Evolution of emerging force-free flux tube to be a flare loop

Solar Physics - Evolution of a filamentary magnetic flux tube emerging from the photosphere is investigated in the assumption that the magnetic field is force-free and unchanged during the...     

Small-scale magnetic flux tube diagnostics in a solar flare

I ±V profiles of the Fei 5247 and 5250 lines in the 2B flare of June 16, 1989 have been analyzed. A bright knot of the flare outside the sunspot where the central intensity of H reached a peak value of 1.4 (relative to the continuum) has been explored. The Fei 5250/Fei 5247 magnetic line ratio based on the StokesV peak separations of these lines at five evolutionary phases of the flare (including the start of the flare, the flash phase, the peak and 16 min after the peak) has been analyzed. It was found that the StokesV peak separation for the Fei 5250 line was systematically larger than that of the Fei 5247 line. This is evidence for the presence in the flare of small-scale flux tubes with kG fields. The flux tube magnetic field strength was about 1.1 kG at the start of the flare and during the flash phase, 1.55 kG during the peak, and 1.38 kG 16 min after the peak. The filling factor,, appears to decrease monotonically during the flare.     

Equilibrium of a thin,isolated, axisymmetric force-free magnetic flux tube in a stratified atmosphere

We show that up to second order in the thinness parameter, the equations of equilibrium of a thin, isolated, axisymmetric, force-free magnetic flux tube in a stratified atmosphere and the other essential constraints form a mathematically closed system. Auxiliary assumptions about temperature of the tube plasma are neither needed nor possible.     

Plasma motions in the solar loop of emerging magnetic flux

The results of analyzing variations in the line-of-sight (LOS) velocities in the solar loop at photospheric and chromospheric levels in the region of emerging magnetic flux for the evolving active region NOAA 11024 are reported. The analysis combines the data of multiwave spectropolarimetric observations that were carried out on July 4, 2009, (Tenerife, Spain) using THEMIS solar telescope and the data obtained with GOES, SOHO, and STEREO cosmic satellites. A complex sequence of active events has been studied: formation of the Ellerman bomb, B1 X-ray microflare, and four chromospheric surges that were formed as a result of magnetic reconnection caused by new emerging magnetic flux. The Ellerman bomb was formed in the vicinity of a growing pore. Variations in the velocity V _LOS of the EB had an oscillation character for chromosphere and photosphere. Before the microflare, the average velocities of the upward and downward plasma fluxes in one leg of the magnetic loop were nearly the same—26 km/s. During the microflare, the velocity V _LOS of the ascending and descending flows increased up to ?33 and 50 km/s, respectively. Variations in line-of-sight velocity of a plasma in the second leg of the magnetic loop correlated well with variations of V _LOS in the region of microflare, but they occurred 1.5 minutes later. During the time of observations, four chromospheric ejections of matter were formed and three of them occurred in the region of Ellerman’s bomb formation. Sharp variations in the soft X-ray intensity occurred during these ejections. At photospheric level, variations in the line-of-sight velocity of plasma in the legs of the loop occurred in the opposite direction. In the region of the first leg, velocity V _LOS diminished from ?1.8 to ?0.4 km/s, while the velocity increased from ?0.6 to ?2.6 km/s in the region of the second leg.     

Characteristics of proton flare active regions and force-free magnetic fields

Proton flare active regions have the common characteristic of an S-shaped neutral line caused by relative movements of sunspots. This observational feature suggested to us a method of estimating the force-free parameter. For three typical active regions we have estimated the variation in the force-free parameter, and assuming a constant force-free parameter, we calculated the structure of the force-free field and the magnetic energy of the potential field. Our calculations show that before the occurence of proton flares, the force-free parameter increases and the magnetic energy of the potential field decreases, this decrease may well be the source for the development of the force-free field, and its magnitude is sufficient for the requirement of the proton flare.     

Oscillations of a loop prominence preceding a limb flare

Observations of a limb flare and an associated loop prominence were obtained in H with the 512 channel magnetograph of the Kitt Peak National Observatory. Simultaneous radial and torsional oscillations with a period near 75 s, wavelength of 37 000 km, and amplitude of 1–2 km s^–1 were detected in the loop approximately 90 min before the onset of the flare. We interpret these coupled oscillations in terms of a kink instability of a current carrying flux tube. The magnitude of the steady-state component of current is estimated to be 6 × 10¹⁰ ampères.Visiting Astronomer, Kitt Peak National Observatory, operated by Aura, Inc., under contract with the National Science Foundation.Visiting Student, Kitt Peak National Observatory.     

The calculation of force-free fields from discrete flux distributions

This paper presents particularly simple mathematical formulas for the calculation of force-free fields of constant α from the distribution of discrete sources on a flat surface. The advantage of these formulas lies in their physical simplicity and the fact that they can be easily used in practice to calculate the fields. The disadvantage is that they are limited to fields of ‘sufficiently small α’. These formulas may be useful in the study of chromospheric magnetic fields by the comparison of high-resolution Hα photographs and photospheric magnetograms.     

The equilibrium shape of slender flux tubes in a linear force-free magnetic field

In this paper we extend previous work of Browning and Priest (1984, 1986) by studying the equilibrium path of twisted and untwisted thin flux tubes in a stratified, isothermal atmosphere using as the ambient field a linear force-free field. When an untwisted flux tube is considered, we find that shearing the magnetic arcade provides a different form to change the parameter which characterizes the external atmosphere, but at the same time this introduces a limitation in the width allowed for the external arcade. Also, the critical width found for the different analytical cases considered is always greater than one arch of the ambient arcade which prevents an eruption inside the arcade. In the case of twisted flux tubes, an analytical solution can be found for the critical _c , which separates regimes of strong and weak gravity, and the shape of the flux tube is now dependent on , a parameter which represents the magnetic field enhancement of the loop at the photosphere.     

A magnetodynamic mechanism for the heating of emerging magnetic flux tubes and loop flares

A new magnetodynamic model for loop flares is proposed to explain the following observational facts obtained from space during the last solar activity maximum: (i) Blueshifted lines of Ca xix and Fe xxv appear in some cases a minute or so before the initiation of impulsive bursts and relax into the unshifted lines with large width by the time of the onset of impulsive bursts, (ii) the hot source is formed by that time at the top of a loop-like structure, and confined there for a considerable time, and (iii) -ray line enhancement occurs at about the same time as hard X-ray spikes.In our model, the supply of energy to the loop top comes from below the chromosphere immediately before the flare (30 s-1 min before the hard X-ray impulsive bursts) in the form of the relaxing fronts of magnetic twist of opposite sign. These packets are thought to be built up in the process of loop emergence, stored at the footpoints of the loop below the photosphere, and released when the part of the feet floats up further. These released packets of magnetic twist drive the mass in the high chromosphere and transition zone into helical flows with pinch heating, and when these collide at the top of the loop, a very hot region appears there with a violent unwinding of the twists, resulting in the rapid dynamical annihilation of the magnetic energy, . Electrons and ions, raised to medium energies in the pinch at the incidence of the packets to the loop, are accelerated further by the Fermi-I mechanism between the approaching fronts of magnetic twist, and when B is weakened by unwinding they are released towards the chromosphere, and cause simultaneous -ray and hard X-ray bursts.     

Electric charge and magnetic flux on rotating black holes in a force-free magnetosphere

The electric charge on rotating black holes is calculated to be ∼ BJ in the force-free configuration of Ghosh, with a horizon flux of ∼ BM ². This charge is gravitationally weak for B ∼10¹⁵ G , so that the Kerr metric applies. Being similar to the electric charge of a magnetar, both electric charge and magnetic flux should be, in sign and order of magnitude, continuous during stellar collapse into a black hole. Extraction of the rotational energy from newly formed black holes may proceed by interaction with the magnetic field.     

Cooling of a coronal flare loop through radiation and conduction

A simple method is proposed for a computation of the cooling of coronal flare loops by radiation and conduction, for various temperatures, densities, and lengths of the loops. The relative importance of conductive and radiative losses is briefly discussed.     

Evolution of fibrils with special reference to flare activity

We show observational results on the pre-flare evolutions of H structures as well as the developments of H flares. It is shown that the chromospheric features are brought to a sheared state before flares due to motions of footpoints which correspond to particular sunspot motions. Generally in evolutions of the chromospheric features it is found that motions and reconnections of the footpoints play essential roles. The following three stages are found for development of the neutral line filament before flares: (1) formation of a filament as a result of reconnection; (2) increase of the shear of the filament due to the shear motion; and (3) reconnection of fine components of the filament to form an elongated component immediately before flares. We further show developments of two particular flares with and without the filament, and point out basic release processes of flares. The flare that occurred at the filament (July 5, 1974) started with the activation of the elongated component of the filament after the process (3). The main phase of a two-ribbon flare is considered as the rises of short components of the filament triggered by the rising motion of the elongated component. The flare of September 10, 1974 occurred at the region where fibrils connect the sunspots in distorted form. Pre-flare distortion was produced by translational rotation of the sunspot. Development of this two-ribbon flare is interpreted as being due to successive rises of the fibrils with a self-trigger mechanism.On leave from Tokyo Astronomical Observatory (present address).     

A single loop of 21 January 1974 flare

A single loop associated with a flare of 21 January 1974 was studied with NRL spectroheliograms in order to understand the phenomenon of evaporation. The loop seen in the emission lines of Fe xv reached its maximum brightness 15 min after the onset. The loop is different from a flare loop because of the time sequence in which it appeared and is different from a post-flare loop prominence system because of its morphology. The electron density in the loop increases gradually to 4 × 10¹⁰ cm^–3. The material of the loop is thought to be supplied from the lower atmosphere of the chromosphere or the photosphere. The loop is an associated phenomenon of the main flare event distinguished by a longer rise time (15 min) and a lower peak temperature (2 × 10⁶ K).     

Radiative transfer across a magnetic flux tube

In the coming years, some solar telescopes will be able to yield the Stokes' parameters of polarized light with a resolution better than 1 arc sec (0.3 arc sec for THEMIS). We have simulated the Stokes' parameters of a solar magnetic flux tube as seen with such a resolution. We have shown that, observing with a line-of-sight not parallel to the axis of the flux tube (assumed vertical and axisymmetric), it is possible to see differences between different configurations of the magnetic field inside the flux tube (presence, and in what direction, of an azimuthal component of the field). Furthermore, along such a line-of-sight, the polarization profiles of any atomic line are strongly absorbed at the line center. We then suggest a strategy to infer the structure of the magnetic field from observations at high spatial resolution.     

Magnetic field lines for a flux tube

Equations for the magnetic field components in a two dimensional cylindrically symmetric flux tube equilibrium have been derived and, in a simple case, solved. The resulting magnetic configuration possesses a strong magnetic field in a thin tube below a reference level (solar photosphere). Above this reference level the field lines spread out in all directions.     

Computation and analysis of the physical fields of a disc flare loop

We carried out a preliminary reduction of the Ha-SSHG spectral data on the large, double-ribbon, class 3B flare of 1981 May 16, obtained at Yunnan Observatory. We used a least-squares fit of a nonlinear function to derive simultaneously from the observed profile, the optical depth at the line centre, the Doppler width, the mean source function and the radial velocity, thus providing an example of a numerical treatment of data having the characteristics of a time-space series.The results of calculation show that matter inside the flare loop was falling along the loop legs to the chromosphere and that it would be more reasonable to use the chromospheric evaporation model to account for the source of the loop matter.     

The stability of line tied force-free cylindrical arcades: Is an active region filament a requirement for a two-ribbon flare?

The MHD stability of force-free, cylindrical arcades is investigated, including the stabilising effect of photospheric line tying. It is found that a wide variety of fields are stable. This suggests that either a departure from a force free equilibrium or suppression of line tying is necessary if a two-ribbon flare is to be triggered. It is postulated that in both circumstances, the existence of an active region filament is an essential preflare requirement for a two-ribbon flare.     

Magnetostatic configuration of a confined magnetic flux tube

In this paper an isolated magnetic flux tube confined in stratified atmosphere is studied for slender and axisymmetric model. The functions of the pressure, density, and temperature are expanded as a Taylor series of magnetic surface function . Several models of an isolated magnetic flux tube confined in a stratified atmosphere are constructed, and the external pressure of the stratified atmosphere decreases reasonably with increasing height. The distribution of thermal dynamic quantities and the magnetic pressure in the flux tube are also obtained.     

The solar soft X-ray background flux and its relation to flare occurrence

Solar Physics - The soft X-ray background flux (XBF) based on GOES 1–8&;nbsp;Å measurements for the period 1975–2003 is studied. There is strong evidence that in the XBF the...