Ca II K line profile of the truly quiet Sun

While evaluating the chromospheric variability (solar cycle related or any other) using the Ca II K line (3933.684 Å) as an indicator, an essential prerequisite is the knowledge of the profile of a truly quiet Sun in the integrated light. Such a profile can serve as a bench mark over which enhancements can be measured, particularly when modelling variability. This paper describes how such a K-line profile has been derived for the quiet Sun using disc-integrated light.     

Solar chromospheric magnetic fields in active regions inferred by monochromatic images of the Stokes' parameterV of the Hβ line

A series of H chromospheric magnetograms was obtained at various wavelengths near the line center with the vector video magnetograph at Huairou Solar Observing Station as a diagnostic of chromospheric magnetic structures. The two-dimensional distribution of the circular polarization light of the H line with its blended lines at various wavelength in active regions was obtained, which consists of the analyses of Stokes' profileV of this line. Due to the disturbance of the photospheric blended line Fei 4860.98 for the measurement of the chromospheric magnetic field, a reversal in the chromospheric magnetograms relative to the photospheric ones occurs in the sunspot umbrae. But in the quiet, plage regions, even penumbrae, the influence of the photospheric blended Fei 4860.98 line is not obvious. As regards the observation of the H chromospheric magnetograms, we can select the working wavelength between -0.20 and -0.24 from the line core of H to avoid the wavelengths of the photospheric blended lines in the wing of H.After the spectral analysis of chromospheric magnetograms, we conclude that the distribution of the chromospheric magnetic field is similar to the photospheric field, especially in the umbrae of the sunspots. The chromospheric magnetic field is the result of the extension of the photospheric field.     

Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.     

High resolution spectroscopy of the disk chromosphere

We compare temporal power spectra of solar atmospheric oscillations in plage and quiet Sun regions occurring on different parts of a time series of high-quality spectrograms. For periods shorter than 300 s, the oscillation amplitude in the photospheric and low chromospheric parts of the plage is reduced. There is a significant increase in long period power in the chromospheric plage. Our results provide no clear evidence that plages are heated by the dissipation of short-period waves.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-78-17292 with the National Science Foundation.     

First phase of active regions and their relation to the chromospheric network

An analysis of 63 young active regions shows that they originate exclusively on the borders of the chromospheric cells. In most cases they do not produce pores or spots, but always arch filament systems and chromospheric faculae. The arch filament systems evolve along the cell border, they are rooted on the network and not in the interior of the chromospheric cells. The sub-photospheric magnetic flux seems to reach the photosphere in units of 10¹⁹ Mx. About 3 hr after the first arch filaments appear pores evolve on the cell border. The pores and spots are probably formed by local aggregations of the emerging flux tubes.Mitteilungen aus dem Fraunhofer-Institut Nr. 132.     

Lifetime of the Hα chromospheric network

Time-lapse photographs of the H chromospheric network were taken at 3-sec intervals with a -Å filter centered at H + 0.65 Å, where the network appears as a system of dark mottles. A nearly continuous sequence of 62 hr duration was made possible by summertime operation of the telescope at Thule Air Base, Greenland, 10° above the Arctic Circle. A quiet region was followed across the center of the disc. The life history of the network, as determined by measurement of the cross-correlation of selected photographs taken at intervals ranging between 3 sec and 29 hr, shows a rather complicated behavior. Initially the correlation falls off quite rapidly, dropping from 0.97 at 3 sec to 0.60 at about 7 min. Thereafter, the decline becomes increasingly more gradual, reaching a value of 0.24 at 4 hr. From this point the correlation falls off quite slowly. A least-squares fit to that portion of the cross-correlation curve for time differences greater than 4 hr yields a mean (1/e) life of 25.0 ± 1.6 hr and a value for twice the half-life of 38.8 ± 4.0 hr. This rate of decline is believed to be characteristic of the lifetime of the chromospheric network. The rapid initial loss of correlation appears to be due to a redistribution of the features, seen in the red wing of the H line, along the network.     

Configuration of the chromospheric magnetic field in a unipolar sunspot region

A set of H chromospheric magnetograms at various wavelengths near the line center, chromospheric Dopplergrams, and photospheric vector magnetograms of a unipolar sunspot region near the solar limb were obtained with the vector video magnetograph at the Huairou Solar Observing Station. The superpenumbral chromospheric magnetic field is almost parallel to the surface at the outside of the sunspot penumbra, where the magnetic lines of force are mainly concentrated in the superpenumbral filaments. In the gaps between the filaments the chromospheric horizontal field is weak.     

Dynamics of Blinkers

The relative Doppler and non-thermal velocities of quiet-Sun and active-region blinkers identified in Ov with CDS are calculated. Relative velocities for the corresponding chromospheric plasma below are also determined using the Hei line. Ov blinkers and the chromosphere directly below, have a preference to be more red-shifted than the normal transition region and chromospheric plasma. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler velocities in these regions. The anticipated ranges of Doppler velocities of blinkers are 10–15 km s^–1 in the quiet Sun (10–20 km s^–1 in active regions) for Hei and 25–30 km s^–1 in the quiet Sun (20–40 km s^–1 in active regions) for Ov. Blinkers and the chromosphere below also have preferentially larger non-thermal velocities than the typical background chromosphere and transition region. Again the increase in magnitude of these non-thermal velocities is no greater than the typical ranges of non-thermal velocities. The ranges of non-thermal velocities of blinkers in both the quiet Sun and active regions are estimated to be 15–25 km s^–1 in Hei and 30–45 km s^–1 in Ov. There are more blinkers with larger Doppler and non-thermal velocities than would be expected in the whole of the chromosphere and transition region. The recently suggested mechanisms for blinkers are revisited and discussed further in light of the new results.     

Systematic photometry of XUV solar images

Units and methods have been devised to express the photometry of solar XUV images. The source and limb-brightened fluxes are given in terms of the sun's quiet central intensity. Measurements made on this system can be meaningfully compared with solar data and with theoretical predictions. Calculations have been made of the XUV distribution for optically thin solar models and results have been converted onto the measurement system. Photometric measurements have been made from four films lent by the Culham and Leicester Laboratories. Certain inconsistencies suggest that the measurement accuracy is not yet good enough for definitive results. However, there is evidence that: (a) the X-ray emission sources are brighter, relative to the quiet sun, than the longer wave EUV sources; (b) X-ray limb photons all escape (i.e. limb optically thin) but EUV limb emission is affected by absorption; and (c) the observed image diameter fits an emission scale height of 0.05 .     

The First DIRECT Distance to a Detached Eclipsing Binary in M33

We present the first direct distance determination to a detached eclipsing binary in M33, which was found by the DIRECT Project. Located in the OB 66 association, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its 4.8938 day period. We obtained follow-up BV photometry and spectroscopy from which we determined the parameters of the system. It contains two O7 main sequence stars with masses of and and radii of and , respectively. We derive temperatures of K and K and determine the reddening . Using HST photometry for flux calibration in the V band, we obtain a preliminary distance modulus of mag ( kpc). The photometry and thus distance is subject to revision in the final paper.     

A UV spectral atlas of a prominence

The ultraviolet spectrum of a large prominence has been observed with the High Resolution Telescope and Spectrograph (HRTS) on SPACELAB 2, 5 August, 1985. The spectrum covers the wavelength range 1335–1670 Å and shows numerous emission lines from gas at chromospheric and transition region temperatures. An Atlas of the prominence spectrum has been made showing absolute calibrated intensities on an accurate wavelength scale. The Atlas includes for comparison the corresponding UV-spectrum from an average quiet solar region.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

A model of the solar atmosphere and wind

Using a combination of solar and interplanetary measurements, a topological model is developed of the overall magnetic and plasma structures.

The influence of chromospheric condensation on Hα line profiles

The influence of the chromospheric condensation on H line profiles for the thermal model of a solar flare has been empirically studied in this paper. The so-called thermal model here means that there is no temperature increase relative to the quiet-Sun chromosphere but with a chromospheric condensation in the lower part of its transition region, which case is assumed to represent the early stage of the impulsive phase. The main results include: when the temperature within the condensation region is assumed to be equal to that in front of it, the influence is to create an additional absorption profile overlapping on the original one; by increasing the condensation strength, the H line profile changes from a little line-center increase to broadened red asymmetry, then to the reversed red asymmetry, and finally to two independent absorption profiles; the thickness of the condensation determines the absorption of the additional profile; descending the transition region has no obvious effect on the basic characteristic of the H line profile except a little increase in the line center. Assuming that the temperature within the condensation is higher than that ahead of the condensation, the calculated H line profiles may be strong enough to be comparable with the observations. This means that if the condensation in the purely thermal model can reach a higher temperature, we may also use only thermal origin to explain a chromospheric flare.Alexander von Humboldt Research Fellow, on leave from Purple Mountain Observatory, Nanjing, China.     

Time-match semi-empirical models of the chromospheric flare on 3 February, 1983

Spectra of a 2B flare on 3 February, 1983 were observed simultaneously at H, H, and Can H, K lines with a multichannel spectrograph in the solar tower telescope of Nanjing University. The flare occurred in an extended region of penumbra at S 17 W07 from 05 : 41 to 07 : 00 UT. By use of an iterative method to solve the equations describing hydrostatic, radiative, and statistical equilibrium for hydrogen and ionized calcium atoms, five semi-empirical models corresponding to different times of the chromospheric flare have been computed. The results show that after the beginning of the flare, the heating of the chromosphere starts and the transition layer begins to be displaced downwards. However, during the impulsive phase the flare chromospheric region has a rapid outward expansion followed by a quick downward contraction. At the same time the transition layer starts to ascend and then descend again. After the H intensity maximum, the flare chromospheric region continues to condense and attains its most dense phase more than ten minutes after the maximum. Finally, the flare chromospheric region returns slowly to the normal chromospheric situation.     

Narrow-band photometry and differential effects in close binary systems

The application of narrow-band photometry to close binary systems is considered, with particular attention to the effects of eclipses. Models are examined, first in which two normal absorption spectra are superposed and then in which a third emission component is present. The purpose of these models is to relate to narrow-band observations of a kind similar to some preliminary results presented for U Sge in H. It is shown that, under the conditions obtaining for such observations, the role of Doppler effects should be small, but that the observations should be sensitive to any difference between the limb darkening in the line and surrounding continuum.In the three-component models, the emitting volume is pictured as (i) a chromospheric shell and (ii) an equational disk about the primary star. The expected modifications corresponding to such models of observations of primary eclipse are calculated. Possible applicability of the models to the preliminary observations is briefly considered.     

Chromospheric absorbing features promising the appearance and the development of an active center

In this paper we study some absorbing features seldom associated with flare occurrence and observed in the wings of H by means of a cinematographic patrol.We describe short lived chromospheric dark features with strong velocity fields and we show their correlation with the birth and the further development of an active center.It is shown that radial velocities precede the modifications of magnetic fields.An attempt to compare these chromospheric velocities and photospheric ones points out that it could be possible to find good correlation between them in a changing but not flaring region.     

Fine structures of chromospheric magnetic field and material flow in a solar active region

In this paper, we analyze the relations between photospheric vector magnetic fields, chromospheric longitudinal magnetic fields and velocity fields in a solar active region. Agreements between the photospheric and chromospheric magnetograms can be found in large-scale structures or in the stronger magnetic structures, but differences also can be found in the fine structures or in other places, which reflect the variation of the magnetic force lines from the photosphere to the chromosphere. The chromospheric superpenumbral magnetic field, measured by the Hline, presents a spoke-like structure. It consists of thick magnetic fibrils which are different from photospheric penumbral magnetic fibrils. The outer superpenumbral magnetic field is almost horizontal. The direction of the chromospheric magnetic fibrils is generally parallel to the transverse components of the photospheric vector magnetic fields. The chromospheric material flow is coupled with the magnetic field structure. The structures of the H chromospheric magnetic fibrils in the network are similar to H dark fibrils, and the feet of the magnetic fibrils are located at the photospheric magnetic elements.     

Impulsive phenomena in a small active region

The temporal and spatial variations of EUV emission from a small growing active region were investigated. Frequent localized short term ( few minutes) fluctuations in EUV emission were observed throughout the 7.2 hr interval when the most continuous observations were acquired. Approximately 20% of the 5 x 5 pixels had intensity variations exceeding a factor of 1.3 for the chromospheric L line, a factor of 1.5 for lines formed in the chromospheric-coronal transition region and a factor of 1.4 for the coronal Mg x line. A subflare in the region produced the largest intensity enhancements, ranging from a factor of 2.3 for the chromospheric L line to 8 for the transition region and coronal lines. The EUV fluctuations in this small active region are similar to those observed in coronal bright points, suggesting that impulsive heating is an important, perhaps dominant form of heating the upper chromospheric and lower coronal plasmas in small magnetic bipolar regions. The responsible mechanism most likely involves the rapid release of magnetic energy, possibly associated with the emergence of magnetic flux from lower levels into the chromosphere and corona.     

Stellar irradiance variations due to surface temperature inhomogeneities

Observations of rotational modulation of continuum brightness and photospheric and chromospheric spectral-line profiles of late-type stars indicate the presence of very inhomogeneous surface temperature distributions. We present three stellar examples (VY Ari, HR 7275, HU Vir) where time-series photometry is used to trace the evolution of spotted regions. Simultaneous spectroscopy and Doppler imaging for one of the three stars (HU Virgo, Fig. 1) makes it possible to compute the temperature distribution of the photosphere and the relative intensity distribution of parts of the chromosphere (from CaII K and H line profiles). The combination of time-series spot modeling and Doppler imaging enabled us to determine thesign and amount of differential surface rotation on HU Vir. We found a big, cool polar spot (see figure below) and a differential (surface) rotation law where higher-latitude regions rotate faster than lower-latitude regions (opposite to what we see on the Sun). Currently, this ensemble of techniques - time-series photometry and photospheric and chromospheric Doppler imaging - is only applicable to stars overactive by approximately a factor of 100 as compared to the active Sun, e.g. the evolved components in RS CVn-type binaries and some rapidly-rotating, single, pre-main sequence stars or giant stars. Stellar rotation is a fundamental parameter for (magnetic) activity. Starspots, or any other surface inhomogeneities, allow one to derive very precise stellar rotation rates and, if coupled with seismological observations of solar-type stars, could provide information on the internal angular momentum distribution in overactive late-type stars.To be published in Astronomy & Astrophysics.