperature of the photosphere near the solar limb. Previous observations had suggested the presence of a polar temperature excess as large as 1.5 K. The RHESSI observations, made with a rotating telescope in space, have great advantages in the rejection of systematic errors in the very precise photometry required for such an observation. This photometry is differential, i.e. relative to a mean limb-darkening function. The data base consists of about 1,000 images per day from linear CCDs with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM) at 670 nm in the red continuum. Each image shows a chord crossing the disk at a different location as the spacecraft rotates and precesses around its average solar pointing. We fit an average limb-darkening function and reassemble the residuals into synoptic maps of differential intensity variations as a function of position angle, relative location from the limb, and time. We further mask these images against SOHO/EIT 284A images in order to eliminate magnetic regions. Temperature distributions from the presumably non-magnetic sun are derived from the measured moments of temperature variations as function of data fraction by applying an EUV threshold with respect to an empirically established latitude dependent EUV base-level. We present results from our new analysis as we see much smaller quadrupolar temperature variations than previously reported.