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Related Theories of Convection. Global Circulation. Global circulation theory. Flux Tubes and Network.

Sunspots, Active Regions, and Global Patterns. Theories of the Solar Magnetic Field. Flux Tubes and Network Active Regions and Sunspots. Solar Dynamo Theory. Concluding Remarks Acknowledgments. Eddington-Sweet Circulation. Rotationally Driver Instabilities. An Internal Magnetic Field. Theory of Waves and Oscillations. Basic Equations. Full hydromagnetic equations.

Physics of the Sun: A First Course

Linearized equations. Nonmagnetic waves. Magnetohydrodynamic waves. General Properties of Solar Oscillations. Equations and spheroidal mode solutions. Cowling approximation. Asymptotic behavior of p- and g-mode frequencies. Radial oscillations. Properties of nonadiabatic solutions. Toroidal oscillations. Excitation and Damping of Solar Pulsations. Excitation and damping mechanisms. Mode lifetimes. Stability of solar pulsation modes.

Detailed Solutions for Frequencies and Frequency Splitting. Effects of structure on unperturbed frequencies. Effects of rotation. Effects of internal magnetic fields. Future Theoretical Needs. Landi, E. Neon and oxygen abundances and abundance ratio in the solar corona. Woods, T. Extreme ultraviolet variability experiment EVE on the solar dynamics observatory SDO : Overview of science objectives, instrument design, data products, and model developments.

Tapping, K. The Space Weather 11 , — Laming, J. Stellar coronal abundances. Baker, D. FIP bias evolution in a decaying active region. Wood, B.

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Living Rev. Audard, M. A study of coronal abundances in RS CVn binaries. Robrade, J. Neon and oxygen in low activity stars: towards a coronal unification with the Sun. Favata, F. The X-ray cycle in the solar-type star HD XMM-Newton observations and implications for the coronal structure. Vidotto, A. Stellar magnetism: empirical trends with age and rotation. Baliunas, S. A dynamo interpretation of stellar activity cycles. Multiple and changing cycles of active stars. Magnetic cycles at different ages of stars. Sanz-Forcada, J. Three years in the coronal life of AB Dor.

Plasma emission measure distributions and abundances at different activity levels. Lalitha, S. X-ray activity cycle on the active ultra-fast rotator AB Doradus A? Implication of correlated coronal and photometric variability. Nordon, R. Variability of a stellar corona on a time scale of days. Evidence for abundance fractionation in an emerging coronal active region.

Drake, J. Warren, H. Measurements of absolute abundances in solar flares.

Volume II: The Solar Atmosphere

Pesnell, W. The solar dynamics observatory SDO. Culhane, J. The EUV imaging spectrometer for Hinode. Brown, C. Brooks, D. Full-sun observations for identifying the source of the slow solar wind. Grevesse, N.


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The solar chemical composition. Space Sci. Kashyap, V. Markov-Chain Monte Carlo reconstruction of emission measure distributions: application to solar extreme-ultraviolet spectra. India 28 , — Dere, K. Del Zanna, G. Version 8. Evidence for the first ionization potential effect in Alpha Centauri. Allende Prieto, C. S4N: A spectroscopic survey of stars in the solar neighborhood. The nearest 15 pc. Telleschi, A. Coronal evolution of the sun in time: High-resolution X-Ray spectroscopy of solar analogs with different ages. Liefke, C. Coronal properties of the EQ Pegasi binary system.

Thermal conductivity and element fractionation in EV Lac. The coronal abundance anomalies of M Dwarfs. The coronal abundances of Mid-F Dwarfs. New insights from old solar X-rays: a plasma temperature dependence of the coronal neon content. Schmelz, J. Anomalous coronal Neon abundances in quiescent solar active regions. Download references. We thank Brian Wood for helpful comments on the paper. The work of D. The analysis software was written by H.

All authors contributed to the discussion of the final results and the presentation of the manuscript. Correspondence to David H. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Reprints and Permissions. The Astrophysical Journal Journal of Atmospheric and Solar-Terrestrial Physics By submitting a comment you agree to abide by our Terms and Community Guidelines.

If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Advanced search. Skip to main content. Subjects Solar physics Stars. Abstract The elemental composition in the coronae of low-activity solar-like stars appears to be related to fundamental stellar properties such as rotation, surface gravity, and spectral type.

Physics of the Sun: Volume I: The Solar Interior / Edition 1

Introduction Knowledge of the elemental composition of the Sun underpins our understanding of the transport of energy from the deep interior, through the outer atmosphere, and into the heliosphere. Results Solar observations Whether the implications for stellar observations are significant or not depends on bridging the solar—stellar connection more directly. Full size image. Discussion Our finding of a coronal composition variation in Sun-as-a-star full disk-integrated spectra, which is correlated with the solar cycle, is intuitively consistent with the area coverage of different features quiet Sun, active region, coronal holes , and their consequent contribution to coronal composition, changing with the solar cycle.

Methods Comparison of coronal composition and F Data sources The F Uncertainties and comparisons with previous work In order to determine the uncertainty in the measurements, we conducted an experiment where we re-computed the FIP bias in separate simulations for the data set used in Supplementary Fig.

Data availability The data that support the findings of this study are available from the corresponding author upon request. References 1. Article Google Scholar ADS Google Scholar Article Google Scholar Download references. Acknowledgements We thank Brian Wood for helpful comments on the paper. Author information Author notes David H. When the plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation.

There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region for a thermal instability , its dark filaments obvious against the solar disk or prominences off the Sun's limb. Coronal loops may have lifetimes in the order of seconds in the case of flare events , minutes, hours or days.

Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops. Coronal loops are very important to our understanding of the current coronal heating problem. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE. An explanation of the coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present i.

In-situ measurements are required before a definitive answer can be had, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions. They were first detected in the June 8, flare observation during a rocket flight. The large-scale structure of the corona changes over the year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration plus a quadrupolar component.

The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare. Some other features of this kind are helmet streamers —large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions.


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Coronal streamers are considered to be sources of the slow solar wind. They were first observed in the two rocket flights which also detected coronal holes. Filament cavities are cooler clouds of gases plasma suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense.

It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere. Bright points are small active regions found on the solar disk. X-ray bright points were first detected on April 8, during a rocket flight.

The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1. The variations in temperature are often correlated with changes in the X-ray emission. Coronal holes are the Polar Regions which look dark in the X-rays since they do not emit much radiation. The high speed solar wind arises mainly from these regions. In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind.

These are the coronal plumes. More exactly, they are long thin streamers that project outward from the Sun's north and south poles. The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun. The equatorial region has a faster rotation speed than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle , while they almost disappear during each minimum.

Therefore, the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle. Approaching the minimum of the solar cycle also named butterfly cycle , the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are coronal holes. A portrait as diversified as the one already pointed out for the coronal features is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve in times very different among them.

Studying the coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table. Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.

Flares are impulsive phenomena, of average duration of 15 minutes, and the most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature. An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.

However, two kinds of basic structures can be distinguished: [17]. As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event:.

Accompanying solar flares or large solar prominences , "coronal transients" also called coronal mass ejections are sometimes released. These are enormous loops of coronal material that travel outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger ejections can propel hundreds of millions of tons of material into space at roughly 1.

Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung—Russell diagram. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group [20] showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun.

The O-B stars , which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths.

Thus they appear to have neither chromospheres nor coronae. According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour. The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization than that found in the photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons.

The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere, but by collisional equilibrium in the optically-thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.

The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.


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However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average for the equipartition theorem , electrons have a mass roughly 1 times smaller than protons, therefore they acquire more velocity.

Metal ions are always slower. This fact has relevant physical consequences either on radiative processes that are very different from the photospheric radiative processes , or on thermal conduction.

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Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields. Magnetohydrodynamic waves MHD waves can also propagate in this plasma, [21] even if it is not still clear how they can be transmitted or generated in the corona.

The plasma is transparent to its own radiation and to that one coming from below, therefore we say that it is optically-thin. The gas, in fact, is very rarefied and the photon mean free-path overcomes by far all the other length-scales, including the typical sizes of the coronal features. Different processes of radiation take place in the emission, due to binary collisions between plasma particles, while the interactions with the photons, coming from below; are very rare.

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Because the emission is due to collisions between ions and electrons, the energy emitted from a unit volume in the time unit is proportional to the squared number of particles in a unit volume, or more exactly, to the product of the electron density and proton density. In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.