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.     

Observations of the inner F and K coronas below λ2220

The inner coronal continuum has been observed and measured below 2220 on the slitless spectra obtained by Speer et al. (1970) at the 7 March 1970 eclipse. These observations set some constraints on the brightness of the inner F corona and hence on the scattering efficiency of the inner interplanetary dust cloud particles in the far ultraviolet. They neither confirm nor reject the possibility that the inner dust cloud has the same sharp upturn in scattering efficiency below 2000 observed in the zodiacal light and in the interstellar medium.On leave from Imperial College, London.This work was supported in part by NASA grant NGL 12-001-011. An I.A.U. travelgrant to one of us (R.J.S.) is gratefully acknowledged.     

Magnetic pukas and the lifetime of the supergranulation

We direct attention to the existence of exceptionally long-lived supergranular patterns (4–7 days) seen on synoptic magnetograms.     

The derivation of vector magnetic fields from stokes profiles: Integral versus least squares fitting techniques

We compare the results of two distinct methods for deriving photospheric vector magnetic fields from the Zeeman effect as observed in the Fe I line at 6302.5 A at high spectral resolution (45 mA.). One method ignores magneto-optical effects but allows for an absolute determination of B from the integral properties of the Stokes profiles, under the assumption of weak field strength. The other method is an iterative least-squares fitting technique developed by Lites and Skumanich which fits the observed Stokes profiles to the profiles predicted by the Unno-Rachkovsky solution to the radiative transfer equation. We find empirically that for sunspot fields above 1500 gauss the two methods agree in derived azimuthal and inclination angles to within ± 20 degs., Furthermore, for such fields, the estimate of the line of sight field and the transverse field derived using the two methods agree to within ± 500 gauss. In weak field strength regions the integral method can be used with little error and computational load in the estimation of the line of sight field, the transverse field and the inclination angle but the disagreement in derived azimuthal angle is considerable ( ± 90 degs.).     

White light events as photospheric flares

We consider the possibility that white light flares are due to heating of the photosphere by a flux of energetic ions and electrons impinging on it from above. Particles with energy in the range 10 MeV to 1 BeV release most of their energy to the ambient gas at about optical depth unity in the photosphere. This increase in energy produces a temperature perturbation of several hundred degrees in the layer and results in a re-radiation of the energy with a radiative relaxation time of several seconds. The consequences of this model are applied to a study of the great flare of May 23, 1967 and to the very fast event of August 11, 1954. Large numbers of very energetic electrons or protons must be produced in the first few minutes of the primary flare event if our interpretation (or one based on synchrotron emission) is correct.After this paper had been submitted and accepted for publication we received from Dr. vestka a copy of his paper, The Phase of Particle Acceleration in the Flare Development, which was then already in proof. (It appeared in Solar Phys. 13, 471.) A number of the arguments contained in our paper, which we had previously presented in abstract form (Najita and Orrall, 1969) and in part in a dissertation (Najita, 1969), are independently discussed by Dr. vestka who was unaware of our earlier work. Although the ground covered by his and our paper touches in places we have left our paper as originally submitted and believe that this independent agreement supports the conclusions of both.     

The derivation of vector magnetic fields from stokes profiles: Integral versus least squares fitting techniques

Solar Physics - We compare the results of two distinct methods for deriving photospheric vector magnetic fields from the Zeeman effect as observed in the Fe I line at 6302.5 A at high spectral...     

The spectrum of the inner corona observed during the total solar eclipse of 30 May 1965

A series of spectrograms of the inner solar corona were obtained at the total solar eclipse of 30 May 1965 using a fast spectrograph with a circular slit that recorded the spectrum from 3000 to 9000 at all position angles around the limb simultaneously. In this paper absolute intensity is given as a function of position angle for the stronger lines and the continuum. In the coronal enhancement or condensation centered at heliocentric position angle 293°, absolute intensity is given for 34 forbidden emission lines and the continuum.     

Coronal holes

Coronal holes are extensive regions of extremely low density in the solar corona within 60° of latitude from the equator. (They are not to be confused with the well-known coronal cavities which surround quiescent prominences beneath helmet streamers.) We have superposed maps of the calculated current-free (potential) coronal magnetic field with maps of the coronal electron density for the period of November 1966, and find that coronal holes are generally characterized by weak and diverging magnetic field lines. The chromosphere underlying the holes is extremely quiet, being free of weak plages and filaments. The existence of coronal holes clearly has important implications for the energy balance in the transition region and the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The interpretation of total line intensities from optically thin gases

We describe a general method for inferring, from the line emission of an optically thin medium, the physical state of the gas along the column in the line of sight which is sampled by the observations. Since it is not possible to infer the distribution of the physical state parameters with position in the line of sight - any arbitrary rearrangement of material giving equivalent line emission - we seek instead to specify the state in another way. A unique specification is found in terms of the bivariate distribution function (n, T), describing the partitioning of the matter in the gas over the density and temperature. We show that, given sufficient observational data, it is in principle possible to determine both (n, T), and the chemical composition. With less complete data the acuity of the analysis is correspondingly reduced.The method is devised for application to the astronomical case, especially for studies of the solar corona, the chromosphere-corona transition region, planetary nebulae and other optically thin sources. We illustrate the formulation for the situation encountered in the solar corona.Presently on leave of absence from the Institute for Astronomy, University of Hawaii.The National Center for Atmospheric Research is sponsored by the National Science Foundation.John Simon Guggenheim Memorial Fellow, 1970.Of the National Bureau of Standards and the University of Colorado.     

Brightness fluctuations in the K-line wings

A power-spectrum and cross-spectrum analysis has been made of measurements of temporal fluctuations of intensity observed in the K-line wing (2.07 Å from line center) and of simultaneous measurements of temporal fluctuations of Doppler displacement of the cores of 3931.122 Fe i and 3933 Ca ii (K₃). The measurements were made in a quiet region near the center of the Sun's disk. We find that the average power spectra of the intensity fluctuations have two significant peaks of about equal strength: one at 0.0033 Hz (300-s period); and one at about 0.001 Hz (1000-s period). The average rms value of these intensity fluctuations is 0.0435±0.0082. Maximum brightness comes before maximum violet displacement of the Fe i line. The mean of the best determined phases is 137° and of all the data 108°. At those places on the Sun where the 300-s oscillations can be identified in the k₃ core, the Doppler displacement of the Fe i line leads that of the K₃ core by a mean phase angle of 27°.