Structural characteristics of eruptive prominences

Nowadays the primordial importance of the magnetic field for coronal plasma physics is well known. However, its determination is only made in cool regions, mainly the photosphere and prominences. The extrapolation to the corona gives some indications of the magnetic structure but is not presently sufficiently reliable. So it is important to consider all the other observable physical effects of the magnetic field.In this puzzle, eruptive prominences may play a key role because the cool plasma is forced to move along field lines, which can then be visualized. In the strongest field regions, flares also give such information, while coronal mass ejections (CME) play such a role at larger scales. The magnetic field, which is at the base of the physical processes, is a common link between these different events.Observed properties of solar prominence eruptions are reviewed, then their relationships with CMEs and flares are discussed, with the help of present models. We emphasize the importance of magnetic measurements in future coordinated observations.Invited paper presented at the IAU Commission 10 Meeting on Dynamics and Structure of Prominences in Buenos Aires.     

Spectral diagnostics for eruptive prominences

The diagnostic of eruptive prominences needs the development of new tools. Here we propose the Lyman and Balmer lines of hydrogen, which are important in the radiative budget. In the NLTE radiative transfer calculations, we include the effect of the outward motion of the structure associated with the eruption of the prominence. The treatment of the resonance scattering of L and L with partial redistribution gives higher intensities, and a higher ionization than the complete redistribution, but the two approaches converge to the same solution as the velocity increases. As a first step in the diagnostic, we present new results concerning the variation of the integrated intensities of hydrogen lines with respect to the radial velocity.     

Physical conditions in eruptive prominences at several solar radii

SMM data from the Corograph/Polarimeter experiment giving intensities of H and continuum emission in eight erupting prominences are analyzed to obtain the physical conditions in the regions of H emission. Since the H intensity depends upon three unknowns whereas only two independent observations are available, it is necessary to assume one additional condition in order to obtain unique solutions. Solutions are chosen that give the maximum expansion of the prominence volume as reflected by minimum values of the electron density. These solutions correspond closely with those giving the best agreement between the gas pressure in the prominence and the ambient coronal pressure. Electron densities are found to be of the order of 10⁸ cm^-3 at temperatures near 2 × 10⁴ K.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.     

Stability of the lundquist field and the physical nature of eruptive prominences with helical structure

The stability of the Lundquist field is analysed in the light of the energy principle. The results show: (1) form = 0 disturbances, the Lundquist field is stable; (2) form = 1 disturbances, the Lundquist field will turn unstable in certain conditions. Whether the field will remain stable or not depends on parameters such as the helical pinch angle and radius and length of the prominence. Comparison between theory and observations reveals that a kink instability in the Lundquist field may be a critical physical reason for eruptive prominences with helical patterns.     

A solar scenario for the associated occurrence of flares, eruptive prominences, coronal mass ejections, coronal holes, and interplanetary shocks

The observation of non-corotating shock fronts in interplanetary space is always associated with the previous occurrence of a coronal mass ejection (CME), which is frequently accompanied by a flare or a prominence eruption. When looking at the solar region of origin of these events, a coronal hole is always found. Here we propose a scenario at the Sun where all these related events can find a place.     

X-ray emission associated with solar prominences,sprays and surges

Using H observations made at the Astronomical Observatory of Wroclaw University, and 3.5–5.5 keV X-ray data from the Hard X-ray Imaging Spectrometer on board the Solar Maximum Mission, sites of solar X-ray emission are identified which are associated with active H features, such as prominences, sprays and surges. The X-ray emission is found to be highly localized within the active (H) structures. For example, in the prominences examined, 3.5–5.5 keV X-rays were found only in compact sites near the feet of the prominences. Models predicting that, during the active phase of these structures, the energy release should be evenly distributed along the structure are clearly brought into question. It is argued that these X-ray sites are indicative of the cause of the expulsion and transport of chromospheric material. Models which satisfy these observations are discussed.This work was started during a visit to the High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, U.S.A. (NCAR is sponsored by the National Science Foundation).     

The dynamical process of a coronal transient associated with an eruptive prominence

The statistical correlation between an eruptive prominence and the coronal transient associated with this prominence implies that there should be a relationship between these two kinds of dynamical processes. This paper analyzes the dynamical effect of a plasma ‘piston’ in the corona, consisting of an eruptive prominence and/or a magnetic flux region (loop or arcade, or blob) in front of the prominence. Ahead of the piston, there is a compressed flow, which produces a shock front. This high-density region corresponds to the bright feature of the transient. Behind the piston, there is a rarefaction region, which corresponds to the dark feature of the transient. Therefore, both the bright and dark features of the transient may be explained at the same time by the dynamical process of the moving piston. Two local solutions, one perpendicular and one parallel to the direction of solar gravitational field, are discussed. The influence of gravity on the gas-dynamical process driven by the piston is discussed in terms of characteristic theory, and the flow field is given quantitatively. For a typical piston trajectory similar to the one for an eruptive prominence, the velocity of the shock front which locates ahead the transient front is nearly constant or slightly accelerated, and the width of the compressed flow region may be kept nearly constant or increased linearly, depending on the velocity distribution of the piston. Based on these results, the major features of the transient may be explained. Some of the fine structure of the transient is also shown, which may be compared in detail with observations.     

A model of ‘disparitions brusques’ (sudden disappearance of eruptive prominences) as an instability driven by mhd waves

A model of ‘disparitions brusques’ (sudden disappearence of eruptive prominences) is discussed based on the Kippenhahn ans Schlüter configuration. It is shown that Kippenhahn and Schlüter's current sheet is very weakly unstable against magnetic reconnecting modes during the lifetime of quiescent prominences. Disturbances in the form of fast magnetosonic waves originating from nearby active regions or the changes of whole magnetic configuration due to newly emerged magnetic flux may trigger a rapid growing instability associated with magnetic field reconnection. This instability gives rise to disruptions of quiescent prominences and also generates high energy particles.     

An eruptive prominence and associated cm-mm emission outside the solar limb

We present radio maps at 22 and 44 GHz which show the emission before and after the eruption of a quiescent prominence located at the west limb. The observed radiation following the eruption is not consistent with thermal bremsstrahlung mechanism. It can be interpreted as due to gyrosynchrotron emission of nonthermal electrons. Our observations appear to be similar to the microwave radiation observed in post-flare loops; this radiation is due to nonthermal electrons trapped in the closed magnetic structures formed after the prominence eruption.     

Study of a large helical eruptive prominence associated with double CME on 21 April 2001

Here we present a preliminary analysis of a helical eruptive prominence at the east limb of the Sun on 21 April 2001. Unusually this eruption is associated with a double CME. We have tried to study the morphology of the event, energy budget of the prominence and associated CMEs. Our analysis shows that the prominence and first CME started simultaneously from the limb and prominence carries sufficient energy to feed both the CMEs. Moreover, it is also concluded that CMEs are magnetically driven and internally powered.     

Radiation transfer in prominences with filamentary structure

Observations concerning the structure of sunspots, obtained during the fourth flight of the Soviet Stratospheric Observatory (SSO), are discussed. Objects brighter than the mean photospheric background inside the sunspot penumbra retaining the stable position sometimes vary within time intervals of a few minutes. The brightness change in pores can be explained by their different location at highest levels of the photosphere. The same mechanism can cause the brightness difference of the penumbra filaments. The gradient of the brightness variation inside the pores is determined. The value of this gradient was found to be practically the same for all dark objects. Most penumbral filaments show no magnetic expansion with growing distance from the spot center.     

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.     

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.     

Motion of ascending prominences

Apparent motions of a number of ascending prominences of the limb-SD type (sudden disappearances) are investigated. The direction, the velocity of ascent and the correlation with flares are studied. The maximum velocity, which seems to deviate systematically from the radial direction, increases with height and shows a clear dependence on the distance to an initiating flare and its importance.A good correlation between ascending prominences and solar radio emission has been found.     

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.     

Structure and stability of prominences with helical structure

Observations of internal structure and development of four helical prominences are presented. We assume that the helically twisted fine structure threads are outlining magnetic field lines and we found that it is possible to describe the magnetic fields by the uniform twist configuration, with the twists ranging between 2 and 7. The estimated lower limits for the magnetic fields were about 20 G which give lower limits for the currents flowing along the prominences in the range between 2 × 10¹⁰ A and 2 × 10¹¹ A and current densities at the axis of the prominences about 10^-4 A m^-2. The upper limit of electron drift velocity could be estimated as 1 m s^-1, which is far below the critical velocities for the onset of plasma microinstabilities.The stability of the studied prominences is discussed and the criteria for the onset of eruptive instability are established for a prominence modelled as a twisted and elliptically curved magnetic flux tube which is anchored in the photosphere and affected by its mirror-current. The eruption starts when the prominence attains a critical height which must be larger than half of the footpoint separation and depends on the values of twist, radius, and footpoint distance of the magnetic flux tube. The observed examples of eruptive prominences agree very well with the predictions. Possible applications to the two-ribbon flare process are outlined.Properties of stable cylindrical prominences in equilibrium are analyzed and a criterion for the distinction between the Kuperus-Raadu and Kippenhahn-Schlüter types of prominences is proposed. According to established criteria, two of the studied prominences were of the Kuperus-Raadu type, while the other two were of the Kippenhahn-Schlüter type.     

Origin of quiescent prominences

For stable equilibrium, prominences must be supported with magnetic lines of force leaning upon the photosphere and concave in their tops; however the general structure may be more complicated. If such a field appears in the corona, the heating of the gas near the upper pit should be low, because Alfvén and slow waves do not propagate across magnetic lines and fast mode waves attenuate because of refraction. The gas of the corona, distributed along the magnetic lines tube, cannot keep balance, it should flow down in the pit, condense there and fall down into the chromosphere in some places. The prominence, therefore, originates in the matter of the chromosphere which is situated at the other end of the magnetic lines and flows through the corona under the effect of a siphon-type mechanism. A similar mechanism for chromospheric structures was earlier suggested by Meyer and Schmidt. A stationary stream along the tube has been calculated with allowance for the heat conductivity and radiative cooling of the corona gas. The stream is subsonic and is about 10¹⁵ cm⁻² sec⁻¹ which corresponds to the prominence formation time of the order of a day.     

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.On leave from Max-Planck Institut für Physik und Astrophysik, München.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The fine structure of prominences

The fine structure of nonspot prominences are studied from H filtergrams. The size of the smallest prominence structures increases with height above the chromosphere. Some prominences contain structures close to 1/2 arc second, which is the spatial resolution in the present data. The effective thickness of many nonspot prominences ranges between 4 × 10⁷ cm and 1.5 × 10⁸ cm. An apparent downward directed motion is observed in the majority of the prominences. No preferred direction of the motion is seen in regions composed of comparatively large diffuse structures. Some bright threads are visible for 1 hr and longer. Bright knots have an average observed lifetime of about 8 min. The process of condensation and subsequent destruction of prominence fine structure appears to take place on a very short time scale compared to the life time of the regions where prominences may exist. The observed H brightness of the prominences in the present data may be accounted for as scattered chromospheric radiation.     

Analysis of two active prominences

We discuss the observations of two eruptive prominences, and the formation of condensations during the phenomena. The density and intensity variations of the condensations are analyzed spectroscopically in one of the events. Some hypothesis about the magnetic field configuration have been used in order to explain the observational data.