5.10 Summary: The young, active Sun

Although the MS Sun started as a ZAMS star with a bolometric luminosity that was significantly lower than today – by about 30% – a very different picture must be drawn from high-energy observations of solar analogs, including UV, FUV, EUV, X-ray, and non-thermal radio emissions. All signatures of magnetic activity, among them dark starspots, chromospheric and transition-region line emission, coronal X-rays, and radio gyrosynchrotron emission in young solar analogs point to levels of magnetic activity orders of magnitude above the present solar level. The present-day Sun in fact resides at the lowest levels a solar-like star can attain – near the end of its MS life, as a spun-down star.

Short-wavelength radiation and non-thermal radio emission show power-law decays during the solar evolution that steepen toward higher energies of the underlying electron population. In parallel, the rotation rate of the Sun also decayed, again in a power-law fashion. Rotation is indeed thought to be the ultimate driver of these decay laws, as it is one of the two fundamental ingredients of the magnetic dynamo, the other being convection which does not strongly change during the Sun’s main-sequence life.

The solar wind is another expression of magnetic activity. The mass loss induced by the wind also decays according to a power-law although this does not seem to hold for the youngest and most active examples which show a suppressed wind.

The high level of magnetic activity in the young Sun must have been a pivotal ingredient in shaping the solar environment, in particular planetary atmospheres. The most extreme expressions of magnetic activity, magnetic flares that appear to be dominating contributors to high-energy photon and particle production in young solar analogs, need to be accounted for in planetary atmospheric modeling. I will discuss some of these aspects in Section 7.

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