This article discusses an unpublished paradigm by Athay that relates the general properties of the solar photosphere, chromosphere, and corona to the stream of photons, kinetic energy, and magnetic fields flowing from the solar interior. Using the Athay paradigm, we discuss the physics of the solar atmosphere and its coupling to the solar dynamo to clarify the connection of observed structures and variations in the three layers to their hydromagnetic interpretation. The details of the eleven-year cycles of solar activity are quite different, but each cycle exhibits two invariant features. First, the chromosphere and corona are always present above the photosphere in its turbulent state maintained by the radiative flux escaping at the surface as the solar luminosity. Second, the solar magnetic field is globally reversed early in each cycle, accompanied by systematic drifts in magnetic activity shown in the sunspot butterfly diagram of each cycle. We describe a scenario for the corresponding systematic changes in the upper solar atmosphere that recover the minimum-activity corona from one cycle to the next. We discuss in some detail the mechanisms that heat the atmosphere and process the magnetic flux continually emerging from the interior, providing a unified view of the interior-atmospheric system.

A chromosphere of partially ionized atoms in a turbulent, magnetic, solar-type star may be subject to a thermal instability due to ionization, which we propose in this paper. The instability occurs when changing magnetic fields cause ion heating so that a proton-electron pair are heated preferentially with respect to neutral atoms. This preferential heating, when partial ionization is present, causes any small temperature change to be amplified. An increase in temperature increases the ionization, which in turn increases the local heating. Similarly, a decrease in temperature decreases the ionization, which decreases the heating. The layer of instability occurs between two stable layers : the chromosphere, in which the ionization is too low, and the corona, in which the ionization is essentially complete. Such an atmosphere tends to be bimodal; it is either fully neutral or fully ionized. We use the models of Fontenla and coworkers to demonstrate the instability, but any magnetic stellar model in which ions are heated preferentially over neutral hydrogen will exhibit the same instability.

We suggest that the waxing and waning of chromospheric and coronal heating leads to a dynamic solar atmosphere which, under the right circumstances, may produce spicules. Little is known about the heating process. However, Anderson and Athay (1989a) concluded from their study of chromospheric heating that the heating rate per gram of chromospheric matter is only a small fraction of the heating rate per gram of coronal matter. We postulate that the increased heating rate in the corona is a consequence of heating charged particles as opposed to heating neutral atoms. This leads to a specific degree of hydrogen ionization at which coronal heating begins to predominate over chromospheric heating. It also introduces the likelihood that the waxing and waning of the heating rates will have relatively large consequences in the levels where hydrogen ionization is becoming significant. It is demonstrated that changes in the heating rates are capable of inducing increases and decreases in coronal mass comparable to the mass contained in a typical spicule.

Observations of O I lines in the solar spectrum are examined to determine whether differences in behavior of lines of the quintet and triplet term systems are consistent with collisional excitation and/or photoexcitation of both quintets and triplets. Intensities, I(IR), in near-infrared emission lines observed above the limb at total eclipse decrease exponentially with height h. The inverse scale heights (d ln I(IR)/dh) for the triplet lines at 844.6 nm and quintet lines at 777.2 nm are found to be in the ratio of 1.45. Ultraviolet O I emission-line intensities I(UV) observed on the solar disk show strong variations, and the distributions of triplet (130.4 nm) and quintet line intensities about the means are different. Variances in ln I(UV) are found to have a triplet-to-quintet ratio of 1.50, in close agreement with the ratio of d ln I(IR)/dh.It is shown that the simple assumption of collisional excitation of quintets and triplets coupled with collisional de-excitation of the quintets leads to the correct ratios for both the UV variances and d ln I(IR)/dh. Also, under this assumption d ln I(IR)/dh for the quintet lines is predicted to have the same value as d ln I/dh at the head of the hydrogen Balmer continuum, which, in fact, it does. On the other hand, Carlsson & Judge (1993) have shown that collision rates computed from the Vernazza, Avrett, & Loeser (1981, hereafter VAL) model chromosphere using current estimates of O I collision strengths are too low to produce the observed mean intensity in O I 130.4 nm. In a similar sense, we find that the predicted intensity of O I 130.4 nm is much too weak relative to O I 135.6 nm, and that the VAL mean models A-F cannot reproduce the observed behavior of these lines, even including photoexcitation by H Lybeta. These difficulties are removed by increasing specific electron-atom collision rates. Such increases could reflect (unacceptably?) large errors in atomic cross sections close to threshold and/or the inadequacy of the assumptions made by VAL for predicting line intensities. The nonlinear dependence of line intensities on temperature and density, especially for far-UV lines, makes the latter alternative a likely factor. We conclude that the O I UV lines are very sensitive to inhomogeneities, much more so than more traditional chromspheric lines (e.g., Mg II k) which are formed over similar regions of the chromosphere. Such lines could therefore provide valuable diagnostics of departures of the chromospheric plasma from mean models and thereby place constraints upon heating mechanisms, once accurate atomic data become available.

Spectroscopic data from two flights of the Naval Research Laboratory's High Resolution Telescope and Spectrograph (HRTS) are analyzed for evidence of variations in relative abundances in the low chromosphere. Comparisons of sunspot, active region, and quiet-Sun data from HRTS II reveal decreases of intensities of sunspot lines from the first ionization stages of elements with low first ionization potential relative to both the active region and the quiet Sun. C I lines, however, are more intense in the sunspot than either the active region or the quiet Sun. Within a sunspot in Spacelab II data, the C I line at 156.1 nm is near its average intensity, whereas the Fe II line at 156.3 is much weaker than average. Both spots suggest a relative high value for the C I/Fe II abundance ratio. Within the zone of the same magnetic polarity as the sunspot (leading polarity) in the Spacelab II data, the brightest plages in C I show large C I/Fe II intensity ratios similar to those found in the sunspot. By contrast, the zones of following polarity on either side of the leading polarity show several well-defined areas of unusually low C I/Fe II intensity ratios associated with dark features in C I. The plages within these same zones have near normal or somewhat enhanced values for the C I/Fe II intensity ratios. It is noteworthy, also, that many of the brightest areas in C I do not coincide spatially with the brightest regions in Fe II. Neither do the darkest areas in C I align well with the darkest areas in Fe II. The association of high C I/Fe II intensity ratios with the zone of leading polarity and low-intensity ratios with zones of following polarity suggests that the iron abundance is dependent on the field polarity and is relatively low in the sunspot and the brighter plages in the zone of leading polarity and relatively high in C I dark flocculi in zones of following polarity. Failure of the brightest and darkest features in C I to align with the brightest and darkest features in Fe II further suggests that in the case of these particular features the Fe II abundance may vary inversely with the gas pressure, as well as with magnetic polarity. Other possible interpretations are discussed but are considered as being less viable.

Velocity data obtained from Doppler shifts of EUV spectral lines observed with the High Resolution Telescope and Spectrograph flown on Spacelab II are analyzed to determine the properties of the mean velocities VBAR and its spatial variance V-sigma, for chromospheric and transition region lines. The lines studied include the ions O I, C I, Fe II, C II, Si III, Si IV, and C IV. Observations were made using 11 radial slit positions covering a rectangular area approximately 850" in length and 30" wide with a spatial resolution of 1" and 3" spacing between slits. The constancy of V-sigma from center to limb together with a systematic maximum redshift in VBAR in central disk regions is interpreted to mean that the plasma flow is divergent from the tops of closed field lines and that the cool plasma more or less fills the magnetic loops from which most of the radiation originates. The same is true of the hotter plasma. It is suggested that the crossfield component of del T is large and that the alternation from cool loops to hot loops occurs on spatial scales much less than 1". Each fine-structure loop within the temperature range characterizing a given ion is assumed to have a peculiar flow velocity, V(i). The dispersion in V(i) is identified with the non-thermal line width, zeta, and the mean V(i) is identified with VBAR and its large-scale spatial fluctuations, V-sigma.

EUV emission-line intensities from the HRTS experiment on Spacelab 2 are analyzed to determine the causes of intensity variations from point to point on the solar surface. Chromospheric lines of O I and transition region lines of C II, Si IV, and C IV exhibit saturation effects that limit the line intensities in the brighter regions. No such effect is found in lines of C I, Fe II, or Si III. The chromospheric lines saturate because the optical thickness exceeds the thermalization depth, whereas the transition region lines saturate because of the geometry of the unresolved fine structure. The latter is assumed to consist of elongated cylinder-like features oriented along magnetic lines of force that are mainly vertical. Saturation occurs when the optical thickness along the axis of the cylinders exceeds unity. Estimates based on the observed saturation effects give fill factors for the fine structure of 5 x 10(-3) and 0.4 at 10(5) K and 3 x 10(4) K, respectively. The corresponding lengths of the cylinders are estimated at 1000 km and 40 km at the two temperatures. It is argued that in both chromospheric and transition region lines changes in density, temperature, and the volume of the emitting material each play important roles in the intensity changes from the one point to another on the solar disk. Evidence suggests that changes in the volume of the emitting material overshadow the effects of density and temperature changes. Intensity ratios of line pairs are shown to provide a reliable basis for density diagnostics if both lines are free from saturation effects and if they are formed at similar temperatures.

view Abstract Citations (18) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Bifurcation in the Low Chromosphere Athay, R. Grant ; Dere, K. P. Abstract Bright chromospheric faculae of diameter d(c) covering an area of the solar disk f(c) smaller than 1 yield a predictable level of limb brightening. For a resolution diameter d(r), the combination of f(c) and d(c) leads, also, to a prediction of the fraction, phi(0), of observing pixels (resolution areas) that are free of facular emission. Data from the first flight of the High Resolution Telescope and Spectrograph of the U.S. Naval Research Laboratory with d(r) about 1 arcsec are used to measure both the level of limb brightening and phi(0) in EUV emission lines of O I, C I, and Fe II. The results favor f(c) about 0.9 and d(c) greater than about 2.5 arcssec. It is concluded that the chromospheric temperature rise is present over about 90 percent of the solar surface. Publication: The Astrophysical Journal Pub Date: August 1990 DOI: 10.1086/169025 Bibcode: 1990ApJ...358..710A Keywords: Chromosphere; Extreme Ultraviolet Radiation; Faculae; Limb Darkening; Line Spectra; Solar Limb; Brightness Temperature; Carbon; Iron; Oxygen; Solar Physics; SUN: CHROMOSPHERE; SUN: FACULAE; SUN: LIMB DARKENING full text sources ADS |

Astrophysicist R. Grant Athay views the sun and stars as the most valuable laboratories for studying the basic physical processes of the universe. He describes the sun's physical properties, including its chemical makeup, core, corona, energy output, and life span. He interprets scriptural creation accounts from Genesis and the book of Moses in the Pearl of Great Price.

view Abstract Citations (39) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Temperature and Center-Limb Variations of Transition Region Velocities Athay, R. G. ; Dere, K. P. Abstract HRTS data from the Spacelab 2 mission are used to derive the center-limb and temperature variations of the mean velocity and the velocity variance in the solar chromosphere and transition zone. The mean velocity is found to vary much more rapidly from center to limb and with temperature than does the velocity variance. Also, the mean velocity shows a characteristic signature at some magnetic neutral lines in accordance with the findings of Klimchuk (1987) from Solar Maximum Mission (SMM) data. The velocity variance does not show a characteristic signature at the neutral lines but shows an inverse correlation with intensity. The latter is interpreted as reduced velocity variance in strong field regions. The results are discussed in terms of downflow along lines of force in magnetic arcades. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/168033 Bibcode: 1989ApJ...346..514A Keywords: Plasma Dynamics; Solar Atmosphere; Solar Limb; Solar Temperature; Chromosphere; Coronal Loops; H Alpha Line; Solar Magnetic Field; Solar Maximum Mission; Sunspots; Solar Physics; SUN: ATMOSPHERIC MOTIONS; SUN: ROTATION; SUN: TRANSITION REGION full text sources ADS |

view Abstract Citations (104) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Chromospheric and Coronal Heating Anderson, Lawrence S. ; Athay, R. Grant Abstract From computations for a theoretical model chromosphere matched to the empirical model of Vernazza, Avrett, and Loeser published in 1981, it is concluded that the required magnitude and mass dependence of the heat input are compatible with heating by sound waves whose velocity amplitude is near the sound speed. However, the required rate of heat input per gram increases markedly at the top of the neutral chromosphere, and a different form of heating appears necessary in the corona and transition region. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/167078 Bibcode: 1989ApJ...336.1089A Keywords: Atmospheric Heating; Chromosphere; Heat Flux; Solar Corona; Sound Waves; Acoustic Excitation; Radiative Heat Transfer; Solar Atmosphere; Temperature Distribution; Solar Physics; RADIATIVE TRANSFER; SUN: CHROMOSPHERE; SUN: CORONA; SUN: TRANSITION REGION full text sources ADS |

view Abstract Citations (129) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Model Solar Chromosphere with Prescribed Heating Anderson, Lawrence S. ; Athay, R. Grant Abstract Computed model solar chromospheres for prescribed departures from radiative equilibrium are specified in terms of the local mechanical (nonradiative) heat input. The computations are fully non-LTE and include millions of spectral lines. From the variety of models considered, the requirements on the heat input for a positive temperature gradient, dT/dh, in different layers of the chromosphere are discussed. The derived radiative cooling function for different models show that the cooling function is model-dependent. By comparing the computed models with the Vernazza, Avrett, and Loeser (1981) models, it is shown that the VAL-C model is characterized by a total heat flux 1.4 x 10 to the 7th ergs/sq cm/per sec, most of which is dissipated near the base of the temperature plateau. Half of the radiation loss is provided by Fe II, with Ca II, Mg II, and H playing important, but secondary, roles. CO molecules and H(-) are of lesser importance even near the temperature minimum, except in cases of minimal heating. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/168083 Bibcode: 1989ApJ...346.1010A Keywords: Atmospheric Heating; Chromosphere; Solar Atmosphere; Calcium; Carbon Monoxide; Lyman Alpha Radiation; Radiative Transfer; Solar Spectra; Stellar Models; Solar Physics; OPACITIES; SUN: CHROMOSPHERE full text sources ADS |

Gradients of Hα and electron scattering intensities derived from instantaneous radial distributions of erupting prominence material observed at several solar radii by Illing and Athay (1986) are often markedly smaller than those inferred by comparing the intensities observed near several radii to average prominence intensities observed near the limb. In this paper, we show that gradients derived by following individual features in their outward progression with time yield values that are consistent with limb observations and that usually exceed the values obtained from instantaneous distributions. We conclude from the diversity of observed gradients that the prominence eruption cannot be described by a self-similar expansion in which the expansion velocity is a function of radius and time only. However, we cannot rule out possible self-similar solutions that allow the expansion velocity to be a function of angular direction.

The existence of the solar wind requires a net outflow of material at all depths in the solar atmosphere. Thus, the observation of a prevalent downflow at temperatures near 105 K1,2 presents a considerable challenge. Two classes of flow models have been suggested, each involving flow along closed magnetic-field lines that extend above the chromosphere into the hotter transition region and corona. Type-(l) models3,4,5 assume unidirectional equilibrium flows along field lines and rely on asymmetry in the flow and plasma properties to produce an apparent downflow. Type-(2) models2,6, by comparison, assume a series of episodic upflows and subsequent downflows along each line of force and rely on asymmetry in the plasma properties and flow durations to produce a statistically prevalent downflow. Here I argue that observational evidence strongly suggests that both types of flow are present but that type-(2) flow is predominant.

view Abstract Citations (13) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Magnetic and Velocity Structure Adjacent to Solar Active Regions Athay, R. Grant ; Klimchuk, J. A. Abstract Results from a number of earlier papers relating velocity patterns observed in the C IV line at 154.8 nm to photospheric magnetic-field patterns are combined to develop a qualitative model of the magnetic-field geometry outside of the strong field areas of active regions. The motion is assumed to originate at the crests of magnetic arcades and flow downward along field lines, which are assumed to be elliptical in shape with the major axis in the photosphere. It is found that the ratio of the major axis to the minor axis of the ellipse must be less than two for fields under 100 G. Also, it is concluded that the magnetic neutral surfaces defined by the loci of horizontal field lines are often tilted at a large angle to the vertical at the altitude of the C IV emission. Publication: The Astrophysical Journal Pub Date: July 1987 DOI: 10.1086/165380 Bibcode: 1987ApJ...318..437A Keywords: Kinetic Energy; Solar Atmosphere; Solar Flares; Solar Magnetic Field; Ultraviolet Spectra; Flow Distribution; H Alpha Line; Line Of Sight; Velocity Distribution; Solar Physics; SUN: ATMOSPHERIC MOTIONS; SUN: MAGNETIC FIELDS; ULTRAVIOLET: SPECTRA full text sources ADS |

view Abstract Citations (10) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic Shear. IV. Hale Regions 16740, 16815, and 16850 Athay, R. G. ; Klimchuk, J. A. ; Jones, H. P. ; Zirin, H. Abstract Dopplergrams made in C IV 1548 A are studied for evidence of velocity shear near H-alpha dark filaments and for large-scale flow convergent on active regions. The three regions studied support earlier conclusions that shear is a common property of active regions and that active regions may be the foci of converging plasma flow. Flow patterns near filaments show divergence or convergence as well as shear. Also the sense of the shear can be either cyclonic or anticyclonic. No preference is noted for convergence or divergence or for a particular sense of shear, and there appears to be no correlation between the sense of the shear and the sign of the velocity gradient normal to the filament. The close association of H-alpha dark filaments with shear lines leads to the suggestion that the filaments may arise from a cooling instability induced by the Bernoulli effect. Publication: The Astrophysical Journal Pub Date: April 1986 DOI: 10.1086/164136 Bibcode: 1986ApJ...303..884A Keywords: Atmospheric Circulation; Solar Atmosphere; Solar Magnetic Field; Sunspots; Doppler Effect; Magnetohydrodynamic Flow; Shear Flow; Velocity Distribution; Solar Physics; SUN: ATMOSPHERIC MOTIONS; SUN: MAGNETIC FIELDS; SUN: SUNSPOTS full text sources ADS | Related Materials (3) Part 1: 1985ApJ...288..363A Part 2: 1985ApJ...291..344A Part 3: 1986ApJ...303..877A

Coronal mass ejections detected with the Solar Maximum Mission coronagraph/polarimeter are often accompanied by erupting prominence material observed both in Hα and in the electron scattering continuum. Hα emission is concentrated in bright filaments moving radially outward. The same filaments are seen in the electron scattering continuum as regions of enhanced brightness. In this paper we develop a diagnostic method based on the observed Hα and continuum brightness to derive the electron density, line of sight thickness, and degree of ionization of hydrogen as functions of the temperature of the prominence filaments. Our method differs from that of Poland and Munro (1976) in the treatment of Ly α excitation. Analysis of data from the event of August 18, 1980, illustrates that the rising prominence material has decreased density, increased temperature, and increased ionization of hydrogen relative to quiescent prominences in the lower corona. Hydrogen is found to be 90–99% ionized, electron densities are near 108 cm−3, and the temperature is near 20,000 K. The increased ionization is due mainly to the decreased density. Use of the results is made here and in an accompanying paper by Illing and Hundhausen to determine the total mass ejected in the event.

view Abstract Citations (73) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation Loss Rates in Lyman Alpha for Solar Conditions Athay, R. G. Abstract Mean radiation loss rates per unit volume in Lyman-alpha are computed for solar conditions as a function of the optical thickness of the radiating region. No significant corrections to the optically thin coronal approximation are found until the Lyman-alpha optical thickness at line center exceeds 100. At larger optical thicknesses the corrections can be large as a result of the increased ionization of hydrogen from absorptions in the Balmer continuum. The correction suggested by McClymont and Canfield (1983) is found to be too large for most solar applications. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164565 Bibcode: 1986ApJ...308..975A Keywords: Energy Dissipation; Lyman Alpha Radiation; Optical Thickness; Radiative Transfer; Solar Radiation; Absorption Spectra; Balmer Series; Hydrogen Ions; Solar Corona; Solar Spectra; Solar Temperature; Solar Physics; RADIATIVE TRANSFER; SUN: CORONA; SUN: SPECTRA; ULTRAVIOLET: SPECTRA full text sources ADS |