Temporal evolution and correlations of optical activity indicators measured in Sun-as-a-star observations

Abstract

Context. Understanding stellar activity in solar-type stars is crucial for the physics of stellar atmospheres as well as for ongoing exoplanet programmes. Aims. We aim to test how well we understand stellar activity using our own star, the Sun, as a test case. Methods. We perform a detailed study of the main optical activity indicators (Ca ii H & K, Balmer lines, Na i D 1 D 2 , and He i D 3 ) measured for the Sun using the data provided by the HARPS-N solar-telescope feed at the Telescopio Nazionale Galileo. We make use of periodogram analyses to study solar rotation, and we use the pool variance technique to study the temporal evolution of active regions. The correlations between the different activity indicators as well as the correlations between activity indexes and the derived parameters from the cross-correlation technique are analysed. We also study the temporal evolution of these correlations and their possible relationship with indicators of inhomogeneities in the solar photosphere like sunspot number or radio flux values. Results. The value of the solar rotation period is found in all the activity indicators, with the only exception being Hδ. The derived values vary from 26.29 days (Hγ line) to 31.23 days (He i). From an analysis of sliding periodograms we find that in most of the activity indicators the spectral power is split into several "bands" of periods around 26 and 30 days, that might be explained by the migration of active regions between the equator and a latitude of ∼ 30 • , spot evolution or a combination of both effects. In agreement with previous works a typical lifetime of active regions of ∼ ten rotation periods is inferred from the pooled variance diagrams. We find that Hα, Hβ, Hγ, H , and He i show a significant correlation with the S index. Significant correlations between the contrast, bisector span, and the heliocentric radial velocity with the activity indexes are also found. We show that the full width at half maximum, the bisector, and the disc-integrated magnetic field correlate with the radial velocity variations. The correlation of the S index and Hα changes with time, increasing with larger sun spot numbers and solar irradiance. A similar tendency with the S index -radial velocity correlation is also present in the data. Conclusions. Our results are consistent with a scenario in which higher activity favours the correlation between the S index and the Hα activity indicators and between the S index and radial velocity variations.

The spectral impact of magnetic activity on disc-integrated HARPS-N solar observations: exploring new activity indicators

“…In conclusion, using data from the HARPS-N solar telescope, we produce 'relative spectra' by dividing high-activity echelle orders by low-activity templates and find, after removing a pattern caused by etaloning, a number of pseudo-emission features. These features correlate well with log R H K and the faculae-filling factors derived from SDO images, but show significantly smaller or no correlation with spot-filling factors or activity indices published by Maldonado et al (2019) for the Hα, Hβ, Hγ , Hδ, Na I D1 D2, and He I D3 lines.…”

“…Furthermore, on short timescales it is known that log R H K and Hα does not correlate strongly. Indeed, Maldonado et al (2019) find that the Pearson's r correlation coefficient between Hα and S-index for the Sun can vary from nearly −0.7 to +0.6 when analysing different ∼90-d observation windows, and for other stars a large dispersion is also seen in the correlations between Hα and the Ca II H & K activity indicators (e.g. Cincunegui, Díaz & Mauas 2007).…”

“…This indicates that the pseudo-emission features being tracked in the solar spectra change in a manner more strongly correlated to changes in the faculae filling factor, rather than the spot filling factor. Maldonado et al (2019) conducted a study of conventional solaractivity indicators (using the same HARPS-N solar telescope data used in this work) and include activity indices of the Hα, Hβ, Hγ , Hδ, Na I D1 D2, and He I D3 lines. We also present the Pearson's r correlation coefficients between the daily medians of these indices and our pseudo-equivalent widths in the final six columns of Table 2.…”

“…The lack of correlation with these other activity indicators is perhaps unsurprising. Maldonado et al (2019) note that measurements of He I, Na I, and Hα are all impacted by telluric lines either contaminating the cores or continua of these indices. We also checked the spectra over the dates analysed, and indeed found substantial telluric contamination of these lines during the dates under study.…”

The ancient main-sequence solar proxy HIP 102152 unveils the activity and rotational fate of our Sun

“…Different activity indicators were used in this work such as Ca ii H (3933.663±0.595Å) and K (3968.469±0.595Å) lines; Balmer lines Hα (6562.8±0.5Å), Hβ (4861.32±0.5 A), Hγ (4340.46±0.4Å), Hδ (4101.76±0.25Å), and H (3970.07±0.3Å); Na i lines (5889.95±0.25Å, 5895.92±0.25 A). We defined their respective pseudo-continuum regions based on Maldonado et al (2019) and Giribaldi et al (2019), albeit with slight modifications: 3901.07 and 4001.07Å (∆λ = 20Å; Ca ii and H ), 6500.625 and 6625.55Å (∆λ = 5Å; Hα), 4845.0 and 4880.0Å (∆λ = 10Å; Hβ), 4318 and 4366 A (∆λ = 10Å; Hγ), 4085 and 4120Å (∆λ = 20Å; Hδ), 5845.0 and 5940Å (∆λ = 5Å; Na i). Instrumental activity indices are given by the ratio between the sum of the line fluxes and their respective continuum regions.…”