Heating of the quiet solar chromosphere

We have analyzed a large number of Caii H line profiles at the sites of the bright points in the interior of the network using a 35-min-long time sequence of spectra obtained at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory on a quiet regon of the solar disc and studied the dynamical processes associated with these structures. Our analysis shows that the profiles can be grouped into three classes in terms of their evolutionary behaviour. It is surmized that the differences in their behaviour are directly linked with the inner network photospheric magnetic points to which they have been observed to bear a spatial correspondence. The light curves of these bright points give the impression that the main pulse, which is the upward propagating disturbance carrying energy, throws the medium within the bright point into a resonant mode of oscillation that is seen as the follower pulses. The main pulse as well as the follower pulses have identical periods of intensity oscillations, with a mean value around 190 ± 20 s. We show that the energy transported by these main pulses at the sites of the bright points over the entire visible solar surface can account for a substantial fraction of the radiative loss from the quiet chromosphere, according to current models.     

Wave propagation in the quiet solar chromosphere

In order to precise previous results about wave propagation in the quiet chromosphere (N. Mein and P. Mein, 1976), we study the behaviour of Doppler shifts and intensity fluctuations in 3 lines of Ca ii. We use the same observation as in our previous work, that is to say a sequence of spectra lasting 27 mn, taken at Sacramento Peak Observatory solar tower. Results can be summarized as follows:

1. Phase-lag between intensity fluctuations and dopplershifts is always near 90° in the Ca ii lines, even for frequencies as high as 15 mHz, and whatever is the location in the chromospheric network.
2. Magneto-acoustic waves propagating vertically in a vertical or horizontal magnetic field could account for the observations only if they were, on one hand reflected in the upper atmosphere, on the other hand propagating with a very high sound or Alfvén speed. The lower limit for the speed (70 km s^-1) does not seem to be realistic. Oblique waves could be investigated for better agreement.

     

The hydrogen balmer lines and the structure of the quiet solar chromosphere

Spectra of Hα, Hβ and Hδ have been taken under good seeing conditions with the vacuum tower telescope of Sacramento Peak Observatory. Intensity curves are given at various wavelengths in these lines to permit further comparison with a theoretical model. Moreover, considering in each case the range of height in which the lines are almost optically thin and using a few approximations, the following results are derived: between 2000 and 6000 km above the limb the average thermal + turbulent velocity of the atoms is found to increase from 20 km s^?1 to 30 km s^?1 and the mean number of hydrogen atoms per cm³ in level 2 is given by $$\log n_2 {\text{ = }}4.5{\text{ }} - {\text{ 0}}{\text{.00056(}}z - 2000)$$ z being the altitude above the limb in km. For line profile computations a new interpolation formula is presented; it gives good profiles with a small number of scans, saving microphotometer time.     

Short-lived flare-like phenomena in the quiet chromosphere

We present a minimum-gradient model to complement a maximum-gradient model (Zwaan, 1974). Both models are compatible with published continuum intensities and within constraints on the temperature gradient; both are adjusted to T _eff=4000K. In order to explain the visual intensities the minimum-gradient model requires an additional line-haze opacity which is only slightly smaller than in the maximum-gradient model.We show that various published models are not compatible with plausible constraints on the temperature gradient.     

Velocity structure and variations in the region of quiet solar filaments

We study the velocity fields in the region of quiet solar filaments using spectral observations at the Sayan Solar Observatory (ISTP, Irkutsk). Once the series of spectral images have been processed, maps of the two-dimensional distribution of the velocity and its variations in the chromosphere (in the Hβ λ = 486.13 nm line) and the photosphere (in the Fe I λ = 486.37 nm line) are constructed. The motions in the filaments have been found to consist of steady and periodic components. Our analysis of the spatial distributions of various oscillation modes shows that the short-period (<10 min) oscillations propagate mainly vertically and are observed at the filament edges, on scales of several arcseconds. The quasi-hour (>40 min) oscillations propagate mostly along the filament at a small angle to its axis. The intensity in the Hβ core in individual fragments of some filaments varies with a period of about one hour. The observed velocity structures in the filaments and the imbalance of steady motions on the opposite sides of the filaments can be explained in terms of the model of a twisted fine-structure magnetic flux tube.     

On the energy dissipation of fast hydromagnetic shock waves in the solar chromosphere

MHD equations including dissipation terms are applied to study the most important irreversible processes occurring in fast hydromagnetic shock waves under the conditions of the outer solar atmosphere. The atmosphere is assumed to be permeated by a nearly horizontal, uniform magnetic field, the magnitude and inclination angle of which being parameters of the analysis. Numerical examples, corresponding to situations which might occur in the upper chromosphere, are computed in order to demonstrate the procedure.     

Mass motion in the solar chromosphere

A high resolution profile of the solar Oi 1304.9 Å line has been measured from rocket spectrograms. The profile is nearly flat-topped, showing only a slight solar reversal after instrument effects and absorption due to atomic oxygen in the earth's atmosphere have been allowed for. A theoretical analysis of this line, under the assumption of non-LTE conditions and a homogeneous, spherically symmetric chromosphere, predicts a rather deep solar reversal. The theoretical profile may be made consistent with the observed profile if mass motion is present in the chromospheric region where the line is formed. A Gaussian distribution of up and down velocities with a root mean square velocity of about 7 km/sec gives best agreement between the predicted and observed profile. This result is consistent with the conclusion made from a study of high resolution profiles of solar lines in the visible spectrum that mass vertical velocities increase with height above the photosphere.     

Effect of small-scale bernstein turbulence on low-frequency plasma waves in the preflare solar chromosphere

The studied region is a part of the current circuit of a magnetic loop in a solar active region in the altitude range of 1400–2500 km above the photosphere. At the earliest stage of development of a flare process, the magnetic field of the loop was assumed to be stationary and uniform in the interval corresponding to weak fields (the so-called deca-hectogauss fields). The conditions for emergence and development of instability of the second harmonic of Bernstein modes in this previously unexamined region were determined. This instability (and low-frequency instabilities emerging later) was assumed to be caused by the sub-Dreicer electric field of the loop, while pair Coulomb collisions were considered to be the major factor hindering its development. The obtained extremely low instability thresholds point to the possibility of subsequent emergence of low-frequency instabilities (and plasma waves corresponding to them) with much higher threshold values against the background of saturated Bernstein turbulence. The frequency of electron scattering by turbulence pulsations in this scenario normally exceeds the frequency of pair Coulomb (primarily ion–electron) collisions. Both the quasistatic sub-Dreicer field in the loop and the weak spatial inhomogeneity of plasma temperature and density were taken into account in the process of derivation and analysis of the dispersion relation for low-frequency waves. It was demonstrated that the solutions of the obtained dispersion relation in the cases of prevalent pair Coulomb collisions and dominant electron momentum losses at pulsations of saturated Bernstein turbulence are morphologically similar and differ only in the boundary values of perturbation parameters. In both cases, these solutions correspond to the two wave families, namely, kinetic Alfven waves and kinetic ion acoustic waves. These waves have their own electric fields and may play the important role in the process of preflare acceleration of energetic electrons.     

High-resolution photography of the solar chromosphere

High-quality photographs of the solar limb and neighbouring regions of the disk at various wavelengths in the H line have been obtained through a tunable 1/8 Å filter used in tandem with a 1 Å Halle filter to eliminate parasitic light. The new observations throw further light on (a) the reappearance of the photospheric limb in the wings of H, and (b) the dark band lying immediately above the photospheric limb described by Loughhead (1969) and Nikolsky (1970). On the other hand, taken in isolation they add very little new information bearing on the question of the identification of spicules with disk structures.As a result of small temperature drifts in the 1/8 Å filter the stated wavelengths of the photographs given in Figures 1–4 may be in error by upwards of a few hundredths of an Ångström. Provision has since been made for setting accurately the zero of the wavelength scale by the method described by Bray and Winter (1970); this involves temporarily sliding the Halle filter out of the beam.     

Wave heating of the solar chromosphere

The nonmagnetic interior of supergranulation cells has been thought since the 1940s to be heated by the dissipation of acoustic waves. But all attempts to measure the acoustic flux have failed to show sufficient energy for chromospheric heating. Recent space observations with TRACE, for example, have found 10% or less of the necessary flux. To explain the missing energy it has been speculated that the nonmagnetic chromosphere is heated mainly by waves related to the magnetic field. If that were correct, the whole chromosphere, magnetic as well as nonmagnetic, would be heated mainly by waves related to the magnetic field. But contrary to expectation, the radiation emerging from the nonmagnetic chromosphere shows none of the signatures of magnetic waves, only those of acoustic waves. Nearly all the heating of the nonmagnetic chromosphere must therefore be due to acoustic waves. In the magnetic network on the boundary of supergranulation cells, on the other hand, the small filling factor of the magnetic field in the photosphere implies that only a small fraction of the wave flux that travels upward to heat the chromosphere can be channeled by the magnetic field. Hence, while some of the energy that is dissipated in the magnetic network is in the form of magnetic waves, most of it must be in the form of acoustic waves. Thus, the quiet solar chromosphere, instead of being heated mainly by magnetic waves throughout, must be heated mainly by acoustic waves throughout. The full wave flux heating the quiet chromosphere must travel through the photosphere. In the nonmagnetic medium, this flux is essentially all in the form of acoustic waves; TRACE registers at most 10% of it, perhaps because of limited spatial resolution.     

High-resolution photography of the solar chromosphere

Computer control of a tunable 1/8 Å filter is described. The filter is installed in the 30-cm chromospheric telescope of the CSIRO Solar Observatory, Culgoora.     

High-resolution photography of the solar chromosphere

High-resolution filtergrams of a sunspot and its surroundings, taken at seven wavelengths in H with the aid of a computer-controlled 1/8 Å filter, have been used to derive the contrast of ten sunspot superpenumbra fibrils as functions of wavelength. The observed contrast profiles are compared with profiles calculated on the basis of three theoretical models, namely, Beckers' (1964) cloud model, Athay's (1970) velocity model, and a pure Doppler shift model. However, none of these models in their present form account for all the observed profiles.Arguments are presented which suggest that, in addition to line-of-sight velocity, the height of a moving chromospheric structure relative to the region over which the H line is formed plays a decisive role in shaping the observed profile. Improvements to existing theoretical treatments are suggested.The nature of the fibril velocity field is discussed briefly.     

High-resolution photography of the solar chromosphere

Measurements have been made of the heights of seven bright mottles photographed beyond the limb on sequences of broad-band Hα filtergrams. They reveal no evidence of a systematic upward or downward motion persisting throughout the lifetime of a mottle, and in no case does the observed change in height exceed 1″ (725 km) over the period of observation. On the other hand, the possibility cannot be excluded that the bright mottles execute vertical oscillations with velocities comparable to those recently deduced from Hα contrast profile analyses by Grossmann-Doerth and von Uexküll (1971) and Bray (1973) provided the periods do not exceed a few tens of seconds.

     

High-resolution photography of the solar chromosphere

High-quality H photographs of the solar chromosphere reveal the presence around isolated sunspots of a pattern of elongated dark elements (fibrils) bearing a strong resemblance to a greatly enlarged version of the white-light penumbra. Individual fibrils have a representative length of some 25 of arc (18000 km) and a typical separation of 2–3. Comparison of pairs of photographs separated by intervals ranging from 0.5 min to 42 min shows that the fibrils undergo continual changes in brightness, size, and shape; their average lifetime is about 17 min. The question is raised whether the fibril structure around a spot is related in any way to the observed inflow of material from the surrounding chromosphere (Evershed effect).     

High-resolution photography of the solar chromosphere

Using photographs of the centre of the solar disk with an effective resolution of 1 sec of arc or better, the size, shape, and evolution of dark mottles are studied.     

High-resolution photography of the solar chromosphere

A method is described for reconstructing the true geometry of a solar loop observed on the disk which takes account of tilt in its own plane. Reconstructions of three H loops yield small tilt angles (14°) and provide further evidence that H loops show a close correspondence to the field lines of a magnetic dipole. The method offers new opportunities for exploring the physics of individual solar loops.     

High-resolution photography of the solar chromosphere

A new telescope designed for high-resolution photography of the chromosphere is described. It is mounted in the open air on a 15-m tower and is provided with air suction shields to cool all surfaces exposed to the sun's rays. The best moments for photography are selected by a seeing monitor. The highest resolution so far attained is 0. 75 of arc, compared with a theoretical resolution limit at H of 0. 55.     

High-resolution photography of the solar chromosphere

High-resolution filtergrams of the quiet chromosphere, taken at seven wavelengths in H with the aid of a computer-controlled 1/8 Å filter, have been used to derive the contrast of ten bright and dark mottles as functions of wavelength. The contrast profiles of bright and dark mottles are strikingly different. They disagree with Athay's (1970) velocity model but, with an appropriate choice of parameters, can be brought into good agreement with Beckers' (1964) cloud model. Comparison between observation and theory yields values for the source function S, optical thickness t ₀, line broadening parameter ₀, and line-of-sight velocity V for both bright and dark mottles. The values of S and t ₀ obtained for dark mottles are consistent with Beckers' (1968) spicule model.