O. Engvold

Counter-streaming gas flows in solar prominences as evidence for vertical magnetic fields

Solar prominences are sheets of relatively cool and dense gas embedded in the surrounding hotter corona. An erupting prominence can inject a mass of up to 1015 g into the solar wind as part of a coronal mass ejection. These eruptions must depend critically on the topology of the prominences magnetic field. In all present models,, the prominence hangs on horizontal or helical field lines, while an overlying magnetic arcade temporarily restrains the prominence from erupting. Such models are inconsistent, however, with the slow upward vertical gas flows that are seen in prominences. Here we report counter-streaming flows along closely spaced vertical regions of a prominence, between its top and the lower solar atmosphere. As the flows must be aligned with the magnetic field, this observation implies that a field connects the prominence directly to the photosphere, contrary to all existing models. These magnetic ‘tethers’ might help prevent a prominence from erupting.

SWAYING THREADS OF A SOLAR FILAMENT

From recent high-resolution observations obtained with the Swedish 1 m Solar Telescope in La Palma, we detect swaying motions of individual filament threads in the plane of the sky. The oscillatory characters of these motions are comparable with oscillatory Doppler signals obtained from corresponding filament threads. Simultaneous recordings of motions in the line of sight and in the plane of the sky give information about the orientation of the oscillatory plane. These oscillations are interpreted in the context of the magnetohydrodynamic (MHD) theory. Kink MHD waves supported by the thread body are proposed as an explanation of the observed thread oscillations. On the basis of this interpretation and by means of seismological arguments, we give an estimation of the thread Alfven speed and magnetic field strength by means of seismological arguments.

Structure and oscillations in quiescent filaments from observations in He I λ10830 Å

Observations of two quiescent filaments show oscillatory variations in Doppler shift and central intensity of the He i λ10830 Å line.The oscillatory periods range from about 5 to 15 min, with dominant periods of 5, 9, and 16 min. The 5-min period is also detected in the intensity variations, after correction for atmospheric effects. Doppler shifts precede intensity variations by about one period. The possibility that the oscillations are Alfvén waves is discussed.The Doppler signals of the filament form fibril-like structures. The fibrils are all inclined at an angle of about 25° to the long axis of the filament. The magnetic field has a similar orientation relative to the major direction of the filament, and the measured Doppler signals are apparently produced by motions along magnetic flux tubes threading the filament.The measured lifetimes of the small-scale fibrils of quiescent disk filaments are very likely a combined effect of intensity modulations and reshuffling of the structures.

Microphotometry of the blood column and the light streak on retinal vessels in fundus photographs

Abstract A scanning microdensitometric technique to be applied directly on negatives of fundus photographs is presented. Computer‐controlled high quality scans are performed across positions of interest on retinal vessels, and the width and the intensity profiles of both the blood column and its central light streak are analysed. From every scan, various parameters of microcirculatory interest are obtained by light reflection from arteries and veins. Different aspects of this technique are discussed in details: calibration of the film, the fundus camera, a marker technique, the scanning microphotometer, data analysis, the parameters from the scan profile, and the reproducibility of every procedural step. Some preliminary results of various light streak parameters are discussed and related to previous ophthalmoscopic observations.

Vertical velocities and oscillations in quiescent filaments

Analysis of He iλ 10 830 Å spectral observations of a large, quiescent filament reveals a pronounced oscillatory behaviour of the vertical mass motion. The filament is situated in a quiet region more than 15° away from the nearest active region.It is concluded that the magnetic field of the quiescent filament, which occurs in the form of long thin flux ropes, moves with the gas and that there is no net mass flow perpendicular to the most frequently observed horizontal field lines. The oscillatory motion is accompanied by phase dependent variation of the He i line intensity which could possibly imply wave induced compression of the plasma.

The fine structure of prominences

Ions falling in vertically aligned magnetic structures of quiescent prominences may experience a vertical Lorentz force as flux ropes are distorted from the force-free condition. The terminal velocity of such ions may be sub-Alfvénic and may correspond to the 5–15 km s−1 velocity of down falling material observed in many quiescent prominences. The higher velocities of down falling material found in active prominences and coronal rain may occur because of higher terminal velocities occurring in stronger magnetic fields.

Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars

High-dispersion IUE spectra of six late-type stars exhibit small but statistically significant differential redshifts of high-temperature emission lines, like Si IV and C IV, with respect to low-temperature lines like S I and O I. A well-exposed, small-aperture spectrum of the active chromosphere binary Capella (Alpha Aurigae A: G6 II+F9 III) establishes that the high-temperature lines are redshifted in an absolute sense with respect to the accurately determined photospheric velocity of the system at single-line phase 0.50. Several possible explanations for the stellar redshifts are discused, including a warm wind (100,000 K) in which aparent redshifts are produced in optically thick lines by an accelerating outfow, and the downflowing component of a vertical circulation system for which the up-leg portion of the flow is too cool, too hot, or too tenuous to be visible in Si IV and C IV. If the second scenario is true, the stellar redshifts may provide an important phenomenological link to the downflows observed in 100,000 K species over magnetic active regions in the sun.

Gas flows in the transition region above sunspots

Strong downflows and moderate upflows in the transition region over a sunspot have been observed with the Spacelab 2 HRTS in 1985. The flows are found to be persistent, and it is suggested that they are common. Data show that the downflows are supersonic and that there is more than one characteristic flow speed in the downflows. Evidence is provided for constant downflows in the 30,000-230,000-K temperature range.

The Prominence-Corona Transition Region in transverse magnetic fields

An emission measure analysis is performed for the Prominence-Corona Transition Region (PCTR) under the assumption that the cool matter of quiescent filaments is contained in long, thin magnetic flux loops imbedded in hot coronal cavity gas. Consequently, there is a transition region around each thread.Comparison of the model and observations implies that the temperature gradient is perpendicular to the magnetic lines of force in the lower part of the PCTR (T < 105 K). It is shown that in this layer the heating given by the divergence of the transverse conduction fails to account for the observed UV and EUV emission by several orders of magnitude. It is, therefore, suggested that the heating of these layers could be due to dissipation of Alfvén waves.In the high-temperature layers (T ≥ 105 K), where the plasma β ≥ 1, the temperature gradient is governed by radiative cooling balancing conductive heating from the surrounding hot coronal gas. Also in these outer layers the presence of magnetic fields reduces notably the thermal conduction relative to the ideal field-free case. Numerical modelling gives good agreement with observed DEM; the inferred value of the flux carried by Alfvén waves, as well as that of the damping length, greatly support the suggested form of heating. The model assumes that about 1/3 of the volume is occupied by threads and the rest by hot coronal cavity matter.The brightness of the EUV emission will depend on the angle between the thread structure and the line of sight, which may lead to a difference in brightness from observations at the limb and on the disk.

Description and Classification of Prominences

Solar prominences are bright cloud-like structures when observed beyond the solar limb and they appear as dark filamentary objects which are termed filaments when seen against the solar disk. The aims of prominence classifications were from the start to establish references and frameworks for understanding the physical conditions for their formation and development through interplay with the solar magnetic environment. The multi-thermal nature of solar prominences became fully apparent once observations from space in UV, VUV, EUV and X-rays could be made. The cool prominence plasma is thermally shielded from the much hotter corona and supported in the field of gravity by small- and large-scale magnetic fields of the filament channels. High cadence, subarcsecond observing facilities on ground and in space have firmly proven the highly dynamic nature of solar prominences down to the smallest observed structural sizes of 100 km. The origin of the ubiquitous oscillations and flowing of the plasma over a variety of spatial and temporal scales, whether the cool dense plasma originates from below via levitation, injections by reconnection or results from condensation processes, are central issues in prominence research today. The unveiling of instabilities leading to prominences eruptions and Coronal Mass Ejections is another important challenge. The objective of this chapter is to review the main characteristics of various types of prominences and their associated magnetic environments, which will all be addressed in details in the following chapters of this book.