Harrison P. Jones

On the Role of the Solar Corona and Transition Region in the Excitation of the Spectrum of Neutral Helium

We investigate the formation of the spectrum of neutral helium in the solar atmosphere by solving the corresponding non-LTE problem in an extended grid of model atmospheres. From the results we infer several general properties and scaling laws that can be used to discriminate the different proposed formation mechanisms. Some of the scaling laws have also been tested in a comparison with previous calculations made with different assumptions. In our models, the excitation of the spectrum by direct photoionization by EUV radiation shortward of 504 A followed by recombinations (PR mechanism), seems to be capable of influencing significantly the resonance continuum and the subordinate lines, even in the presence of other (collisional) excitation mechanisms. While this influence is almost unavoidable in most atmospheric features, it is hardly justifiable as the only possible contribution. Moreover, the resonance lines seem inclined to respond much more effectively to the conditions of the lower transition region, even in the presence of a significant coronal EUV illumination. With the help of the detailed non-LTE calculations and of the derived scaling laws, we explore the interplay of the possible formation mechanisms and their effect on the individual spectral features.

The NASA/NSO spectromagnetograph

The NASA/NSO Spectromagnetograph is a new focal plane instrument for the National Solar Observatory/Kitt Peak Vacuum Telescope which features real-time digital analysis of long-slit spectra formed on a two-dimensional CCD detector. The instrument is placed at an exit port of a Littrow spectrograph and uses an existing modulator of circular polarization. The new instrument replaces the 512-channel Diode Array Magnetograph first used in 1973. Commercial video processing boards are used to digitize the spectral images at video rates and to separate, accumulate, and buffer the spectra in the two polarization states. An attached processor removes fixed-pattern bias and gain from the spectra in cadence with spatial scanning of the image across the entrance slit. The data control computer performs position and width analysis of the line profiles as they are acquired and records line-of-sight magnetic field, Doppler shift, and other computed parameters. The observer controls the instrument through windowed processes on a data control console using a keyboard and mouse. Early observations made with the spectromagnetograph are presented and plans for future development are discussed.

The Solar Acoustic Spectrum and Eigenmode Parameters

The Global Oscillation Network Group (GONG) project estimates the frequencies, amplitudes, and linewidths of more than 250,000 acoustic resonances of the sun from data sets lasting 36 days. The frequency resolution of a single data set is 0.321 microhertz. For frequencies averaged over the azimuthal order m, the median formal error is 0.044 microhertz, and the associated median fractional error is 1.6 × 10−5. For a 3-year data set, the fractional error is expected to be 3 × 10−6. The GONG m-averaged frequency measurements differ from other helioseismic data sets by 0.03 to 0.08 microhertz. The differences arise from a combination of systematic errors, random errors, and possible changes in solar structure.

GONG Observations of Solar Surface Flows

Doppler velocity observations obtained by the Global Oscillation Network Group (GONG) instruments directly measure the nearly steady flows in the solar photosphere. The suns differential rotation is accurately determined from single observations. The rotation profile with respect to latitude agrees well with previous measures, but it also shows a slight north-south asymmetry. Rotation profiles averaged over 27-day rotations of the sun reveal the torsional oscillation signal—weak, jetlike features, with amplitudes of 5 meters per second, that are associated with the sunspot latitude activity belts. A meridional circulation with a poleward flow of about 20 meters per second is also evident. Several characteristics of the surface flows suggest the presence of large convection cells.

The three-dimensional structure of atmospheric magnetic fields in two active regions

The magnetic field above two unrelated active regions on 11 and 12 September, 1974 has been studied using magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and Hα. In C I 9111, originating low in the photosphere, the fields are strong and sharply defined. In Ca II 8542 and Hα they are very diffuse, with significant diffuseness also in Fe I 8688, due to the spreading of the field with height to form almost horizontal magnetic canopies over regions free of field at lower levels.Within a region between two small sunspots some 140 Mm apart, the canopy height found is typically 300–400 km. Within a small superpenumbra, the canopy height is ∼ 150–250 km. In extensive areas surrounding the active regions, over one-half the canopy bases are less than 400–500 km above the τc = 1 level, and over 80% less than 700 km.Arguments are given that the chromospheric fibrils (e.g., in Hα), taken to delineate the field configuration, are not due primarily to lateral variations in field but rather to differences in density or excitation of gas across the lines of force.

He I 10830 Å Wing Asymmetry in Polar Coronal Holes: Evidence for Radial Outflows

Imaging spectroscopy of the Sun, carried out across the the solar poles, yielded several thousand profiles of the He I 10830 A chromospheric absorption line with effective spatial pixels of 1.1 × 2 arcsec2. Profiles of He I 10830 A show the relative blue-wing absorption is stronger in the coronal holes than in the quiet Sun, creating an asymmetric profile indicative of mass outflow. Within the coronal holes, blueshifted line wings are found where He I absorption is weak, corresponding to the center of supergranular cells. However, in the quiet Sun, there is no line wing shift in supergranular centers. Spatially compact regions of strong red-wing absorption also occur across the disk. Within the polar coronal holes, the amplitude of the wing shift shows a linear dependence with cos θ (where the angle θ is measured with respect to an outward normal to the Suns surface), suggesting that a radial outflow occurs with a characteristic speed of ~8 km s-1. These observations represent the first detection of systematic outflows near the chromosphere transition region interface that appear to mark the origin of the high-speed wind acceleration from the solar surface.

Simultaneous observations of solar plage with the solar extreme ultraviolet rocket telescope and spectrograph (SERTS), the VLA, and the kitt peak magnetograph

We obtained simultaneous images of solar plage on 1991, May 7 with SERTS, the VLA,4 and the NASA/National Solar Observatory spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Using intensity ratios of Fe XVI to Fe XV emission lines, we find that the coronal plasma temperature is (2.3-2.9) x 10 exp 6 K throughout the region. The column emission measure ranges from 2.5 x 10 exp 27 to l.3 x 10 exp 28 cm exp -5. The calculated structure and intensity of the 20 cm wavelength thermal bremsstrahlung emission from the hot plasma observed by SERTS is quite similar to the observed structure and intensity of the 20 cm microwave emission observed by the VLA. Using the Meyer (1991, 1992) revised coronal iron abundance, we find no evidence either for cool absorbing plasma or for contributions from thermal gyroemission. Using the observed microwave polarization and the SERTS plasma parameters, we calculate a map of the coronal longitudinal magnetic field. The resulting values, about 30-60 G, are comparable to extrapolated values of the potential field at heights of 5000 and 10,000 km.

Magnetic canopies in unipolar regions

Base-height statistics are presented for magnetic canopies in six unipolar magnetic regions which were observed near the limb with the Kitt Peak Vacuum Telescope and Diode Array Magnetograph during the period 25 April–3 July, 1980. As in earlier studies, extensive areas are found to be covered by low-lying canopies.

Magnetograph response to canopy-type fields

The response of longitudinal-field magnetographs to magnetic fields which are semi-infinite or confined to a horizontal layer is discussed with respect to the interpretation of solar diffuse fields, observed towards the limb, in terms of magnetic canopy models. Numerical results are presented for several reference solar models and typical ‘calibration’ curves are shown for the C I 9111 Å, Fe I 8688 Å, and Ca II 8542 Å lines in magnetostatic atmospheres derived from a mean model. A procedure is developed for determining the base heights of magnetic canopies from observations with an uncertainty not exceeding the order of a pressure scale height. Until definitive information regarding atmospheric structure inside flux tubes can be developed from theory or observation, reliable field strengths cannot be derived from the data.

Correlation of He II lyman alpha with He I 10830 Å, and with chromospheric and EUV coronal emission

This paper describes the results of comparing SERTS-3 images obtained in the transition region line of Heii 304 Å with chromospheric Hei 10830 Å absorption, with strong coronal lines of Mgix 368 Å, Fexv 284 Å and 417 Å, and Fexvi 335 Å and 31 Å, with Hα, with Caii 8542 Å, and with magnetograms in Fei 8688Hα. All of the images are illustrated, and the image reconstruction techniques used are described and evaluated. The high correlation of the Heii 304 Å and Hei 10830 Å images, originally found by Harvey and Sheeley (1977), is confirmed and is put on a quantitative basis. We find that the supergranulation network has greater contrast, and that filaments appear darker, in 10830 Å than in 304 Å. In active regions, the 304 Å line follows more closely the behavior of Hα and Caii 8542 Å than the 10830 Å line.