Ilaria Ermolli

Full-disk observations of the Sun in the Ca ii K line have been carried out since the late 19th century at various observatories worldwide. These long-term records of solar activity are crucial for reducing discrepancies among solar irradiance reconstructions and for advancing our understanding of the solar dynamo. To construct a consistent composite record, data from different observatories must be cross-calibrated to account for variations in spectral passband and spatial resolution, which are the primary sources of discrepancies between archives. In this study, we use high spectral and spatial resolution observations in the Ca ii K line from the state-of-the-art S unrise iii mission to emulate different passbands and derive empirical contrast-contrast relationships between them. We find that these relationships are well described by a power law and provide coefficients for different combinations of passband widths in the range 0.1–9 Å and spatial resolutions between 1″ and 6″. Applying such a relationship to observations from two major Ca ii K archives demonstrates its potential to improve their cross-calibration. The results provide a foundation for the construction of a consistent, century-long time series of solar activity from historical and modern Ca ii K observations.

We present a detailed analysis of the chromospheric response during a small-scale flaring event observed in the active region NOAA 13394 on 8 August 2023. By utilizing coordinated high-resolution spatial and spectral observations from the GRIS spectropolarimeter at the GREGOR telescope, targeting the He I 1083.0 nm triplet, and the IRIS spectrograph, covering the near- and far-ultraviolet spectral regions, we investigate the plasma dynamics and heating processes within the solar atmosphere. We observe complex, asymmetric He I Stokes profiles; inversions performed with the state-of-the-art HAZEL code reveal that these profiles are characterized by significant redshifts, with velocities reaching ∼30 km s ⁻¹ , which are interpreted as signatures of chromospheric condensation. These findings are supported by simultaneous IRIS observations, which show redshifts in transition region lines (Si IV , C II ) along the footpoints of the flaring event, suggesting a coherent, multi-layer response to the heating. The evolution of the Mg II h&k line profiles, transitioning from a central reversal to a single-peaked morphology, suggests a rising source function associated with reduced opacity. These features provide compelling evidence of deep heating in the lower chromosphere during the flaring event.

Angelo Secchi, a pioneer in solar physics, provided an extensive and regular record of sunspot observations spanning the period 1858–1878. This study analyzes the handwritten records by Secchi and collaborators in the form of drawings and an observation diary. We focus on the systematic annotations of group and spot counts, and sunspot area measurements. We present a machine-readable dataset that provides valuable information on historical solar activity, adding new data to limited series such as sunspot area measurements. This work compares Secchi’s sunspot records with modern solar activity series, evaluates the consistency of his records, and highlights their importance in extending long-term solar indices.

The manuscript entitled “Solar flare spectroscopy at Pic du Midi Observatory (1956-1968) in the frame of the International Geophysical Year” aims at presenting two spectrographs employed for spectroscopic studies of solar flares at the Pic du Midi Ob...

Bipolar emerging flux regions (EFRs) form active regions (ARs) that generally evolve into a pre-existing magnetic environment in the solar atmosphere. Reconfiguration of the small- and large-scale magnetic connectivities is invoked to explain a plethora of energy-release phenomena observed at the sites of EFRs. These include brightening events, surges, and jets, whose triggers and relationships are still unclear. In this context, we study the formation of a proto-spot in AR NOAA 11462 by analyzing spectropolarimetric and spectroscopic measurements taken by the Interferometric Bidimensional Spectrometer along the Fe I 630.2 nm and Ca II 854.2 nm lines on 17 April 2012. We complement these high-resolution data with simultaneous SDO satellite observations. The proto-spot forms from magnetic flux and emerges into the photosphere, coalescing following plasma flows in its surroundings. The chromospheric and higher atmosphere observations show that flux emergence occurs in a pre-existing magnetic environment, with small- and large-scale coronal arcades that seemingly shape the proto-spot formation in the upper atmospheric layers. In addition, in the chromosphere, we observe an arch filament system and repeated intense brightening events and surges, likely due to magnetic interactions of the new flux with the pre-existing overlying coronal field. These phenomena have been observed since the early stages of the new flux emergence.

The Sun’s polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future solar cycles. However, direct measurements of the polar field have only been available since the 1970s. The chromospheric Ca ii K polar network index (PNI; the fractional area of the chromospheric network regions above a certain latitude) has recently emerged as a reliable proxy for polar magnetic fields. In this study, we derive PNI estimates from newly calibrated, rotation-corrected Ca ii K observations from the Kodaikanal Solar Observatory (1904–2007) and modern data from the Rome Precision Solar Photometric Telescope (2000–2022). We use both of those Ca ii K archives to identify polar network regions with an automatic adaptive threshold segmentation technique and calculate the PNI. The PNI obtained from both the archives shows a significant correlation with the measured polar field from the Wilcox Solar Observatory (Pearson correlation coefficient r > 0.93) and the derived polar field based on an Advective Flux Transport Model (r > 0.91). The PNI series also shows a significant correlation with faculae counts derived from Mount Wilson Observatory observations (r > 0.87) for both Kodaikanal Solar Observatory and Rome Precision Solar Photometric Telescope data. Finally, we use the PNI series from both archives to reconstruct the polar magnetic field over a 119 yr long period, which includes the last 11 solar cycles (Cycles 14–24). We also obtain a relationship between the amplitude of solar cycles (in 13 month smoothed sunspot number) and the strength of the reconstructed polar field at the preceding solar cycle minimum to validate the prediction of the ongoing solar cycle, Cycle 25.

The provision of solar forcing datasets for CMIP7 includes a dataset with scenarios from the present to 2300. This dataset contains daily values of the same variables as in the historical solar forcing for CMIP7, namely: solar spectral irradiance, medium energy electrons, solar energetic protons and galactic cosmic rays. In contrast to CMIP6, which had only two scenarios, for CMIP7 we will provide a large ensemble of scenarios to avoid selection bias.Let us stress that we are providing scenarios, not forecasts: the reconstructions vary randomly in time, but their statistical and spectral properties are fully consistent with historical variations, providing realistic surrogates for solar forcing.In this presentation we explain how historical observations are used to build these surrogate reconstructions. This process involves several steps, starting with the 14C reconstructions of past solar activity. These will be described in detail, together with the first version of the dataset.

Back in 2017, solar forcing recommendations for the 6th round of the Coupled Model Intercomparison Project (CMIP) were provided which covered, for the first time, all relevant solar irradiance and energetic particle contributions. Since then, this dataset has been extensively used in climate model experiments and has been tested in various intercomparison studies. Further, new datasets have been come available. An International Space Sciene Institute (ISSI) Working Group has been established to review these recent achievements in order to define the strategy for building a revised solar forcing dataset for the 7th round of CMIP. After receiving community feedback on this strategy, a historical solar forcing dataset for CMIP7 has been recently constructed. Major changes with respect to CMIP6 include the adoption of the new Total and Spectral Solar Irradiance Sensor (TSIS-1) solar reference spectrum for solar spectral irradiance and an improved description of top-of-the-atmosphere energetic electron fluxes, as well as their reconstruction back to 1850 by means of geomagnetic proxy data. Solar irradiance varaibility in the reference forcing dataset is based on historical reconstructions generated with the new empirical NASA NOAA LASP (NNL) Solar Spectral Irradiance Version 1 model, NNLSSI1. In adition, an alternative solar irradiance dataset, based on SATIRE, is provided for sensitivity experiments. In this talk we will discuss the applied modifications with respect to CMIP6 and their implication for climate modeling. Ongoing activities on solar forcing uncertainty quantification and the construction of future solar forcing scenarios will also be summarized.

The Sun's polar magnetic field is pivotal in understanding solar dynamo processes and forecasting future solar cycles. However, direct measurements of the polar field is only available since the 1970s. The chromospheric Ca II K polar network index (PNI; the fractional area of the chromospheric network regions above a certain latitude) has recently emerged as a reliable proxy for polar magnetic fields. In this study, we derive PNI estimates from newly calibrated, rotation-corrected Ca II K observations from the Kodaikanal Solar Observatory (1904-2007) and modern data from the Rome Precision Solar Photometric Telescope (2000-2022). We use both of those Ca II K archives to identify polar network regions with an automatic adaptive threshold segmentation technique and calculate the PNI. The PNI obtained from both the archives shows a significant correlation with the measured polar field from WSO (Pearson correlation coefficient r > 0.93) and the derived polar field based on an Advective Flux Transport Model (r > 0.91). The PNI series also shows a significant correlation with faculae counts derived from Mount Wilson Observatory observations (r > 0.87) for both KoSO and Rome-PSPT data. Finally, we use the PNI series from both archives to reconstruct the polar magnetic field over a 119-year-long period, which includes last 11 solar cycles (Cycle 14-24). We also obtain a relationship between the amplitude of solar cycles (in 13-month smoothed sunspot number) and the strength of the reconstructed polar field at the preceding solar cycle minimum to validate the prediction of the ongoing solar cycle, Cycle 25.