Top Of Loop Research Articles

Analysis of Magnetic Shear in An X17 Solar Flare on October 28, 2003

An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28, 2003 is analyzed in order to determine the relationship between the timing of the impulsive phase of the flare and the magnetic shear change in the flaring region. EUV observations made by the Transition Region and Coronal Explorer (TRACE) show a clear decrease in the shear of the flare footpoints during the flare. The shear change stopped in the middle of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly sheared core rope during the early phase of the flare. We have also investigated the temporal correlation between the EUV emission from the brightenings observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV) observed by the anticoincidence system (ACS) of the spectrometer SPI on board the ESA INTEGRAL satellite. The correlation between these two emissions is very good, and the HXR sources (RHESSI) late in the flare are located within the two EUV ribbons. These observations are favorable to the explanation that the EUV brightenings mainly result from direct bombardment of the atmosphere by the energetic particles accelerated at the reconnection site, as does the HXR emission. However, if there is a high temperature (T > 20 MK) HXR source close to the loop top, a contribution of thermal conduction to the EUV brightenings cannot be ruled out.

Evolution of the Loop‐Top Source of Solar Flares: Heating and Cooling Processes

We present a study of the spatial and spectral evolution of the loop-top (LT) sources in a sample of 6 flares near the solar limb observed by {\it RHESSI}. A distinct coronal source, which we identify as the LT source, was seen in each of these flares from the early ``pre-heating'' phase through the late decay phase. Spectral analyses reveal an evident steep power-law component in the pre-heating and impulsive phases, suggesting that the particle acceleration starts upon the onset of the flares. In the late decay phase the LT source has a thermal spectrum and appears to be confined within a small region near the top of the flare loop, and does not spread throughout the loop, as is observed at lower energies. The total energy of this source decreases usually faster than expected from the radiative cooling but much slower than that due to the classical Spitzer conductive cooling along the flare loop. These results indicate the presence of a distinct LT region, where the thermal conductivity is suppressed significantly and/or there is a continuous energy input. We suggest that plasma wave turbulence could play important roles in both heating the plasma and suppressing the conduction during the decay phase of solar flares. With a simple quasi-steady loop model we show that the energy input in the gradual phase can be comparable to that in the impulsive phase and demonstrate how the observed cooling and confinement of the LT source can be used to constrain the wave-particle interaction.

Quasi-periodic Reversals of Radio Polarization at 17 GHz Observed in the 2002 April 21 Solar Event

We investigate high spatial resolution radio polarization data obtained by the Nobeyama Radioheliograph (NoRH) and high time resolution data observed with the Nobeyama Radio Polarimeters (NoRP) during the well-studied flare/CME event of 2002 April 21. A 17 GHz radio source at the loop top was seen by NoRH to move upward together with the expanding flare loops at a speed of around 10 km s(-1). In the 5 minutes before the source began its upward motion, the Stokes V of the radio signals at 17 GHz showed quasi-periodic reversals between left-circular polarization (LCP) and right-circular polarization (RCP). Following this interval, the polarizations gradually turned to LCP. During this period, the polarization of the corresponding footpoint source maintained the RCP sense. The reversal of Stokes V between RCP and LCP was also detected at lower frequencies (1-2 GHz) by NoRP, without spatial resolution. The observed reversals between RCP and LCP of the radio signals from the top of the flare loop system can be taken as evidence that magnetic energy is released or energetic particles are produced at the magnetic reconnection site in a quasi-periodic fashion.

X-ray sources and magnetic reconnection in the X3.9 flare of 2003 November 3

Context. Recent RHESSI observations indicate an apparent altitude decrease of flare X-ray loop-top (LT) sources before changing to the commonly observed upward growth of the flare loop system. Aims. We performed a detailed study of the LT altitude decrease for one well observed flare in order to find further hints on the physics of this phenomenon and how it is related to the magnetic reconnection process in solar flares. Methods. RHESSI X-ray source motions in the 2003 November 3, X3.9 flare are studied together with complementary data from SXI, EIT, and Kanzelhohe Ha. We particularly concentrate on the apparent altitude decrease of the RHESSI X-ray LT source early in the flare and combine kinematical and X-ray spectral analysis. Furthermore, we present simulations from a magnetic collapsing trap model embedded in a standard 2-D magnetic reconnection model of solar flares. Results. We find that at higher photon energies the LT source is located at higher altitudes and shows higher downward velocities than at lower energies. The mean downward velocities range from 14 km s -1 in the RHESSI 10-15 keV energy band to 45 km s -1 in the 25-30 keV band. For this flare, the LT altitude decrease was also observed by the SXI instrument with a mean speed of 12 km s -1 . RHESSI spectra indicate that during the time of LT altitude decrease the emission of the LT source is thermal bremsstrahlung from a superhot plasma with temperatures increasing from 35 MK to 45 MK and densities of the order of 10 10 cm -3 . The temperature does not significantly increase after this early (pre-impulsive superhot LT) phase, whereas the LT densities increase to a peak value of (3-4) x 10 11 cm -3 . Conclusions. Modeling of a collapsing magnetic trap embedded in a standard 2D magnetic reconnection model can reproduce the key observational findings in case that the observed emission is thermal bremsstrahlung from the hot LT plasma. This agrees with the evaluated RHESSI spectra for this flare.

Self‐Consistent Magnetohydrodynamic Modeling of a Coronal Mass Ejection, Coronal Dimming, and a Giant Cusp‐shaped Arcade Formation

We performed magnetohydrodynamic simulations of coronal mass ejections (CMEs) and associated giant arcade formations, and the results suggest new interpretations of observations of CMEs. We performed two cases of the simulation: with and without heat conduction. Comparing between the results of the two cases, we found that the reconnection rate in the conductive case is a little higher than that in the adiabatic case and that the temperature of the loop top is consistent with the theoretical value predicted by the Yokoyama-Shibata scaling law. The dynamical properties such as velocity and magnetic field are similar in the two cases, whereas thermal properties such as temperature and density are very different. In both cases, slow shocks associated with magnetic reconnection propagate from the reconnection region along the magnetic field lines around the flux rope, and the shock fronts form spiral patterns. Just outside the slow shocks, the plasma density decreases greatly. The soft X-ray images synthesized from the numerical results are compared with the soft X-ray images of a giant arcade observed with the Soft X-Ray Telescope aboard Yohkoh; it is confirmed that the effect of heat conduction is significant for the detailed comparison between simulation and observation. The comparison between synthesized and observed soft X-ray images provides new interpretations of various features associated with CMEs and giant arcades. (1) It is likely that the Y-shaped ejecting structure, observed in the giant arcade on 1992 January 24, corresponds to slow and fast shocks associated with magnetic reconnection. (2) Soft X-ray twin dimming corresponds to the rarefaction induced by reconnection. (3) The inner boundary of the dimming region corresponds to the slow shocks. (4) The three-part structure of a CME can be explained by our numerical results. (5) The numerical results also suggest that a backbone feature of a flare/giant arcade may correspond to the fast shock formed by the collision of the downward reconnection outflow.

The CDR1 of the human λVI light chains adopts a new canonical structure

We performed a comparative analysis of the conformation of the CDR1 of the human lambdaVI variable domains JTO and WIL and the equivalent loop of the lambdaI light chains RHE and KOL, which are representative of the type I canonical structure for lambda light chains. On the basis of the differences found in the main chain conformation, as well as the identity of the residues at key positions, we showed that the L1 of some lambdaVI light chains adopts a conformation that represents a new type of canonical structure. The conformation of the L1 of those lambdaVI light chains, is primarily determined by the presence of an Arg residue at position 25. The analysis of the lambdaVI light chain sequences so far reported, showed that near 25% of those proteins have Gly at position 25 instead of Arg, which represents an allotypic variant of the lambdaVI variable locus. The presence of Gly at position 25 in the L1 of lambdaVI light chains would imply a different conformation for this loop. Additionally, the position 68 in lambdaVI light chains, which is at the top of the FR3 loop, showed such spatial orientation and variability that suggested its participation in the conformation of the antigen recognition surface in this subgroup of lambda chains.

Coronal Loop Heating by Nanoflares: The Impact of the Field‐aligned Distribution of the Heating on Loop Observations

Nanoflares occurring at subresolution strands with repetition times longer than the coronal cooling time are a promising candidate for coronal loop heating. To investigate the impact of the spatial distribution of the nanoflare heating on loop observables, we compute hydrodynamic simulations with several different spatial distributions (uniform, loop top, randomly localized, and footpoint). The outputs of the simulations are then used to calculate density and temperature diagnostics from synthetic TRACE and SXT observations. We find that the diagnostics depend only weakly on the spatial distribution of the heating and therefore are not especially useful for distinguishing among the different possibilities. Observations of the very high temperature plasmas that are present only in the earliest stages of nanoflares can shed more light on the field-aligned distribution of the heating.

Spatially resolved microwave pulsations of a flare loop

A microwave burst with quasi-periodic pulsations was studied with high spatial resolution using observations with the Nobeyama Radioheliograph (NoRH). We found that the time profiles of the microwave emission at 17 and 34 GHz exhibit quasi-periodic (with two well defined periods 14–17 s and 8–11 s) variations of the intensity at different parts of an observed flaring loop. Detailed Fourier analysis shows the P1 spectral component to be dominant at the top, while the P2 one near the feet of the loop. The 14–17 s pulsations are synchronous at the top and in both legs of the loop. The 8–11 s pulsations at the legs are well correlated with each other but the correlation is not so obvious with the pulsations at the loop top. For this P2 spectral component, a definite phase shift, s, between pulsations in the northern leg and loop top parts of the loop have been found. The length of the flaring loop is estimated as Mm (≈) and its average width at half intensity at 34 GHz as about 6 Mm (). Microwave diagnostics shows the loop to be filled with a dense plasma with the number density cm-3, penetrated by the magnetic field changing from G near the loop top up to G near the north footpoint. A comparative analysis of different MHD modes of the loop demonstrates the possibility of the simultaneous existence of two modes of oscillations in the loop: the global sausage mode, with the period 14–17 s and the nodes at the footpoints, and a higher harmonics mode (possibly with the radial wave number ), with 8–11 s.

Non-thermal emission from AGN coronae

Accretion disk coronae are believed to account for X-ray emission in Active Galactic Nuclei (AGNs). In this paper the observed emission is assumed to be due to a population of relativistic, non-thermal electrons (e.g. produced in a flare) injected at the top of an accretion disk magnetic loop. While electrons stream along magnetic field lines, their energy distribution evolves in time essentially because of inverse Compton and synchrotron losses. The corresponding time-dependent emission due, in the X-ray energy range, to the inverse Compton mechanism, has been computed. Since the typical decay time of a flare is shorter than the integration time for data acquisition in the X-ray domain, the resulting spectrum is derived as the temporal mean of the real, time-dependent emission, as originated from a series of consecutive and identical flares. The model outcome is compared to both the broad band BeppoSAX X-ray data of the bright Seyfert 1 NGC 5548 and to a few general X-ray spectral properties of Seyfert 1s as a class. The good agreement between model and observations suggests that the presently proposed non-thermal, non-stationary model could be a plausible explanation of AGN X-ray emission as an alternative to thermal coronae models.

Energetic Electrons in Loop Top and Footpoint Microwave Sources

We have studied two microwave events with one-loop top (LT) and two-footpoint (FP) sources observed at 17 and 34 GHz by the Nobeyama Radioheliograph (NoRH). The microwave brightness peak is located near the FPs of the flare loop for one event, but near the LT for the other event. The microwave spectra of the FP sources are considerably softer (by Δα≈2.0) than that of the LT’s for both events. We assume that the microwave emission is gyro-synchrotron radiation from energetic electrons trapped in a magnetic dipole field and the energetic electron distribution is isotropic in pitch angle and power law. In the gyro-synchrotron calculations, the self-absorption and gyro-resonance absorption are taken into account simultaneously. The numerical calculations based on the general equation of radiative transfer show that the distributions of energetic electrons along a flare loop are highly inhomogeneous: accelerated electrons are concentrated in the FPs for both events. Even for the event with brightness maximum near the LT the electron number density of the LT source is still an order of magnitude lower than that of the FP sources. The emission peak near LT results mainly from the much harder spectral index of the energetic electrons in the LT source.

Relationship between impulsive and post-eruptive processes in solar X-ray flares

Powerful solar flares contain one or more impulsive events, plasma ejection, and the subsequent development of gigant post-eruptive loops. In the middle of the 1980s, Jakimiec proposed an analysis of the flare loops based on log T-1/2log EM diagrams constructed from the observed soft X-rays (the so-called Jakimiec model). We have used this method to construct and analyze these diagrams not for various arbitrary events, but instead for similar flares within a single center of activity; in other words, for homological flares (two-ribbon flares observed in November 2000, powerful prolonged events observed in October–November 2003, etc.). This eliminated the effect of differences in the magnetic configurations, enabling us to find a new relationship: the slope (tan α) of the logT-1/2log EM line during the flare decay depends on the maximum temperature Tmax at the source of the soft X-rays. The dependence of tan α on Tmax gradually evolves from a series of short flares to a series of powerful, prolonged, nonstationary processes. Our results support the idea that the development of post-eruptive loops depends on the energy of the impulsive events for the phenomenon as a whole. Explosive evaporation simultaneously increases both the temperature and the density of the plasma at the loop top. The subsequent evolution of the post-eruptive formations depends on the difference in the initial conditions and on the degree of opening of the magnetic configuration. The importance of our analysis for the duration of flares and differences between dimmings is briefly discussed.

The Origin of Postflare Loops

We apply a tracking technique, previously developed to study motions in the outer corona by Sheeley, Walters, Wang, and Howard, to 195 A filtergrams obtained with the Transition Region and Coronal Explorer (TRACE) satellite and obtain height-time maps of the motions in the hot (10-20 MK) plasma clouds above postflare loop systems. These maps indicate the following two main characteristics. (1) Within the plasma cloud, the motions are downward at speeds of approximately 4 km s-1. The cloud itself grows with time, its upper layers being replenished by the arrival and deceleration of fast inflows and its lower layers disappearing when they cool to form the tops of new postflare loops. (2) Early in these events, the inward motions are turbulent, showing a variety of dark elongated features resembling tadpoles and some bright features. Later, the inflows are visible as dark collapsing loops, changing from initially cusp-shaped features to rounder loops as they move inward. Their speeds initially lie in the range 100-600 km s-1 but decrease to 4 km s-1 in about 3 minutes, corresponding to an average deceleration ~1500 m s-2. Combining these observations with similar observations obtained at reconnection sites in the outer corona by the Large Angle Spectrometric Coronagraph (LASCO), we conclude that postflare loops are the end result of the formation, filling, deceleration, and cooling of magnetic loops produced by the reconnection of field lines blown open in the flare. The formation of collapsing loops occurs in the dark tadpoles; the filling of these initially dark loops occurs via chromospheric evaporation, which also contributes to the deceleration of the loops; and the radiative cooling ultimately resolves the loops into sharply defined structures.

Multi-Wavelength Observation Results of the C5.6 Limb Flare of 1 August 2003

We obtained a complete set of Hα, Ca Π 8542 A and He I 10830 A spectra and slit-jaw Hα images of the C5.6 limb flare of 1 August 2003 using the Multi-channel Infrared Solar Spectrograph (MISS) at Purple Mountain Observatory. This flare was also observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and partially by the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO. This flare underwent a rapid rising and expanding episode in the impulsive phase. All the Hα, Ca Π 8542 A and He I 10830 A profiles of the flare are rather wide and the widest profiles were observed in the middle bright part of the flare instead of at the flare loop top near the flare maximum. The flare manifested obvious rotation in the flare loop and the decrease of the rotation angular speed with time at the loop-top may imply a de-twisting process of the magnetic field. The significant increases of the Doppler widths of these lines in the impulsive phase reflect quick heating of the chromosphere, and rapid rising and expanding of the flare loop. The RHESSI observations give a thermal energy spectrum for this flare, and two thermal sources and no non-thermal source are found in the reconstructed RHESSI images. This presumably indicates that the energy transfer in this flare is mainly by heat conduction. The stronger thermal source is located near the solar limb with its position unchanged in the flare process and spatially coincident with the intense EUV and Hα emissions. The weaker one moved during the flare process and is located in the Hα dark cavities. This flare may support the theory of the magnetic reconnections in the lower solar atmosphere.

A Model for Bright Extreme‐Ultraviolet Knots in Solar Flare Loops

EUV observations often indicate the presence of bright knots in flare loops. The temperature of the knot plasma is of the order of 1 MK, and the knots themselves are usually localized somewhere near the loop tops. We propose a model in which the formation of EUV knots is due to the spatial structure of the nonflare active region heating. We present the results of a series of one-dimensional hydrodynamic, flare-loop simulations, which include both an impulsive flare heating and a background, active region heating. The simulations demonstrate that the formation of the observed knots depends critically on the spatial distribution of the background heating during the decay phase. In particular, the heating must be localized far from the loop apex and have a magnitude comparable to the local radiative losses of the cooling loop. Our results, therefore, provide strong constraints on both coronal heating and postflare conditions.

A novel neurological mechanism to explain the adverse effect of depression on coronary artery disease

The strong antecedent effect of depression as a risk factor in the development of coronary artery disease (CAD) has been demonstrated in many robust epidemiological studies. However, the underlying causative mechanisms are incompletely understood: this hypothesis proposes one possibility. A variety of histological and radiological techniques have been used to demonstrate structural and functional abnormalities in the prefrontal cortex (PFC) of depressed subjects. In addition, this limbic region has been inextricably implicated in the modulation of autonomic tone. Cardio-specifically, stimulation of PFC has arrhythmogenic effects. Reciprocally, cardiac stress tests produce activation of this cortical region. These observations place PFC at the top of a hierarchical autonomic loop involved in sensing and modulating cardiac variables. Finally, the reduced heart rate variability, higher heart rates and elevated cerebrospinal fluid catecholamine levels in depressed patients, compared to non-depressed matched controls, suggest sympathetic overdrive in these patients. These observations lead the author to propose that a primary defect in the PFC of depressed subjects destabilises the autonomic neurocardiac axis, accounting for the proven adverse effect of depression on CAD. This novel neurological mechanism can help to develop other current theories and to design and trial future therapies to reduce the adverse effect of depression on coronary artery disease.

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.

Stochastic Fermi acceleration of subrelativistic electrons and its role in impulsive solar flares

We re-examine stochastic Fermi acceleration (STFA) in the low-energy (Newtonian) regime in the context of solar flares. The particle energization rate depends on a dispersive term and on a coherent gain term. The energy dependence of pitch angle scattering is important for determining the electron energy spectrum. For scattering by whistler wave turbulence, STFA produces a quasi-thermal spectrum. A second well-constrained scattering mechanism is needed for STFA to match the observed 10‐100 keV non-thermal spectrum. We suggest that STFA most plausibly acts as phase one of a two-phase particle acceleration engine in impulsive flares: STFA can match the thermal spectrum below 10 keV, and possibly the power-law spectrum between 10 and 100 keV, given the proper pitch angle scattering. However, a second phase, such as shock acceleration at loop tops, is probably required to match the spectrum above the observed knee at 100 keV. Understanding this knee, if it survives further observations, is tricky.

Loop Density Enhancement by Nonlinear Magnetohydrodynamic Waves

We study the possibility that waves in coronal loops can produce density enhancements at loop tops. The mechanism involved is the ponderomotive force of standing magnetohydrodynamic (MHD) waves. We study the effect of large-amplitude waves in loops by first using a simple second-order one-dimensional MHD model for which analytical expressions are available. This simple model shows how Alfven waves can excite density fluctuations on slow timescales. Next, we solve the time-dependent nonlinear 2.5-dimensional MHD equations in an arcade model, in order to study the effect of nonuniform Alfven speed along the loop on large-amplitude waves. Finally, we solve the nonlinear three-dimensional MHD equations in a flux-tube configuration. Waves are launched by an initial transverse velocity profile in the loop, with footpoints fixed in the photosphere. We find that large initial disturbances produce a pressure imbalance along the loop, which results in an upflow from its legs. The accumulation of mass at the top of the oscillating loop can produce significant density enhancements for typical coronal conditions. In a later stage, the pressure gradient becomes dominant and inhibits the concentration of mass at the loop top. We find that oscillating loops observed by the EUV telescope on board TRACE on 1998 July 14 exhibit emission measure enhancements consistent with this mechanism.

RHESSI Observations of a Simple Large X-Ray Flare on 2003 November 3

We present data analysis and interpretation of a simple X-class flare observed with RHESSI on 2003 November 3. In contrast to other X-class flares observed previously, this flare shows a very simple morphology with well-defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward motion of the LT source and increase in separation of the two FP sources are consistent with magnetic reconnection models proposed for solar flares. In addition, we find that the source motions are relatively slower during the more active phases of hard X-ray emission; the emission centroid of the LT source shifts toward higher altitudes with the increase of energy; and the separation between the LT emission centroids at two different photon energies is anticorrelated with the FP flux. Nonuniformity of the reconnecting magnetic fields could be a possible explanation of these features.

The configuration of simple short-duration solar microwave bursts

Using data from the Nobeyama Radioheliograph (NoRH) we study the source configuration of four simple short-duration 17 and 34 GHz bursts which have also been observed partially by the Yohkoh Soft X-ray Telescope (SXT). Two events are consistent with a single flaring loop configuration. In one of them the flaring loop is resolved in the SXT im- ages. We derive a self-consistent model for this event by comparing the radio observations with gyrosynchrotron model loop calculations. Our best-fit model is able to reproduce both the observed flaring loop shape as well as the fluxes and structures of the radio emission at the peak of the event. The flaring loop is relatively small having a footpoint separation of 16 �� and maximum height of 7.7 �� . The variation of the magnetic field along the loop is small (800 G at the footpoints and 665 G at loop top) and the loop is filled with electrons with energies up to 10 MeV. The other two bursts show two radio sources; one source being cospatial with a compact bright soft X-ray loop associated with a patch of parasitic magnetic polarity whose photospheric magnetic flux increases before the flares while the other source is not prominent at any other wavelength range. The two sources are connected with diffuse loop-like soft X-ray emission. We infer that these bursts originate from the interaction of two sets of loops with different sizes. Therefore the simple short duration microwave bursts we studied do not always appear in the same configuration. Contrary to previous results not all of them appear as single-loop events. It is possible that some events are caused by two interacting loops.