Latitudinal variation of the solar photospheric intensity

TL;DR

This study investigates latitudinal variations in the solar photospheric intensity, revealing a subtle polar brightening likely of thermal origin, distinguished from magnetic effects, supported by careful analysis despite measurement challenges.

Contribution

It provides evidence for a small, thermal polar brightening in the solar photosphere, distinguishing it from magnetic influences, through detailed analysis of high-precision solar images.

Findings

Weak polar intensity enhancement (~0.1-0.2%) detected

Polar brightening likely of thermal origin

Measurement robust against systematic errors

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 $\sim0.1 - 0.2%$ corresponding to a brightness temperature enhancement of $\sim2.5{\rm K}$). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and CaIIK intensity distributions shifted towards higher values with little change in shape from their mid-latitude 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.