The formation of spicules in the course of the chromospheric network magnetic field reconnection

We investigate the physical processes occurring in the supergranule boundary cylinder layer (SBCL). Taking into account the Coriolis force, we obtain an expression for the component of the magnetic field and velocity in the SBCL. Within the framework of linear MHD, we consider the formation and coalescence of magnetic tubes, i.e. spicules, in the course of the reconnection of the SBCL magnetic field. The estimated number of spicules appearing on each supergranule cell is in agreement with observations. This number depends on the solar latitude : (1) if the normal component of the magnetic fieldB _z is assumed to be independent of , then the maximum number of spicules should be at = 71°; (2) ifB _z is assumed to be the component of the dipolar fieldB _z sin , then the maximum number should be at the pole: = 90°. The timescale of the formation and the coalescence of the magnetic tubes is 10–20 min, which is of the order of the observed lifetime of the spicules.     

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.     

The effects of magnetic structure on the conduction cooling of flare loops

A model of the sheared magnetic field in a coronal loop is used to evaluate the average cross-field suppression of axial thermal conduction. If the energy source is uniform in radius, this can lead to heat-flux reduction by a factor greater than three. When the source is annular, in a region of radius where the current density and shear are peaked, the effect can be significantly larger. In one extreme case, however, in which magnetic tearing provides the heating in a very narrow layer, the spatial resonance of the source excitation in a long loop leads to approximately axial conduction.     

Magnetic reconnection as a driver of chromospheric surges

The basal structure of a surge precisely on the limb has been photographed with 1-resolution in the core and wings of H. The dynamics observed in the fine structures are consistent in general with reconnection theory, but they also display flows more complicated than those predicted by 2D-reconnection models. The magnetic topology of the surrounding long-lived plage indicates that flux cancellation rather than its emergence is the key factor in promoting recurrent surges at this site.Dedicated to Cornelis de Jager     

Structures of chromospheric magnetic fields in the solar flare producing active region 5747

In this paper, the formation and the measurement of the H line in chromospheric magnetic fields are discussed. The evolution of the chromospheric magnetic structures and the relation with the photospheric vector magnetic fields and chromospheric velocity fields in the flare producing active region AR 5747 are also demonstrated.The chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric field are found in the flare-producing region. This probably reflects the complication of the magnetic force lines above the photosphere in the active region. The evolution of the chromospheric magnetic structures in the active region is caused by the emergence of magnetic flux from the sub-atmosphere or the shear motion of photospheric magnetic fields. The filaments separate the opposite polarities of the chromospheric magnetic field, but only roughly those of the photospheric field. The filaments also mark the inversion lines of the chromospheric Doppler velocity field which are caused by the relative motion of the main magnetic poles of opposite polarities in the active region under discussion.     

An evidence of flare energy buildup and release related to magnetic shear and reconnection

We study series of homologous flares, observed in the active region NOAA 2372 by the HXIS on the Solar Maximum Mission and ground based observatories. Changes in the flare homology, particularly those related to the location of the hard X-ray emission, show clear correlation with the development of magnetic shear within the active region. Following our early study (Machado et al., 1983) we propose that magnetic shear and reconnection are necessary for high power energy release, but the former may not be a sufficient condition in an isolated magnetic loop. These results are discussed within the context of a broader study, in order to explore their generality.     

Evidence of two-stage magnetic reconnection in the 2005 January 15 X2.6 flare

We analyze in detail the X2.6 flare that occurred on 2005 January 15 in the NOAA AR 10720 using multiwavelength observations. There are several interesting properties of the flare that reveal possible two-stage magnetic reconnection similar to that in the physical picture of tether-cutting, where the magnetic fields of two separate loop systems reconnect at the flare core region, and subsequently a large flux rope forms, erupts, and breaks open the overlying arcade fields. The observed manifestations include: (1) remote Hα brightenings appear minutes before the main phase of the flare; (2) separation of the flare ribbons has a slow and a fast phase, and the flare hard X-ray emission appears in the later fast phase; (3) rapid transverse field enhancement near the magnetic polarity inversion line (PIL) is found to be associated with the flare. We conclude that the flare occurrence fits the tether-cutting reconnection picture in a special way, in which there are three flare ribbons outlining the sigmoid configuration. We also discuss this event in the context of what was predicted by Hudson et al. (2008), where the Lorentz force near the flaring PIL drops after the flare and consequently the magnetic field lines there turn to be more horizontal as we observed.     

Optical properties of bipolar flare kernels associated with asymmetric magnetic loops

Observations are presented for nine flares containing two principal patches of emission, or kernels, in which the kernel associated with weaker magnetic field has the greater H line width. The observations are interpreted in terms of an asymmetric bipolar magnetic loop from which high energy electrons precipitate predominantly at the loop footpoint of weaker field. Calculations are presented which indicate that, for an isotropic distribution of electron velocity vectors at their initial point of injection, the observations are consistent with a location for the injection in the upper part of the loop. The same type of model predicts the associated microwave burst to be stronger near the opposite (strong field) footpoint (Kundu and Vlahos, 1979).Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Kink unstable coronal loops: current sheets,current saturation and magnetic reconnection

The kink instability in a coronal loop is a possible explanation of a compact loop flare as it may cause a current sheet to form allowing reconnection to take place and release the free magnetic energy stored in the loop. However, current sheets do not form in all cases. Ali and Sneyd (2001) investigated three different classes of equilibrium (determined by the form of the twist) using a magneto-frictional code. They searched for the equilibria to which the loop might evolve once it had become unstable to the kink instability. They found indications of current-sheet formation for only one class of equilibrium studied. However, as they pointed out, since their code searched for equilibria they were unable to say for certain that the loop would evolve in this way. In this paper we have considered the same three classes of equilibria but have used a code which follows the non-linear 3D MHD (magnetohydrodynamic) evolution of the loop. We have investigated whether or not there are indications of current-sheet formation. In the cases where there is evidence of this we have found that reconnection does occur and releases sufficient magnetic energy to explain a compact loop flare.     

Coronal heating by magnetic reconnection

The theory of magnetic reconnection has advanced substantially over the past few years. There now exists a new generation of fast two-dimensional models known as almost-uniform reconnection and nonuniform reconnection, depending on the boundary conditions. Also, we are beginning to explore the uncharted region of three-dimensional reconnection, where regimes of “spine reconnection” and “fan reconnection” have been discovered. Furthermore, part of the coronal heating problem appears to have been solved with recent observational support for the Converging Flux Model in which heating is produced by coronal reconnection driven by footpoint motions.     

Influence of magnetic field structure on the conduction cooling of flare loops

A simple model facilitates calculation of the influence of magnetic field configuration on the conduction cooling rate of a hot post-flare coronal plasma. The magnetic field is taken to be that produced by a line dipole or point dipole at an arbitrary depth below the chromosphere. For the high temperatures (T 10⁷ K) produced by flares, the plasma may remain static and isobaric. The influence of the field is such as to increase the heat flux (per unit area) into the chromosphere, but to decrease the total conduction cooling of the flare plasma. This leads to a significant enhancement of the total energy radiated by the flare plasma.     

Structure and dynamics of cool flare loops

MSDP observations of the 16 May, 1981 two-ribbon flare are used to study the physical structure and the dynamical behaviour of cool flare loops. The loops have been detected in the H line just after the flare maximum and they appeared in absorption against the disk. Using the first-order differential cloud model (DCM1) technique, we derive empirically some basic plasma parameters at 15 points along one loop leg. The flow velocities and the true heights have been reconstructed with respect to a geometrical projection. Subsequently, detailed non-LTE models of cool loops have been constructed in order to fit H source function values previously derived from DCM1 analysis. It is demonstrated that this source function is rather sensitive to the radial component of the flow velocity (the so-called Doppler brightening) and to enhanced irradiation of the loops from the underlying flare ribbons. In this way, we have been able to estimate quantitatively all plasma parameters which determine the physical structure of cool loops (i.e., the temperature, pressure, density), as well as the momentum-balance condition within the loops. For these dark loops we have arrived at relatively low gas pressures of the order of 0.1–0.5 dyne cm^-2 with corresponding electron densities around 10¹¹ cm^-3. Pressure-gradient forces have been found to be of small importance in the momentum-balance equation, and thus they cannot explain departures from a free-fall motion found in our MSDP data analysis. We propose three possible solutions to this problem.     

Initial phase of chromospheric evaporation in a solar flare

In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 10⁷ K was moving toward the observer with a mean velocity of about 80 km s^-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s^-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 10¹⁰ erg s^-1 cm^-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona.     

Magnetic reconnection driven by recombination during collision of two current loops

It is shown by using a 3-D resistive MHD simulation code, taking into account the recombination effect, that magnetic reconnection during collision of two current loops can be enhanced by recombination. It is also shown that the temperature in the thin current sheet formed between two loops increases from few to about thirty times larger than a case of no recombination, depending on both the plasma beta and the strength of recombination. The simulation results obtained here may be applicable for a mechanism of chromospheric heating and as an explanation of X-ray bright points as well as solar flares observed in the chromosphere.Dedicated to Cornelis de Jager     

On the occurrence of blue asymmetry in chromospheric flare spectra

We present observations of optical spectra of a flare in which blue line asymmetry was seen for more than 4 min close to the flare onset. The maximum blue asymmetry coincided with the maximum of a hard X-ray and microwave burst. We discuss possible interpretations of the blue asymmetry and conclude that the most plausible one is electron-beam heating with return current. Although this process predicts downflows in the lower transition region and upper chromosphere, its ultimate effect on the line profiles can be blue asymmetry: the upper layers moving away from us absorb the radiation of the red peak thus lowering its intensity in comparison to the blue one.     

Combined study between the chromospheric flare models and hard X-ray observation

By comparison between SMM HXRBS observation and ground observation of H and Caii K lines for the 2B flare on February 3, 1983, we found that there was a temporal correlation between H intensity and hard X-ray flux at the early stage of the impulsive phase while different peaks in the hard X-ray flux curve represented bursts at different locations. When we combined SMM HXRBS observation with chromospheric flare models, we further found that the temporal coincidence between H intensity and hard X-ray flux could be explained quantitatively by the fact that the H flare was indeed due to the heating by non-thermal electron beams responsible for the emission of hard X-rays. Together with the discussion on coronal density based on chromospheric flare models, it was also shown that the source of electrons seemed to be situated around the top of the flare loop and the column density at the top of the chromosphere in semi-empirical flare models could not be taken as the total material above the top of the chromosphere.     

The relations between chromospheric features and photospheric magnetic fields

High resolution photographic magnetograms are compared with H filtergrams (both on- and off - band) for a wide variety of solar features. It is verified that H filaments overlie neutral lines or bands and that H plages always occur at magnetic field clumps. However, the brightness of H plages bear no relation to magnetic field strength or polarity, and the direction of the magnetic field with respect to threads and filaments remains obscure. Counter-examples can be found for virtually every rule that has been formulated so far.Basic questions about the usefulness and final research goal of filtergrams and magnetograms are raised. It is shown that neither filtergram or magnetogram alone is capable of furnishing a unique solution. It is suggested that the proper direction for research is to use magnetograms, together with (as yet unspecified) additional sources of data, to understand H structures.     

Model for flare loops,fast motions,and opening of magnetic field in the corona

A model is presented for the penetration into the corona of a new magnetic field of a developing bipolar region and for its interaction with an old large-scale coronal field. An important feature of the model is a reconnection of the old and new fields inside the current sheet arising along the zero line of the total magnetic field calculated in the potential approximation. The magnetic reconnection and accumulation of plasma inside the current sheet can explain the appearance of dense coronal loops and the energy source at their tops. The plasma together with the magnetic lines is flowed into the sheet from both its sides. This fact explains the appearance of coronal cavities above the loops. If the large-scale field gradually decreases with the height, the loop motion is slowed down. The account of the dipolar structure of the magnetic field at large heights explains the possibility of a rapid break of the new field through the corona and the appearance of transients and open field regions - the coronal holes. In this case a fast rising current sheet can be a source of accelerated particles and of type II radio burst, instead of the shock wave considered usually.     

Filament activation and magnetic reconnection

Solar Physics - A curved filament in a decaying active region (AR&;nbsp;8329) was observed on 9 September 1998 with a combination of several instruments. The main data base is a 4-hour long time...     

Heating of chromospheric magnetic features by direct current dissipation

It is shown that the direct current dissipation is very unlikely to be the heat source of the coronal loop, because it accompanies unacceptably high heating rate in the chromospheric portion of the loop. This also suggests that a rather weak current density can supply the heat to a small (R < 10⁷ cm) chromospheric magnetic features. A larger magnetic element may be heated by the direct current dissipation only if the current changes directions within a single element so that the generated magnetic field is sufficiently weak to insure MHD stability.