On the origin of the prolate solar chromosphere

A simple geometric model is proposed to explain the recently reported effect of the prolateness of the solar chromosphere. We assume that a specific dynamical part of the solar atmosphere above the 2 Mm level, being a mixture of moving up and down jets of chromospheric matter with the coronal plasma between them, is responsible for the solar prolateness. Due to the dynamic nature of this layer, the magnetic field is considered to play a very important role in the density distribution with the height, guiding the mass flows along the field lines. The difference of the magnetic field topology in the polar and the equatorial regions leads to different heights of the chromospheric limb. Calculations show a satisfactory coincidence with observations when the mean separation between opposite polarity concentrations is about 9 Mm. The possible observational signature of this network in low photospheric and chromospheric layers is discussed.     

On the size of the structure elements in the solar chromosphere

Photometric reductions of the spectrograms obtained during the third flight of the Soviet Stratopsheric Solar Station are discussed. A comparison of photometric scans in H and its far wings near to the continuum leads to the conclusion that chromospheric mottles are at least several times broader than photospheric granules. The optimum size of mottles is about 0.8–1.1. The H profiles of mottles are practically the same as those obtained from the ground observations. The broadening of mottles is considered as an effect of expansion of magnetic arcs growing up to chromospheric levels.     

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

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.     

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.     

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).     

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

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

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

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

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.     

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.     

Velocity waves in the quiet solar chromosphere

Propagation of velocity waves are investigated in the solar chromosphere, with a special view to high frequencies (periods 60 s). Four line profiles have been observed during 27 mn with the Sacramento Peak vacuum telescope (H, 3933, 8498 and 8542 Ca ii). Three Fourier analysis are performed according to the location in the cells of the chromospheric network. Phase-shifts and amplitude ratios between the line Doppler shifts are computed as functions of frequency. The pollution of high frequency results by energetic low frequency oscillations is investigated.H Doppler shifts are probably affected by the large width of line formation layers (low transfer function). Using formation altitudes for Doppler shifts previously computed for the infra-red lines, we show that acoustic waves propagating upwards cannot account for the observations. In particular, the phase-shifts between oscillations in different chromospheric layers are much smaller than theoretical predictions. As a first attempt for a qualitative agreement, we suggest that most of the high frequency oscillations (10–15 mHz) are magnetoacoustic waves, travelling in layers where the gradient of the Alfvén-speed cannot be neglected, and reflected at the top of the chromosphere. The amplitudes of these waves are probably underestimated as derived from the observed Doppler shifts.