The Sun’s magnetic activity has steadily declined throughout its MS lifetime. This is a direct consequence of the declining dynamo as a star spins down by losing angular momentum through its magnetized wind. The distinguishing property of magnetic activity in the context of stellar radiation is excess emission beyond the photospheric thermal spectrum. Because magnetic activity expresses itself by releasing energy, the most relevant radiation signatures of magnetic activity are at wavelengths shorter than the dominant optical light, i.e., at UV wavelengths (emission from active regions in the chromosphere and the transition region), at far-ultraviolet wavelengths (from the transition region), and the extreme-ultraviolet and X-ray ranges (from coronal active regions). Apart from electromagnetic radiation, particles are accelerated as a consequence of magnetic energy release. These particles are either measured in-situ in the solar system, or can be indirectly inferred from non-thermal radio emission. The solar wind of course is another particle stream that requires acceleration related to open magnetic fields.
The present section summarizes our knowledge of these various photon and particle losses from the “Sun in Time”, contrasting the young Sun’s behavior with the contemporaneous Sun by describing the long-term evolution of the radiation and wind signatures. High-energy emission may have been much more important in heating and ionizing planetary atmospheres or, at still earlier stages, circumstellar accretion disks. To understand the young Sun’s influence on its environment, the spectral output in lines and the continuum must be observed for solar analogs with different rotation periods (corresponding to different magnetic activity levels) across the ultraviolet-to-X-ray range. This “Sun in Time” program, introduced in Section 3, has been conducted during the past decade by various groups (e.g., Dorren and Guinan 1994a; Dorren et al. 1995; Güdel et al. 1994, 1995b, 1997b; Ayres 1997; Gaidos et al. 2000; Guinan et al. 2003; Ribas et al. 2005; Telleschi et al. 2005).
I will start with the solar wind that is responsible for the declining solar spin rate, which in turn controls the dynamo; the latter leads to surface activity, and hence to radiation that is again changing on evolutionary time scales. I will discuss consequences of the elevated high-energy output of the young Sun in the subsequent chapters.
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