The study of the past of our Sun and its solar system has become an interdisciplinary effort between stellar astronomy, astrophysics of star and planet formation, astrochemistry, solar physics, geophysics, planetology, meteoritical science and several further disciplines. The interest in understanding the past evolution of our star is obvious; the Sun’s radiative energy, the solar wind, and various forms of transient phenomena (e.g., shock waves, high-energy particle streams during flares) are key factors in the formation and evolution of the planets and eventually the biosphere on Earth.
The Sun is, like almost all cool stars, a “magnetic star” that produces magnetic fields through dynamo operation in the interior. These fields reach the surface where their presence is noticed in the form of sunspots. However, magnetic activity has much more far-reaching consequences: Solar magnetic fields control essentially the entire outer solar atmosphere, they heat coronal gas to millions of degrees, they produce flares whose by-products such as shock waves and high-energy particles travel through interplanetary space to eventually interact with planetary atmospheres; the solar wind is guided by open magnetic fields; this magnetized-wind complex forms a large bubble, an “astrosphere” in interstellar space containing the entire solar system and protecting it from a high dose of cosmic rays.
Was the Sun’s magnetic activity different in its infancy when planets and their atmospheres formed, or when it was still surrounded by an accretion disk? Accumulated direct and indirect evidence indeed points to a much higher level of magnetic activity in the young Sun, in particular in its pre-main sequence (PMS) phase and the subsequent epoch of its settling on the main sequence (MS). Direct evidence includes meteoritic traces and isotopic anomalies that require much higher proton fluxes at early epochs at least partly from within the solar system (Section 6.5 below); indirect evidence comes from systematic comparisons of the contemporary Sun with solar analogs of younger age that unequivocally show a strong trend toward elevated activity at younger ages (Section 5 below). Interestingly, planetary atmospheres offer further clues to strongly elevated activity levels: Evidence for a warmer early climate on Mars or the extremely arid atmosphere of Venus – a sister planet of the water-rich Earth – call for explanations, and such explanations may be found in the elevated activity of the young Sun (see Section 7.2 below). The study of the early solar activity is the theme of the present review article.
The main goal of this article is therefore to demonstrate evidence for a much more active young Sun, and to study the consequences this might have had for the development of the solar environment, including the formation and evolution of planets. Our discussion will therefore take us through the following three major issues:
The discovery of extrasolar planets in particular around Sun-like stars has also spurred interest in star-planet interactions, e.g., erosion of atmospheres or photochemical reactions, that profit from detailed studies in the solar system.
The focus of this review is therefore, on the one hand, on signatures of magnetic activity across the electromagnetic spectrum, representing physical processes in the photosphere, the chromosphere, and the thermal and non-thermal corona of a solar-like star. I will mostly use young solar-like stars to infer conditions that – by analogy – might have prevailed on the young Sun. On the other hand, I will also discuss traces that the elevated activity of the young Sun might have left behind in meteorites and in planetary atmospheres, thus collecting “in-situ” information about the distant past of our own solar system.
While this article focuses on the conditions on the young Sun and in the early solar system, it has proven convenient to study the solar evolution in time systematically from young to old, because a number of trends become evident that can be calibrated with the contemporaneous Sun. We thus not only learn about the young Sun, but we uncover the systematics that made it different from what it is today. This is the approach I adopt in the present work.
This article will not address issues on the formation and evolution of the Sun that are related to its internal constitution, with the exception of cursory reference to the magnetic dynamo that is, of course, at the origin of all solar magnetic activity. I will treat the PMS Sun in separate chapters for three related reasons: First, fundamental properties of the PMS Sun were largely different from those of the contemporaneous Sun (for example, its spectral type, or its photospheric effective temperature). Second, new features not present in the modern Sun become dominant key players related to activity and environment, among them accretion disks, accretion streams, star-disk magnetospheres, outflows, and jets. And third, the PMS behavior of the Sun cannot be assessed in detail judged from the present-day solar parameters; we can only discuss the range of potential evolutionary scenarios now observed in a wide sample of PMS stars (e.g., with respect to mass accretion rate, disk mass, disk dispersal time, rotation period, etc.).
Numerous review articles are available on subjects related to the present one. Without intention to be complete, I refer here in particular to the collection of papers edited by Sonett et al. (1991) and Dupree and Benz (2004), the Cool Stars Workshop series (the latest volume edited by van Belle 2007), and the Protostars and Planets series (in particular the latest volumes by Mannings et al. 2000 and Reipurth et al. 2007). An early overview of solar variability (including that of its activity) can be found in Newkirk Jr (1980). Walter and Barry (1991) specifically reviewed knowledge of the long-term evolution of solar activity as known in the early nineties, in a similar spirit as the present review; numerous older references can be found in that work. For summaries of stellar X-ray and radio emission, I refer to Güdel (2004) and Güdel (2002), respectively. Feigelson and Montmerle (1999) and Feigelson et al. (2007) have summarized PMS aspects of magnetic activity. Glassgold et al. (2005) have reviewed the influence of the magnetic activity of the PMS Sun on its environment, in particular on its circumstellar disk where our planets were forming. Wood (2004) has discussed evidence for winds emanating from solar-like stars, and Goswami and Vanhala (2000) have summarized findings related to radionuclides in meteorites and inferences for the young solar system; the most recent developments in this field have been reviewed by Wadhwa et al. (2007). Kulikov et al. (2007) and Lundin et al. (2007) have provided summaries on interactions between solar high-energy radiation and particles with planetary atmospheres, in particular those of Venus, Earth, and Mars.
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