Solar corona

The comparison of coronal and photospheric abundances in cool stars is a very important tool in the interpretation of the physics of the corona. Active stars show a very different pattern to that followed by low activity stars such as the Sun, being the First Ionization Potential (FIP) the main variable used to classify the elements. The overall solar corona shows the so-called FIP effect the elements with low FIP (<10 eV, like Ca, N, Mg, Fe or Si), are enhanced by a factor of 4, while elements with higher FIP (S, C, O, N, Ar, Ne) remain at photospheric levels. The physics that yields to this pattern is still a subject of debate. In the case of the active stars (see [2] for a review), the initial results seemed to point towards an opposite trend, the so called Inverse FIP effect , or the MAD effect (for Metal Abundance Depletion). In this case, the elements with low FIP have a substantial depletion when compared to the solar photosphere, while elements with high FIP have same levels (the ratio of Ne and Fe lines of similar temperature of formation in an X-ray spectrum shows very clearly this effect). However, most of the results reported to date lack from their respective photospheric counterparts, raising doubts on how real is the MAD effect. 

The signals in the oxygen isotope record from the S. gigan-tea s rings at 6.69 and 37.4 years may or may not be related to periodicities, of the sun. The sun has many ways to vary, apart from the sun spot cycle, such as fluctuations in frequency of solar flares and plages, and misbehaviors of the overall magnetic field of the sun and of the solar corona. 

Equilibrium (i.e. local steady-state) ionization leads in this regime to solar-corona-like conditions where col-lisional ionization is balanced by recombination and the degree of ionization is fixed by the temperature alone, the electron density cancelling out. However, here departures from equilibrium occur because the time taken to establish ionization equilibrium is not negligible with respect to the timescale of expansion. 

In stellar astronomy, spectroscopic studies are indirectly revealing the structure deep down in stars where one could not see otherwise. An internal structure shows up in the emergent radiation spectrum both kinematically and through abundance anomalies. The solution of the solar corona line problem in 1942 may serve as a typical example of astrophysical applications of atomic spectroscopy [256]. 

Investigations on the doubly excited states of two electron systems under weakly coupled plasma have been performed by several authors. Such states usually occur as resonance states in electron atom collisions and are usually autoionizing [225]. Many of these states appear in solar flare and corona [226,227] and contribute significantly to the excitation cross-sections required to determine the rate coefficients for transitions between ionic states in a high temperature plasma. These are particularly important for dielectronic recombination processes which occur in low density high temperature plasma, occurring e.g. in solar corona. Coronal equilibrium is usually guided by the balance between the rates of different ionization and... 

A wide variety of plasma diagnostic applications is available from the measurement of the relatively simple X-ray spectra of He-like ions [1] and references therein. The n = 2 and n = 3 X-ray spectra from many mid- and high-Z He-like ions have been studied in tokamak plasmas [2-4] and in solar flares [5,6]. The high n Rydberg series of medium Z helium-like ions have been observed from Z-pinches [7,8], laser-produced plasmas [9], exploding wires [8], the solar corona [10], tokamaks [11-13] and ion traps [14]. Always associated with X-ray emission from these two electron systems are satellite lines from lithium-like ions. Comparison of observed X-ray spectra with calculated transitions can provide tests of atomic kinetics models and structure calculations for helium- and lithium-like ions. From wavelength measurements, a systematic study of the n and Z dependence of atomic potentials may be undertaken. From the satellite line intensities, the dynamics of level population by dielectronic recombination and inner-shell excitation may be addressed. 

Fig. 2. Sun-grazer Comet Ikeya Seki 1965 VIII passing through solar corona. Photographed with a coronagrph of Norikura Corona Station in Japan on October 21, 1965...

Emission spectroscopy of the solar corona, solar energetic particles (SEP) and the composition of the solar wind yield information on the composition of the Sun. Solar wind data were used for isotopic decomposition of rare gases. Coronal abundances are fractionated relative to photo-spheric abundances. Elements with high first ionization potential are depleted relative to the rest (see Anders and Grevesse, 1989 for details). 

During a solar eclipse in 1868, a new emission line, matching no known element, was found in the spectrum of the solar corona. J. N. Locklear and E. Frankland proposed the existence of a new element named, appropriately, helium (Greek, helios, sun). The same spectral line was subsequently observed in the gases of Mount Vesuvius. 

Plasma comprises the majority of the universe the solar corona, solar wind, nebula, and Earth s ionosphere are all plasmas. The best known natural plasma phenomenon in Earth s atmosphere is lightning. The breakthrough experiments with this natural form of plasma were performed long ago by Benjamin Franklin (Fig. 1-1), which probably explains the special interest in plasma research in Philadelphia, where the author of the book works at the Drexel Plasma Institute (Drexel University). 

Hydrogen atom emission spectra measured from the solar corona indicated that the 4s orbital was 102823.8530211 cm , and 3s orbital 97492.221701 cm , respectively, above the Is ground state. (These energies have tiny uncertainties, and can be treated as exact numbers for the sake of this problem.) Calculate the difference in energy (J) between these levels on the basis of these data, and compare this difference to that... 

Svante August Arrhenius (1859-1927), Swedish physical chemist, astrophysicist, professor at the Stockholm University, and originator of the electrolytic theory of ionic dissociation, measurements of the temperature of planets and of the solar corona, and also of the theory deriving life on Earth from outer space. In 1903, he received the Nobel Prize in chemistry for the services he has rendered to the advancement of chemistry by tis e/ec-trolytic theory of dissociation ... 

The physics definition of a Plasma is an ionized gas with an essentially equal density of positive and negative charges. It can exist over an extremely wide range of temperature and pressure. The solar corona, a lightning bolt, a flame, and a neon sign are all examples of electrical plasma. 

Coronal holes Regions in the solar corona in which the solar magnetic fields are not closed but are open into the interplanetary medium. 

Coronal mass ejection Expulsion of hot gas from a localized region in the solar corona into the interplanetary medium. 

Solar corona Region, beginning about 2000 km above the photosphere, with a temperature of over 1 milhon degrees Kelvin. 

Solar wind Expansion of the solar corona, primarily hydrogen ions, into the interplanetary mediiun. 

Solar flares are one example of violent solar activity. The other important one is the episodic injection into the interplanetary medium of large masses of matter from the solar corona. In a so-called coronal mass ejection (CME) event, approximately 10 kg or more mass of ions can be expelled from the corona. Such coronal transients were first identified from data acquired during the Sky lab space station era of solar research in the mid-1970s, and their studies were continued with other missions such as the Solar Maximum Mission (SMM) in the 1980s. The launch of the Solar and Heliospheric Observatory (SOHO) spacecraft in the mid-1990s revolutionized the studies of the sun, and especially the phenomena of CMEs. 

Measurements of the solar atmosphere by atomic spectroscopy techniques reveal that the outermost region—the solar corona from which CMEs are expelled from the sun—has a much lower density and a much higher temperature than the underlying surface (the photosphere). The... 

The spectrum of the solar corona and solar flares has been a rich source of accurate information on the energy structure of the ground configurations through the forbidden transitions which appear in these low-density sources. The information is limited, however, to elements of high cosmic abundance. This limitation does not exist for the tokamak plasma, which is similar to that of flares in temperature and density and where a large number of forbidden lines in elements up to Z = 42 have been discovered recently. 

Figure 7. Levels of the ground configuration in the silicon sequence with the wavelengths (A) of forbidden transitions observed in the solar corona (Z = 24 28) and in tokamaks (Z = 29>42) indicated.

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