The onset of solar cycle 24: What global acoustic modes are telling us

TL;DR

This study uses helioseismic data to detect the onset of solar cycle 24 through low-degree p-mode frequency shifts, revealing internal magnetic activity changes before surface activity becomes observable.

Contribution

It demonstrates that helioseismic p-mode frequency shifts can indicate the start of a solar cycle prior to surface activity signs, highlighting internal magnetic field dynamics.

Findings

l=0 and l=2 modes increased in frequency from late 2007 despite low surface activity

l=1 modes continued decreasing, aligning with surface activity trends

Helioseismic data reveals solar cycle 24 began in late 2007 before surface signs appeared

Abstract

We study the response of the low-degree, solar p-mode frequencies to the unusually extended minimum of solar surface activity since 2007. A total of 4768 days of observations collected by the space-based, Sun-as-a-star helioseismic GOLF instrument are analyzed. A multi-step iterative maximum-likelihood fitting method is applied to subseries of 365 days and 91.25 days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2 p-mode frequencies are then obtained from April 1996 to May 2009. While the p-mode frequency shifts are closely correlated with solar surface activity proxies during the past solar cycles, the frequency shifts of the l=0 and l=2 modes show an increase from the second half of 2007, when no significant surface activity is observable. On the other hand, the l=1 modes follow the general decreasing trend of the solar surface activity. The different behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted as different geometrical responses to the spatial distribution of the solar magnetic field beneath the surface of the Sun. The analysis of the low-degree, solar p-mode frequency shifts indicates that the solar activity cycle 24 started late 2007, despite the absence of activity on the solar surface.