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Stellar flares cannot be spatially resolved, which complicates ascertainingthe physical processes behind particular spectral signatures. Due to theirproximity to Earth, solar flares can serve as a stepping stone forunderstanding their stellar counterparts, especially when using a Sun-as-a-starinstrument and in combination with spatially resolved observations. We aim tounderstand the disk-integrated spectral behaviors of a confined X2.2 solarflare and its eruptive X9.3 successor as measured by HARPS-N. The behavior ofmultiple photospheric and chromospheric spectral lines are investigated bymeans of activity indices and contrast profiles. A number of differentphotospheric lines were also investigated by means of equivalent widths, andradial velocity measures, which are then related to physical processes directlyobserved in high-resolution observations made with the Swedish 1-meter SolarTelescope and SDO Our findings suggest a relationship between the evolvingshapes of contrast profile time and the flare locations, which assists inconstraining flare locations in disk-integrated observations. In addition, anupward bias was found in flare statistics based on activity indices derivedfrom the Ca II H & K lines. In this case, much smaller flares cause a similarincrease in the activity index as that produced by larger flares.H$\alpha$-based activity indices do not show this bias and are therefore lesssusceptible to activity jitter. Sodium line profiles show a strongly asymmetricresponse during flare activity, which is best captured with a newly definedasymmetrical sodium activity index. A strong flare response was detected in MnI line profiles, which is unexpected and calls for further exploration.Intensity increases in H$\alpha$, H$\beta$, and certain spectral windows of AIAbefore the flare onset suggest their potential use as short-term flarepredictors.

astronomy and astrophysics

A comparative study of two X2.2 and X9.3 solar flares observed with  HARPS-N: Reconciling Sun-as-a-star spectroscopy and high-spatial resolution  solar observations in the context of the solar-stellar connection