Latitudinal dependence of variations in the frequencies of solar oscillations above the acoustic cut-off

TL;DR

This study investigates how pseudo-mode frequency shifts in the Sun vary with latitude over 22 years, revealing latitudinal differences and a consistent four-year periodicity, with shifts anti-correlated to solar magnetic activity.

Contribution

It provides the first detailed analysis of latitudinal dependence of pseudo-mode frequency shifts using extensive GONG data and wavelet analysis, uncovering new spatial and temporal patterns.

Findings

Latitudinal differences in pseudo-mode shifts are most pronounced during solar maximum.

Frequency shifts are strongly anti-correlated with solar activity proxies.

A four-year periodicity is observed across all latitudes.

Abstract

At high frequencies beyond the acoustic cut-off, a peak-like structure is visible in the solar power spectrum. Known as the pseudo-modes, their frequencies have been shown to vary in anti-phase with solar magnetic activity. In this work, we determined temporal variations in these frequencies across the solar disc, with the aim of identifying any potential latitudinal dependence of pseudo-mode frequency shifts. We utilised nearly 22 years of spatially resolved GONG data for all azimuthal orders, $\textit{m}$, for harmonic degrees 0 $\leq$ $\textit{l}$ $\leq$ 200, and determined shifts using the resampled periodogram method. Periodogram realisations were created from overlapping, successive 216d-long segments in time, and cropped to 5600-6800$\mu$Hz. Cross-correlation functions were then repeatedly generated between these realisations to identify any variation in frequency and the uncertainty. We categorised each mode by its latitudinal sensitivity and used this categorisation to produce average frequency shifts for different latitude bands (15$^\circ$ and 5$^\circ$ in size) which were compared to magnetic proxies, the $F_{\mathrm{10.7}}$ index and GONG synoptic maps. Morphological differences in the pseudo-mode shifts between different latitudes were found, which were most pronounced during the rise to solar maximum where shifts reach their minimum values. At all latitudes, shift behaviour was strongly in anti-correlation with the activity proxy. Additionally, periodicities shorter than the 11-year cycle were observed. Wavelet analysis was used to identify a periodicity of four years at all latitudes.