The Sun in Time Activity and ...

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The Sun in Time: Activity and Environment
Manuel Gudel
Paul Scherrer Institute, Wurenlingen and Villigen,
CH-5232 Villigen PSI, Switzerland
and
Max-Planck-Institute for Astronomy, Konigstuhl 17,
D-69117 Heidelberg, Germany
email: guedel@astro.phys.ethz.ch
ABSTRACT
The Sun’s magnetic activity has steadily declined during its main-sequence life. While
the solar photospheric luminosity was about 30% lower 4.6 Gyr ago when the Sun arrived
on the main sequence compared to present-day levels, its faster rotation generated enhanced
magnetic activity; magnetic heating processes in the chromosphere, the transition region, and
the corona induced ultraviolet, extreme-ultraviolet, and X-ray emission about 10, 100, and
1000 times, respectively, the present-day levels, as inferred from young solar-analog stars.
Also, the production rate of accelerated, high-energy particles was orders of magnitude higher
than in present-day solar ares, and a much stronger wind escaped from the Sun, permeating
the entire solar system. The consequences of the enhanced radiation and particle uxes from
the young Sun were potentially severe for the evolution of solar-system planets and moons.
Interactions of high-energy radiation and the solar wind with upper planetary atmospheres
may have led to the escape of important amounts of atmospheric constituents. The present dry
atmosphere of Venus and the thin atmosphere of Mars may be a product of early irradiation
and heating by solar high-energy radiation. High levels of magnetic activity are also inferred
for the pre-main sequence Sun. At those stages, interactions of high-energy radiation and
particles with the circumsolar disk in which planets eventually formed were important. Traces
left in meteorites by energetic particles and anomalous isotopic abundance ratios in meteoritic
inclusions may provide evidence for a highly active pre-main sequence Sun. The present
article reviews these various issues related to the magnetic activity of the young Sun and
the consequent interactions with its environment. The emphasis is on the phenomenology
related to the production of high-energy photons and particles. Apart from the activity on
the young Sun, systematic trends applicable to the entire main-sequence life of a solar analog
are discussed.
1
 Contents
1
Introduction
4
2
What is a Solar-Like Star?
7
3
The Sun in Time
9
3.1
Goals of the “Sun in Time” Project . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
3.2
Overview of Stellar Sample
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
4
The Solar Magnetic Field in Time
13
4.1
The Young Solar Photosphere: Large, Polar Spots
. . . . . . . . . . . . . . . . . .
13
4.1.1
Doppler Imaging of Young Solar Analogs
. . . . . . . . . . . . . . . . . . .
13
4.1.2
Polar Spots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
4.2
Coronal Structure of the Young Sun
. . . . . . . . . . . . . . . . . . . . . . . . . .
15
4.2.1
Magnetic Loop Models and Active Regions
. . . . . . . . . . . . . . . . . .
17
4.2.2
Inferences from Coronal Density Measurements . . . . . . . . . . . . . . . .
18
4.2.3
Inferences from Rotational Modulation . . . . . . . . . . . . . . . . . . . . .
18
4.2.4
Inferences from Eclipses . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
4.2.5
Photospheric-Field Extrapolation to the Corona
. . . . . . . . . . . . . . .
20
4.2.6
Summary on Coronal Structure . . . . . . . . . . . . . . . . . . . . . . . . .
20
4.3
Activity Cycles in the Young Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
4.3.1
Starspot Cycles of Solar Analogs
. . . . . . . . . . . . . . . . . . . . . . . .
22
4.3.2
X-Ray Cycles of Solar Analogs
. . . . . . . . . . . . . . . . . . . . . . . . .
25
5
Solar Radiation and Wind in Time
26
5.1
The Solar Wind in Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
5.2
The Solar Spin in Time
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
5.3
The Ultraviolet Sun in Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
5.4
The Far-Ultraviolet Sun in Time
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
5.5
The Extreme-Ultraviolet and X-Ray Sun in Time . . . . . . . . . . . . . . . . . . .
30
5.5.1
The Solar X-Ray Corona in Time . . . . . . . . . . . . . . . . . . . . . . . .
31
5.5.2
The Coronal Temperature in Time . . . . . . . . . . . . . . . . . . . . . . .
33
5.6
Putting it all Together: The XUV Sun in Time . . . . . . . . . . . . . . . . . . . .
37
5.7
The Radio Sun in Time
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
5.7.1
Overview
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
5.7.2
Observational Results
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
5.8
Coronal Flares in Time
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
5.8.1
Flare Energy Distributions and Coronal Heating
. . . . . . . . . . . . . . .
46
5.8.2
Phenomenological Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
5.8.3
Stellar Flare Energy Distributions
. . . . . . . . . . . . . . . . . . . . . . .
47
5.8.4
Stochastic Flares and Coronal Observations . . . . . . . . . . . . . . . . . .
48
5.8.5
Summary: The Importance of Stochastic Flares . . . . . . . . . . . . . . . .
49
5.9
The Solar Coronal Composition in Time . . . . . . . . . . . . . . . . . . . . . . . .
50
5.9.1
Abundances in Stellar Coronae . . . . . . . . . . . . . . . . . . . . . . . . .
50
5.9.2
The Composition of the Young Solar Corona
. . . . . . . . . . . . . . . . .
51
5.9.3
The Ne/O Abundance Ratio: Subject to Evolution?
. . . . . . . . . . . . .
51
5.10 Summary: The Young, Active Sun
. . . . . . . . . . . . . . . . . . . . . . . . . . .
53
2
6
Further Back in Time: The Pre-Main Sequence Sun
54
6.1
Where was the Cradle of the Sun?
. . . . . . . . . . . . . . . . . . . . . . . . . . .
54
6.2
New Features in the Pre-Main Sequence Sun
. . . . . . . . . . . . . . . . . . . . .
54
6.2.1
Evolutionary stages: Overview
. . . . . . . . . . . . . . . . . . . . . . . . .
54
6.2.2
New Features: Accretion, Disks, and Jets
. . . . . . . . . . . . . . . . . . .
55
6.2.3
New Emission Properties: Solar-Like or Not?
. . . . . . . . . . . . . . . . .
55
6.3
The T Tauri Sun
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
6.3.1
The Magnetic Field of the T Tauri Sun
. . . . . . . . . . . . . . . . . . . .
56
6.3.2
The Ultraviolet T Tauri Sun
. . . . . . . . . . . . . . . . . . . . . . . . . .
58
6.3.3
The X-ray T Tauri Sun in Time
. . . . . . . . . . . . . . . . . . . . . . . .
59
6.3.4
Coronal Excesses and Decits Induced by Activity?
. . . . . . . . . . . . .
60
6.3.5
X-Ray Flaring of the T Tauri Sun
. . . . . . . . . . . . . . . . . . . . . . .
62
6.3.6
The Radio T Tauri Sun in Time
. . . . . . . . . . . . . . . . . . . . . . . .
63
6.3.7
The Composition of the T Tauri Sun’s Corona
. . . . . . . . . . . . . . . .
63
6.4
The Protostellar Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
6.4.1
Magnetic Activity in the Protostellar Sun . . . . . . . . . . . . . . . . . . .
64
6.4.2
Magnetic Flaring of the Protostellar Sun . . . . . . . . . . . . . . . . . . . .
65
6.4.3
Radio Emission from the Protostellar Sun . . . . . . . . . . . . . . . . . . .
65
6.5
The Pre-Main Sequence Sun’s Environment in Time
. . . . . . . . . . . . . . . . .
66
6.5.1
Circumstellar Disk Ionization . . . . . . . . . . . . . . . . . . . . . . . . . .
66
6.5.2
Circumstellar Disk Heating
. . . . . . . . . . . . . . . . . . . . . . . . . . .
67
6.5.3
Observational Evidence of Disk Irradiation
. . . . . . . . . . . . . . . . . .
68
6.6
The T Tauri Sun’s Activity and Meteoritics . . . . . . . . . . . . . . . . . . . . . .
69
6.7
Summary: The Violent Pre-Main Sequence Sun . . . . . . . . . . . . . . . . . . . .
71
7
The Solar System in Time: Solar Activity and Planetary Atmospheres
72
7.1
The Faint Young Sun Paradox: Greenhouse or Deep Freeze? . . . . . . . . . . . . .
72
7.1.1
The Relevance of Greenhouse Gases
. . . . . . . . . . . . . . . . . . . . . .
73
7.1.2
A Bright Young Sun?
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
7.1.3
Cosmic Rays and a Stronger Solar Wind . . . . . . . . . . . . . . . . . . . .
76
7.2
The Sun’s Activity in the Young Solar System
. . . . . . . . . . . . . . . . . . . .
76
7.2.1
Planetary Atmospheric Chemistry Induced by High-Energy Radiation
. . .
76
7.2.2
High-Energy Radiation and the Planetary Biospheres in Habitable Zones
.
78
7.2.3
Planetary Atmospheric Loss and High-Energy Radiation and Particles . . .
79
7.2.4
Mercury: Erosion of Atmosphere and Mantle? . . . . . . . . . . . . . . . . .
81
7.2.5
Venus: Where has the Water Gone?
. . . . . . . . . . . . . . . . . . . . . .
81
7.2.6
Earth: A CO
2
Atmosphere and Magnetic Protection . . . . . . . . . . . . .
83
7.2.7
Mars: Once Warmer and Wetter? . . . . . . . . . . . . . . . . . . . . . . . .
83
7.2.8
Venus, Earth, Mars: Similar Start, Dierent Results?
. . . . . . . . . . . .
84
7.2.9
Titan: Early Atmospheric Blow-O? . . . . . . . . . . . . . . . . . . . . . .
85
7.2.10 Hot Jupiters: Mass Evolution by Evaporation?
. . . . . . . . . . . . . . . .
86
7.3
Summary: The High-Energy Young Solar System . . . . . . . . . . . . . . . . . . .
87
8
Summary and Conclusions
89
9
Acknowledgements
92
3
1
Introduction
The study of the past of our Sun and its solar system has become an interdisciplinary eort be-
tween stellar astronomy, astrophysics of star and planet formation, astrochemistry, solar physics,
geophysics, planetology, meteoritical science and several further disciplines. The interest in un-
derstanding 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
ares) 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 elds through
dynamo operation in the interior. These elds reach the surface where their presence is noticed
in the form of sunspots. However, magnetic activity has much more far-reaching consequences:
solar magnetic elds control essentially the entire outer solar atmosphere, they heat coronal gas to
millions of degrees, they produce ares 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 elds; 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 dierent 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 uxes 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 oer 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 young Sun’s more rapid rotation induced an internal magnetic dynamo that was much
more ecient than the present-day Sun’s. Consequently, stronger surface magnetic elds
and/or higher surface magnetic lling factors should have induced enhanced “activity” in
all its variations, from larger surface spots to a stronger, extended solar wind. If we can
observationally probe the outer magnetic activity of the Sun, we obtain invaluable diagnostics
for a deeper theoretical understanding of the internal dynamo.
•
The solar output largely controls planetary atmospheres and their climates. While this is
obviously true for the dominant optical and infrared emissions of a star like the Sun, the
irradiation of planetary atmospheres by higher-energy ultraviolet and X-ray photons as well
as interactions with high-energy particles and the solar wind leads to atmospheric alterations
that have been recognized and numerically simulated only recently. The much higher mag-
netic activity of the young Sun and the resulting higher levels of ultraviolet, X-ray, and
particle irradiation were therefore of prime importance for the early evolution of the planets.
4
 The discovery of extrasolar planets in particular around Sun-like stars has also spurred inter-
est in star-planet interactions, e.g., erosion of atmospheres or photochemical reactions, that
prot from detailed studies in the solar system.
•
Similarly, at still younger stages of a star’s evolution, its environment is rich in molecular
gas and dust, both in the form of a large envelope (in the youngest, protostellar phases
of evolution) and a circumstellar gas and dust disk (including the T Tauri phase when
planets were forming). Star-disk interactions are manifold, and their role is fundamental in
various respects. The optical and ultraviolet radiation heats the disk and therefore primarily
determines disk structure and the formation and evolution of planetary systems.
High-energy emission, in particular X-ray radiation, further heats and ionizes parts of the
circumstellar disk. Even moderate disk ionization will lead to accretion instabilities if weak
magnetic elds are present. Disk heating by X-rays may also produce extreme temperature
gradients across the disk that drive complicated chemical networks relevant for the later
processing of the disk material into forming planets and planetary atmospheres.
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 chromo-
sphere, 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 dierent
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 dierent from
those of the contemporaneous Sun (for example, its spectral type, or its photospheric eective
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, outows, 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) specically 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 Gudel (2004) and Gudel (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 inuence of the magnetic activity of the PMS Sun on its
environment, in particular on its circumstellar disk where our planets were forming. Wood (2004)
5
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