Transient brightenings of interconnecting loops

We discuss three different kinds of dynamic events related to interconnecting loops observed in soft X-rays aboard Skylab: (1) A newly born transequatorial loop that was either emerging from subphotospheric layers or gradually filled in with hot plasma. (2) Large-scale twists of interconnecting loops which never relax, and often only form, after the loop brightenings. (3) Three events where the loop that later interconnected two active regions had been visible long before one of the interconnecting regions was born. Several impacts this observation might have upon our understanding of the process of flux emergence are suggested.     

On long-term forecasts of proton flares

174 proton flares which were observed during the period from 1956 to 1965, occurred in 81 different active regions. It is shown that these active regions formed in complexes of activity, which stayed on the solar surface for many months, and in some cases even for several years. Since the proton-flare regions develop very rapidly and reach the proton-flare active stage within a few days, these complexes of activity represent the areas on the sun, where proton-flare regions can form at any time. Reference is made to contributions by Bumba and Howard, who investigated the birth of active regions and detected some properties of complexes of activity; nevertheless, at the present time, we do not know any method to predict when a proton-flare region begins to develop in such a complex of activity.On the other hand, there is a chance of predicting the dangerous longitudes on the sun, as soon as such a complex of activity has been well recognized or, from the opposite point of view, to predict the safe proton-flare free periods on the sun. If, however, all the complexes on both the hemispheres are taken into account and every complex is considered proton-dangerous from 2 days before to 7 days after the central meridian passage, one can prove that no proton-flare free periods existed for more than 3 years around the maximum of the last solar cycle. Applying this result to the present cycle, one can conclude that no safe forecasts of proton-flare free periods can be made from the beginning of 1968 to the end of 1970. During the remaining 7 or 8 years of the solar cycle, long-term forecasts of proton flares could be made provided that our knowledge of the formation and development of the complexes of activity is improved.It is of interest to notice some properties of the complexes formed in the last solar cycle. While the complexes on the Northern solar hemisphere remained at fairly constant heliographic longitudes for many years, the complexes formed on the Southern hemisphere seemed to travel in two rows around the sun, in the direction opposite to the solar rotation. Another interesting fact is a yearly periodicity in the formation of proton-flare regions in the complexes of activity, with a maximum in the summer period and a deep minimum in the winter season. Such a seasonal variation also appears, if one considers the flare activity, type-IV bursts, PCA's, great magnetic storms, and magnetic crochets. Therefore, one can reasonably believe that this yearly variation, even when similar to the seasonal variation at the earth, is of solar origin.Invited Lecture given at the COSPAR meeting in London, July 1967.     

Models of dense stars

The aim of this paper is to present the results of a construction of five models of composite stellar configurations, consisting of an energy-generating convective core surrounded by source-free envelope in radiative equilibrium, and of sufficient density for the coefficient of absorption to vary approximately as inverse square of the local temperature. The principal characteristic of such models proved to be their very high degree of central condensation; their central densities being 10³–10⁴ times as large as the mean density of the composite configuration. The relevance of such models to the internal structure of subdwarf components of close binary systems with periods less than a day is briefly pointed out.Investigation supported in part by Contract N5 ori-07843 with the Office of Naval Research of the U.S. Navy Department.     

An analysis of the radial velocity curves of close binary systems,I

The aim of the present paper will be to develop a theory of the radial-velocity changes of the components of close binary systems, with special attention to phenomena arising from finite dimensions of such components and their mutual distortion as well as irradiation. It is particularly stressed that the deformability of fluid stars and gas motions in their atmospheres can give rise to systematic differences between the observed radial velocities of such stars and those of their mass centres.In Section 2 (which follows a brief statement of the problem outlined in Section 1) we shall introduce the coordinate systems subsequently employed to treat various aspects of our problem: Section 3 will be concerned with an extraction of information from the radial-velocity component of absolute motions of the mass-centres of such stars; and in Section 4 we shall generalize the classical work by an investigation of radial velocities at any point of the apparent disks of distorted components, and their relation to the motion of their centres of mass. Section 5 will contain an evaluation of the effects of distortion, on radial velocity, averaged over the entire visible disk of the respective star at different phases; and in Section 6 we shall extend the same treatment to stars undergoing eclipses.An investigation of the effects, on the observed radial velocities, of atmospheric streaming caused by mutual irradiation of the two stars is being postponed for a subsequent communication.     

Locations of Footpoints of Transequatorial Interconnecting Loops

We discuss footpoints of loops seen by Yohkoh in soft X-rays that connect active regions across the equator (transequatorial interconnecting loops – TILs). While most TILs are rooted in moderately strong fields at peripheries of active regions, there are also cases when these loops are anchored in very weak or very strong fields, ranging from < 30 G to several hundred gauss. Some have their footpoints near sunspot penumbrae, creating `X-ray fountains' in a combination with active region loops. But TILs are never rooted in sunspots. The most likely explanation is that magnetic field lines leave spots almost vertically so that TILs rooted in them extend high into the corona and density in them is below the limit of visibility in X-rays. The fact that in force-free modeling some TILs are rooted in sunspots is most probably due to the difference between field-line connections in `vacuum' and in the highly conductive plasma on the Sun. Some TILs end before they reach active regions which sometimes may indicate the real situation, but mostly this `gap' is probably due to a temperature decrease near the loop footpoints which makes them invisible in X-rays. In that case the fact that these cool lowest parts of TILs are never found in TRACE or SOHO EIT images indicates that plasma density in TILs must be very low. Still, the total absence of any counterparts of X-ray TILs in TRACE and EIT images is puzzling and, therefore, other possible interpretations of the `gap' origin are also briefly mentioned.     

Dynamical tides in close binary systems,I

The aim of the present paper will be to develop from the fundamental equations of hydrodynamics a theory of dynamical tides in close binary systems, the components of which are regarded to consist of heterogeneous viscous fluid, and to revolve around their common centre of gravity in eccentric orbits; moreover, the equatorial planes of their axial rotation and the orbital plane need not be co-planar, but all may be inclined to the invariable plane of the system of arbitrary amounts. The changes in the pressure or density invoked by time-dependent deformation will be regarded as adiabatic; but, in the equilibrium state, both the density and viscosity of the material of our components may be arbitrary functions of the radial distance.Following a brief exposition in Section 2 of the fundamental equations linearized to small oscillations — be these free or forced — in Section 3 we shall particularize them to describe spheroidal deformations; with due regard to all terms arising from viscosity. Section 4 will contain a specification of the boundary conditions to be imposed upon such oscillations; and in Section 5 we shall solve the problem of non-radial oscillations of self-gravitating inviscid configurations in terms of hypergeometric series. The remaining Sections 6–8 will be devoted to a discussion of the phenomena arising from viscosity: in particular, we shall solve in a closed form the problem of non-radial oscillations of incompressible viscous globes in the terms of Bessel functions. It will be shown that the effect of viscosity — like those of compressibility — tend to de-stabilize all non-radial oscillations of homogeneous configurations.At the other extreme, a similar treatment of a mass-point model — as well as of one exhibiting high but finite degree of central condensation — is being postponed for a subsequent communication.     

Slow-mode oscillations of large-scale coronal loops

On several occasions, repetitive X-ray brightenings, sometimes accompanied by mass injections into adjacent loops, appeared quasi-periodically with mean periods close to 20 minutes. In all cases when X-ray images were available, the sites of these brightenings were in active regions which were associated with large-scale coronat loops of length (2 – 3) × 10⁵ km. Therefore, the primary source of these long-periodic pulsations might be slow-mode oscillations in these large-scale loops. Free MHD oscillations, proposed earlier by Roberts, Edwin, and Benz (1984), may fit the observed data.