Formation Of Solar Prominences Research Articles

The influence of flux rope heating models on solar prominence formation

Context. Prominences are cool, dense clouds suspended within the solar corona. Their in situ formation through the levitation-condensation mechanism is a textbook example of the thermal instability, where a slight energy imbalance leads to a runaway process resulting in condensed filamentary structures embedded within the concave-up portions of a flux rope. The detailed interplay between local radiative losses and the global heating of the solar corona is investigated here for prominence-forming flux rope structures. Aims. We begin by exploring the influence of two classes of commonly adopted heating models on the formation behaviour of solar prominences. These models consider either an exponential variation dependent on height alone, or local density and magnetic field conditions. We highlight and address some of the limitations inherent to these early approximations by proposing a new, dynamic 2D flux rope heating model that qualitatively accounts for the 3D topology of the twisted flux rope field. Methods. We performed 2.5D grid-adaptive numerical simulations of prominence formation via the levitation-condensation mechanism. A linear force-free arcade is subjected to shearing and converging motions, leading to the formation of a flux rope containing material that may succumb to thermal instability. The eventual formation and subsequent evolution of prominence condensations was then quantified as a function of the specific background heating prescription adopted. For the simulations that consider the topology of the flux rope, reduced heating was considered within a dynamically evolving ellipse that traces the flux rope cross-section. This ellipse is centred on the flux rope axis and tracked during runtime using an approach based on the instantaneous magnetic field curvature. Results. We find that the nature of the heating model is clearly imprinted on the evolution and morphology of any resulting prominences: one large, low-altitude condensation is obtained for the heating model based on local parameters, while the exponential model leads to the additional formation of smaller blobs throughout the flux rope which then relocate as they tend towards achieving hydrostatic equilibrium. Finally, a study of the condensation process in phase space reveals a non-isobaric evolution with an eventual recovery of uniform pressure balance along flux surfaces.

A Unified Model of Solar Prominence Formation

Abstract Several mechanisms have been proposed to account for the formation of solar prominences or filaments, among which direct injection and evaporation–condensation models are the two most popular ones. In the direct injection model, cold plasma is ejected from the chromosphere into the corona along magnetic field lines; in the evaporation–condensation model, the cold chromospheric plasma is heated to over a million degrees and is evaporated into the corona, where the accumulated plasma finally reaches thermal instability or nonequilibrium so as to condensate to cold prominences. In this paper, we try to unify the two mechanisms: The essence of filament formation is the localized heating in the chromosphere. If the heating happens in the lower chromosphere, the enhanced gas pressure pushes the cold plasma in the upper chromosphere to move up to the corona, such a process is manifested as the direct injection model. If the heating happens in the upper chromosphere, the local plasma is heated to 1–2 million degrees, and is evaporated into the corona. Later, the plasma condensates to form a prominence. Such a process is manifested as the evaporation–condensation model. With radiative hydrodynamic simulations we confirmed that the two widely accepted formation mechanisms of solar prominences can really be unified in such a single framework. A particular case is also found where both injection and evaporation–condensation processes occur together.

The Effect of Heat Conduction on the Formation of Coronal Condensations in the Solar Atmosphere

In this work, the evolution of oscillation instability in large-scale magnetic fields in the upper layers of the Sun’s convective zone is studied. The nonlinear phase of the Parker instability evolution is studied in the thin magnetic tube approximation for slow oscillation modes up to the saturation stage. The dynamics of magnetic structures emerging from the convective zone to the solar atmosphere was calculated taking into account the heating of the magnetic tube gas due to the effect of heat conduction along magnetic field lines. Two phases of the formation of solar prominences (coronal condensations) under conditions of the anomalously heated solar atmosphere are distinguished: the explosive phase of the magnetic field emission into the solar chromosphere with hypersonic velocities (about 300 km/s) and the phase of deceleration of emerging magnetic fields in the Sun’s corona. The stability of coronal condensations is preliminarily analyzed depending on their horizontal size in the solar atmosphere. The physical processes leading to the implementation of coronal transients (coronal mass ejections) in the solar atmosphere after the explosive phase of the magnetic field emission to the chromosphere are revealed.

Reconnection–Condensation Model for Solar Prominence Formation

Abstract We propose a reconnection–condensation model in which topological change in a coronal magnetic field via reconnection triggers radiative condensation, thereby resulting in prominence formation. Previous observational studies have suggested that reconnection at a polarity inversion line of a coronal arcade field creates a flux rope that can sustain a prominence; however, they did not explain the origin of cool dense plasmas of prominences. Using three-dimensional magnetohydrodynamic simulations, including anisotropic nonlinear thermal conduction and optically thin radiative cooling, we demonstrate that reconnection can lead not only to flux rope formation but also to radiative condensation under a certain condition. In our model, this condition is described by the Field length, which is defined as the scale length for thermal balance between radiative cooling and thermal conduction. This critical condition depends weakly on the artificial background heating. The extreme ultraviolet emissions synthesized with our simulation results have good agreement with observational signatures reported in previous studies.

Solar prominence dynamic model

The possibility of solar prominence formation due to the motion of magnetized plasma particles has been studied. As a result of such motions, chromospheric and coronal plasma can locally condense to density ∼1011 cm−3 and to appearance of the current of unmagnetized protons to ∼1012 A, which corresponds to the values typical of prominences. The model calculations of the studied process indicated that plasma is condensed on a neutral magnetic surface in the form of a stable formation with prominence geometric parameters. This process can proceed if the magnetic field octupole harmonic is significant to altitudes of ≥0.1RSun and the magnetic field strength continuously decreases. The latter condition corresponds to observational data and is considered a necessary condition for the appearance of a prominence.

Extreme‐Ultraviolet Transition‐Region Line Emission during the Dynamic Formation of Prominence Condensations

We calculated the emission expected in EUV transition-region lines during the process of dynamic formation of prominence condensations in coronal loops, as predicted by the thermal nonequilibrium model of Antiochos et al. We selected some lines emitted by ions of carbon and oxygen because they are among the most intense and representative in the temperature range corresponding to the solar transition region. We present and discuss the principal characteristics of the line intensities and profiles synthesized from the hydrodynamic model at different times during the loop evolution. The ionization balance is computed in detail and the deviations from the ionization equilibrium caused by plasma flows and variations of temperature and density are accounted for. The atomic physics is treated using the latest atomic coefficients and the collisional-radiative theory approach. The synthesized carbon and oxygen lines exhibit a behavior significantly dependent on the variations of the plasma parameters inside the magnetic flux tube and therefore are suitable observational signatures of the processes giving rise to prominence condensations. In particular, a sizeable increase of line intensity as well as small blueshifts are expected from the loop footpoints during the first part of the evaporation phase that fills the loop with the material which subsequently condenses into the prominence. Once the condensation appears, line intensities decrease in the footpoints and simultaneously increase at the transition regions between the cool plasma of the condensation and the coronal portion of the loop. Line shifts are quite small in our symmetric model, and during most of the condensation's lifetime, the nonthermal widths are relatively small. These results can be compared with detailed ultraviolet observations of filament/prominence regions obtained by recent space missions in order to test the model proposed for the formation of solar prominences.

Formation of Solar Prominences by Flux Convergence

Observations have found three main conditions that have to be fulfilled before a prominence will form. These are the presence of a magnetic arcade, a transport of flux toward the polarity inversion line under the arcade, and cancellation of magnetic flux that approaches the polarity inversion. We have set up a three-dimensional model that initially contains two bipolar regions, representing new and old regions of flux on the Sun. The regions are forced together by an imposed velocity flow on the lower boundary. As they approach one another, the magnetic field reaches a state where a thin current distribution is created above the polarity inversion line. When the current becomes strong enough, magnetic reconnection starts. In the right parameter regime it is found that the reconnected field lines are able to lift plasma several pressure scale heights against gravity. The lifted plasma forms a region with enhanced density above the current sheet, along the polarity inversion line and with a length that is longer than the length of the current sheet. Different types of field line connectivity are found that provide the force to lift mass as the reconnection proceeds.

Simulation Studies of Solar Prominence Formation

AbstractA series of numerical simulations are presented for formation of solar prominences. We have investigated the dynamic and thermodynamic evolution of coronal plasma in response to photospheric horizontal motions. In three different setups of field configurations and footpoint motions, different types of prominences are found to be formed by thermal instability.

Formation of Solar Prominences with Normal and Inverse Polarities

AbstractNumerical simulations of solar prominence formation are presented employing photospheric horizontal motions as boundary conditions. Three different combinations of magnetic field configurations and footpoint motions are considered: (1) a bipolar arcade with a footpoint shear, (2) an arcade exposed to a shearing and converging motion, (3) two adjacent arcades undergoing a shearing and converging motion. In each case, it is found that the dynamic evolution of magnetic fields can force the plasma into a thermal instability leading to the formation of a prominence.

Formation of solar prominences by photospheric shearing motions

A numerical simulation is performed to investigate the prominence formation in a magnetic arcade by photospheric shearing motions. A two-and-a-half-dimensional magnetohydrodynamic (MHD) code is used, in which the gravitational force, radiative cooling, thermal conduction and a simplified form of coronal heating are included. It is found that a footpoint shear induces an expansion of the magnetic arcade and cooling of the plasma in it. Simultaneously the denser material from the lower part of the arcade is pulled up by the expanding field lines. A local enhancement of radiative cooling is thus effected, which leads to the onset of thermal instability and the condensation of coronal plasma. The condensed material grows vertically to form a sheet-like structure making dips on field lines, leading to the formation of the Kippenhahn- Schluter type prominence. The mass of the prominence is found to be supplied not only by the condensation of the material in the vicinity but also by the siphon-type upflows. The upward growth of the vertical sheet-structure of the prominence is saturated at a certain stage and the newly condensed material is found to slide down from above the prominence along magnetic field lines. This drainage of material leads to the formation of an arc-shaped cavity of low density and low pressure around the prominence. The problem of force and heat balance is addressed and the prominence is found to be not in a static equilibrium but in a dynamic interaction with its environment.

A model for the formation of solar prominences

We propose a model for the formation of prominence condensations in hot coronal loops. Previous studies have concentrated on cooling the hot plasma by decreasing the coronal heating rate. The difficulty with such models is that when the heating decreases, most of the loop mass is lost by draining onto the chromosphere. We argue that a prominence condensation is likely to be due to an increase in the heating. The key idea of our models is that the heating increase is spatially dependent so that it is localized nearer to the chromospheric footpoints than to the loop midpoint. We present results of numerical simulations of hot loops that are initially heated uniformly, and then undergo heating increases that are concentrated away from the loop midpoint

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).

The structure of coronal arcades and the formation of solar prominences

The temperature and density are obtained for coronal plasma in thermal and hydrostatic equilibrium and located in a force-free magnetic arcade. The isotherms are found to be inclined to the magnetic field lines and so care should be taken in inferring the magnetic structure from observed emission. When the coronal pressure becomes too great, the equilibrium ceases to exist and the material cools to form a quiescent prominence. The same process can be initiated at low heating rates when the width or shear of the arcade exceeds a critical value. We suggest that the prominence should be modelled as a dynamic structure with plasma always draining downwards. Material is continually sucked up along field lines of the ambient arcade and into the region lacking a hot equilibrium, where it cools to form new prominence material.

The formation of solar prominences by thermal instability in a current sheet

The energy balance equation for the upper chromosphere or lower corona contains a radiative loss term which is destabilizing, because a slight decrease in temperature from the equilibrium value causes more radiation and hence a cooling of the plasma; also a slight increase in temperature has the effect of heating the plasma. In spite of this tendency towards thermal instability, most of the solar atmosphere is remarkably stable, since thermal conduction is very efficient at equalizing any temperature irregularity which may arise. However, the effectiveness of thermal conduction in transporting heat is decreased considerably in a current sheet or a magnetic flux tube, since heat can be conducted quickly only along the magnetic field lines. This paper presents a simple model for the thermal equilibrium and stability of a current sheet. It is found that, when its length exceeds a certain maximum value, no equilibrium is possible and the plasma in the sheet cools. The results may be relevant for the formation of a quiescent prominence.

Formation of Solar Prominences

A non-linear treatment of condensation of quiescent type prominence, from the solar corona, by means of thermal instability is presented. It is found that the prominence stage is reached after times of the order of 10 6 s. The condensation process is inhibited due to heat conduction by free electrons if the initial magnetic field strength is less than 10 −4 gauss. Magnetic fields greater than 0.1 gauss do not allow any significant density increase. However, for magnetic fields >0.1 gauss significant density increase will take place due to secondary processes after the initial (thermal instability) prominence temperature is attained.

Solution of Flow Problems in Unidimensional Lagrangian Hydromagnetics

A problem of considerable interest in many branches of astrophysics is that of the subsequent behaviour of a current which at an initial time t = 0 is largely concentrated within a given region of an ionized gas of infinite extent. In particular, it has been suggested by Alfv�n (1950) that a high current discharge in an ionized gas is likely to constrict because of the electromagnetic attraction' between parallel currents and that this constriction effect may be involved in the formation of solar prominences. Similar considerations may also be of importance in studies of magnetic fields in the spiral arms of the Galaxy.