Flare Loops Research Articles

PLASMA HEATING IN SOLAR FLARES AND THEIR SOFT AND HARD X-RAY EMISSIONS

In this paper, the energy budgets of two single-loop-like flares observed in X-ray are analyzed under the assumption that nonthermal electrons (NTEs) are the only source of plasma heating during all phases of both events. The flares were observed by RHESSI and GOES on 2002 February 20 and June 2, respectively. Using a one-dimensional (1D) hydrodynamic code for both flares, the energy deposited in the chromosphere was derived applying RHESSI observational data. The use of the Fokker–Planck formalism permits the calculation of distributions of the NTEs in flaring loops and thus spatial distributions of the X-ray nonthermal emissions and integral fluxes for the selected energy ranges that were compared with the observed ones. Additionally, a comparative analysis of the spatial distributions of the signals in the RHESSI images was conducted for the footpoints and for all the flare loops in selected energy ranges with these quantities' fluxes obtained from the models. The best compatibility of the model and observations was obtained for the 2002 June 2 event in the 0.5–4 Å GOES range and total fluxes in the 6–12 keV, 12–25 keV, 20–25 keV, and 50–100 keV energy bands. Results of photometry of the individual flaring structures in a high energy range show that the best compliance occurred for the 2002 June 2 flare, where the synthesized emissions were at least 30% higher than the observed emissions. For the 2002 February 20 flare, synthesized emission is about four times lower than the observed one. However, in the low energy range the best conformity was obtained for the 2002 February 20 flare, where emission from the model is about 11% lower than the observed one. The larger inconsistency occurs for the 2002 June 2 solar flare, where synthesized emission is about 12 times greater or even more than the observed emission. Some part of these differences may be caused by inevitable flaws of the applied methodology, like by an assumption that the model of the flare is symmetric and there are no differences in the emissions originating from the feet of the flares loop and by relative simplicity of the applied numerical 1D code and procedures. No doubt a significant refinement of the applied numerical models and more sophisticated implementation of the various physical mechanisms involved are required to achieve a better agreement. Despite these problems, a collation of modeled results with observations shows that soft and hard X-ray emissions observed for analyzed single-loop-like events may be fully explained by electron-beam-driven evaporation only.

MAGNETOHYDRODYNAMIC SIMULATION OF THE X2.2 SOLAR FLARE ON 2011 FEBRUARY 15. I. COMPARISON WITH THE OBSERVATIONS

We performed a magnetohydrodynamic (MHD) simulation using a nonlinear force-free field (NLFFF) in solar active region 11158 to clarify the dynamics of an X2.2-class solar flare. We found that the NLFFF never shows the drastic dynamics seen in observations, i.e., it is in stable state against the perturbations. On the other hand, the MHD simulation shows that when the strongly twisted lines are formed at close to the neutral line, which are produced via tether-cutting reconnection in the twisted lines of the NLFFF, consequently they erupt away from the solar surface via the complicated reconnection. This result supports the argument that the strongly twisted lines formed in NLFFF via tether-cutting reconnection are responsible for breaking the force balance condition of the magnetic fields in the lower solar corona. In addition to this the dynamical evolution of these field lines reveals that at the initial stage the spatial pattern of the footpoints caused by the reconnection of the twisted lines appropriately maps the distribution of the observed two-ribbon flares. Interestingly, after the flare the reconnected field lines convert into the structure like the post flare loops, which is analogous to EUV image taken by SDO. Eventually, we found that the twisted lines exceed a critical height at which the flux tube becomes unstable to the torus instability. These results illustrate the reliability of our simulation and also provide an important relationship between flare-CME dynamics.

NEW ASPECTS OF A LID-REMOVAL MECHANISM IN THE ONSET OF AN ERUPTION SEQUENCE THAT PRODUCED A LARGE SOLAR ENERGETIC PARTICLE (SEP) EVENT

We examine a sequence of two ejective eruptions from a single active region on 2012 January 23, using magnetograms and EUV images from the Solar Dynamics Observatory's (SDO) Helioseismic and Magnetic Imager (HMI) and Atmospheric and Imaging Assembly (AIA), and EUV images from STEREO/EUVI. This sequence produced two coronal mass ejections (CMEs) and a strong solar energetic particle event (SEP); here we focus on the magnetic onset of this important space weather episode. Cheng et al. showed that the first eruption's (Eruption 1) flux rope was apparent only in hotter AIA channels, and that it removed overlying field that allowed the second eruption (Eruption 2) to begin via ideal MHD instability; here we say that Eruption 2 began via a lid removal mechanism. We show that during Eruption 1's onset, its flux rope underwent a tether weakening (TW) reconnection with field that arched from the eruption-source active region to an adjacent active region. Standard flare loops from Eruption 1 developed over Eruption 2's flux rope and enclosed filament, but these overarching new loops were unable to confine that flux rope/filament. Eruption 1's flare loops, from both TW reconnection and standard-flare-model internal reconnection, were much cooler than Eruption 2's flare loops (GOES thermal temperatures of ~7.5 MK and 9 MK, compared to ~14 MK). The corresponding three sequential GOES flares were, respectively, due to TW reconnection plus earlier phase Eruption 1 tether-cutting reconnection, Eruption 1 later-phase tether-cutting reconnection, and Eruption 2 tether-cutting reconnection.

PLASMA DYNAMICS ABOVE SOLAR FLARE SOFT X-RAY LOOP TOPS

We measure non-thermal motions in flare loop tops and above the loop tops using profiles of highly ionized spectral lines of Fe XXIV and Fe XXIII formed at multimillion-degree temperatures. Non-thermal motions that may be due to turbulence or multiple flow regions along the line of sight are extracted from the line profiles. The non-thermal motions are measured for four flares seen at or close to the solar limb. The profile data are obtained using the Extreme-ultraviolet Imaging Spectrometer on the Hinode spacecraft. The multimillion-degree non-thermal motions are between 20 and 60 km s–1 and appear to increase with height above the loop tops. Motions determined from coronal lines (i.e., lines formed at about 1.5 MK) tend to be smaller. The multimillion-degree temperatures in the loop tops and above range from about 11 MK to 15 MK and also tend to increase with height above the bright X-ray-emitting loop tops. The non-thermal motions measured along the line of sight, as well as their apparent increase with height, are supported by Solar Dynamics Observatory Atmospheric Imaging Assembly measurements of turbulent velocities in the plane of the sky.

Contribution of thermal bremsstrahlung to microwave emission of solar flare loops

The contribution of thermal bremsstrahlung to the total microwave flux of a solar flare loop is considered. The total-flux data were obtained on the Nobeyama Radio Heliograph. The calculation of the thermal bremsstrahlung radio flux was based on determining the integrated temperature and number density of the hot flare-loop plasma from its soft X-ray flux, obtained using data from the GOES-10 and GOES-12 satellites. The effect of thermal bremsstrahlung on the total flux and the spectral index of the microwave radiation is insignificant at the burst maximum (Fth/Ftot < 3%, Δα < 0.2), while the contribution of bremsstrahlung can be substantial during the decay phase of the burst (up to 80%). This results in an appreciable decrease in the observed spectral index (to Δα ∼ 1.5). Therefore, when diagnosing the parameters of the accelerated electrons based on the characteristics of their gyrosynchrotron radiation, the most accurate results can be obtained using the emission characteristics obtained near the burst maximum.

DIFFERENTIAL EMISSION MEASURE ANALYSIS OF A LIMB SOLAR FLARE ON 2012 JULY 19

We perform Differential Emission Measure (DEM) analysis of an M7.7 flare that occurred on 2012 July 19 and was well observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamic Observatory. Using the observational data with unprecedented high temporal and spatial resolution from six AIA coronal passbands, we calculate the DEM of the flare and derive the time series of maps of DEM-weighted temperature and emission measure (EM). It is found that, during the flare, the highest EM region is located in the flare loop top with a value varying between 8.4 10^28 cm^-5 and 2.5 10^30 cm^-5. The temperature there rises from 8 MK at about 04:40 UT (the initial rise phase) to a maximum value of 13 MK at about 05:20 UT (the hard X-ray peak). Moreover, we find a hot region that is above the flare loop top with a temperature even up to 16 MK. We also analyze the DEM properties of the reconnection site. The temperature and density there are not as high as that in the loop top and the flux rope, indicating that the main heating may not take place inside the reconnection site. In the end, we examine the dynamic behavior of the flare loops. Along the flare loop, both the temperature and the EM are the highest in the loop top and gradually decrease towards the footpoints. In the northern footpoint, an upward force appears with a biggest value in the impulsive phase, which we conjecture originates from chromospheric evaporation.

SLIPPING MAGNETIC RECONNECTION DURING AN X-CLASS SOLAR FLARE OBSERVED BYSDO/AIA

We present SDO/AIA observations of an eruptive X-class flare of July 12, 2012, and compare its evolution with the predictions of a 3D numerical simulation. We focus on the dynamics of flare loops that are seen to undergo slipping reconnection during the flare. In the AIA 131A observations, lower parts of 10 MK flare loops exhibit an apparent motion with velocities of several tens of km/s along the developing flare ribbons. In the early stages of the flare, flare ribbons consist of compact, localized bright transition-region emission from the footpoints of the flare loops. A DEM analysis shows that the flare loops have temperatures up to the formation of Fe XXIV. A series of very long, S-shaped loops erupt, leading to a CME observed by STEREO. The observed dynamics are compared with the evolution of magnetic structures in the "standard solar flare model in 3D". This model matches the observations well, reproducing both the apparently slipping flare loops, S-shaped erupting loops, and the evolution of flare ribbons. All of these processes are explained via 3D reconnection mechanisms resulting from the expansion of a torus-unstable flux rope. The AIA observations and the numerical model are complemented by radio observations showing a noise storm in the metric range. Dm-drifting pulsation structures occurring during the eruption indicate plasmoid ejection and enhancement of reconnection rate. The bursty nature of radio emission shows that the slipping reconnection is still intermittent, although it is observed to persist for more than an hour.

TEMPERATURE EVOLUTION OF A MAGNETIC FLUX ROPE IN A FAILED SOLAR ERUPTION

In this presentation, we report for the first time the detailed temperature evolution process of the magnetic flux rope in a failed solar eruption. Occurred on January 05, 2013, the flux rope was impulsively accelerated to a speed of ~ 400 km/s in the first minute, then decelerated and came to a complete stop in two minutes. The failed eruption resulted in a large-size high-lying (~ 100 Mm above the surface) high-temperature "fire ball" sitting in the corona for more than two hours. The time evolution of the thermal structure of the flux rope was revealed through the differential emission measure analysis technique, which produced temperature maps using observations of the Atmospheric Imaging Assembly on board Solar Dynamic Observatory. The average temperature of the flux rope steadily increased from ~ 5 MK to ~ 10 MK during the first nine minutes of the evolution, which was much longer than the rise time (about three minutes) of the associated soft X-ray flare. We suggest that the flux rope be heated by the energy release of the continuing magnetic reconnection, different from the heating of the low-lying flare loops, which is mainly produced by the chromospheric plasma evaporation. The loop arcade overlying the flux rope was pushed up by ~ 10 Mm during the attempted eruption. The pattern of the velocity variation of the loop arcade strongly suggests that the failure of the eruption be caused by the strapping effect of the overlying loop arcade.

A SMALL-SCALE ERUPTION LEADING TO A BLOWOUT MACROSPICULE JET IN AN ON-DISK CORONAL HOLE

We examine the three-dimensional magnetic structure and dynamics of a solar EUV-macrospicule jet that occurred on 2011 February 27 in an on-disk coronal hole. The observations are from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) and the SDO Helioseismic and Magnetic Imager (HMI). The observations reveal that in this event, closed-field-carrying cool absorbing plasma, as in an erupting mini-filament, erupted and opened, forming a blowout jet. Contrary to some jet models, there was no substantial recently emerged, closed, bipolar-magnetic field in the base of the jet. Instead, over several hours, flux convergence and cancellation at the polarity inversion line inside an evolved arcade in the base apparently destabilized the entire arcade, including its cool-plasma-carrying core field, to undergo a blowout eruption in the manner of many standard-sized, arcade-blowout eruptions that produce a and coronal mass ejection. Internal reconnection made bright flare loops over the polarity inversion line inside the blowing-out arcade field, and external reconnection of the blowing-out arcade field with an ambient open field made longer and dimmer EUV loops on the outside of the blowing-out arcade. That the loops made by the external reconnection were much larger than the loops made by the internal reconnection makes this event a new variety of blowout jet, a variety not recognized in previous observations and models of blowout jets.

Flare on 2011 September 22 and coronal mass ejection from observations in the He i 10830-Å line

In this paper, we present the results from a study of the X1.4/2N flare and the accompanying coronal mass ejection (CME) on 2011 September 22 from observations of the spectral region around the He i 10830-Å line. In the study, 20 He i 10830-Å line images of the full disc of the Sun and 20 dopplerograms were obtained. Parts of the active region (AR) 11302 with steady blueshift were detected on the dopplerograms. The velocities of the flows were 10–40 km s−1. Expansion was a characteristic feature of this event; it was observed for more than 2 h and was associated with the development of the CME. A transient coronal hole (TCH) was found in the unipolar magnetic field near the AR from the images in the He i 10830-Å line and from the dopplerograms. The profiles of the He i 10830-Å line in the flare loops were obtained for different moments of the process. The half-width of the helium line at the limb was ≈4.2 Å at the beginning of the observations, and remained ≈3.5 Å until 11:45 ut. On subsequent scans, the half-width slowly decreased to ≈2 Å. The time dependence of the power of energy release in the He i 10830-Å line for the process at the limb was found to have several peaks associated with the development of the CME. We conclude that in the period 11:15–11:50 ut, the non-thermal impact ionization of helium accompanied by recombination was the prevailing mechanism by which the triplet levels were filled corresponding to the He i 10830-Å line. The total energy released in the active process of 2011 September 22 at the limb in the He i 10830-Å line is estimated to be ≈1030 erg.

HEATING AND DYNAMICS OF TWO FLARE LOOP SYSTEMS OBSERVED BY AIA AND EIS

We investigate heating and evolution of flare loops in a C4.7 two-ribbon flare on 2011 February 13. From SDO/AIA imaging observations, we can identify two sets of loops. Hinode/EIS spectroscopic observations reveal blueshifts at the feet of both sets of loops. The evolution and dynamics of the two sets are quite different. The first set of loops exhibits blueshifts for about 25 minutes followed by redshifts, while the second set shows stronger blueshifts, which are maintained for about one hour. The UV 1600 observation by AIA also shows that the feet of the second set of loops brighten twice. These suggest that continuous heating may be present in the second set of loops. We use spatially resolved UV light curves to infer heating rates in the few tens of individual loops comprising the two loop systems. With these heating rates, we then compute plasma evolution in these loops with the "enthalpy-based thermal evolution of loops" (EBTEL) model. The results show that, for the first set of loops, the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines, and the computed mean enthalpy flow velocities also agree with the Doppler shift measurements by EIS. For the second set of loops modeled with twice-heating, there are some discrepancies between modeled and observed EUV light curves in low-temperature bands, and the model does not fully produce the prolonged blueshift signatures as observed. We discuss possible causes for the discrepancies.

PARTICLE DENSITIES WITHIN THE ACCELERATION REGION OF A SOLAR FLARE

The limb flare SOL2012-07-19T05:58 (M7.7) provides the best example of a non-thermal above-the-loop-top hard X-ray source with simultaneous observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. By combining the two sets of observations, we present the first direct measurement of the thermal proton density and non-thermal electron density within the above-the-loop-top source where particle acceleration occurs. We find that both densities are of the same order of magnitude of a few times 109 cm–3, about 30 times lower than the density in the underlying thermal flare loops. The equal densities indicate that the entire electron population within the above-the-loop-top source is energized. While the derived densities depend on the unknown source depth and filling factor, the ratio of these two densities does not. Within the uncertainties, the ratio is one for a low energy cutoff of the non-thermal electron spectrum between 10 and 15 keV. RHESSI observations only constrain the cutoff energy to below ~15 keV, leaving the spectral shape of the electrons within the above-the-loop-top source at lower energies unknown. Nevertheless, these robust results strongly corroborate earlier findings that the above-the-loop-top source is the acceleration region where a bulk energization process acts on all electrons.

Evolution of the Source of Quasi-Periodic Microwave Pulsations in a Single Flaring Loop

Quasi-periodic pulsations (QPP) of the microwave emission of a solar flare on 2002 July 3, observed with the Nobeyama Radioheliograph (Japan), were investigated with emphasise on the spatial structure of the emission source. It was shown that the time profiles of the variation of the distance between two main sources of the emission and the microwave flux are similar, with a common quasi-periodicity. The most significant QPP is found to be at 30 s. The increase in the microwave emission flux from the Northern source, as well as the increase in the spatiallyintegrated signal, is accompanied by a decrease in the distance between the sources. In contrast, the decrease in the flux corresponds to the decrease in the source separation. This behaviour, together with the observed spatial localisation of the QPP, is very consistent with the vertically polarised global kink mode of the flaring loop.

Oscillating Magnetic Trap and Non-Thermal Emission from Solar Flares

Abstract A comparative analysis of the fine time structure of microwave and hard X-ray emissions from the flare event on 1992 November 5 is given. Based on wavelet analysis, quasi-periodic pulsations with a fundamental period of 6 s in both wave bands have been revealed. The anticorrelation of time profiles of microwave and hard X-ray emissions as well as time delays between the emission peaks in different wave bands are explained by the excitation of sausage oscillations of a flare loop modelling by a magnetic trap. It has been shown that the intermediate pitch angle diffusion regime of trapped electrons into the loss-cone should be realized in flare loops. The relation between time delays of emission peaks at different wave bands and the pulse duration has been investigated.

Microwave and EUV Observations of an Erupting Filament and Associated Flare and Coronal Mass Ejections

Abstract A filament eruption was observed with the Siberian Solar Radio Telescope (SSRT) on 2012 June 23, starting at around 06:40 UT, beyond the west limb. The filament could be followed in SSRT images to heights above 1 R⊙, and coincided with the core of the CME, seen in LASCO C2 images. We briefly discuss the dynamics of the eruption: the top of the filament showed a smooth acceleration up to an apparent velocity of $\sim 1100 \textrm {km} \enspace \textrm s^{-1}$. Images behind the limb from STEREO-A show a two-ribbon flare and the interaction of the main filament, located along the primary neutral line, with an arch-like structure, oriented in the perpendicular direction. The interaction was accompanied by strong emission and twisting motions. The microwave images show a low-temperature component, a high-temperature component associated with the interaction of the two filaments and another high-temperature component apparently associated with the top of flare loops. We computed the differential emission measure from the high-temperature AIA bands and from this the expected microwave brightness temperature; for emission associated with the top of the flare loops, the computed brightness was 35% lower than the observed value.

Extremely Microwave-Rich Solar Flare Observed with Nobeyama Radioheliograph

A compact flare was observed with Nobeyama Radioheliograph (NoRH) slightly behind the west limb on 2011 March 10. The microwave peak flux values at 17 and 34 GHz were about 210 and 133 sfu, respectively. From the correlation between the 17 GHz peak flux and the GOES 1-8 Å soft X-ray peak flux, M1.5-class is expected for this microwave flux. However, only the B1-level enhancement was detected in the GOES 1-8 Å soft X-ray light curve on the C1-level background during the flare period. In addition to microwaves, Suzaku detected hard X-ray emissions, even in the energy range above 100 keV. It is clear that high-energy electrons were effectively produced in this flare, while the thermal emission was very weak. Why did this flare have this unique feature? The following two cases are considered. One is the case that a magnetic trap for electrons works effectively, and that each electron continues to emit microwaves in its relatively long lifetime. The other is that the magnetic field around the looptop region is intense, and relatively a large number of lower-energy electrons emit microwaves. Considering the observational facts, such as the short duration and the small flare loop, the latter case is more plausible.

Observations of subterahertz radiation of solar flares with an RT-7.5 radiotelescope

We analyzed unique observations of two flare events at frequencies of 93 and 140 GHz. The observations were carried out with an RT-7.5 radiotelescope at the Bauman State Technical University (Moscow) using the method of continuous active region tracking with spatial resolutions of 2.5 (at a frequency of 93 GHz) and 1.5 arc-minutes (at 140 GHz). The light curves of the bursts were analyzed and compared with the time profiles of soft and hard X-ray emission obtained by the GOES and RHESSI spacecraft. The radio delete this word flux density spectra were plotted. It was found that the radiation flux at a frequency of 140 GHz exceeded the flux at 93 GHz. This constitutes a new independent confirmation of the presence of a subterahertz flare component, the appearance of which may be associated with the thermal radiation of the hot plasma at the base of flare loops.

Spatial brightness distribution of hard X-Ray emission along flare loops

The work is devoted to the simulation of recently discovered hard X-ray and gamma radiation sources localized near the top of solar flare loops. The calculations were performed in the context of a model of a flare magnetic loop inhomogeneous with respect to the magnetic field. Two cases of injection were considered: isotropic and anisotropic along the loop axis. The distributions of electrons along the loop are found by solving the nonstationary relativistic kinetic equation in the Fokker-Planck form. Based on the calculated electron distribution functions, the spatial brightness distribution of hard X-ray and gamma radiation has been calculated. Radiation characteristics are compared for different sets of injection parameters.

Magnetic fields in a limb flare on July 19, 2012

The magnetic field intensity in the limb solar flare of July 19, 2012, was measured by the Zeeman bisector splitting in the Hα line. An average magnetic field is established to attain 200 G at a measurement error of ±100 G in the top of a radiant flare loop at a height of about 40 Mm. The confinement of a strong magnetic field in the high coronal arcade at a relatively weak external field of the corona (about 1–2 G) is considered using a model of a force-free magnetic flux rope with a fine magnetic structure at a scale of about 300 km.

EVIDENCE OF MULTIPLE SLOW ACOUSTIC OSCILLATIONS IN THE STELLAR FLARING LOOPS OF PROXIMA CENTAURI

We present the first observational evidence of multiple slow acoustic oscillations in the post flaring loops of the corona of Proxima Centauri using XMM-Newton observations. We find the signature of periodic oscillations localized in the decay phase of the flare in its soft (0.3-10.0 keV) X-ray emissions. Using the standard wavelet tool, we find the multiple periodicities of 1261 s and 687 s. These bursty oscillations persist for durations of 90 minutes and 50 minutes, respectively, for more than 4 cycles. The intensity oscillations with the period of 1261 s may be the signature of the fundamental mode of slow magnetoacoustic waves with the phase-speed of 119 km s$^{-1}$ in the loop of the length 7.5$\times 10^{9}$ cm heated initially to obtain the flare peak temperature of 33 MK and later cooled down in the decay phase maintained at the average temperature of 7.2 MK. The other period of 687 s may be associated with the first overtone of slow magnetoacoustic oscillations in the flaring loop. The fundamental mode oscillations show a dissipation with damping time of 47 min. The period ratio P$_{1}$/P$_{2}$ is found to be 1.83 indicating that such oscillations are most likely excited in longitudinal density stratified stellar loops. We estimate the density scale height of stellar loop system as 22.6 Mm, which is smaller than the hydrostatic scale height of the hot loop system, and implies the existence of non-equilibrium conditions.