Asymmetric Distribution of Weak Photospheric Magnetic Field Values

Abstract

We use the synoptic maps of the photospheric magnetic field observed at Wilcox Solar Observatory, Mount Wilson Observatory, Kitt Peak, SOHO/MDI, SOLIS/VSM, and SDO/HMI to study the distribution of weak photospheric magnetic field values in 1974–2018. We fit the histogram distribution of weak field values for each synoptic map of the six data sets separately with a parameterized Gaussian function in order to calculate the possible shift (to be called here the weak-field asymmetry) of the maximum of the Gaussian distribution from zero. We estimate the statistical significance of the weak-field asymmetry for each rotation. We also calculate several versions of lower-resolution synoptic maps from the high-resolution maps and calculate their rotational weak-field asymmetries. We find that the weak-field asymmetries increase with decreasing map resolution. A very large fraction of weak-field asymmetries are statistically significant, with the fraction of significant weak-field asymmetries increasing with decreasing resolution. Significant weak-field asymmetries of high- and low-resolution maps mainly occur at the same times and have the same sign. Weak-field asymmetries for the different data sets and resolutions vary quite similarly in time, and their mutual correlations are very high, especially for low-resolution maps. These results give strong evidence for weak-field asymmetries reflecting a real feature of weak field values, which is best seen in medium- and low-resolution synoptic maps and is most likely related to the supergranulation scale of the photospheric field.