The Sun Through Time
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The Sun Through Time Manuel Güdel1
Received: 24 December 2019 / Accepted: 17 November 2020 © The Author(s) 2020
Abstract Magnetic activity of stars like the Sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. These magnetic fields are generated by an internal dynamo driven by convection and differential rotation. Spin-down therefore converges at an age of about 700 Myr for solar-mass stars to values uniquely determined by the stellar mass and age. Before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. This non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. I present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models. Keywords Stellar activity evolution · Stellar wind evolution · Stellar mass-loss evolution · Stellar rotation · Stellar spin-down
1 Introduction Solar activity was recognized as an important property of the Sun in observations of magnetic fields in sunspots (Hale 1908), the occurrence of giant outbursts initially in white light (Carrington 1859; Hodgson 1859), the presence of a very hot (millions of K) corona initially in optical lines (Grotrian 1939; Edlén 1942), and later in radio waves (Hey 1946) and X-rays (Burnight 1949). We now know that magnetic fields are at the origin of all these phenomena, as they can store energy derived from convective motions at and below the photospheric level, but can also continuously or episodically release energy in higher atmospheric layers (the chromosphere, the transition region, the corona, and the solar-wind region). Reading Terrestrial Planet Evolution in Isotopes and Element Measurements Edited by Helmut Lammer, Bernard Marty, Aubrey L. Zerkle, Michel Blanc, Hugh O’Neill and Thorsten Kleine
B M. Güdel
[email protected]
1
Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
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M. Güdel
Magnetic fields originate in the dynamo deep inside the convection zone; differential rotation and convective motion act together to amplify magnetic fields that rise buoyantly to the surface where they form magnetic active regions with “magnetic loops” reaching out into the corona. A solar coronal wind was inferred initially from observations of comets (Biermann 1951) and explained theoretically by Parker (1958), followed by in situ observations by space probes in the early 1960s. In what sense solar/stellar rotation itself matters for “magnetic activity” was less clear; it was stellar astronomy that made important contributions to this question. Kraft (1967) reported that the average rotational velocities of stars with strong Ca II emission are higher than velocities for weak Ca emitters. This pointed to an evolutionary spin-down of stars because stronger Ca II emitters are you
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