The Scientific Rationale

Stellar variability phenomena, such as supergranulation and magnetically active regions, can produce significant signals in radial velocity (RV) measurements. We do not yet have any model that can properly account for and reproduce these signals down to the tens of centimeter per second level. This barrier prevents the detection of any Earth-like exoplanet. Thus, a better understanding of these stellar processes is required.

Variability phenomena on the surfaces of solar- and later-type stars produce signals in the measured RVs. These signals evolve on timescales ranging from minutes, hours and days — induced by acoustic pulsations and convective granulation (with disk-integrated amplitudes of ~1 m/s, e.g., Meunier et al. 2015) - to months and years, associated with magnetically active regions and their growth patterns over stellar magnetic cycles (amplitudes up to 10 m/s, e.g. Meunier et al. 2020).
To understand and model these signals to an accuracy within a few centimeters per second would be a remarkable feat in and of itself. However, in recent years, this challenge has additionally become a necessity in the field of exoplanetary sciences (Crass et al. 2021).

As pressure-stabilized, high-resolution spectrographs have become capable of measuring RVs with a precision of tens of centimeters per second, the main obstacle to detecting small-mass planets on long orbits is no longer instrumental. Instead, it is of stellar nature, as variability can drown out or mimic the periodic signals of potential companions. Earth-like planets, for instance, exhibit a semi-amplitude of only about 10-40 cm/s. The inability to thoroughly account for stellar variability also impacts the accuracy of the inferred planetary properties and their subsequent atmospheric studies with, e.g., JWST.

While some activity components can be averaged out through the observational strategy (such as pulsations; Dumusque et al. 2011 Chaplin et al. 2019) or modeled quite accurately with the aid of simultaneously measured activity indicators (such as active regions; see review by Aigrain & Foreman-mackey 2023), others have proven to be more difficult to account for. This includes granulation at several temporal and spatial scales (cf. supergranulation at the larger end; Rincon & Rieutord 2018), and especially its interplay with the evolving surface magnetic field structure. It has already been well established that magnetically active regions (i.e., faculae and spots) locally inhibit convective flows, causing asymmetries in the surface velocity field, which produces distinct RV signatures. Since these variations are quasi-periodic, the community is currently capable of isolating their contribution. Supergranulation on the other hand, due to its stochasticity at hard-to-probe timescales and its lack of known activity indicators, has been much more difficult to mitigate down to desired precision levels.

Topic and Goals

This splinter will focus on the connection between convection and magnetism, mainly through talks on recent studies and discussions regarding next steps. These could include comparisons between observations and simulations, and lessons learned from solar telescopes connected to high-resolution spectrographs.

The goals of the proposed splinter session would be as follows:

  • Summarize the recent developments in the interplay between convective granulation and stellar surface magnetic fields.
  • Identify the most urgent actions needed to improve our ability to isolate and de-correlate granulation effects in RV time series, enabling surveys of moderately active stars to achieve sub-40 cm/s precision.
The first goal will be achieved by hosting talks about advancements in theoretical and simulated granulation studies. This could include recent work which has attempted to model the RV signatures of solar granulation (Palumbo et al. 2024), developed statistical frameworks to study the evolution of its characteristics over a solar cycle (O'Sullivan et al. 2025), or performed complex magneto-hydrodynamical simulations of the Sun to deduce which spectral features are the most sensitive to granulation (Sowmya et al. 2025).

The second goal will be achieved by discussing what the aforementioned studies are currently missing and how future RV surveys can be designed to facilitate more effective activity mitigation. Since granulation and supergranulation are primarily stochastic processes, and operate on timescales difficult to probe due to observing constraints, it becomes important to understand how their temporal properties change with spectral type and magnetic cycle phase (Anna John et al. 2025).

Another aspect to consider is the extrapolation of knowledge from the Sun to other stars. A few years ago, there existed few solar telescopes connected to an ultra-stable, broad-band, high-resolution spectrograph. Today, the HARPS-N solar telescope has made available a decade of disk-integrated Sun-as-a-star observations, and similar instruments (namely the solar telescopes connected to EXPRES, HARPS, KPF and NEID) have since gained popularity, while more advanced solar telescopes like PoET (Santoset al. 2025) and ABORAS (Farret Jentink et al. 2022), designed to additionally capture disk-resolved and polarized spectra, respectively, are soon to follow. Together, these facilities provide high-cadence, high-S/N spectra which enable us to disentangle solar signals from instrumental systematics in the measured RVs (Zhao et al. 2023), and to benchmark novel RV extraction and activity mitigation methods.