A magnetodynamic mechanism for loop flares

Loop flares are given a new magnetodynamic interpretation. In this model, the top of the magnetic loop is heated up by a collision of magnetic twist-wave packets (non-linear torsional Alfven wave) which are produced in the process of the loop emergence, and stored and released from the footpoints of the loop with some retardation. The appearance of the blueshifted component in CaXIX and FeXXV lines a minute or so before the impulsive phase, and the so-called instantaneous acceleration of ions deduced from the nearly simultaneous (with a delay of seconds) occurrence of -ray line emission with the impulsive hard X rays, are very naturally explained in the present model which originally aims at providing an explanation of the source of energy, a blackbox located at the top of the loop in the loop flare theories discussed thus far.     

Dynamical interpretation of the very hot region appearing at the top of the loop

In order to explain the appearance of a hard X-ray source at the top of a loop, we present a model in which the dynamical effects of the dark filament mass infallng along the loop in association with the “disparition brusque” plays an important role. The crash of the infalling mass produces high temperature regions in the low corona above the two footpoints of the loop, and the up-going shocks, created in the crash and strengthened in propagating upwards along the steep density gradient in the tail of the infalling mass, produce a very high temperature (10⁸ K) region upon colliding with each other near the top of the loop. Successive occurrence of this process in successively higher loops in magnetic arcade may account for the sources of gradual hard X-ray bursts appearing at the top of the loop-like structure.     

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.     

Magnetic field structures of hard X-ray flares observed by HINOTORI spacecraft

The magnetic field structure of five flares observed by HINOTORI spacecraft is studied. The double source structure of impulsive flares seems to indicate hard X-ray emission from the two footpoints of a flaring loop, but the potential field computation does not reproduce a loop connecting the two sources. Therefore the magnetic field could be in a sheared configuration and the force-free field modeling would be the next step to examine. On the other hand gradual flares are characterized by hard X-ray sources located in the corona, 2–4 x 10⁴ km above the photosphere. The potential field modeling is found to give a reasonable fitting in this type of flares, and the hard X-ray sources are located at the top of the magnetic loop or arcade. This configuration is consistent with the thick-target trap model of the hard X-ray bursts.     

Plasma waves caused by transient heat conduction in a coronal loop and anomalous resistivity

Numerical simulation is made of the transient heat conduction during local heating in a model coronal magnetic loop with an axial electric current. It is assumed that a segment near the top of the normal coronal loop is heated to above 10⁷ K by a sufficiently small heat input as compared with the total flare energy. A hump appears in the velocity distribution of electrons moving down the temperature gradient with speeds slightly below the thermal one. Consequently, electron plasma waves are excited. The high intensity of the waves persists in the upper region of the loop for more than a second until the termination of the simulation. The energy density of the plasma waves normalized with respect to thermal density is 10^–3.5 at maximum. A theoretical estimate gives an anomalous resistivity 5 orders of magnitude greater than an initial value. Based on the above result, we propose a model for impulsive loop flares.     

A model of hot loops associated with solar flares

A dynamical model is proposed for the formation of soft X-ray emitting hot loops in solar flares. It is examined by numerical simulations how a solar model atmosphere in a magnetic loop changes its state and forms a hot loop when the flare energy is released in the form of heat liberation either at the top part or around the transition region in the loop.When the heat liberation takes place at the top part of the loop which arches in the corona, the plasma temperature around the loop apex rises rapidly and, as the result, the downward thermal conductive flux is increased along the magnetic tube of force. Soon after the thermal conduction front rushes into the upper chromosphere, a local peak of pressure is produced near the conduction front and the chromospheric material begins to expand into the corona to form a high-temperature (10⁷ K-3 × 10⁷ K at the loop apex) and high-density (10¹⁰ cm^–3-10¹¹ cm^–3 at the loop apex) loop. The velocity of the expanding material can reach a few hundred kilometres per second in the coronal part. The thermal conduction front also plays a role of piston pushing the chromospheric material downward and gives birth to a shock wave which propagates through the minimum temperature region into the photosphere. If, on the other hand, the heat source is placed around the transition region in the loop, the expansion of the material into the corona occurs from the beginning of the flare and the formation process of the hot loop differs somewhat from the case with the heat source at the top part of the loop.Thermal components of radiations emitted from flare regions, ranging from soft X-rays to radio wavelengths, are interpreted in a unified way by using physical quantities obtained as functions of time and position in our flare loop model as will be discussed in detail in a following paper.     

Initial Fe/O Enhancements in Large, Gradual, Solar Energetic Particle Events: Observations from Wind and Ulysses

Shocks driven by fast coronal mass ejections (CMEs) are the dominant particle accelerators in large, “gradual” solar energetic particle (SEP) events. In these events, the event-integrated value of the iron-to-oxygen ratio (Fe/O) is typically ～?0.1, at least at energies of a few MeV/nucleon. However, at the start of some gradual events, when intensities are low and growing, initially Fe/O is ～?1. This value is also characteristic of small, “impulsive” SEP events, in which particle acceleration is due to magnetic reconnection. These observations suggested that SEPs in gradual events also include a direct contribution from the flare that accompanied the CME launch. If correct, this interpretation is of critical importance: it indicates a clear path to interplanetary space for particles from the reconnection region beneath the CME. A key issue for the flare origin is “magnetic connectedness”, i.e., proximity of the flare site to the solar footpoint of the observer’s magnetic field line. We present two large gradual events observed in 2001 by Wind at L1 and by Ulysses, when it was located at >?60^° heliolatitude and beyond 1.6 AU. In these events, transient Fe/O enhancements at 5?–?10 MeV/nucleon were seen at both spacecraft, even though one or both is not “well-connected” to the flare. These observations demonstrate that an initial Fe/O enhancement cannot be cited as evidence for a direct flare component. Instead, initial Fe/O enhancements are better understood as a transport effect, driven by the different mass-to-charge ratios of Fe and O. We further demonstrate that the time-constant of the roughly exponential decay of the Fe/O ratio scales as R ², where R is the observer’s radial distance from the Sun. This behavior is consistent with radial diffusion. These observations thus also provide a potential constraint on models in which SEPs reach high heliolatitudes by cross-field diffusion.     

Electron beams and associated rapid fluctuations in solar flares

Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares.     

High-resolution observations of solar radio bursts at 2, 6, and 20 cm wavelength

The Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe eight solar bursts at 2, 6, or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10⁷ K and 2 x 10⁸ K, and degrees of circular polarization between 10 and 90%. A series of 10 s snapshot maps are presented for the more intense bursts, and superimposed on photospheric magnetograms or Hα photographs. The impulsive phase of the radio bursts is located near the magnetic neutral line of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. The impulsive phase of one 6 cm burst was smaller and spatially separated from both the preburst radio emission and the gradual decay phase of the burst. Another 6 cm burst exhibited preburst heating of the coronal loop in which the burst occurred. The plasma was probably heated at a lower level in the loop, while the burst energy was released several minutes later at a higher level. A multiple-spike 20 cm burst exhibited polarity inversions with degrees of circular polarization of 90%. The rapid changes in circular polarization are attributed to either a magnetically complex region or the emersion of new magnetic flux at coronal heights where magnetic field strengths H ≈ 300 to 400 G.     

Evaluating Mean Magnetic Field in Flare Loops

We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection rate in terms of V _r B _r by measuring the ribbon-expansion velocity (V _r) and the chromospheric magnetic field (B _r) swept by the ribbons. We also measure the velocity (V _t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B _t) at the top of newly-formed flare loops using the relation 〈V _t B _t〉≈〈V _r B _r〉, namely, conservation of reconnection flux along flare loops. For this flare, B _t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore, the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event.     

A search for the origin of very low electron temperatures

VLET's (Very Low Electron Temperature) are regions in the solar wind (lasting 12–30 h) in which the electron temperature is abnormally low. Because it is generally believed that the thermal conductivity parallel to the interplanetary magnetic field lines is high for electrons, i.e. the contact with the Sun should be ideal, VLET's are surprising observations. In this work a statistical analysis of many of these events is made with respect to the dependence of this phenomenon on interplanetary plasma and field parameters. In contrast with earlier work it was found that VLET's exist not only after a shock front. The statistical analysis showed further that a VLET is always associated with a VLPT (Very Low Proton Temperature) event and that, whilst on average the temperature of the electrons and protons outside the low temperature regions is about the same, the mean proton temperature inside is three times lower than that of the electrons.A particular model for VLET's is investigated in detail; the closed magnetic loop or “blob” model. By assuming: (a) pure adiabatic expansion in a radially streaming solar wind without pressure equalization across the “blob” boundary, and (b) rapid pressure equalization across the boundary, an attempt was made to quantitatively investigate the feasibility of the “blob” model in the light of plasma and field data. It can be shown that the closed magnetic loop model is unlikely to be the major cause of VLET's.     

Observations of preburst heating and magnetic field changes in a coronal loop at 20 cm wavelength

The Very Large Array (VLA) has been used at 20 cm wavelength to study the evolution of a burst loop with 4 resolution on timescales as short as 10 s. The VLA observations show that the coronal loop began to heat up and change its structure about 15 min before the eruption of two impulsive bursts. The first of these bursts occurred near the top of the loop that underwent preburst heating, while the second burst probably occurred along the legs of an adjacent loop. These observations evoke flare models in which coronal loops twist, develop magnetic instabilities and then erupt. We also combine the VLA observations with GOES X-ray data to derive a peak electron temperature of T _e = 2.5 × 10⁷ K and an average electron density of N _e 1 × 10¹⁰ cm^–3 in the coronal loop during the preburst heating phase.     

A family of ionospheric models for different uses

Empirical models of three dimensional electron density distributions in the ionosphere have been constructed for global as well as regional use. The models differ by their degree of complexity and calculation time and therefore have different uses. All are based on “ionogram parameter” (critical frequencies foE, foF1, foF2 and the F2 region transfer parameter M(3000)F2). The models allow the use of global or regional maps for foF2 and M(3000)F2 and use built-in formulations for foE and foF1. Update (instantaneous mapping / nowcasting) versions exist which take foF2 and M(3000)F2 or F2 region peak height and electron density as input. The ground to F2 layer peak part of the profile is identical for all three models and is based on an Epstein formulation. The “quick calculationr” model NeQuick uses a simple formulation for the topside F layer, which is essentially a semi-Epstein layer with a thickness parameter which increases linearly with height. The “ionospheric model” COSTprof is the model which was adopted by COST 251 in its regional “monthly median” form. Its topside F layer is based on O⁺-H⁺ diffusive equilibrium with built-in maps for three parameters, namely the oxygen scale height at the F2 peak, its height gradient and the O⁺-H⁺ transition height. The “ionosphere-plasmasphere” model NeUoG-plas uses a magnetic field aligned “plasmasphere” above COSTprof Typical uses of the models and comparison among them are discussed.     

Interpreting Yohkoh hard and soft X-ray flare observations

A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE _c , where 15 keV <E _c < 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources.     

A model of impulsive loop flares caused by anomalous heat conduction

Numerical simulation is made of the impulsive loop flare caused by transient heat conduction along the loop with an applied axial electric current.It is assumed that a segment near the top of the coronal loop is heated to above 10⁷ K by a heat input that is small compared with the total flare energy, which is given by the magnetic energy of the initial current. Due to the heat conduction, a hump appears in the velocity distribution of electrons, which may excite electron plasma waves with a sufficiently high intensity to cause an anomalous resistivity, as shown theoretically in a previous paper. In that paper, an effect of the plasma waves on the dynamics of electrons was taken into account consistently, but an anomalous heating due to an ohmic dissipation of the initial current under the anomalous resistivity was not taken into account.The aim of the present study is to study the subsequent dynamics of the heated gas caused by the anomalous heating, but in order to avoid an unpractically long computation time, the energy density of the plasma waves is estimated by the energy density of electrons in the velocity hump, without taking into account the effect of the plasma waves consistently in the dynamics of the electrons.The initial current starts to decay gradually by an ohmic dissipation under the anomalous resistivity occurring near the top of the loop to heat this region more. The enhanced heat conduction causes the velocity humps in a wider location. Consequently, the anomalous heating continues and spreads in a self-generating way even after the end of the initial minor heating. Thus the temperature near the loop top becomes above 10⁸ K and the high-temperature region spreads in both directions along the loop with such a high speed as (2–3) × 10⁴ km s^–1, which is nearly equal to the speed of flux-limited heat conduction. On the other hand, induced electric field estimated from the anomalous resistivity is 3.3 × 10⁷ V at the termination of the present simulation, under the modest initial current of 1.5 A m^–2.X-ray emissions expected from the present model loop, show three sources, two footpoints with unequal brightness and a coronal source expanding along the loop in both directions.     

YOHKOH/HXT EVIDENCE FOR A HYPERHOT LOOP-TOP SOURCE IN THE PRE-IMPULSIVE PHASE OF A LOOP FLARE

A loop flare that occurred on 22 April 1993 near the disk center is examined using the Yohkoh Hard X-ray Telescope (HXT). We specifically looked into the faint early phase of the flare prior to the start of the strong impulsive phase. The pre-impulsive phase, though weak in intensity, is expected to contain essential clues to the mechanism of loop flares according to the causality principle, but it has not received attention previously, probably due to the insufficient dynamic range and cadence of observations by the instruments on earlier satellites. Observations with Yohkoh/HXT can clarify what occurs in this phase. This flare, like many other flares of this type, shows a relatively weak emission with a smooth and gradual increase during this pre-impulsive phase, followed by impulsive bursts, and then turns into a smooth decay phase without impulsive bursts. First, we found that the spectrum for the initial smooth rise part is consistent with a thin-thermal source at a temperature around 80 MK. Imaging of this phase in the HXT/L and M bands shows a single source between the footpoint sources that will come up in the impulsive phase following this phase, suggesting that this hyperhot source is located at a high part of the loop between the footpoints, since this flare takes a form of a loop. Furthermore, as we go up to the earliest times of the flare before this `hyperhot' source phase, two fainter sources are found near the footpoint sources that will appear later in the impulsive phase. The spectra of these sources at this earliest time of the flare, in contrast to the `hyperhot' source, cannot be determined from the HXT because the instrument was not in flare mode, and HXT/M1, M2, and H-band data are, unfortunately, not available at this very initial time. We can guess, however, that they are also of thermal character because the time profile is smooth without any spikes just as in the following `hyperhot' thermal phase, and in the post-impulsive `superhot' thermal phase coming up much later. These findings suggest that there is an important, and probably dynamic, early phase in loop flares that has been unnoticed in the still dark pre-impulsive phase, because the very early footpoint sources change into the loop top source in a matter of 20–30 s, comparable to the dynamic Alfvén time scale. Some implications of our new findings are discussed.     

Microwave Neutral Line Associated Source and a Current Sheet

Neutral Line associated Sources (NLSs) are quasi-stationary microwave sources projected onto vicinities of the neutral line of the photospheric magnetic field. NLSs are often precursors of powerful flares, but their nature is unclear. We endeavor to reveal the structure of an NLS and to analyze a physical connection between such a source with a site of energy release in the corona above NOAA 10488 (October/November 2003). Evolution of this AR includes emergence and collision of two bipolar magnetic structures, rise of the main magnetic separator, and the appearance of an NLS underneath. The NLS appears at a contact site of colliding sunspots, whose relative motion goes on, resulting in a large shear along a tangent. Then the nascent NLS becomes the main source of microwave fluctuations in the AR. The NLS emission at 17 GHz is dominated by either footpoints or the top of a loop-like structure, an NLS loop, which connects two colliding sunspots. During a considerable amount of time, the emission dominates over that footpoint of the NLS loop, where the magnetic field is stronger. At that time, the NLS resembles a usual sunspot–associated radio source, whose brightness center is displaced towards the periphery of a sunspot. Microwave emission of an X2.7 flare is mainly concentrated in an ascending flare loop, initially coinciding with the NLS loop. The top of this loop is located at the base of a non-uniform bar-like structure visible in soft X-rays and at 34 GHz at the flare onset. We reveal i) upward lengthening of this bar before the flare onset, ii) the motion of the top of an apparently ascending flare loop along the axis of this bar, and iii) a non-thermal microwave source, whose descent along the bar was associated with the launching of a coronal ejection. We connect the bar with a probable position of a nearly vertical diffusion region, a site of maximal energy release inside an extended pre-flare current sheet. The top of the NLS loop is located at the bottom of this region. A combination of the NLS loop and diffusion region constitutes the skeleton of a quasi-stationary microwave NLS.     

Flat microwave spectra seen at X-class flares

We report peculiar spectral activity of four large microwave bursts as obtained from the Solar Arrays at the Owens Valley Radio Observatory during observations of X-class flares on 1990 May 24 and 1991 March 7, 8, and 22. Main observational points that we newly uncovered are: (1) flat flux spectra over 1–18 GHz in large amounts of flux ranging from 10² to 10⁴ s.f.u. at the maximum phase, (2) a common evolutionary pattern in which the spectral region of dominant flux shifts from high frequencies at the initial rise to low frequencies at the decaying phase, and (3) unusual time profiles that are impulsive at high frequencies but more extended at lower frequencies.In an attempt to elucidate these new properties, we carry out the model calculations of microwave spectra under assumptions of gyrosynchrotron mechanism and a dipole field configuration to reproduce the observational characteristics. Our results are summarized as follows. First, a flat microwave spectrum reaching up to 10²–10⁴ s.f.u. may occur in a case where a magnetic loop is extended to an angular size of (0.7–7.0) × 10^–7 sterad and contains a huge number (N(E > 10 keV) 10³⁶– 10³⁸) of nonthermal electrons with power-law index 3–3.5 over the entire volume. Second, the observed spectral activity could adequately be accounted for by the shrinking of the region of nonthermal electrons to the loop top and by the softening of the power-law spectrum of electrons in a time scale ranging 3–45 min depending on the event. Third, the extended microwave activity at lower frequencies is probably due to electrons trapped in the loop top where magnetic fields are low. Finally, we clarify the physical distinction between these large, extended microwave bursts and the gradual/post-microwave bursts often seen in weak events, both of which are characterized by long-period activity and broadband spectra.     

Investigation of the X-Ray Emission of the Large Arcade Flare of 2 March 1993

A large arcade flare, occurring on 2 March 1993, has been investigated using X-ray observations recorded by the Yohkoh and GOES satellites and the Compton Gamma Ray Observatory. We analyzed the quasi-periodicity of the hard-X-ray (HXR) pulses in the impulsive phase of the flare and found a close similarity between the quasi-periodic sequence of the pulses to that observed in another large arcade flare, that of 2 November 1991. This similarity helped to explain the strong HXR pulses which were recorded at the end of the impulsive phase as due to the inflow of dense plasma (coming from the chromospheric evaporation) into the acceleration volume inside the cusp. In HXR images a high flaring loop was seen with a triangular cusp structure at the top, where the electrons were efficiently accelerated. The sequence of HXR images allowed us to investigate complicated changes in the precipitation of the accelerated electrons toward the flare footpoints. We have shown that all these impulsive-phase observations can be easily explained in terms of the model of electron acceleration in oscillating magnetic traps located within the cusp structure. Some soft-X-ray (SXR) images were available for the late decay phase. They show a long arcade of SXR loops. Important information about the evolution of the flare during the slow decay phase is contained in the time variation of the temperature, T(t), and emission measure, EM(t). This information is the following: i) weak heating occurs during the slow decay phase and it slowly decreases; ii) the decrease in the heating determines a slow and smooth decrease in EM; iii) the coupling between the heating and the amount of the hot plasma makes the flare evolve along a sequence of quasi-steady states during the slow decay phase (QSS evolution).