Library

Notes: Abstract We analyze the pre-flare and impulsive phase of an eruptive (two-ribbon) flare at several wavelengths. The total energy (mechanical plus radiative) released by the flare is 8 x 1030 erg, about a factor 6 higher than the free magnetic energy (1.3 1030 erg) estimated from the non-potentiality of the magnetic field configuration in the flare area. During the impulsive phase, we find a very good time coincidence between the hard X-ray light curve and the light curves for 2 small areas (≃ 4″ in size) in the red wing of the Hα line and in the He-D3 line center. This temporal coincidence is compatible with the interpretation that hard X-ray emission is produced by bremsstrahlung of accelerated electron beams striking these dense areas. For the other regions of the Hα ribbons we find more gradual light curves, suggesting a different energy transport mechanism such as conduction.

Notes: Abstract The results of an exploratory work on the use of a high-speed, computer-controlled flying-spot for quantitative photometry of solar Hα images are given. The used method is briefly illustrated and applied to the study of the photometric structure and evolution of some chromospheric active regions. Evolutive curves and isophotes for the 27 October 1969 plages (McMath 10381 and 10385) are presented. The brightness fluctuations of the center, border and node of the quiet chromospheric network are also analyzed; only a marked periodicity at 300 s is present, with a rms relative intensity of 6% for the center, 8% for the border, and 13% for the node of the quiet chromospheric cell. Some conclusions concerning the constraints on the analysis method and the observational material quality needed for an efficient, computer controlled, interactive photometry are drawn.

Notes: Abstract We describe a method of solar bidimensional spectroscopy exploiting the performances of a Universal Birefringent Filter (UBF) like that of the Sacramento Peak Observatory, which enable an estimate of the profile of some chromospheric lines with moderate spectral resolution ( $$(\lambda /\Delta \lambda = 2.5 x 10^4 )$$ ). The numerical inversion technique of Backus and Gilbert has been used to retrieve the estimated line profiles; the capabilities of the proposed method is fully analyzed with some numerical tests and examples. Correction procedures for errors in the positions of the UBF passband, random fluctuations of the exposure times and non-uniform brightness distribution on the filtergrams are also presented. The whole method has been tested on the recovery of quiet atmosphere line profiles and the results derived for the Na D2 line show that the proposed method is completely suitable for many investigations in solar physics.

Notes: Abstract We develop an automatic, computer controlled procedure to select and to analyze the Network Bright Points (NBPs) on solar images. These have been obtained at the Sac Peak Vacuum Tower Telescope by means of the Universal Birefringent Filter and Zeiss Hα filters, tuned, respectively, along the profiles of the Hβ, Mg-b1, Na-D2, and Hα lines. A structure is identified as an NBP if at the wavelength Hα- 1.5 A its maximum intensity is greater than 〈I〉 + 3σ and its area is greater than 1.5 arc sec2 at 〈I〉 + 1.5σ, where 〈I〉 is the mean value and σ the standard deviation of the intensity distribution on the image. Each detected NBP is then searched and confirmed in all the remaining 31 images at different wavelengths. For each NBP several parameters are measured (position, area, mean and maximum contrast, Dopplergram velocity, compactness, and so on) and some identification constraints are applied. The statistical analysis of the various parameter distributions, for NBPs present within an active region and its surroundings, shows that two types of NBPs can be identified according to the value of their mean contrast C min the Hα- 1.5 Å image (C m ≤ 0.1 → type I, C m〉 0.1 → type II). The type I NBPs (all occurring on the boundaries of the supergranular network) appear to be much more frequent (180/26) than the type II ones. The size 〈A〉 of type I NBPs is less than 1.0 arc sec for Hα/Hβ wings but of the order of 1.2 arc sec for Na-D2 and Mg-bl. The mean contrast C m is around the value of 10% along the Na-D2 and Mg-bl profiles and of 20% along the Hα/Hβ wings. The C m - 〈A〉 scatter diagrams show, for the photospheric radiation (h 〈 100 km), a narrow range of variability for C min correspondence with a wide range for 〈A〉. For radiation orginated at higher levels (h 〉 200 km), the C m- 〈A〉 scatter diagrams seem to indicate, even if with a large variance, that the highest C m's tend to correspond to the highest 〈A〉 values. The mean Doppler shift is close to zero for Na-D2 and Mg-bl lines but negative (downward motion) for Hα and Hβ lines. The type II NBPs tends to be preferentially located in the neighbourhood of small, compact sunspots and their detectability is almost constant through all the 4 studied line profiles. No conclusions can be derived on the mean size, contrast and Doppler shift values because their distributions are too dispersed. The only positive information is that its C m- 〈A〉 scatter diagram, in Hα and Hβ wings, indicates a wide range of variability for C m in correspondence with very narrow range of variability for 〈A〉.

Notes: Abstract We present preliminary results of solar bi-dimensional spectroscopy observations obtained with the new 20 mÅ NSO-Sacramento Peak tunable filter. The procedures of image destretching and the corrections for the modulation of the 5-min oscillations are briefly discussed.

Notes: Abstract In the last years we have gained some experience in the diagnostics of small-scale structures, both on the interpretative and on the observational point of view. We report here the conclusions and the suggestions for future developments attained in two main fields of interest.

Notes: Abstract Using the method of solar bidimensional spectroscopy based on the Universal Birefringent Filter (UBF), we have determined the bidimensional maps of moments of some chromospheric lines. The observational material, referring to a quiet region on the disk center, have been acquired with the UBF of the NSO at Sacramento Peak on Aug. 27,1985. In this note we present the work in progress and the new observational aspects arising from this diagnostic method.

Notes: Abstract Inference of magnetic fields from very high spatial, spectral, and temporal resolution polarized images is critical in understanding the physical processes that form and evolve fine scale structures in the solar atmosphere. Studying high spectral resolution data also helps in understanding the limits of lower resolution spectral data. We compare three different methods for calibrating the line-of-sight component of the magnetic field. Each method is tested for varying degrees of spectral resolution on both synthetic line profiles computed for known magnetic fields and real data. The methods evaluated are: (a) the differences in the center of gravity of the right and left circular components for different spectral resolution, (b) conversion of circular polarization, at particular wavelengths, to magnetic fields using model-dependent numerical solutions to the equations of polarized radiative transfer, and (c) the derivative method using the weak field approximation. Each method is applied to very high spatial and spectral resolution circular polarization images of an active region, acquired in the Fei 5250 Å Zeeman-sensitive spectral line. The images were obtained using the 20 mÅ pass-band tunable filter at NSO/Sacramento Peak Observatory Vacuum Tower Telescope. We find that the center-of-gravity separation offers the best way of inferring the longitudinal magnetic field.