Latitudinal Variation of the Solar Photospheric Intensity

Abstract

We have examined images from the Precision Solar Photometric Telescope ( PSPT ) at the Mauna Loa Solar Observatory ( MLSO) in search of latitudinal variation in the solar photospheric intensity. Along with the expected brightening of the solar activity belts, we have found a weak enhancement of the mean continuum intensity at polar latitudes (continuum intensity enhancement $0.1%Y0.2%, corresponding to a brightness temperature enhancement of $2.5 K). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and Ca ii K intensity distributions shifted toward higher values with little change in shape from their midlatitude distributions. Since the enhancement is of low spatial frequency and of very small amplitude, it is difficult to separate from systematic instrumental and processing errors. We provide a thorough discussion of these and conclude that the measurement captures real solar latitudinal intensity variations.

Convection and Differential Rotation in F-Type Stars

“…The temperatures range from the hot poles at 840 K to the cool equator at −280 K with yet cooler mid-latitudes. This equator-to-pole temperature contrast is slightly more than two orders of magnitude greater than the observational limits on the latitudinal temperature profile of the Sun (Rast et al 2008). However, these variations are taken relative to the mean temperature that varies between 1.3×10 6 K at the bottom of the domain and 10 5 K at the top, so they are still relatively small, being of order 1%.…”

The Intensity Profile of the Solar Supergranulation

“…Dotted fiducial lines indicate the average intensity of the unmasked pixels in the images. Profiles found using the least severe mask (M0) are at top of each plot, with each curve below found using progressively more severe activity masks (mask M7 used for the bottommost profile; see Table 3 in Rast et al 2008 for mask definitions). The peak at r/r 0 = 1 for weak masking reflects the contribution from magnetic network.…”

“…This interpretation is suggested by the apparent ubiquitous presence of inter-network flux elements (e.g., de Wijn et al 2008, and references therein), and the degree of masking required to remove the magnetic element signature from the measurement. Average profiles with enhanced cell center intensities are observed at the red and blue continuum wavelengths only after more than 95% of the pixels have been masked, leaving only pixels with magnetic flux density 0.7 G (see Table 3 in Rast et al 2008 for tabulated properties of the eight masks employed). The Ca ii K intensity, on the other hand, remains enhanced at network cell boundaries even after more severe masking (dark cell centers and brighter boundaries are still seen in Ca ii K after masking 99.96% of the brightest pixels, down to average magnetic flux densities of ∼0.4 G).…”

“…ξ is a tunable parameter. 8 The contrast at each wavelength is defined as I c ≡ δI I 0 = I −I 0 I 0 , where I is the observed gain-corrected intensity and I 0 is the measured center-to-limb function at that wavelength (Rast et al 2008). For this study, we take L = 40 pixels (∼ 30 Mm), a typical supergranular length scale, and explore the sensitivity of the measured supergranualar properties to the value of ξ (see Table 1).…”

“…Selection was based on the quality of the red image (the images used in this study have maximum red and blue continuum limb widths of 2.5 and 3.5 pixels, respectively; see Rast et al 2008) and the availability of images in all three wavelength bands. Since our final measurement of the average radial intensity profile of supergranular cells makes use of only a small fraction of the pixels in any given image, those remaining after masking out magnetic element contributions, the signal from many images must be combined into a statistically significant measurement.…”