One- and multi-component models of the upper photosphere based on molecular spectra

Spectroheliograms taken in the CN(0, 0) violet band near 3883 Å show very small scale network and cell structures with high contrast. The bandhead itself, which is a broad feature due to overlap of several CN lines, allows the diagnostic simplicity of a continuum since motions, magnetic fields, and broadening mechanisms are unimportant. We have obtained spectroheliograms in the bandhead and center-to-limb photoelectric spectra of CN(0, 0) at Kitt Peak National Observatory. From the photoelectric spectra and a detailed analysis of the formation of the CN(0, 0) spectrum we derive a best-fit one-component upper photospheric model differing from that of the HSRA and recommend a change in solar carbon abundance from the HSRA value of logA _c = 8.55 to logA _c = 8.25. From the calibrated spectroheliograms we consider a multi-component model to account for the observed fine structure intensity variations.Staff Member, Laboratory Astrophysics Division, National Bureau of Standards.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

One- and multi-component models of the upper photosphere based on molecular spectra

We have obtained center-to-limb photoelectric spectra of the CN(1,1) B-X bandhead region λ3868–3872 Å at Kitt Peak National Observatory. From these spectra and a detailed analysis of the formation of the CN (1, 1) spectrum we derive a best-fit upper photospheric model differing from the HSRA which is consistent with our previous CN(0, 0) λ3883 spectra. We derive a solar carbon abundance of log A _c = 8.30 ± 0.10 compared to the HSRA value of log A _c = 8.55 ± 0.10. In addition we specify the regions of formation for the CN(0, 0) λ3883.35 and CN(1, 1) λ 3871.38 bandheads at disc center and limb.     

On the occurrence of convective motions in the upper photosphere

We have examined whether the motion field in the photosphere in the range of optical depths 0.25< ₀< 0.6 is dominated by thermal convection or by vibrations. The observed asymmetries of infrared Fraunhofer lines indicate the presence of motions, and the fact that the asymmetry is zero for lines of low excitation and increases with the excitation potential shows that these motions are chiefly convective in this part of the photosphere: upward moving elements appear to be hotter than downward moving ones.Assuming furthermore that the photosphere can be described by a three-column model, with temperature differences as given by Edmonds (1967), we find that in the range of optical depths given above, where T seems to vary between 80 and 160 °K, average convective velocities of 2.3 to 3.2 km/sec should occur. This result is in numerical agreement with (a) a previous one by the present authors (1967) derived from the variation of line asymmetry with depth in lines of one multiplet, (b) a finding by Lambert and Mallia (1968) deduced from absolute wavelength measurements of Fraunhofer lines, and (c) a recent result of Beckers (1968) found from a comparison of two granulation pictures obtained simultaneously with a narrow-band filter centred on the two wings of a faint line.     

Temperature variation with latitude in the upper solar photosphere: Relevance to solar oblateness measurements and facular models

Solar Physics - Altrock and Canfield's observations of temperature variation with latitude in the upper solar photosphere refer to higher levels (smaller optical depths) than those to which...     

Thermal bifurcation in the upper photosphere inferred from heterodyne spectroscopy of OH rotational lines

We have obtained low noise (S/N > 10³), high spectral resolution (/ 10 ⁶) observations of two pure rotation transitions of OH from the solar photosphere. The observations were obtained using the technique of optically null-balanced infrared heterodyne spectroscopy, and consist of center-to-limb line profiles of a = 1 and a = 0 transition near 12 m. These lines should be formed in local thermodynamic equilibrium (LTE), and are diagnostics of the thermal structure of the upper photosphere. We find that the = 0R22(24.5)e line strengthens at the solar limb, in contradiction to the predictions of current one-dimensional photospheric models. Our data for this line support a two-dimensional model in which horizontal thermal fluctuations of order ±800 K occur in the region ₅₀₀₀ 10^–3–10^–2. This thermal bifurcation may be maintained by the presence of magnetic flux tubes, and may be related to the solar limb extensions observed in the 30–200 m region.Observations of the = 1R11(29.5)f line, at 885.643 cm^–1, show that it is anomalously weak in the photospheric spectrum. We argue that the source function in the core of this line has been substantially increased by interaction with the 9j-7i transition of Mgi at 885.524 cm^–1, which is itself too weak to appear in the disk center spectrum.Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.     

The current sheet and Joule heating of a slender magnetic tube in the upper photosphere

Joule heating in a slender magnetic flux tube is investigated. The distribution of the magnetic field and electric sheet current encircling a vertical cylindrical magnetic tube is determined by equating the converging magnetic flux, which results from the converging and downward flow of the granulation, and the dissipative expanding magnetic flux due to Ohmic decay. Here, to ensure the mass flux conservation, an overshooting convective flow pattern resembling recent simulations was assumed. Even with the electrical resistivity from neutral hydrogen, the width of the current sheet was found to be 2 km, being much smaller than the tube diameter of 150 km, either from an exact or approximate (Gaussian) field distribution.The resultant energy flux density due to Joule heating averaged over the cylindrical cross sectional area, is 1 × 10⁹ erg cm^-2 s^-1 for an assumed photospheric magnetic field of 1500 G. This amount may supply enough energy to heat the temperature minimum region of the flux tube by T = 300 K in accord with observations, though our estimation of the excess radiation loss which should be supplied by the Joule heating to keep T = 300 K is rather uncertain.A possible role of the Joule heating on spicule formation is briefly discussed together with discussions on the slab geometry, general flow patterns, and non-constant field distributions inside the flux tube.     

Two-component photosphere models of a 2N/M2-class solar flare

Physical state of the photosphere during a 2N/M2 solar flare on July 18, 2000, was studied. We used Echelle Zeeman spectrograms obtained by V. G. Lozitsky in orthogonal circular polarizations with a solar spectrograph. Semiempirical photospheric models were constructed for three moments in time in the initial and main phases of the flare using the SIR code applied to Stokes I and V profiles of seven iron and chromium lines. The photospheric model of the flare contains two components: a magnetic-field component and nonmagnetic environment. The height distributions of the temperature, magnetic field, and line-of-sight velocity were derived. The temperature in the nonmagnetic component had a nonmonotonous run with height. The models include layers in the middle and upper photosphere in which temperature is enhanced relative to an unperturbed photosphere model. As the flare developed, the temperature in the lower layers was increasing by 500–800 K. The magnetic field increased by 0.05 T and 0.08–0.1 T in the lower and upper photosphere during the flare, respectively, with the vertical temperature gradient decreasing from 0.0012 to 0.0008 T/km. The model for the onset phase of the flare indicates that there were upflows and downflows of substance in the lower and upper photosphere, respectively. The flow velocities decreased appreciably in the main phase of the flare. The model parameters of the nonmagnetic environment were only slightly different from those of the unperturbed photosphere.     

Inhomogeneous model of the photosphere

A new method for the construction of empirical models of photosphere is developed starting with a first approximation called ‘multicolumn’. The old idea of two or three column models is thus kept but completed by the following ideas: - A good photograph of granulation is a fair representation of topological features along x and y. - Vertical distributions T(z), P(z) cannot be very much different from distributions given by stellar atmosphere models in radiative and hydrostatic equilibrium. The sampling interval (200 km) is such that a ‘granule’ is represented by 10 to 30 columns. Horizontal transfer is computed explicitly at each point. Corrections are introduced in the model but are not of significant importance: temperature differences of 1000 K or more at the same geometrical depth in the τ = 1 region are compatible with a life time of the order of 10 min. The importance of pressure is emphasized: significant pressure excess in the granule is suggested a priori. Results tend to confirm this hypothesis.     

Solar force-free magnetic fields on and above the photosphere

If the problem of a magnetic field being force-free with = constant ( 0) is solved by some previously published methods, then the field obtained in the whole exterior of the Sun cannot have a finite energy content and the solution cannot be determined uniquely from only one magnetic field component given at the photosphere. A magnetic field in the volume between two parallel planes has been investigated by us (Chen and Wang, 1986).Based on observational data we present in this paper a suitable physical model for a half-space and adopted an integral transform established by us (Chen, 1980, 1983) to solve this problem. We then obtain a unique analytical solution of the problem from only one magnetic field component (longitudinal field observed) given at the photosphere. Not only the uniqueness of the solution has been proved but also the finiteness of magnetic energy content in the half-space considered has been verified. We have demonstrated that there is no singular point in the solution. It enables us to describe analytically the configurations of magnetic fields on and above the photosphere.     

Density models for the upper atmosphere

Modeling the effects of atmospheric drag is one of the more important problems associated with the determination of the orbit of a near-earth satellite. Errors in the drag model can lead to significant errors in the determination and prediction of the satellite motion. The uncertainty in the drag acceleration can be attributed to three separate effects: (a) errors in the atmospheric density model, (b) errors in the ballistic coefficient, and (c) errors in the satellite relative velocity. In a number of contemporary satellite missions, the requirements for performing the orbit determination and predictions in near real-time has placed an emphasis on density model computation time as well as the model accuracy. In this investigation, a comparison is made of three contemporary atmospheric density models which are candidates for meeting the current orbit computation requirements. The models considered are the analytic Jacchia-Roberts model, the modified Harris-Priester model, and the USSR Cosmos satellite derived density model. The computational characteristics of each of the models are compared and a modification to the modified Harris-Priester model is proposed which improves its ability to represent the diurnal variation in the atmospheric density.This investigation was supported by the NASA Goddard Spaceflight Center under contract NAS5-20946 and Contract NSG 5154.     

Location of energy source for coronal heating on the photosphere

It is reported that ultra-fine dynamic ejections along magnetic loops of an active region originate from intergranular lanes and they are associated with subsequent heating in the corona. As continuing work, we analyze the same set of data but focus on a quiet region and the overlying EUV/UV emission as observed by the Atmospheric Imaging Assembly(AIA) on board Solar Dynamics Observatory(SDO). We find that there appear to be dark patches scattered across the quiet region and the dark patches always stay along intergranular lanes. Over the dark patches, the average UV/EUV emission at 131, 171, 304 and 1600 (middle temperature) is more intense than that of other regions and EUV brightness is negatively correlated with 10830  intensity, though, such a trend does not exist for high temperature lines at 94, 193, 211 and 335 . For the same quiet region, where both Ti O 7057  broad band images and 10830  filtergrams are available, contours for the darkest lane areas on Ti O images and dark patches on 10830  filtergrams frequently differ in space. The results suggest that the dark patches do not simply reflect the areas with the darkest lanes but are associated with a kind of enhanced absorption(EA) at 10830 . A strict definition for EA with narrow band 10830  filtergrams is found to be difficult. In this paper, we define enhanced absorption patches(EAPs) of a quiet region as the areas where emission is less than ~90% of the mean intensity of the region. The value is equivalent to the average intensity along thin dark loops connecting two moss regions of the active region. A more strict definition for EAPs, say 88%, gives even more intense UV/EUV emission over those in the middle temperature range. The results provide further observational evidence that energy for heating the upper solar atmosphere comes from the intergranular lane area where the magnetic field is constantly brought in by convection motion in granules.     

On the differential rotation of the solar photosphere

It is shown that sunspots as tracers can give the same results for the differential rotation of the solar photosphere as the Doppler-shift measurements, if the sunspots used have only insignificant motion relative to their immediate photospheric surroundings.     

The Bilderberg model of the photosphere and low chromosphere

From 17 through 21 April 1967, an international study week was held in the Bilderberg near Arnhem, Netherlands, with the aim of obtaining an internationally acceptable model of the solar photosphere and low chromosphere. It was found that such a model, based on observed intensities and center-to-limb observations of the solar continuous spectrum, could indeed be established. This model, henceforth called the Bilderberg Continuum Atmosphere (BCA), is shown in Table I, which gives the temperature, gas and electron pressures, and other data as functions of the continuous optical depth at 5000 Å between 5000 = 10^–7 and 25. The model is characterized by a flat temperature minimum of 4600 °K between 5000 10^–2 to 10^–4. The model is homogeneous, and in hydrostatic equilibrium. A hydrogen-helium ratio of 10 has been assumed.Much divergence remains in the interpretation of line-profile observations with regard to the establishment of a photospheric model (Section 4). It proved to be as yet impossible to obtain reliable information on the variation with depth of the following functions: temperature fluctuations, turbulence velocities, convective velocities, and vibrational velocity amplitudes (Section 5). Provisionally, it is assumed that _macro = 2 km/sec and _micro = 1 km/sec, isotropic and independent of depth.     

An analytic model for upper atmosphere densities based upon Jacchia's 1970 models

An analytic model is presented which gives upper atmospheric densities as a function of the exospheric temperature and the altitude. The densities produced are identical to those produced by Jacchia's 1970 models (1970) for altitudes between 90 and 125 km and closely approximate Jacchia's values for altitudes greater than 125 km.     

Mars: Far-infrared spectra and thermal-emission models

Spectra of Mars from 100 to 360 cm^?1 were obtained during three different observation periods from NASA's Kuiper Airborne Observatory. Also, a new thermal model was constructed for the surface of Mars, and synthetic spectra were computed from the models to compare with the observations. The models include the effects of a dusty atmosphere which absorbs, scatters, and reradiates energy. The synthetic spectra show significant effects on disk-averaged brigthness temperatures, as well as absorption features, due to silicate dust. The spectra of Mars, which are ratios of Mars to the Moon, do not fit the synthetic spectra unless the surface emissivities of Mars and the Moon have different dependencies on wavelenght. A possible explanation for this behavior is a difference in soil particle-size distributions between Mars and the Moon, with Mars being depleted in large particles compared to the Moon. Small particles are consistent with clay minerals which have been suggested elsewhere as constituents of the Martian surface.     

Resistive effects in the turbulent photosphere and chromosphere

We have investigated the role of finite resistivity effects in the photosphere and chromosphere. We demonstrate that turbulence in the photospheric conductivity gives rise to a resistive instability, as does the gradient in resistivity between the chromospheric layer of the Sun and the photospheric layer, which latter unstable mode is the well known tearing mode of Furth, Killeen and Rosenbluth. In both cases the calculations indicate time scales of the order of seconds or minutes, and we therefore believe that solar flares and spicules can be produced by finite conductivity instabilities. We also demonstrate that the finite resistive diffusion makes it difficult to maintain an initially force-free flux tube in the chromosphere unless the Alfvén speed is sufficiently high and/or the flux tube is sufficiently thick. We also demonstrate that the magnetic fields in the turbulent photosphere becomes trapped by high conductivity regions and this leads to enhancement of the resistive instabilities.Our analysis does not explain the origin of the high-energy particles in solar flares—for this the problem of dynamical acceleration must be investigated.     

Electrical conductivity in sunspots and the quiet photosphere

The electrical conductivity, thermal conductivity and viscosity of models of the quiet photosphere and the umbra of a sunspot have been calculated using LTE ionization equilibria and the Chapman-Enskog theory of transport coefficients. The results are presented in tabular form, and compared with previous calculations.