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.     

Helium emission in the middle chromosphere

Slitless spectrograms obtained during the eclipse of 10 June 1972 have been analyzed to determine the height distribution of the D₃ He line intensity.For undisturbed regions the maximum of D₃ line intensity is confirmed to exist at about 1700km above the limb. Besides the above mentioned maximum, in plages a considerable intensity may be observed at low heights (h < 1000 km).An analysis of these observations for h > 1000 km has been carried out within the low temperature mechanism of triplet helium emission taking into account the helium ionization by XUV radiation. The density dependence of the 2³ S level population at different XUV flux values has been calculated. Our observations give N _e 2 × 10¹⁰ cm^–3 in the chromosphere at h = 2000 km. The probable coincidence of the H and He emission small filaments in the middle chromosphere is discussed.     

The intensity distribution of the D3 helium line near the solar limb

Cinematographic observations of the slitless flash spectrum near the D₃ helium line were performed in Yurgamysh at the total solar eclipse of September 22, 1968. The intensity distribution of the D₃ line was obtained with a height resolution of 44 km within the height interval between –3400 and +1700 km above the limb. The absorption line D₃ on the disk near the limb was discovered.     

Physical conditions in the chromosphere of a two-ribbon solar flare accompanied by a surge: 1

We studied changes in thermodynamic parameters of the chromosphere at the initial stage of the two-ribbon solar flare accompanied by a surge that occurred on September 4, 1990. The inhomogeneous semiempirical models of the flare chromosphere and surge are constructed for four observation moments. The spectra were obtained with the ATsU-26 horizontal solar telescope of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (Terskol Peak). Photometric transections of the spectra passed through two bright kernels of one of the flare ribbons and through the surge. The comparison of the observed profiles of the line H_α in the solar active and quiet-Sun regions reveals the substantial emission in the line wings (up to 1–1.2 nm) with a residual intensity of 0.6–0.77 at the center of the line profiles. Calculations within the two-component models of the chromosphere have shown that this may be the evidence of the existence of the details (unresolved by the telescope and occupying 5–12% of the total area) with a deep heating of the chromosphere layers. A strong asymmetry of the line profiles and the shift with respect to the line profile for the quiet-Sun region are explained by peculiarities of the line-of-sight velocity distribution over the height. It is found that the motion is directed to the observer in the upper chromosphere (10–30 km/s) and from the observer in the lower chromosphere (5–20 km/s) for the larger part of the active region under study. According to the models calculated for the surge, the line-of-sight velocities reach a value of 70 km/s.     

On the large scale brightness fluctuations in the solar atmosphere

The October 1976 spectroscopic observations of the solar chromosphere with the great coronagraph of the Kislovodsk Station of the Pulkovo Observatory were analysed using the autocorrelation method. The autocorrelation function and the power spectrum were calculated. It was found that the brightness distribution in the chromosphere is essentially bimodal and has characteristic scales of 1.2 × 10⁴ km and 2.4 × 10⁴ km. The supergranulation (3.5 × 10⁴ km) in the brightness field was found to show up more faintly at all heights.     

Measurements of the helium 584 Å airglow during the Cassini flyby of Venus

The helium resonance line at 584 Å has been observed with the UltraViolet Imaging Spectrograph (UVIS) Extreme Ultraviolet channel during the flyby of Venus by Cassini at a period of high solar activity. The brightness was measured along the disk from the morning terminator up to the bright limb near local noon. The mean disk intensity was ∼320 R, reaching ∼700 R at the bright limb. These values are slightly higher than those determined from previous observations. The sensitivity of the 584 Å intensity to the helium abundance is analyzed using recent cross-sections and solar irradiance measurements at 584 Å. The intensity distribution along the UVIS footprint on the disk is best reproduced using the EUVAC solar flux model and the helium density distribution from the VTS3 empirical model. It corresponds to a helium density of 8×10⁶ cm⁻³ at the level of where the CO₂ is 2×10¹⁰ cm⁻³.     

A model of the inhomogeneous chromosphere of the sun

A new model is proposed for the solar chromosphere, which is assumed to be an instable inhomogeneous formation, consisting of numerous elements (filaments), each with different temperature and density. Fluctuations of the magnetic field may give rise to chromospheric turbulence and may also cause the chromospheric inhomogeneities.The chromosphere is suggested to consist of four discrete groups of filaments: (1) metallic filaments where the conditions for the emission of lines of neutral metals are optimal, (2) hydrogenic elements, with optimum conditions for the emission of the Balmer series of hydrogen, (3) helium filaments, with optimum conditions for the appearence of the neutral helium lines, (4) the subcoronal filaments, representing a transition from chromospheric to coronal formations.The metallic filaments may be further subdivided, first into filaments where the emission arises from scattering of photospheric radiation - these emit lines of neutral metals and of some metallic ions (CaII, SrII, and others), and further into filaments where the emission is farther from LTE conditions; the latter filaments are characterized by a somewhat higher electron temperature and by an electron density at least exceeding that of the other elements by an order of 10. Computations of the optimum conditions for the emission of the neutral helium lines were made with the aid of new tables of Sobolev. The helium filaments in the low chromosphere have lower temperatures and are denser than those in the upper chromosphere; for a part they may also be considered as hydrogen filaments. The derivation of the physical parameters of the subcoronal filaments was based on data on the Heii4686 chromospheric line emission and also on rocket observations of the ultraviolet solar spectrum. In order to evaluate the relative distribution of the various filaments between heights of 0 and 5000 km, data on the radio emission of the Sun at 8 mm are also used. Characteristics of the proposed model of the chromosphere are given in Table III and Figure 1.     

Relationship between the Parameters of Coronal Holes and High-Speed Solar Wind Streams over an Activity Cycle

The comparison of the brightness and area of coronal holes (CH) to the solar wind speed, which was started by Obridko et al. (Solar Phys. 260, 191, 2009a) has been continued. While the previous work was dealing with a relatively short time interval 2000 – 2006, here we have analyzed the data on coronal holes observed in the Sun throughout activity Cycle 23. A catalog of equatorial coronal holes has been compiled, and their brightness and area variations during the cycle have been analyzed. It is shown that CH is not merely an undisturbed zone between the active regions. The corona heating mechanism in CH seems to be essentially the same as in the regions of higher activity. The reduced brightness is the result of a specific structure with the magnetic field being quasi-radial at as low an altitude as 1.1R _⊙ or a bit higher. The plasma outflow decreases the measure of emission from CH. With an adequate choice of the photometric boundaries, the CH area and brightness indices display a fairly high correlation (0.6 – 0.8) with the solar wind velocity throughout the cycle, except for two years, which deviate dramatically – 2001 and 2007, i.e., the maximum and the minimum of the cycle. The mean brightness of the darkest part of CH, where the field lines are nearly radial at low altitudes, is of the order of 18 – 20% of the solar brightness, while the brightness of the other parts of the CH is 30 – 40%. The solar wind streams originate at the base of the coronal hole, which acts as an ejecting nozzle. The solar wind parameters in CH are determined at the level where the field lines are radial.     

Brightness temperature distribution in solar corona based on RATAN-600 observations of the maximum phase of the March 29, 2006 solar eclipse

We describe the technique and results of modelling the solar radio emission during the maximum phase of the solar eclipse of March 29, 2006 on the RATAN-600. The aim of modelling is to refine the brightness temperature of the solar corona at the distances up to two solar radii from the center of the optical disk of the Sun. We obtained the distribution of brightness temperature in the vicinity of the coronal hole above the solar North Pole at the wavelength of 13 cm. The results of modelling showed that brightness temperatures of the coronal hole at the distances greater than 1.02 R_C (here R_C is the radius of the optical disk of the Sun) is substantially lower than the expected average brightness temperature of a typical coronal hole, and that of the quiescent Sun (below 30000 K) at the wavelength of 13 cm. The classical Baumbach-Allen formula for electron density in a spherically symmetric corona agrees with the results of observations starting at distances of (1.4–1.5) R_C.     

Radial velocity field in the lower atmosphere of the solar active region during a flare with an ejection: The initial phase of the flare

Horizontal motion has been studied of the matter along the active region at different heights of the photosphere (115–580 km) in the initial phase of the two-ribbon solar flare on September 4, 1990, near the solar limb, accompanied by the ejection. Photospheric velocities varied in the range −3.5 ... 2.5 km/s. The direction of motion in the photosphere and the chromosphere was mainly toward the observer. Kinematic elements have been discovered in the structure of the horizontal velocity field. Their size reduced as they approached the maximum of the flare from 7–12 to 4–5 Mm, and the velocity amplitude decreased. Throughout the whole investigated active region, vortex motions were observed in the photosphere and chromosphere. Temporal changes in the horizontal velocity field in node areas and in their vicinity were oscillatory in nature and occurred almost simultaneously along the entire height of the photosphere.     

Detection of small scale structure in metal lines at the extreme solar limb

We describe a tangential limb spectrum at 5870 Å which geometrically probes the high photosphere through the low chromosphere. Velocity and brightness structures with sizes ranging from 500 to 1500 km are present in the stronger emission lines. Such structure is consistent between the Fe i and Ba ii lines, and emission knots in these lines coincide with continuum bright streaks. But no correlation is evident between structure in the He i D₃ line, emission in the Na i D₂ line, and emission in the Fe i and Ba ii lines as a group. Two classes of near-horizontal velocity structure are seen in the height range from 0 to 500 km above the limb: υ ? 1 km s^?1 for the weaker metals and υ ～ 7–10 km s^?1 for the Na i line. Differences in line opacity are suggested as the cause of the low correlation between the fine structure in the various lines.

     

The helium radiation in prominences

It is shown that the emission of quiescent and loop prominences in the helium D₃ line and in the 4686 Å line of He⁺ respectively, occurs at low temperatures, of the order of 7000 K.The ionization of neutral helium is produced by short-wave solar radiation, which is absorbed in the outer layers of filaments composing a prominence. The population of helium triplet levels in prominences is determined by recombinations and subsequent resonance scattering of photospheric radiation. Transitions from triplet to singlet levels caused by electron collisions considerably reduce the line brightness.Emission of ionized helium in the 4686 Å line arises in prominence surface layers as well. In quiescent prominences the emission is very faint and is due to recombination; the second ionization is caused by the far ultraviolet radiation.In flare-like events ionized helium emits due to charge-exchange collisions. The symmetrical resonance charge-exchange of -particles is caused by helium ions in corpuscular streams which are probably generated in photospheric layers. Due to increased radiation losses the temperature of the prominence under the action of the stream is negligibly increased. With a stream density equal to 5 × 10⁸ cm^-3 and velocity 300 km/s the theoretical intensity of the 4686 He⁺ line is some hundreds of microängströms and agrees with observations of Goldberg-Rogozinskaya (1962, 1965) and others.     

Long-Term and Medium-Term Variations of Solar Radio Emissions at Different frequencies

Plots of 12-month moving averages of the radio emission values for 1947–2002 indicated that the ratios (maximum/minimum) of the solar cycles 19–23 were low (∼ 1.2) in the upper chromosphere and lower corona (frequencies near 15 000 MHz), rose to maximum levels of ∼ 3.5 in the middle corona (frequencies ∼ 2000±500 MHz), and dropped thereafter to ∼ 2.5. In some cycles, there were two maxima separated by about 2 years. In cycles 20 and 23, mostly the second maximum was larger than the first maximum, but in cycles 21 and 22, some parameters showed the first maximum larger while others showed the second maximum larger. There was no systematic shift from first maximum to second maximum, with frequency or temperature (or altitude).     

Horizontal convective velocity field obtained from the observations of the solar limb

Structure of horizontal convective currents in the solar atmosphere has been investigated using profiles of the λ ≈ 532.42 nm neutral iron line which were observed at the solar limb with high spatial resolution. The asymmetry of the observed line was shown to arise when approaching the solar limb. The spatial and time velocity variations were simulated using the λ-meter technique. Acoustic waves were removed using the k-ω filters. The convection currents on various spatial scales were distinguished, namely, those connected with granulation, mesogranulation, and supergranulation. The spatial and time distribution of the convection velocities in the photosphere and in the low chromosphere has been analyzed. The horizontal currents were shown to exist on granulation, mesogranulation, and supergranulation scales as low as h ≈ 250 km, and the granulation and mesogranulation horizontal velocities increase with height. In the photospheric layers, the supergranulation vertical-velocity field appears almost invariable, while the supergranulation horizontal-velocity field can vary with height. The horizontal velocity distribution within large convection currents is found to be asymmetric on granulation, mesogranulation, and supergranulation scales.     

D3 spicules and the lower chromosphere

High resolution filtergrams of the solar limb in D₃ and off-band H have been used to investigate the spatial structure of the D₃ chromosphere. It was found that spicules provide the major contribution to the intensity of the D₃ emission band observed above the limb, with the remainder of the emission coming from a semi-homogeneous background component at low heights.The observations can be understood on the basis of the photoionization model, whereby it is found that helium is only slightly ionized at the height of peak intensity in the D₃ emission band, and that spicules are at least 3 times denser than their surroundings at this height.In coronal holes, the D₃ emission is confined to isolated emission patches, and these patches contain a fine structure resembling normal chromospheric spicules.     

A one-dimensional numerical model of H<Subscript>2</Subscript>O cloud formation in the Martian atmosphere

A one-dimensional numerical model with a size distribution of aerosol particles in Martian atmosphere is developed. The model incorporates detailed microphysics and turbulent transport. Dust particles suspended in the Martian atmosphere play a role of cloud condensation nuclei. Diurnal cycle of condensational processes is obtained on the basis of GCM temperature profiles. An effective radius of ice particles is 1–2 μm near the lower boundary of cloud layer and 0.2–0.3 μm at the altitude of 50–60 km. These results are consistent with solar infrared occultations by SPICAM experiment on Mars-Express. Near-surface fogs may form under specific conditions. The connections of condensational processes and cloud macroscopic parameters on microphysical properties of aerosol particles are main focus of this paper. In particular, the dependence on variations of cloud condensation nuclei contact parameter is analyzed, taking into account new experimental data of adsorption properties of minerals at low temperatures.     

High-Dispersion Spectroscopy of the Sodium (NA) Emission within the Inner Coma of C/1995 O1 (Hale-Bopp)

Very-high spectral resolution observations of the neutral Na emission have enabled measurements of the velocity dispersions of the Na atoms within ∼40,000 km of the opto center of Hale-Bopp. Asymmetric Na D line profiles imply both an in situ or core Na source and a secondary Na source at locations within the inner coma. The core velocity distribution had a FWHM of 2 km s^-1. The extended source FWHM increased with distance from the nucleus (up to ∼6 km s^-1, but appeared smaller in the more dusty regions (∼2.5–3.0 km s^-1) of the inner coma. The D₂/D₁ line strength ratio was consistent with an optically thin inner coma. Within 5,000 km of the opto center the continuum spatial intensity profiles decreased as ∼r^-1 while the Na D emission decreased at less than r^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

1983年日全食闪光谱中氦D3线的研究

我们对１９８３年日食闪光谱中的Ｄ３线进行了光度测量，加上比较可靠的定标系数，得到了氦Ｄ３线的绝对表面亮度分布，由此还求出了氦的密芳随高度的分布。其结果与以前观测结果大体一致。Ｇｕｌｙａｅｖ由统计发现，Ｄ３线的绝对亮度有着随太阳黑子数增加而下降这一趋势。我们的结果符合这一趋势。     

Height of helium emission in the chromosphere

Filtergrams of the limb show the He i D₃ chromosphere as a shell which is separated from the limb by a gap. The height of maximum D₃ contribution occurs at about 1350 km above the limb and is independent of the intensity of the D₃ emission. We interpret this effect as the height to which coronal EUV radiation is capable of penetrating the atmosphere.     

Emission of helium in prominences and the chromosphere

The excitation of the levels (2¹ P, 3¹ D, 5¹ D, 6¹ D and 7¹ D) of helium in quiescent prominences and the chromosphere is considered using the observations made during the 1952, 1961 and 1970 solar eclipses (Thomas and Athay, 1961; Athay and Orrall, 1957; Rigutti and Russo, 1964; Kubota et al., 1972) as well as by means of high altitude coronographs (Jefferies and Orrall, 1962). It is shown that the lower levels of the singlet series of helium, like the lower levels of hydrogen, are excited by resonance (λ = 584 Å) and photospheric radiation. The higher levels are populated by transitions from the triplet system, so that a relative state LTE is established between the systems for n ? 6.