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.

     

Vertical and horizontal heating in solar chromosphere

Anisotropy of the stress tensor of turbulent waves is used in this paper to explain the effects of Alfven waves in a magnetic tube. Under the conditions of a force-free field with negligible thermal conduction and convection, the equations governing the behaviour of a one-dimensional steady flow are derived. The calculated results give a reasonable explanation of the observations that the density in a plage is of the same order as, and slightly higher than the density in the surroundings while the temperature is higher by a few hundred degrees. The model of horizontal heating explains the temperature excess in “plage bridges”. The fact that, in the quiet regions, bright patches are often where the vertical field is strong while dark filaments correspond to locations with predominent horizontal field can be attributed to the different amounts of absorption of the turbulent waves by magnetic field of different strengths.     

Radiation loss and mechanical heating in the solar chromosphere

The raditation loss of the solar chromosphere is evaluated on the basis of the Harvard Smithsonian Reference Atmosphere. The total radiative flux is found to be between 2.5 and 3.3 E6 erg cm^?2 s^?1. A discussion of possible heating mechanisms shows that the short period acoustic wave theory is the only one able to balance the chromospheric radiation loss and is consistent with observation.     

Radiation loss and mechanical heating in the low solar chromosphere

We discuss Ulmschneider's claim to have established that the short-period acoustic wave energy input is the only one able to balance the chromospheric radiation loss. We first review the range of uncertainty in empirical and radiative equilibrium models, on which estimates of the excess radiation coming from mechanical heating rest. We then show that Ulmschneider's estimate of this excess radiation from such models uses an over-simplified computational method. The resultant of uncertainty in models and in computational methods suggests that Ulmschneider's results on excess radiation from heating in the low chromosphere is subject to overestimation by an order of magnitude.     

Wave systems in the chromosphere

Observations are reported of two, possibly three, distinct wave systems in the Hα chromosphere.

1. Velocity films show waves propagating predominantly outwards along mottles and fibrils from as close as 2000 km to the network axis at velocities of the order of 70 km s^-1. The line-of-sight component of the velocity amplitude is estimated to be typically 5 km s^-1. The velocities are accompanied by propagating intensity fluctuations. The system is interpreted as one of basically Alfvén waves. Similar waves are observed propagating predominantly outwards along superpenumbral fibrils radiating from a small sunspot.
2. The velocities in the chromospheric granulation undergo fluctuations of an oscillatory character but without any observable horizontal propagation. The intensities show a close correlation with the velocities, maximum intensity occurring about T/4 after maximum downward velocity. The period is variable across the surface (2.5 min upwards). The intensity-velocity correlation is characteristic of a standing compressional wave.
3. Intensity cinefilms at Hα line centre show in places a horizontal drift of the chromospheric granulation pattern at about 12 km s^-1 without any accompanying vertical velocity fluctuations. It is not known whether this is due to a gas stream at sonic velocities, or to a horizontally propagating sound wave.

The Alfvén wave system is shown to make a significant contribution to coronal heating. Whether the velocity fluctuations in the chromospheric granulation also make an important contribution depends on whether there are upwardly propagating or standing waves; this is not yet established despite the intensity-velocity correlation.     

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

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.     

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.     

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.     

A numerical model of a solar flare based on electron beam heating of the chromosphere

There are two parts to this paper. In the first we calculate the hydrodynamic response of the solar atmosphere to the injection of an intense beam of electrons in a numerical simulation of a solar flare. In the second we predict the spectroscopic consequences of the hydrodynamic behaviour calculated in the first part. The hydrodynamics is predicted by solving the equations of conservation of mass, momentum, and energy. The latter is expressed as two temperature equations; one for the electrons and the other for the neutral atoms and positive ions of hydrogen. The equations are solved in one dimension and the geometric form is of a semi-circular loop having its ends in the photosphere. The results show how the loop is filled at supersonic speed with plasma at temperatures characteristic of flares. At the same time a compression wave is predicted to propagate down towards the photosphere. After the heating pulse stops, the plasma that has risen into the loop, starts to decay and return to the condition it was in before the pulse started. In predicting the spectrum that would be emitted by such a plasma calcium was chosen for illustration. The first and main part of this calculation was setting up and solving the time-dependent equations of ionization/recombination. In order to provide a standard for comparison the same ionization and recombination rate coefficients are used to predict the steady-state distribution of populations of ionization stages. This is then compared with the distribution found from the time-dependent solution and shows that there is a negligibly small time lag predicted by the time-dependent result. However the more significant comparisons to make are between the temperatures of the peak abundances of the various ions under the assumptions of steady-state and time-dependent ionization. For the particular circumstances chosen here the temperature differences are predicted to be in the neighbourhood of 10% or less and in view of the overall accuracy of the atomic data are not significant. It would appear therefore that the much simpler assumption of steady-state ionization balance leads to results of acceptable accuracy for the particular case considered.     

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

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

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).     

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.     

High-resolution photography of the solar chromosphere

Bright chromospheric mottles observed at the H line centre are found to have sizes ranging from 1450 to 4400 km and lifetimes of about 11 min. They occur in close juxtaposition to dark mottles which, at intermediate heliocentric angles ( 60°), are found to be displaced towards the limb relative to the associated bright mottles. The magnitude of the displacement indicates a height difference of 4300 km. In conjunction with height measurements of bright mottles beyond the limb (Loughhead, 1969), this implies that bright and dark mottles are phenomena of the lower ( 3300 km) and upper ( 5000–7600 km) chromosphere respectively.     

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

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.     

High-resolution photography of the solar chromosphere

High-resolution filtergrams of an active region loop taken at seven wavelengths in Hα have been used to derive the contrast at eleven locations along its length as a function of wavelength. With an appropriate choice of parameters, theoretical curves calculated on the basis of the ‘cloud” model give a reasonable fit to the observed contrast profiles. The inferred line-of-sight components of the mass velocity range from 27 km s^?1 upward to 78 km s^?1 downward. However, more accurate profiles and a more rigorous theory are needed to confirm the validity of this application of the cloud model.     

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.