Les Trente Glorieuses: 29 years of helioseismic observations with the Luminosity Oscillations Imager

TL;DR

This work delivers a comprehensive 29-year helioseismic dataset from the LOI instrument aboard SoHO, detailing a rigorous calibration and time-series-generation pipeline to extract p-mode parameters in intensity. It combines global and local MLE-based fits (progFIT/guiFIT) with collapsogram techniques to detect low-frequency modes and explores mode visibilities across Sun-as-a-star, resolved, EW/NS, HV, and guiding signals. The study confirms solar-activity–driven frequency shifts and linewidth-height variations, quantifies how these depend on degree, frequency, and signal type, and highlights limb-darkening and radius-related effects on visibilities. The results provide a critical baseline for stellar seismology using intensity data, inform limb and rotation-inclination considerations, and establish LOI as a definitive reference for the final SoHO data archive.

Abstract

The Luminosity Oscillations Imager (LOI) of the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) instrument aboard the SoHO mission has been operating for almost the past 30 years. I report on the effect of solar activity upon mode frequencies, linewidths, height and energy rate. I report on the variation as a function of frequency for frequency, $a_2$ coefficient and linewidth changes, as well as the average over the degree and the frequency of these changes. Using the 29-year time series, I report on the frequencies, linewidths and mode height fitted with \texttt{progFIT}. Using the collapsogram technique, I also report on the detection of modes below 1600 $μ$Hz, making the lowest frequencies detected with an instrument observing the Sun in intensity. I also report on the detection of p mode in the high voltage and guiding pixel signals with a mode height about 5 to 10 times larger than what is observed in the Sun-as-a-star signal for $l=1$. The ratios of the observed mode visibilities for the different signals are provided following a calibration of the size of the guiding pixels. While the visibility ratios for the signals excluding the limb are in good agreement with theory, those covering the solar limb are in strong disagreement.

Figures (22)

• Figure 1: Relative pixel intensity as a function of time in minutes for two pixels: raw (Left), identified attractors in red (Right)
• Figure 2: Compilation of all attracted bits over the 12 scientific pixels over 29 years as a function of the binary value (left) and as a function of the inverse binary value (right). For the latter in addition, shown in green and mirrored the same compilation but as a function the inverse binary value multiplied by a factor 2.
• Figure 3: Duty cycle as a function of time for the 12-pixel data (continuous line), for the fraction of time any pixel having at least one attractor (dashed line), for the fraction of time at least one pixel having an attractor lasting longer than 5 minutes (orange line), for the fraction of time at least two pixels having an attractor lasting longer than five minutes (green lines).
• Figure 4: power spectrum as a function of frequency for the full disk signal for 1996 (top) and 2024 (bottom) for one year of data.
• Figure 5: (Left) power spectrum as a function of frequency for the Y high voltage for 1996 (top) and 2024 (bottom) for one year of data. (Right) power spectrum as a function of frequency for the Z high voltage for 1996 (top) and 2024 (bottom) for one year of data.