The mystery of the chromosphere

We discuss many aspects of the solar chromosphere from an observational point of view, and show that most existing models are in direct contradiction to radio and eclipse measurements. We plead for attention to the actual observed radio temperatures and density gradients, as well as images of the chromosphere. We find that the chromosphere is not in hydrostatic equilibrium and suggest that the support is due to the tangled intranetwork fields.Dedicated to Cornelis de Jager     

The brightness of the helium D3 line in the undisturbed chromosphere from eclipse observations

Slitless spectra of the chromosphere, observed cinematographically at the total solar eclipse of 10 July 1972, were reduced. The surface brightness distribution of the helium D₃ line in the undisturbed chromosphere was obtained in agreement with results by other observers. The available eclipse data on the D₃ absolute brightness was analysed by means of theoretical curves of growth. Intensity data by some observers were found to be certainly too high. A trend was found that the D₃ absolute brightness in the quiet chromosphere decreases with the increasing solar activity (sunspot number). This perhaps indicates a variation of the spicule number over the solar surface during a sunspot cycle.     

Spectroscopic Coronal Observations During the Total Solar Eclipse of 11 July 2010

The flash spectra of the solar chromosphere and corona were measured with a slitless spectrograph before, after, and during the totality of the solar eclipse of 11 July 2010, at Easter Island, Chile. This eclipse took place at the beginning of Solar Cycle 24, after an extended minimum of solar activity. The spectra taken during the eclipse show a different intensity ratio of the red and green coronal lines compared with those taken during the total solar eclipse of 1 August 2008, which took place toward the end of Solar Cycle 23. The characteristic coronal emission line of forbidden Fe xiv (5303 Å) was observed on the east and west solar limbs in four areas relatively symmetrically located with respect to the solar rotation axis. Subtraction of the continuum flash-spectrum background led to the identification of several extremely weak emission lines, including forbidden Ca xv (5694 Å), which is normally detected only in regions of very high excitation, e.g., during flares or above large sunspots. The height of the chromosphere was measured spectrophotometrically, using spectral lines from light elements and compared with the equivalent height of the lower chromosphere measured using spectral lines from heavy elements.     

Solar chromosphere in the D<Subscript>3</Subscript> helium line from spectra of the eclipse of March 29, 2006

We present the processing results of slitless chromospheric spectra in a D₃ helium line obtained during the solar eclipse of March 29, 2006. A “Cascade-Photometrics 650” CCD matrix photodetector with a rate of 16 frames/s was used for the observations. Altitude distribution of absolute brightness of the helium chromosphere has been mapped in a latitude interval 30 N-10 S on the western limb. A correlation between absolute brightness of the helium chromosphere and a solar cycle phase has been confirmed. It has been revealed that the altitude distribution of helium emission has one maximum at an altitude of approximately 2000 km. In the lower chromosphere, helium emission is undetectable. This confirms our assumption about absence of an additional maximum at 300 km in the D₃ line brightness distribution in the chromosphere during sunspot maximum periods. The results also prove that X-ray coronal radiation with λ < 6 nm is the source of helium excitation in the lower chromosphere.     

Scientific observations at total solar eclipses

The occasion of the longest totality of an eclipse in the 18 yr 111/3 d saros cycle leads to taking stock of the scientific value of ground-based eclipse observations in this space age. Though a number of space satellites from the U.S., Europe, Japan, and Russia study the Sun, scientists at eclipses can observe the solar chromosphere and corona at higher spatial resolution, at higher temporal resolution, and at higher spectral resolution than are possible aloft. Furthermore, eclipse expeditions can transport a wide variety of state-of-the-art equipment to the path of totality. Thus, for at least some years to come, solar eclipse observations will remain both scientifically valuable and cost-effective ways to study the outer solar atmosphere.     

Detection of Departure from Thermal Dynamic Equilibrium in the Solar Middle and Lower Atmosphere UsingTotal Solar Eclipse Data

There are abundant emission and absorption lines superimposed on the continuum spectra of the different solar atmospheric layers. The chemical composition and physical state of the solar atmosphere can be probed by the inversion of the profiles of these spectral lines. Due to the low density and large temperature difference in the chromosphere and transition region of the Sun, it is hard to establish the thermal dynamic equilibrium. It is necessary to adopt the theory of Non-Local Thermodynamic Equilibrium (N-LTE) to construct the corresponding atmospheric model. In this paper, the departure from the Local Thermodynamic Equilibrium (LTE) in the middle and lower atmosphere of the sun is investigated with the well-defined relative departure factor and the relevant calculations. We first make an inversion of the spectral lines formed at the different heights in the chromosphere and transition region during a total solar eclipse, to obtain the parameters of the observed spectral lines, such as the continuum source function, line source function, Doppler width, and thus the equivalent kinetic temperature. According to these line parameters obtained by the inversion, we calculate the quantitative results about the departure from the LTE at each space sampling point in the 2D field of view. Secondly, we reconstruct the 2D distributions of the radiation intensity, equivalent kinetic temperature, and relative departure factor according to the alignment of the optical fiber array in the integrated field unit (IFU) used by the telescope. The results show that there is a stronger correlation in the distributions of the temperature and relative departure factor existed in local small regions, but without evident correlation with the distribution of radiation intensity. There is an obvious difference between the distributions of the equivalent temperature and relative departure factor derived from two spectral lines, which shows a strong structurization and complexity existed in some local small regions of the solar atmosphere, and provides a new perspective for us to further understand the physics of the middle and lower atmosphere of the Sun.     

Observations of the eclipsing binary ζ aurigae during its 1979/1980 eclipse

This paper gives the results of our photometric observations of ζ Aur during its 1979/80 eclipse in 6 narrow wave bands and in the UBV colours. Useful data were obtained on 31 nights covering all phases of the eclipse. Our narrow band observations enable us to determine accurately the times of the second and third contacts, and hence a new formula for the epoch Minl = JD 2444219.643+972.176E. Our observations clearly show that eclipse depth increases with decreasing wavelength. Observations in the 3940 A band agrees with the variation in the spectral K-line of Ca II in showing a high chromosphere activity during this period. Our UBV results agree well with previous measurements.     

用日全食光谱探测太阳中低层大气对局部热动平衡的偏离

在太阳大气不同层次连续光谱中叠加有丰富的发射线或吸收线,对这些谱线轮廓进行反演分析可以探测太阳大气的化学成分和物理状态.太阳大气的色球及过渡区由于其密度低难以建立热动平衡,建立相应的大气模型需要采用非局部热动平衡(Non-Local Thermodynamic Equilibrium,N-LTE)理论.根据相对偏离因子计算来研究太阳中低层大气偏离局部热动平衡(Local Thermodynamic Equilibrium,LTE)分布的情况.首先对日全食观测过程中得到色球和过渡区不同高度形成的两条光谱数据进行反演,得到确定观测谱线的参数信息,如连续谱源函数、谱线源函数、多普勒宽度和由此推出的等效动力学温度;根据这些反演出的谱线参量计算出二维视场内每个空间采样点偏离LTE状态的定量结果;其次,根据用于观测的积分视场单元光纤排布阵列重构出辐射强度、等效动力学温度和相对偏离因子二维分布.结果显示:在局部小区域,温度分布和相对偏离因子的分布存在较强相关性,而与辐射强度分布无明显相关.从两条谱线导出的等效温度和相对偏离因子分布存在明显的差异.这两种二维分布揭示出太阳大气某些小尺度区域具有较强的结构性和复杂性,为进一步理解太阳中低层大气物理提供了一种新的视角.     

Spectral lines from photosphere to chromosphere,observed during the March 1970 eclipse; A first comparison with theory

A brief report is presented on observations of line profile variations in the transition region from the photosphere to the chromosphere, based on high-resolution eclipse spectrograms. We also show some computations for several lines whose behaviour agrees in general with the observations.     

2009年7月22日昆明日偏食太阳色球观测

阐述了云南天文台太阳全日面Hα色球望远镜对特殊天象日食的观测及资料情况。同时为局部射电源特征分析研究、CME理论、地球物理效应等相关学科研究提供资料共享信息。     

Airborne total eclipse observation of the extreme solar limb at 400 μm

The total solar eclipse of February 26, 1979 was monitored at far infrared wavelengths from the NASA Lear Jet Observatory flying at 12.9 km in the eclipse shadow. The resultant eclipse curve for radiation within a bandwidth of 20 cm^–1 centered upon 25 cm^–1 (400 m) was measured and analysed at an equivalent angular resolution of 1 arc sec over a 100 arc sec region adjacent to the limb to provide information on the intensity distribution of continuum radiation close to this limb. The curve has been compared to predictions derived from models of the solar atmosphere for the specific geometry of this eclipse, and is shown to match most closely that derived from a uniform distribution of radiation across the disk. This is in distinct contrast to the result obtained in the only other comparable experiment, carried out over Africa in 1973 from a supersonic Concorde aircraft, in which an intense but narrow spike of far infrared radiation at the extreme solar limb was inferred from the data. The absence also in the present observations of the significant limb brightening predicted by the HSRA model (in which homogeneity within the source region is assumed) is in substantial agreement with lower resolution results from mountain altitudes. This result is interpreted as further evidence for the presence in the Sun's lower chromosphere of significant inhomogeneity with a scale size of at least 1000 km at this depth.     

The abundance of helium in prominences and in the chromosphere

The abundance of helium relative to hydrogen is spectroscopically determined in prominences and in the chromosphere by using 1952, 1958, 1962 and 1966 eclipse data. Care is taken in the intensity calibration of emission lines, the self-absorption, and the departure from local thermodynamic equilibrium. We find from the line profiles and intensities of prominences and the chromosphere that the neutral helium lines are emitted in the metal-hydrogen emitting region where the kinetic temperature is low enough, 6000 8000 K, so that only the ionization due to UV radiation from the corona can explain the intensity of neutral helium emission. Also we find that the intensity ratio of Hei 3888.65 to H₈ 3889.05 increases towards the upper boundaries of prominences and of the chromosphere and that it approaches to a universal limiting value, both in various prominences or in the chromosphere, where it is considered that the ionization of neutral helium and hydrogen is nearly complete. From these facts the helium to hydrogen number ratio is found to be 6.5 ± 1.5%.A new schematic model of the chromosphere is presented where spicules have no hot region of emitting neutral helium lines. Here it is suggested that the kinetic temperature of spicules, 6000 8000 K, would be primarily determined by the radiation temperature of the corona and the transition region beyond the Lyman continuum of hydrogen which happens to be around those temperatures.     

The chromosphere in continuum emission observed at the total solar eclipse on 7 March 1970

Direct images of the Sun were photographed in continuum emission centered at 6900 Å by the jumping film method near the second contact of the Mexico eclipse on 7 March 1970. The band width was 150 Å defined by a combination of a sharp cut filter and KODAK IV F film. The intensity distribution of the solar outer layers obtained shows a steep decrease by a factor of 0.9 in logarithmic units around 2500 km. This is interpreted as the boundary of the chromosphere and corona. Spicules observed at 3500 km are explained by log n _e = 11.25 and T _e 6000 K. Discussions are made in relation to the other observations and some chromosphere models.     

Three-minute oscillations in sunspots: Seismology of sunspot atmospheres

We show that no eigenmodes of sunspot oscillations with periods of ～ 3 min or shorter exist. A complex spectrum of the 3-min oscillations arises, because the sunspot atmosphere is a multiband filter for slow MHD waves. To ascertain why the filter transmission bands appear, we have investigated the propagation of waves through a sunspot atmosphere using both multilayered isothermal model atmospheres and various empirical model atmospheres. It turns out that there are several different mechanisms responsible for the appearance of transmission bands in the atmospheric filter for slow waves. The filter lowest-frequency transmission band arises from the effect of a Fabry-Perot interference filter at the resonance frequency of the temperature plateau. The frequency of this band is always lower than the cutoff frequency of the temperature minimum. The next (in frequency) transmission band appears at the cutoff frequency. The higher-frequency transmission bands result from the antireflection of the atmosphere, an effect well-known in optics and acoustics. The nonlinearity of the 3-min oscillations observed in the upper chromosphere and transition region has only an indirect effect on the properties of the filter, increasing its transmission in most bands due to a decrease in the amplitude of the wave reflected from the upper atmosphere caused by nonlinear wave absorption. Knowledge of the formation mechanisms for the 3-min oscillation spectrum has allowed us to suggest a technique for estimating the parameters of sunspot atmospheres from the 3-min oscillation spectrum, i.e., to lay the foundations for the seismology of sunspot atmospheres.     

Line-intensities in the photosphere-chromosphere transition region

We present results from eclipse spectra in the wavelength region 4588–4682 Å, taken during the eclipse of February 15, 1961 at Bra, Yugoslavia. Absolute line intensities have been determined in the photosphere-chromosphere transition region. The method of reduction is discussed in Section 2 and the observed variation of the total intensity as a function of the height is shown for a number of lines in Figures 5–8. The results for the Fe i lines are compared with computations (as described in Paper I: van Dessel, 1970) for various solar model atmospheres (Section 4). The model, which combines Holweger's (1967) temperature distribution for the excitation temperature with the HSRA model (Gingerich et al., 1971) for the electron temperature, yields a better agreement than all pure LTE-models.     

1991年7月11日墨西哥日全食红外光谱观测及初步结果

本文概述１９９１年７月１１日日全食时中国日食观测队在墨西哥ＬａＰａｚ所作的近红外光谱观测情况及得到的初步结果。观测是用一强光力望远镜和一中小色散的光谱仪，配以ＣＣＤ摄象机、图象处理器、微机等组成的探测系统进行的。日全食时，在λ１０７１２－１０９７２波区共摄得极上层光球和色球的无缝光谱４１帧。日面外约１．０９６Ｒ处的有缝光谱３５帧。文中还介绍了拍得的色球、日珥资料概况。     

Waves in Sunspots: Resonant Transmission and the Adiabatic Coefficient

We investigate linear acoustic-gravity waves in three different semi-empirical model atmospheres of large sunspot umbrae. The sunspot filter theory is applied, that is, the resonant transmission of vertically propagating waves is modelled. The results are compared with observed linear sunspot oscillations. For three umbral models we present the transmission coefficients and the energy density of the oscillations with the maxima of transmission. The height dependence of the adiabatic coefficient (the ratio of specific heats) strongly influences the calculated resonance frequencies. The variable can explain the observed closely spaced resonance period peaks. The first resonance in the 3 min range is interpreted as a resonance of the upper chromosphere only, while the higher order peaks are resonances of the whole chromosphere.     

Modeling of the R CrB star chromosphere

A model of the R CrB star chromosphere is calculated on the basis of the observed profiles of the Ca II H and K lines and IR triplet and D lines of Na I and H-alpha. The calculated profiles of Ca II H and K lines and IR triplet and H-alpha are in good agreement with the observed ones both for an undisturbed state and for the light minimum. The line profiles for the light minimum are calculated under the assumption that the minimum is attributed to obscuring of the star disc with a dust cloud. In this case, the chromosphere is not hydrostatic since the column density at the chromosphere base is two orders of magnitude higher as compared to that in a hydrostatic model. The model proposed is more extended, less dense at the chromosphere base, and denser in the upper chromosphere. The extension of the calculated chromosphere is about 3 star radii. The density in the chromosphere is 108–1010 atoms per 1 cm³ and the temperature is 5000–7000 K. Agreement of the calculated and observed profiles of Na I D absorption lines is possible if we assume that, around the star, there is a cold envelope containing Na I atoms which expands with a velocity of about 30 km/s. This envelope is beyond the chromosphere, but near enough for the star and the envelope to be observed as a single whole. The optical thickness of the envelope in the Na I D2 line is 1.8. At the brightness minimum, this envelope illuminated with the star light yields additional emission attributed to resonant scattering in the Na I D lines.