Einar Tandberg-Hanssen

The arch systems, cavities and prominences in the helmet streamer observed at the solar eclipse, November 12, 1966

Arch systems lying above quiescent prominences in the solar corona have long drawn the attention of eclipse observers, and such formations have been investigated since the end of the last century. Almost every eclipse photograph shows one or more arches, and in most cases the arch system is accompanied by a quiescent prominence below it and a helmet streamer above it. Also, in some cases there is a dark cavity between the arch system and the prominence.On large-scale photographs obtained at the November 12, 1966 eclipse, detailed photometry has been carried out on a formation in the corona composed of a helmet streamer straddling two multiple-arch systems each with a dark cavity and a quiescent prominence. The excess of electrons in the arches and the deficiency in the cavities are evaluated. We find that the formation of a prominence requires much more material than available in the cavity before depletion. Consequently the condensation theory of coronal matter into prominences seems to have difficulties explaining the necessary amount of matter in the cases where coronal arches - delineating magnetic field lines above the cavity - may exclude inflow of material from the corona. We comment on the low velocity of solar wind in the helmet streamer.

The nature of quiescent solar prominences

Quiescent prominences occur as long-lasting cool sheets of matter in the surrounding hot corona at the base of coronal streamers. Seen on the disk they appear as dark filaments dividing regions of opposite magnetic polarity.In this paper a theoretical model is presented, which describes the general appearance of quiescent prominences.It is shown that the neutral sheet between two regions of oppositely directed magnetic fields is thermally unstable. This gives rise to compression and cooling of coronal material to prominence material in a characteristic time of the order of one day for a field strength of 0.5 gauss in the lower corona.It is assumed that due to the finite electrical resistivity of the plasma, filamentary structures are formed by the ‘tearing-mode’ resistive plasma instability. These structures are thermally insulated from the hot surroundings by the newly formed closed azimuthal magnetic field configuration.It has been shown that for fine structures with a diameter of 300 km the growth rate of the ‘tearing-mode’ instability is of the same order as the cooling time. The occurrence of fine structures within the prominence is of vital importance for their origin.

A model for quiescent prominences with helical structure

We present a model for quiescent prominences with helical structure. The model is described by two magnetic fields, one produced by photospheric or subphotospheric currents, the other due to currents along the cylindrical model prominence.

The orientation of magnetic fields in quiescent prominences

We have measured the longitudinal component, B∥, of the magnetic field in quiescent prominences and obtained a relationship between B∥ and θ, where θ is the angle between the long axis of the prominence and the north-south direction on the sun. From this relationship we deduce a distribution function for the magnetic field vector in quiescent prominences in terms of the angle α between the field and the long axis of the prominence. The mean angle, α, for our data is small, ∼ - 15°, indicating that the magnetic field traverses quiescent prominences under a small, but finite angle.

Hydrogen ionization and n=2 population for model spicules and prominences

Using slab model atmospheres that are irradiated from both sides by photospheric, chromospheric, and coronal radiation fields we have determined the ionization and excitation equilibrium for hydrogen.The model atom consists of two bound levels (n = 1 and n = 2) and a continuum. Ly-α was assumed to be optically thick with the transition in detailed radiative balance. The Balmer continuum was assumed to be optically thin with the associated radiative ionization dominated by the photospheric radiation field (Trad = 5940 K). The ionization equilibrium was determined from an exact treatment of the radiative transfer problem for the internally generated Ly-c field and the impressed chromospheric and coronal field (characterized by Trad = 6500K).Our calculations corroborate the hypothesis that N2, the n = 2 population density, is uniquely determined by the electron density 〈Ne〉. We also present ionization curves for 6000K, 7500K, and 10000K models ranging in total hydrogen density from 1 × 1010/cm3 to 3 × 1012/cm3. Using these curves it is possible to obtain the total hydrogen density from the n = 2 population density in prominences and spicules.

Dynamic response of an isothermal static corona to finite-amplitude disturbances

The velocity evolution of sprays, surges and fast ejections is characterized by a rapid acceleration followed by a slowdown. In contrast, eruptive prominences show a velocity evolution with a slow increase followed by a rapid acceleration. We examine the physical causes which differentiate these two characteristic velocity evolutions, and study the dynamic responses of the solar corona. For simplicity, the ascending disturbances are modelled as purely radial adiabatic flows caused by finite-amplitude perturbations (pulses) in an initially isothermal, static corona. It is shown that the resultant flow depends strongly on the nature of the disturbing causes. In particular, the coronal response to sprays and fast ejections can be identified with temperature and velocity pulses at the bottom of the corona, with surges related to shorter pulses, while the slow moving eruptive prominences are correlated with density pulses. It is shown that for large flare sprays 5 × 1039 particles can be injected into the solar wind.

Magnetic fields in flares and active prominences

We study the longitudinal magnetic field in a number of active limb prominences showing fields in excess of 30 G. The objects fall into three groups: surges, caps and active region prominences. There appears to be an upper limit of 150–200 G for the field strength in prominences.A model of surges is presented in which a pre-surge axi-symmetric magnetic field is established by a line current in the corona. We observe particle acceleration in surges that indicates v×B≠0 in these objects during periods comparable to the Alfvén transit time.The strong fields observed in caps seem to run between parts of active regions in accordance with Hales law of sunspot group polarities.

MAGNETIC FIELDS IN QUIESCENT PROMINENCES.

We discuss the longitudinal component of the magnetic field, B∥, based on data from about 135 quiescent prominences observed at Climax during the period 1968–1969. The measurements are obtained with the magnetograph which records the Zeeman effect on hydrogen, helium and metal lines. Use of the following lines, Hα; Hei, D3, Hei, 4471 Å; Nai, Di and D2, leads to the same value for the observed magnetic field component in these prominences. For more than half of the prominences their mean field, B∥, satisfy the inequalities 3 G ⩽ B∥ ⩽ 8 G, and the overall mean value for all the prominences is 7.3 G. As a rule, the magnetic field enters the prominence on one side and exits on the other, but in traversing the prominence material, the field tends to run along the long axis of the prominence.

PROFILES OF THE H AND K LINES OF CaII IN DISK FLARES.

The profiles of the resonance lines of Caii have been studied in two large disk flares and in the surrounding plage. In the brightest portions of the flares no self-reversal in the central emission core was detected; self-reversed cores were present in the less bright portions of the flares. We find that as the intensity of the emission core increases the separation of the H2 and K2 peaks decreases monotonically, becoming unobservable at intensities near to 0.90 the local continuum. Possible reasons for the behavior of the H and K lines in flares are considered. It is suggested that the largest density enhancements in flares are found near the strongest magnetic field.

Stability of Prominences

In discussing models of prominences, and the way these objects form, we have repeatedly touched on the problem of their stability. In this chapter we shall discuss in more detail different equilibria for prominences, and see how stable they are against various perturbations.