Flare development in active region NOAA 6891, 30 October, 1991

A great sunspot group of October 1991 (NOAA 6891 atL = 189,B = –10) was observed, in which an eruptive flare of importance (X2.5/31B) has been reported. A Caii-line 3933 filter was used, and during a time interval of 3 hours more than 90 frames were taken, using a CCD camera. The progress and the distribution in size of the two main flare points have been obtained; they covered an area of about 1800 millionths of the solar disk within a time interval of about 24 min. The average apparent horizontal velocity was 70 km s^–1.     

Extended dust shells around carbon stars resolved by “HIRAS”

We have examined forty-two carbon stars which show excess emission at 60 and/or 100µm by applying maximum-entropy image reconstruction techniques to the IRAS 60µm survey data. Thirteen stars are found to be extended in the reconstructed images. Four of them show a detached ring centered on the stellar position. In particular, U Ant may have a double detached dust shell. The implications of our results are discussed concerning the variation of mass loss on the AGB evolution.     

A revised listing of the number of sunspot groups made by Pastorff, 1819 to 1833

J. W. Pastorff of Drossen, Germany, made about 1477 observations of sunspots between 1819 and 1833. These observations were erroneously interpreted by A. C. Ranyard in 1874 and then used by Rudolf Wolf in his calculations of the Wolf Sunspot Numbers. The result is a noisier daily time series and overestimation of the monthly and yearly means for these years. Pastorff was actually a very good observer. In this paper, Pastorff's original observations are reexamined and more nearly correct values for the number of sunspot groups are tabulated. We show some examples of the problems created by Ranyard's interpretation and the consequences for the history of solar activity that a correct interpretation of Pastorff's observations will have. Pastorff's observations provide valuable information on the first strong cycle after the Dalton Minimum (1795–1823).     

A measurement of the O2 ultra-violet nightglow emission

New measurements of the Herzberg I emission height profile in the night airglow are reported and indicate a peak emission height near 96 km in agreement with previous measurements. Using an atomic oxygen concentration profile determined from the oxygen green line profile measured on the same rocket it is concluded that the O₂(A³Σ_u⁺) state is not excited in the direct three body recombination of atomic oxygen. It is suggested that the excitation mechanism is a two step process, similar to the Barth mechanism for the atomic oxygen green lineand that the excited intermediate state is C³Δ_u.     

Mass flow and the validity of ionization equilibrium on the Sun

Ionization equilibrium is a useful assumption which allows temperatures and other plasma properties to be deduced from spectral observations. Inherent to this assumption is the premise that the ion stage densities are determined solely by atomic processes which are local functions of the plasma temperature and electron density. However, if the time scale of plasma flow through a temperature gradient is less than the characteristic time scale for an important atomic process, deviations from the ionization stage densities expected for equilibrium will occur which could introduce serious errors into subsequent analyses. In the past few years, significant flow velocities in the upper solar atmosphere have been inferred from observations of emission lines originaing in the transition region (about 10⁴–10⁶ K) and corona. In this paper, three models of the solar atmosphere (quiet Sun, coronal hole, and a network model) are examined to determine if the emission expected from these model atmospheres could be produced from equilibrium ion populations when steady flows of several kilometers per second are assumed. If the flows are quasi-periodic instead of steady, spatial and temporal averaging inherent in the observations may allow for the construction of satisfactory models based on the assumption of ionization equilibrium. Representative emission lines are analysed for the following ions: C iii, iv, O iv, v, vi, Ne vii, viii, Mg ix, x, Si xii, and Fe ix–xiv. Two principle conclusions are drawn. First, only the iron ions are generally in equilibrium for steady flows of 20 km s^–1. For carbon and oxygen, ionization equilibrium is not a valid assumption for steady flows as small as 1 km s^–1. Second, the three models representing different solar conditions behave in a qualitatively similar manner, implying that these results are not particularly model dependent over the range of temperature gradients and electron densities thus far inferred for the Sun. In view of the flow velocities which have been reported for the Sun, our results strongly suggest caution in using the assumption of ionization equilibrium for interpreting spectral lines produced in the transition region.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On depth-dependence of photospheric oscillations

We report on results from photographic observations of photospheric oscillations as a function of depth. Using rms-values and power-spectra from shifts of entire line-profiles, we find qualitatively an increase of the velocity-amplitude with increasing height. We get more quantitative informations by comparing measured asymmetries of line-profiles with calculated ones derived from Voigt-functions containing a depth dependent velocity-field.We find the scale-height H ₀ of photospheric velocity oscillations to be 930±100 km. This result is to be compared with H ₀ = 1100±200 km obtained by Canfield (1976), who used velocity weighting functions of the line centres.Further, we show that a general observed line asymmetry of medium strong lines (c-shape) does not depend on the phase of oscillations.Mitt. aus dem Kiepenheuer-Institut Nr. 178.     

Spatial distribution of large-scale solar magnetic fields and their relation to the interplanetary magnetic field

The spatial organization of the observed photospheric magnetic field, as well as its relation to the polarity of the interplanetary field, have been studied using high resolution magnetograms from Kitt Peak National Observatory. Systematic patterns in the large scale field have been found to be due to contributions from both concentrated flux and more diffuse flux. It is not necessary to assume, as has often been done in previous studies, that there is a weak background solar magnetic field causing the large-scale patterns in the photosphere, although the existence of such a field cannot be excluded. The largest scale structures in the photosphere correspond to the expected pattern at the base of a warped heliomagnetic equator.The polarity of the photospheric field, determined on various spatial scales, correlates with the polarity of the interplanetary field, with the most significant correlation due to mid-latitude fields. However, because the interplanetary field is likely to be rooted in concentrated photospheric regions, rather than across an entire polarity region, both the strength and polarity of the field are important in determining the interplanetary field. Thus studies of the interplanetary field which are based on either instrumental or numerical averaging of fields in the solar photosphere are subject to serious inherent limitations.Analyses based on several spatial scales in the photosphere suggest that new flux in the interplanetary medium is often due to relatively small photospheric features which appear in the photosphere up to one month before they are manifest at the Earth. The evolution of the over-all photospheric pattern may be due to individual sub-patterns which have slightly different rotation properties and which alternate in their relative dominance of the interplanetary medium.     

Dependence of the correlation of small scale photospheric structures upon resolution

Correlations between continuum intensity, velocity, and equivalent widths of two Mn i lines as observed with two different entrance apertures tend to deteriorate with improvement in spatial resolution. The KPNO multichannel magnetograph was used to make area scans at the center of the disk with entrance apertures 3.5×2.5 arc and 1×1 arc. A coherence analysis shows that this effect is caused by marked differences of fluctuations in temperature and temperature gradients as well as in the velocity structure of photospheric elements of various sizes.     

High resolution polarimetric measurements in a sunspot

A Fabry-Perot interferometer is being used for two-dimensional spectropolarimetric measurements. We demonstrate the suitability of the setup for the measurement of the magnetic field and present some preliminary results from first observations.     

Magnetic reconnection induced by Kelvin Helmholtz instability

MHD simulation study is performed to investigate magnetic reconnection induced by the Kelvin Helmholtz instability in the initially sheared magnetic field geometry as well as in the uniform magnetic field geometry. Slow mode rarefaction structures seen in the uniform field case are not observed in the sheared field case. Dynamo action is less prominent and the conversion of plasma flow energy into the other forms of energy is also smaller in the sheared field case than in the uniform field case. Momentum transport is mostly due to the hydrodynamic stress in the sheared field case, while the electromagnetic stress is dominant in the uniform field case. The long term evolutions are also markedly different in the two cases. In the uniform field geometry, the magnetic field lines twisted due to the Kelvin Helmholtz instability become reconnected and flattened so that they resume the straight field line structure which resembles the initial field geometry. The magnetic field, however, is not uniform with smaller intensity in the central region where the pressure balance is partially maintained by the enhanced thermal pressure. In the initially sheared magnetic field geometry, magnetic reconnection continues to operate until the end of the simulation and the conversion of the flow energy into the thermal energy is still seen.