Measurement of the prominences magnetic field

A new type of magnetograph has been built capable to measure weak magnetic fields in the chromosphere and corona. Measurements of magnetic field in two prominences are demonstrated as examples.     

Doppler velocity measurements made with a scanning photoelectric magnetograph

We discuss the problems connected with the measurements and evaluation of line-of-sight velocities, obtained with a scanning photoelectric magnetograph using a line-shifter with enhanced sensitivity. We bring arguments for the validity of the results of our photoelectric Doppler velocity recordings. We have found a network of cellularly shaped patterns in the distribution of photo-electrically measured line-of-sight motions, upflowing in the magnetically quiet (blue-shifted) and downflowing in magnetically active (red-shifted) areas of the photosphere, if the mean velocity level is estimated for a sufficiently large measured area. The features of both directions are mutually complementary. We demonstrate the effect of the shift of the reference zero velocity level on the topology of the line-of-sight velocity maps, and the dependence of this level on the size of the area from which it is estimated.     

The magnetic field in the prominences of the polar crown

The Hanle effect method has been applied to the determination of the magnetic field in 120 prominences of the polar crown observed during the 1974–1980 period, which is the ascending phase of cycle XXI. The average field strength which was about 6 G at the beginning of the cycle reached twice this value just before the maximum. There is also a clear trend for a increase of the prominence field with the altitude. We confirm the fact that the magnetic vector makes a small angle (25 °) with the long axis of the prominence. As to the field orientation, we show that the most striking feature lies in the regular pattern of the component which is parallel to the axis of the filament; its direction seems to depend closely on the polarities of the high latitude photospheric field.     

The representation of magnetic field lines from magnetograph data

Several methods currently used to extrapolate the structure of the solar magnetic field from surface measurements are examined and compared. In particular, the differences between the methods of Schmidt for potential fields and of Nakagawa and Raadu for force-free fields are explained. Suggestions are made regarding the use of these theoretical procedures in making physical conclusions.     

The formation of solar prominences by magnetic reconnection and condensation

A model for solar quiescent prominences nested in a Figure 8 magnetic field topology is developed. This topology is argued to be the natural consequence of the distention of bipolar regions upward into the corona. If this distention is slow enough so that hydrostatic equilibrium holds approximately along the field lines, the transverse gas pressure forces fall exponentially with height whereas the inward Lorentz forces fall as a power law. At a low height in the corona, the pressure forces cannot balance the Lorentz forces provided the field lines remain tied to the photosphere and an inward collapse with subsequent reconnection at the point of closest approach should occur. Because of initial shear in the magnetic field, the reconnection would produce isolated helices above the point of reconnection since field lines would not interact with themselves but with their neighbors. This resulting topology produces a field above the elevated neutral line which is opposite in polarity to that of the photospheric field as in the current sheet models of Kuperus and Tandberg-Hanssen (1967). Raadu and Kuperus (1973), Kuperus and Raadu (1974), and Raadu (1979) and in agreement with recent observations of Leroy (1982), and Leroy et al. (1983).Assuming the isolated helices formed by reconnection are insulated from coronal thermal conduction and heating, the radiative cooling process and condensation is considered for the temperature range of 10⁴-6000 K. This condensation results in a steady downflow to the bottom of the helices as the temperature scale-height falls, thus forming a dense, cool, prominence at the bottom of the helical configuration resting on the elevated neutral line with the remainder of the helix being essentially evacuated of material. We identify this neutral line at the bottom of the prominence with the sharp lower edge often seen when viewing quiescent prominences side-on and the evacuated helix with the coronal cavity observed around prominences when seen during total eclipses.Downflow speeds associated with the condensation process are calculated for prominence temperatures and yield velocities in the range of the observed downflows of about 1 km s^–1.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A note on magnetic fields and electric currents in solar prominences

We determine magnetic fields and electric currents in three prominences without simplifications used in our previous papers.     

Parameters of quiescent prominences derived from magnetic field measurements

Based upon the new magnetic field vector measurements of Bommieret al. (1994) we construct prominence models which are in magnetohydrostatic equilibrium. We compare these new models with earlier ones and find that they are on average more massive and also considerably narrower.     

Kinematics of solar prominences

Two sequences of OSO-4 spectroheliograms in Mg x and Si xii obtained during October–November 1967 and covering the intervals of 83 and 22 hr, respectively, have been analyzed to reveal quasi-periodic oscillations of EUV flux from solar sources with a periodicity of 5–14 hr. The oscillation periods of the emission flux from local sources over sunspots and magnetic field enhancements in plages without spots have been investigated in correlation with characteristics of the respective AR and plages. The greatest periods (> 8 hr) are shown to be peculiar of small sunspots or sunspot groups at the initial or final stage of their development, whereas the smallest periods ( 5–6 hr) are observed in the case of large well-developed groups at the maximum stage of development. In quiet regions on the Sun and plages without spots, the oscillation periods are 6–8 hr. The surface areas in which the oscillations are synchronous and coincide in phase have typical dimensions of 1 in quiet and 1 to 5 in active regions. These areas form a spatial structure similar to the chromospheric network and supergranules. The characteristic lifetime of the structure elements is 1.5–2 days.     

Measurements of line-of-sight velocities in prominences

Measurements of line-of-sight velocities of quiescent and sunspot prominences on the limb made during the years 1966 and 1968 at Swedish Astrophysical Station in Anacapri, Italy are discussed. Several statistical properties of the velocity field, in particular its connection with close McMath plages are investigated. Results are interpreted in terms of oscillatory motion in prominences.     

Physics of solar prominences

Summary Conclusion This colloquium on solar prominences - the first ever held - has shown that a major part of activity in prominence research in recent years concentrated on both observation and computation of the magnetic conditions which were found to play a crucial role for the development and the maintainance of prominences. Remarkable progress was made in fine-scale measurements of photospheric magnetic fields around filaments and in internal field measurements in prominences. In addition, important information on the structure of the magnetic fields in the chromosphere adjacent to the filaments may be derived from high resolution photographs of the H fine structure around filaments which have become available recently; unfortunately, an unambiguous determination of the vector field in the chromosphere is not yet possible.It is quite clear, now, that stable filaments extend along neutral lines which divide regions of opposite longitudinal magnetic fields. Different types of neutral lines are possible, depending on the history and relationship of the opposite field regions. There is convincing evidence that the magnetic field in the neighbouring chromosphere may run nearly parallel to the filament axis and that there are two field components in stable prominences: an axial field dominant in the lower parts and a transverse field dominant in the higher parts.Methods for the computation of possible prominence field configurations from measured longitudinal photospheric fields were developed in recent years. In a number of cases (e.g. for loop prominences) the observed configuration could be perfectly represented by a force-free or even a potential field; poor agreement was found between computed and measured field strengths in quiescent prominences. In order to reconcile both of them it is necessary to assume electric currents. Unambiguous solutions will not be found until measurements of the vector field in the photosphere and in the prominences are available.The two-dimensional Kippenhahn-Schlüter model is still considered a useful tool for the study of prominence support and stability. However, a more refined model taking into account both field components and considering also thermal stability conditions is available now. It was proposed that quiescent prominences may form in magnetically neutral sheets in the corona where fields of opposite directions meet.As for the problem of the origin of the dense prominence material there are still two opposite processes under discussion. The injection of material from below, which was mainly applied to loop prominences, has recently been considered also a possible mechanism for the formation of quiescent prominences. On the other hand, the main objections against the condensation mechanism could be removed: it was shown that (1) sufficient material is available in the surrounding corona, and that (2) coronal matter can be condensed to prominence densities and cooled to prominence temperatures in a sufficiently short time.The energy balance in prominences is largely dependent on their fine structure. It seems that a much better radiative loss function for optically thin matter is now available. The problem of the heat conduction can only be treated properly if the field configuration is known. Very little is known on the heating of the corona and the prominence in a complicated field configuration. For the optically thick prominences the energy balance becomes a complicated radiative transfer problem.Still little is known on the first days of prominence development and on the mechanism of first formation which, both, are crucial for the unterstanding of the prominence phenomenon. As a first important step, it was shown in high resolution H photographs that the chromospheric fine structure becomes aligned along the direction of the neutral line already before first filament appearance. More H studies and magnetic field measurements are badly needed.Recent studies have shown that even in stable prominences strong small-scale internal rotational or helical motions exist; they are not yet understood. On the other hand, no generally agreed interpretation of large-scale motions of prominences seems to exist. A first attempt to explain the ascendance of prominences, the Disparitions Brusques, as the result of a kink instability was made recently.New opportunities in prominence research are offered by the study of invisible radiations: X-rays and meterwaves provide important information, not available otherwise, on physical conditions in the coronal surroundings of prominences; EUV observations will provide data on the thin transition layer between the cool prominence and the hot coronal plasma.Mitt. aus dem Fraunhofer Institut No. 111.     

Magnetic field of solar prominences: The procedure of radio observations

The first successful radio astronomical results of measurements of the magnetic field of solar prominences are presented. The accuracy of polarization and magnetic field observations is 3 · · 10⁻⁴ and 2 G, respectively. The observations were made by the 22-meter radio telescope of the Crimean Astrophysical Observatory at wavelengths of 8 and 13.5 mm. It has been found that the value of the magnetic fields coincides with the optical one (from 7 to 30 G), but the image of radio polarization differs from the intensity.     

Measurements of the magnetic field and the gradient of temperature in the solar atmosphere above a flocculus using radio observations

Simultaneous observations made at several wavelengths in microwave range using the high spatial resolution of radiotelescope RATAN-600 make it possible to develop methods of measuring the magnetic fields in the solar corona and the chromosphere. In this paper we develop a method of measuring the magnetic fields from thermal bremsstrahlung and demonstrate it, using observations of a flocculus (plage) during August 1–3, 1977. The observations show that the flocculus under investigation possessed bipolar magnetic structure with peak to peak amplitude of magnetic field strength of about 40 G at the level of the upper chromosphere and the transition region (with a r.m.s. error of 5.7 G for favourable conditions). The radio astronomical map of the magnetic field is in agreement with the Mt. Wilson magnetic field map to within the experimental error. It follows that the average longitudinal magnetic field above the flocculus does not drop significantly with height above the photosphere up to the CCTR (chromosphere-corona transition region). An analysis of the spectra of polarized radio emission also gives an opportunity to determine the temperature gradient in the CCTR (which proved to amount to about 1000 K km^-1 and to follow their variation with height.     

The magnetic field in some prominences measured with the He I, 5876 Å line

Observations of the longitudinal magnetic field of several prominences were made with the D₃, H, and H lines. There is no significant difference in the magnetic field measured with the helium and hydrogen lines. The possibility of a true difference in the fields on a scale much finer than that of our observations is not excluded.     

Helical structures around quiescent solar prominences computed from observable magnetic fields

We analyse the magnetic support of solar prominences in two-dimensional linear force-free fields. A line current is added to model a helical configuration, well suited to trap dense plasma in its bottom part. The prominence is modeled as a vertical mass-loaded current sheet in equilibrium between gravity and magnetic forces.We use a finite difference numerical technique which incorporates both vertical photospheric and horizontal prominence magnetic field measurements. The solution of this mixed boundary problem generally presents singularities at both the bottom and top of the model prominence. The removal of the singularities is achieved by superposition of solutions. Together with the line current equilibrium, these three conditions determine the amplitude of the magnetic field in the prominence, the flux below the prominence and the current intensity, for a given height of the line current. A numerical check of accuracy in the removal of singularities, is done by using known analytical solutions in the potential limit.We have investigated both bipolar and quadrupolar photospheric regions. In this mixed boundary problem the polarity of the field component orthogonal to the prominence is mainly fixed by the imposed height of the line current. For bipolar regions above (respectively below) a critical height the configuration is inverse (respectively normal). For quadrupolar regions the polarity is reversed if we refer the prominence polarity to the closest photospheric polarities. We introduce the polarity of the component parallel to the prominence axis with reference to a sheared arcade. Increasing the shear with fixed boundary conditions can increase or decrease the mass supported depending on the configuration.     

Modern observations of solar prominences

We review observational studies of solar prominences with some reference to theoretical understandings. We lay emphasis on the following findings: (1) An important discovery was made by Leroy, Bommier, and Sahal-Bréchot concerning the direction of the magnetic field inside some high-altitude, high-latitude prominences, where the field vector points in the opposite direction from the one which would be expected from the potential field calculated from the observed photospheric magnetic field. (2) Landman suggests the possibility of a high total density of 10^–11 g cm ^–3 for the main body of quiescent prominences, 50 times higher than the value hitherto believed. (3) Flow patterns, nearly parallel to the magnetic neutral lines, were detected in the 10⁵ K plasma near and in prominences. (4) Coronal loop structures were found overlying prominences as viewed from X-ray photographs. We propose also an evolutionary scheme by taking the magnetic field topologies into account.The fundamental question why a prominence is present remains basically unanswered.     

Preflare activity of solar prominences

The preflare activity of a plage filament is analysed from H observations made with the Multichannel Subtractive Double Pass Spectrograph (MSDP) of the Meudon Solar Tower. The June 22, 1980 event is studied and interpreted in terms of preflare heating of a filament, connected to the rise of emerging flux, and the relative approach of pores of different magnetic polarity, prior to the onset of a two-ribbon flare.The region with enhanced magnetic field, around the filament, begins to brighten slowly 20 min before the triggering of the flare, in the center of H. Filament dark material begins to rise rapidly while the brightest point on one side drifts towards it, 6 min before the onset of the two-ribbon flare. Simultaneously the absorbing material separates from the remaining part of the filament.In the discussion, we suggest that most of the observed features may be the consequence of emergence of new magnetic flux and the related reconnection processes.     

The magnetic field in the solar atmosphere

This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.     

Observations of magnetic field evolution in a solar flare

I±V and I±Q profiles of nine spectral lines of Fei, Feii, and Hi in the 2B flare of 16 June 1989 have been analyzed. Two bright flare knots outside and inside of a spot were investigated. To measure the true magnetic field strength in the flare, two different methods were applied. In addition to these data, the magnetic field and thermodynamic conditions were determined using the non-LTE program for line profile synthesis. According to the measurements, the magnetic field in both flare knots changed in synchronism and non-monotonically, and reached its peak (nearly 1.6 kG for non-spot areas and approximately 4.0 kG for sunspot locations) at the time of flare peak. For the flare knot outside the spot, a background field component was also detected; the magnetic field in this component was found to have mixed polarity and remained practically unchanged during the flare. The non-LTE calculations show that the unique local magnetic field peak existed near the temperature minimum zone in the flare peak too. The observed perturbations do not exclude such phenomena as a magnetic field transient in flare.     

Changes in the photospheric magnetic field associated with solar flares

The flare-related, persistent and abrupt changes in the photospheric magnetic field have been reported by many authors during recent years. These bewildering observational results pose a challenge to the current flare theories in which the photospheric magnetic field usually remains unchanged in the eruption. In this paper, changes in the photosphere magnetic field during the solar eruption are investigated based on the catastrophe model. The results indicate that the projection effect is an important source that yields the change in the observed photospheric magnetic field in the line-of-sight. Furthermore one may observe the change in the normal component of magnetic field if the spectrum line used to measure the photospheric magnetic field does not exactly come from the photospheric surface. Our results also show that the significance of selecting the correct spectral lines to study the photospheric field becomes more apparent for the magnetic configurations with complex boundary condition (or background field).     

Vector magnetic fields in prominences

A preliminary discussion is presented of measurements of the polarization of the He i D₃ multiplet in a quiescent prominence, observed with a wavelength-scanning Stokes polarimeter. For a series of 43 observations in the same prominence, the linear polarization of the major component of D₃ lies primarily in the range 1 to 2% and of the wing component, the range 2 to 5%; the polarization vector angle lies primarily in the range 10–25° for the major component, and 25–35° for the other component. From a more limited data set, the polarization of both components is found to first increase as a function of height in the prominence, and then to decrease; the polarization angles of the major component vary in a random-like way with height, while the wing component shows a systematic change. The amount of polarization and the angle of polarization are governed by the Hanle effect. The collective effect of the group of lines at the peak of D₃ evidently has a different sensitivity to the Hanle effect than does the wing component, thus yielding at least four independent measurements - two polarizations and two angles. With some redundancy, the vector magnetic field can then be established using the detailed theory of the Hanle effect. Since the wing component of D₃ is a simple triplet, an initial estimate of the magnetic field strength and its horizontal orientation, 0, relative to the line of sight, is simply obtained. Examples of such calculations are presented.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.