Solar and Stellar Activity

TL;DR

The chapter surveys solar and stellar magnetic activity, connecting photospheric and coronal manifestations to dynamo processes and exoplanet environments.For the Sun, it reviews the solar dynamo framework, including the $\alpha\Omega$ and Babcock–Leighton mechanisms, tachocline role, and the 11-year cycle, supported by MHD simulations.Extending to other stars, it discusses observational diagnostics (e.g., $S$-index, $\log R'_ ext{HK}$, Zeeman broadening/intensification, ZDI) and the dependence of activity on rotation quantified by the Rossby number $Ro=P_\mathrm{rot}/\tau$.The chapter highlights the diversity of stellar cycles across spectral types, the challenges of dynamo modeling for partly- vs fully-convective stars, and the implications for exoplanet atmospheres and space weather.

Abstract

This chapter provides an overview of the magnetic activity of the Sun and stars, discussing its underlying physical origin, manifestations, and fundamental role in exoplanet studies. It begins with a summary of the Sun's magnetic activity from the surface towards the outer atmospheric layers, highlighting features such as sunspots, faculae, chromospheric structures, and their temporal modulation known as the activity cycle. These phenomena are sustained throughout the lifetime of the Sun by the magnetic dynamo, which is driven by differential rotation and convective flows. Furthermore, extending these concepts to other stars, the chapter examines the diagnostics that are typically employed to track and quantify the magnetic activity level of stars, and it reviews spectropolarimetry, an observational technique with which to characterise stellar magnetic fields. We finally outline results from both observations and theoretical modelling of stellar activity across distinct spectral types, and we describe the variety of methods used to search for stellar activity cycles, underscoring the multi-wavelength nature of this field of research.

Figures (10)

• Figure 1: Images of the Sun in December 2022 for the flattened brightness (left), line of sight velocity (middle), and magnetic field (right) acquired with the Solar Dynamic Orbiter. Examples of visual stellar activity manifestations are photospheric inhomogeneities, like faculae and spots which are hotter and cooler than the rest of the surface, respectively. These are connected to the underlying magnetic field. Credit: NASA/SDO and the AIA, EVE, and HMI science teams.
• Figure 2: The butterfly diagram illustrates the distribution of sunspots and magnetic flux on the Sun and their change over time. The figure shows the sunspots area coverage as a function of latitude, for which it is possible to see the Spörer law, that is the equatorward drift of the sunspot emergence through a magnetic cycle. Credit: Dr. D. Hathaway http://solarcyclescience.com/solarcycle.html.
• Figure 3: Potential Field Source Surface (PFSS) model of the Sun during cycle maximum and minimum. The images are built using the SolarSoft package from solar magnetograms collected by means of SOHO/MDI and SDO/HMI instruments, and the extrapolation is performed from the photosphere out to about 2.5 solar radii, which is where the source surface is located. The left image (November 2001) illustrates the magnetic field of the Sun at activity maximum (during cycle 23) and the right image (December 2008) illustrates the magnetic field at the activity minimum (and the start of cycle 24). Purple and green colours indicate open field lines of negative and positive polarity, while the white colour indicates closed field lines. The large-scale magnetic field has a simple, mostly dipolar configuration at activity minimum, and a complex one at activity maximum. Credit: NASA's Goddard Space Flight Center Scientific Visualization Studio/Bridgman and Duberstein.
• Figure 4: Mechanisms at the base for the solar dynamo model. The $\Omega$-effect transforms a poloidal field into a toroidal field via differential rotation. Restoring a poloidal field from a toroidal field is accomplished in two ways: the $\alpha$-effect sees cyclonic turbulence twisting toroidal field lines into small-scale poloidal fields which, on average, constitute a global poloidal field. The Babcock-Leighton (BL) mechanism sees the formation of bipolar regions on the surface; those close to equator diffuse and reconnect between hemispheres, whereas the remaining ones are transported towards the pole by meridional circulation and produce a large-scale poloidal field. Image credit: Sanchez2014.
• Figure 5: Spectrum around the Ca ii H&K lines and spectral windows defining the $S$-index. Top row: the Ca ii H&K lines (left and right, respectively) with triangular windows whose full width at half maximum is 1.09 Å. Bottom row: two continuum sections on either side of the H and K lines, identified as V (centred at 4000 Å) and R (centred at 3905 Å), with two 20 Å windows. Image credit: Isaacson2024.