Ionized helium in prominences and in the chromosphere

Quiescent prominences It is found that Heii 4686 is emitted in the same cold region of 10000 K as hydrogen, metal and neutral helium emission lines. This conclusion is based on the finding that the observed width of 4686 is the same as the calculated width of 4686. The calculated width is derived from the observed widths of hydrogen and metallic lines. The large intensity of Heii 4686 in 10000 K can be explained by the ionization of Heii due to the UV radiation below 228 Å that comes from the corona and the transition region.Loop prominences The very broad width (30 to 50 km s^–1) of 4686 for two post-flare loop prominences shows that the Heii line is emitted in hot regions different from regions of hydrogen and metal emission. From the widths of the Balmer lines and many metallic lines the kinetic temperature for one loop is found to be 16000 K in one part and 7600 K in another part. The electron densities are 10^12.0 cm^–3 and less than 10^11.0 cm^–3 respectively.Chromosphere The intensity of 4686 in the chromosphere can be interpreted in terms of a temperature of 10000 K with the ionization due to UV radiation. But, since observations of the width of 4686 are not available, a definitive conclusion for the chromosphere cannot be reached.     

Theoretical model of flares and prominences

A theoretical model of flare which explains observed quantities in H, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10³² erg, the electron density of 10^13.5 cm^–3 in H flare, the temperature of the X-ray flare of 10^7.3K and the time to attain maximum H brightness (600 s) are derived consistent with observations. It is shown that the top height of the H flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences.The observed small thickness of the H flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.     

Nature of the fine structure of the middle chromosphere

Fine dark H filaments fibrils form at the limb, apparently in most of the middle chromosphere corresponding to an altitude between 1500–2000 km and 4000 km. The space in between filaments is corona and the transition layer. The cool gas in fibrils is protected by the magnetic field against the conductive flux out of the hot corona. Therefore the fibrils stretch up to 4000 km where their temperature is about 18 000 K and the density about 5 × 10⁹ cm^–3. The gas in the fibrils is ionized by electronic collisions and by the external ultraviolet radiation. The second level of the hydrogen atoms in fibrils is populated by recombinations, electronic collisions and by Ly- quanta. The calculated optical thickness of the fibrils in H is about 1, it explains the absorption features on the spectroheliograms. The gas pressure in fibrils is lower than the coronal pressure, and the pressure equilibrium is achieved by a magnetic field of about 1.5–2 G. In the active regions the photospheric fields are stronger, therefore the fibrils in active regions are wider and show more contrast. The emission of the fibrils at the limb is explained by the scattering of the solar radiation. The temperature in arches reaching as high as 5000–6000 km, is stabilized near the top by the HeII emission. Thus the middle chromosphere is essentially a collection of magnetic arches.     

Observations of prominences at 3.5 millimeter wavelength

At 3.5 mm wavelength absorption features are observed in correspondence with H dark filaments on the disk; beyond the limb the prominences correspond to emissive regions. The absorption features are larger (2–3 arc) than the corresponding H dark filaments; the emissive regions at the limb have similar angular sizes. The emissive regions at the limb have electron temperatures of 5500±500 K; the amount of absorption observed on the disk leads to mean electron densities of about 5 × 10¹⁰ per cm³.     

The prominence-corona interface compared with the chromosphere-corona transition region

The intensities of 52 EUV emission lines from each of 9 hedgerow prominences observed at the limb with the Harvard experiment on ATM-Skylab have been compared with intensities from the interior of network cells at the center of the disk, in order to compare the prominence-corona (P-C) interface with the chromosphere-corona (C-C) transition region. The intensity ratio I _cell/I _prominence for each line varies systematically (in all of the prominences observed), with the temperature of formation of the line as T ^–0.6. The density sensitive C iii (formed at T 9 × 10⁴ K) line ratio I ₁₁₇₅/I ₉₇₇ implies an average density 1.3 × 10⁹ electrons cm^–3 in the P-C interface and 4 times this value in the C-C transition of the cells. The total optical thickness at the head of the Lyman continuum is 10 in most of the prominences studied; in two of the prominences, however, we cannot reject the possibility that _o is large. Methods of analysis of these EUV data are developed assuming both a resolved and an unresolved internal prominence structure. Although the systematic differences between the P-C interface and the C-C transition are stressed, the similarities are probably more remarkable and may be a result of fine structure in the C-C transition.Currently on leave from the Institute of Astronomy, Hawaii; at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, 80309.     

Testing MHD models of prominences and flares with observations of solar plasma electric fields

We present measurements of electric fields in quiescent prominences and in a small flare surge, obtained with the CRI electrograph at the NSO/SP 40 cm coronagraph, in 1993 and 1994. Our results on the 9 brightest quiescent prominences enable us to place r.m.s. upper limits ofE _t < 2 – 5 V cm^–1 on the component ofE transverse to the line of sight. We show that these upper limits may be difficult to reconcile with non-ideal MHD models of quiescent prominences formed in extended neutral sheets, whether or not the tearing mode instability is present. They do, however, seem consistent with ideal MHD models of prominence support. We point out also that these upper limits are within a factor 4 of the minimum value of anistropic electric field that exists due to motional Stark effect in any thermal plasma permeated by a directed magnetic field.Our data on the flare surge suggest an electric field of intensityE 35 V cm^–1, oriented approximately parallel to the inferred magnetic field. This detection ofE needs to be verified in other flares. But we note that a detectableE would not be expected in the current interruption flare mechanism, if only a single double layer is present. We show further that the observed relatively narrow, approximately-Gaussian, and only slightly Doppler-shifted Paschen lines, seem inconsistent with the multiple double layers invoked in other models based on the current interruption mechanism. Our detection ofE does seem consistent with reconnection (including tearing-mode) models of flares, provided the field-aligned electrical conductivity is anomalous over substantial volumes of the plasma circuit joining the reconnecting domain to the photosphere.     

The Harvard-Smithsonian reference atmosphere

We present a model of the solar atmosphere in the optical depth range from ₅₀₀₀ = 10^–8 to 25. It combines an improved model of the photosphere that incorporates recent EUV observations with a new model of the quiet lower chromosphere. The latter is based on OSO 4 observations of the Lyman continuum, on infrared observations, and on eclipse electron densities.Our model differs from the Bilderberg Continuum Atmosphere (BCA) in the low chromosphere (₅₀₀₀ < 10^–4), where deviations from local thermodynamic equilibrium in hydrogen and carbon have been taken into account. It also differs in the transition region between the chromosphere and the photosphere (10^–4 < ₅₀₀₀ < 10^–2), where the temperature is lower than in the BCA, and in the convective region (₅₀₀₀ 2), where the temperature is higher than in the BCA.     

The internal motion of quiescent prominences

A study has been made of fine structure wavelength shift in the K line spectra from quiescent prominences. A persistent small scale motion is found in the prominence main body. In places where we see the characteristic thread like fine structure in the accompanying H filtergrams the average line-of-sight velocity amplitude is about 1 km s^–1. A higher velocity ( 4 km s^–1) is associated with a slightly coarser, mottled prominence fine structure. In the low lying regions, connecting the prominence body and the chromosphere, we do not detect any fine structure line shift (v 1/2 km s^–1).     

A time evolution study of limb spicule spectra

Time sequences of simultaneous spectra of limb spicules, obtained using the Sacramento Peak Observatory's tower telescope and echelle spectrograph are analyzed. Intensity determinations of H and K, H, 8498 and 8542 of calcium are tabulated for three observing heights. Electron densities averaged over the entire visible lifetimes of spicules are -6 × 10¹⁰ cm^–3 at observing heights of 6000km, while maximum and minimum values were -1.1 × 10¹¹ cm^–3 at 6000km and - 2 × 10¹⁰ cm^–3 at 10000km. Electron temperatures range between 12 000 K and 16 000 K. Profile halfwidths indicate turbulent velocities of 12 to 22 km s^–1, and spectral tilts are interpreted as caused by differential velocity fields of -3 km s^–1 per 1000 km. No large scale spicule expansions or contractions are observed, although possible expulsions or accretions of material are observed. Spicules may be wider in the calcium K and H lines than in H.Now at School of Science and Engineering, The University of Alabama in Huntsville, Huntsville, Alabama.     

Airborne total eclipse observation of the extreme solar limb at 400 μm

The total solar eclipse of February 26, 1979 was monitored at far infrared wavelengths from the NASA Lear Jet Observatory flying at 12.9 km in the eclipse shadow. The resultant eclipse curve for radiation within a bandwidth of 20 cm^–1 centered upon 25 cm^–1 (400 m) was measured and analysed at an equivalent angular resolution of 1 arc sec over a 100 arc sec region adjacent to the limb to provide information on the intensity distribution of continuum radiation close to this limb. The curve has been compared to predictions derived from models of the solar atmosphere for the specific geometry of this eclipse, and is shown to match most closely that derived from a uniform distribution of radiation across the disk. This is in distinct contrast to the result obtained in the only other comparable experiment, carried out over Africa in 1973 from a supersonic Concorde aircraft, in which an intense but narrow spike of far infrared radiation at the extreme solar limb was inferred from the data. The absence also in the present observations of the significant limb brightening predicted by the HSRA model (in which homogeneity within the source region is assumed) is in substantial agreement with lower resolution results from mountain altitudes. This result is interpreted as further evidence for the presence in the Sun's lower chromosphere of significant inhomogeneity with a scale size of at least 1000 km at this depth.     

Lyman continuum observations of solar flares

A study is made of Lyman continuum observations of solar flares, using data obtained by the Harvard College Observatory EUV spectroheliometer on the Apollo Telescope Mount. We find that there are two main types of flare regions: an overall mean flare coincident with the H flare region, and transient Lyman continuum kernels which can be identified with the H and X-ray kernels observed by other authors. It is found that the ground level hydrogen population in flares is closer to LTE than in the quiet Sun and active regions, and that the level of Lyman continuum formation is lowered in the atmosphere from a mass column density m 5/sx 10^–6 g cm^–2 in the quiet Sun to m 3/sx 10^–4 g cm^–2 in the mean flare, and to m 10^–3g cm^–2 in kernels. From these results we derive the amount of chromospheric material evaporated into the high temperature region, which is found to be - 10¹⁵g, in agreement with observations of X-ray emission measures. A comparison is made between kernel observations and the theoretical predictions made by model heating calculations, available in the literature; significant discrepancies are found between observation and current particle-heating models.     

Coronal environment of quiescent prominences

With thespectro-coronagraph and themultichannel subtractive double pass spectrograph (MSDP) at the Pic du Midi Observatory two quiescent prominences were observed simultaneously. From the spectro-coronagraph observations 2D maps of Hei 10830 , Fexiii 10798 and 10747 line intensities were obtained. In addition, we obtained 2D maps of the ratioR of the two iron lines. This ratio is used as a diagnostic for determining the density of the hot coronal plasma surrounding prominences. We found that the electron density is higher at the location of the prominences than in the corona, whereas small regions (40) of lower electron density are unevenly distributed around the prominences indicating that the surrounding corona is highly inhomogeneous. The density of the cavity is reduced by a factor 1.5 compared to the density of the prominence environment (5 × 10⁸ cm^–3). We discuss the existence of cavities around these prominences according to the orientation of their axes relative to the line of sight and according to the velocity field inside the prominences. Constraints on models for prominence formation are derived.     

Hydrogen and helium excitation by EUV radiation for the production of white-light flares

White-light flares are defined as those flares that produce significant enhancement of emission in the visible light continuum. The source of energy for this emission has not yet been identified with several possibilities being suggested: heating of the lower chromosphere by some mechanical or magnetic means, or by soft X-ray or extreme ultraviolet radiation from coronal loops being absorbed in the lower chromosphere and re-emitted in the visible.Using non-LTE radiative transfer calculations for hydrogen and helium in a simple model atmosphere we show that EUV ( < 912 Å) radiation cannot be the main energy source for white-light flares. Estimates of the observed energy emitted in the visible and the EUV indicate that there may be enough energy in the EUV to account for the white light flare with this mechanism. Using enhancements in the wavelength region below 912 Å of up to 7 × 10⁹ ergs cm^–2 s^–1 ster^–1 (5 × 10⁵ times the estimated q radiation field) to represent flare EUV emission from above we investigated the non-LTE level populations for hydrogen and helium and the lower atmospheric heating resulting from this radiation. The basic result is that the opacities in the Lyman continuum and the helium I and II continua are so much larger than even the enhanced opacity in the visible hydrogen continuum that the EUV radiation is absorbed before it can have a significant effect in the visible light continuum. However, the EUV radiation can cause a significant enhancement of H emission.Operated by the Association of Universities for Research in Astronomy Inc. for the National Aeronautics and Space Administration.     

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.     

Mechanism of formation of a dipole magnetic field in the central regions of active galaxies

A model of the formation of large-scale magnetic fields of dipole configuration in the central regions (r 100 pc) of active galaxies is studied. It is assumed that these regions contain a rapidly rotating, highly ionized gas ( 5·10^–15 sec, N_e 10³ cm^–3). Ionized matter escapes from the center of the region with a velocity of several hundred km/sec and is entrained by the rotation of the surrounding medium. Biermann's "battery" effect [L. Biermann, Z. Naturforsch., 5a, 65 (1950)] operates under such conditions, and circular electric currents are formed in the medium, which amplify the dipole magnetic fields. During the active phase of a galaxy, about 10⁸ years, the magnetic field strength at the boundary of this region may reach 10^–4–10^–3 G.Translated from Astrofizika, Vol. 39, No. 1, pp. 111–119, January–March, 1996.     

The abundance of helium in prominences and in the chromosphere

The abundance of helium relative to hydrogen is spectroscopically determined in prominences and in the chromosphere by using 1952, 1958, 1962 and 1966 eclipse data. Care is taken in the intensity calibration of emission lines, the self-absorption, and the departure from local thermodynamic equilibrium. We find from the line profiles and intensities of prominences and the chromosphere that the neutral helium lines are emitted in the metal-hydrogen emitting region where the kinetic temperature is low enough, 6000 8000 K, so that only the ionization due to UV radiation from the corona can explain the intensity of neutral helium emission. Also we find that the intensity ratio of Hei 3888.65 to H₈ 3889.05 increases towards the upper boundaries of prominences and of the chromosphere and that it approaches to a universal limiting value, both in various prominences or in the chromosphere, where it is considered that the ionization of neutral helium and hydrogen is nearly complete. From these facts the helium to hydrogen number ratio is found to be 6.5 ± 1.5%.A new schematic model of the chromosphere is presented where spicules have no hot region of emitting neutral helium lines. Here it is suggested that the kinetic temperature of spicules, 6000 8000 K, would be primarily determined by the radiation temperature of the corona and the transition region beyond the Lyman continuum of hydrogen which happens to be around those temperatures.     

The effect of two-dimensional macroscopic velocity fields on models of the lower solar chromosphere

Two-dimensional macroscopic velocity fields are featured in the calculation of two-dimensional models of the lower solar chromosphere. Relative rms line centre intensity fluctuation data and mean limb darkening data obtained in Mg b and Na D are used together with values of the cross-correlation between line centre brightness and line of sight velocities. It is found that the large scale fluctuation data can be explained by models of the lower solar chromosphere in which the inhomogeneous effects arise only from horizontal, two-dimensional macroscopic velocity fields. It is also shown, however, that the corresponding small scale fluctuation data cannot be explained in a similar manner.The cross-correlation data is found to be a powerful constraint in the computation of two-dimensional models of these regions.     

A spectroscopic study of solar spicules in Hα, Hβ and K

Simultaneous spectra of spicules in H, H and the Ca ii K have been analyzed. These spectra were obtained with the échelle spectrograph of Sacramento Peak Observatory and have a resolution of 1.5 s of arc. Data include absolute intensities, velocities, linewidths and the optical depth at the center of H. Electron densities range between 6 × 10¹⁰cm^–3 and 1.2 × 10¹¹cm^–3, while the range of the electron temperature is between 12000K and 15000K at a height of 5400 km above the limb. The analysis of the variation of the line of sight velocity across spicules shows that their rotation is very small. There is very good correlation in the structure of all three lines.     

Physics of solar prominences

Summary Conclusion This colloquium on solar prominences - the first ever held - has shown that a major part of activity in prominence research in recent years concentrated on both observation and computation of the magnetic conditions which were found to play a crucial role for the development and the maintainance of prominences. Remarkable progress was made in fine-scale measurements of photospheric magnetic fields around filaments and in internal field measurements in prominences. In addition, important information on the structure of the magnetic fields in the chromosphere adjacent to the filaments may be derived from high resolution photographs of the H fine structure around filaments which have become available recently; unfortunately, an unambiguous determination of the vector field in the chromosphere is not yet possible.It is quite clear, now, that stable filaments extend along neutral lines which divide regions of opposite longitudinal magnetic fields. Different types of neutral lines are possible, depending on the history and relationship of the opposite field regions. There is convincing evidence that the magnetic field in the neighbouring chromosphere may run nearly parallel to the filament axis and that there are two field components in stable prominences: an axial field dominant in the lower parts and a transverse field dominant in the higher parts.Methods for the computation of possible prominence field configurations from measured longitudinal photospheric fields were developed in recent years. In a number of cases (e.g. for loop prominences) the observed configuration could be perfectly represented by a force-free or even a potential field; poor agreement was found between computed and measured field strengths in quiescent prominences. In order to reconcile both of them it is necessary to assume electric currents. Unambiguous solutions will not be found until measurements of the vector field in the photosphere and in the prominences are available.The two-dimensional Kippenhahn-Schlüter model is still considered a useful tool for the study of prominence support and stability. However, a more refined model taking into account both field components and considering also thermal stability conditions is available now. It was proposed that quiescent prominences may form in magnetically neutral sheets in the corona where fields of opposite directions meet.As for the problem of the origin of the dense prominence material there are still two opposite processes under discussion. The injection of material from below, which was mainly applied to loop prominences, has recently been considered also a possible mechanism for the formation of quiescent prominences. On the other hand, the main objections against the condensation mechanism could be removed: it was shown that (1) sufficient material is available in the surrounding corona, and that (2) coronal matter can be condensed to prominence densities and cooled to prominence temperatures in a sufficiently short time.The energy balance in prominences is largely dependent on their fine structure. It seems that a much better radiative loss function for optically thin matter is now available. The problem of the heat conduction can only be treated properly if the field configuration is known. Very little is known on the heating of the corona and the prominence in a complicated field configuration. For the optically thick prominences the energy balance becomes a complicated radiative transfer problem.Still little is known on the first days of prominence development and on the mechanism of first formation which, both, are crucial for the unterstanding of the prominence phenomenon. As a first important step, it was shown in high resolution H photographs that the chromospheric fine structure becomes aligned along the direction of the neutral line already before first filament appearance. More H studies and magnetic field measurements are badly needed.Recent studies have shown that even in stable prominences strong small-scale internal rotational or helical motions exist; they are not yet understood. On the other hand, no generally agreed interpretation of large-scale motions of prominences seems to exist. A first attempt to explain the ascendance of prominences, the Disparitions Brusques, as the result of a kink instability was made recently.New opportunities in prominence research are offered by the study of invisible radiations: X-rays and meterwaves provide important information, not available otherwise, on physical conditions in the coronal surroundings of prominences; EUV observations will provide data on the thin transition layer between the cool prominence and the hot coronal plasma.Mitt. aus dem Fraunhofer Institut No. 111.     

Calculated profiles of H I lines of interest for solar plasma electric field measurements

We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 10³ V cm^–1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12–8 and 15–9 in H i, observed in prominences at wavelengths around 11. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far.