Multi-Wavelength Analysis of the Flare on 2 October 1993

By use of Yohkoh hard X-ray flux and soft X-ray images, and of vector magnetograms and 2D spectral observations, a 1N/C6.5 flare observed on 2 October 1993 is analysed in detail. Evidence is provided not only morphologically but also quantitatively that the dynamics at kernels A and C of the flare in the impulsive phase were controlled mainly by electron beam bombardment, while the heating of kernel B is mainly due to heat conduction. By plotting the energy gradient of the electron energy flux as a function of energy for the various spectral indexes observed during the flare, the acceleration mechanism is found to be such that there is a constant energy E₀, close to 20 keV, for which the electron flux d F₁/dE is constant. It is shown that such a conclusion can be reached more directly by using the photon flux, which in that case must be constant for E=E₀, whatever the value of the power index. This result implies also that the electron spectrum is represented by a power law and that the X-ray photons are produced in a thick target. Instantaneous momentum balance is shown to exist between the upflowing soft X-ray-emitting and the downflowing Hα- emitting plasma at the kernels of the flare. The observed Hα red asymmetry is well reproduced by the non-LTE computation, with the down-moving condensation included. The observation of the magnetic field suggests that the flare was triggered probably by magnetic flux emergence.     

Multi-Wavelength Analysis of the 29 September 2002 M2.6/2N Flare

Using TRACE EUV 171 Å line, Hα line, Zürich radio, RHESSI, and HXRS observations the 29 September 2002 flare (M2.6), which occurred in AR NOAA 0134, was analyzed. Flaring structures were compared with a potential magnetic field model (field lines and quasi-separatrix layers) made from SOHO/MDI full-disk magnetogram. Series of high-resolution SOHO/MDI magnetograms and TRACE white-light images were used to find changes in the active region at the photosphere during the flare. The flare began with a rising of a small dark loop followed by the flare brightening observed in 171 Å with TRACE and Hα lines. In radio wavelengths, first type III bursts were observed 5 min prior to the start of hard X-ray emission, indicating a pre-flare coronal activity. The main hard X-ray emission peak (at 06:36 UT) was associated with the second type III burst activity and several slowly negatively drifting features, all starting from one point on the radio spectrum (probably a shock propagating through structures with different plasma parameters). After this time a huge loop formed and three minutes later it became visible in absorption both in Hα and 171 Å EUV lines. The phase of huge dark loop formation was characterized by long-lasting, slowly negatively drifting pulsations and drifting continuum. Finally, considering this huge loop as a surge an evolution of the event under study is discussed.     

A Multi-Wavelength Study of the 3B/X1.2 Flare Observed on 2003 October 26

We report results from a multi-wavelength study of the 3B/X1.2 two-ribbon disk flare (S15E44), which was well observed by both ground-based and space-borne instruments. Two pairs of conjugate kernels - K1 and K4, and K2 and K3 - in the Ha images are identified. These kernels are linked by two different systems of EUV loops. Kl and K4 correspond to the two 17 GHz and 34 GHz microwave sources observed by the Nobeyama Radioheliograph (NoRH), while K2 and K3 have no corresponding microwave sources. Optical spectroscopic observations suggest that all the four kernels are possible precipitating sites of non-thermal electrons. Thus the energy of electron deposited in K2 and K3 should be less than 100 keV. Two-dimensional distributions of the full widths at half maximum (FWHM) of the Ha profiles and the line-of-sight (LOS) velocities derived from the Ca Ⅱ 8542 (?) profiles indicate that the largest FWHM and LOS velocity tends to be located near the outer edges of Ha kernels, which is consistent with the scenario of current two-ribbon flare models and previous results. When non-thermal electron bombardment is present, the observed Ha and Ca Ⅱ 8542 (?) profiles are similar to previous observational and theoretical results, while the He I 10830 A profiles are different from the theoretical ones. This puts some constraints on future theoretical calculation of the He I 10830 (?) line.     

Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare

A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ₁ = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.     

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

We obtained a complete set of H, Ca 8542 and He I 10830 spectra and slit-jaw H 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 H, Ca 8542 and He I 10830 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 H emissions. The weaker one moved during the flare process and is located in the H dark cavities. This flare may support the theory of the magnetic reconnections in the lower solar atmosphere.     

The Gradual Phase of the X17 Flare on October 28, 2003

We present SOHO/CDS observations taken during the gradual phase of the X17 flare that occurred on October 28, 2003. The CDS data are supplemented with TRACE and ground-based observations. The spectral observations allow us to determine velocities from the Doppler shifts measured in the flare loops and in the two ribbon kernels, one hour and a half after the flare peak. Strong downflows (>70 km s⁻¹) are observed along the loop legs at transition-region temperatures. The velocities are close to those expected for free fall. Observations and results from a hydrodynamic simulation are consistent with the heating taking place for a short time near the top of the arcade. Slight upflows are observed in the outer edges of the ribbons (<60 km s⁻¹) in the EUV lines formed at log T < 6.3. These flows could correspond to the so-called “gentle evaporation.” At “flare” temperatures (Fe xix, log T = 6.9), no appreciable flows are observed. The observations are consistent with the general standard reconnection models for two-ribbons flares.     

Multi-Wavelength Observation of the 3n/x3.3 Flare of 28 November 1998

The 3N/X3.3 flare of 28 November 1998 was observed in multiple wavelength simultaneously. The available data include H images, spectra in Hei 1083 nm and Caii 854.2 nm from Purple Mountain Observatory (PMO), soft X-ray (SXR) and hard X-ray (HXR) images and flux from Yohkoh. Morphological relationship investigation and spectral analysis of these data show: (1) The sudden brightening at loop top above the active region and the steep increase of SXR flux before flare onset suggest that the corona there had already been heated to some extent in the preflare phase. (2) The scales of the Caii 854.2 nm emission areas are very similar to those of the H line, but the emission profiles look like those of the Caii K line. Most of the Hei 1083 nm emissions exist in the bright H kernels and can last to the decay phase. (3) Flare spectra show that line shift and asymmetry are very common in this flare not only in the impulsive phase but also in the decay phase. The difference in the line shifts or asymmetry between Caii 854.2 nm and Hei 1083 nm, as well as the difference between the line center and wings of Caii 854.2 nm imply the existence of a velocity gradient in the line-of-sight direction. (4) Post-flare loops with very deep absorption (70%) and very-high-velocity red shifts (30–90 km s^–1) were observed in Hei 1083 nm during the decay phase. However, only a slight dip can be found in the Caii 854.2 nm profile.     

Radio, Hard X-Ray, EUV and Optical Study of September 9, 2002 Solar Flare

The multi-wavelength analysis is performed on a flare on September 9, 2002 with data of Owens Valley Solar Arrays (OVSA), Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Extreme UV Imager Telescope (EIT), and The Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The radio sources at 4.8 and 6.2 GHz located in the intersection of two flaring loops at 195 of SOHO/EIT respectively with two dipole magnetic fields of SOHO/MDI, in which one EIT loop was coincident with an X-ray loop of RHESSI at 12–25 keV, and two H_αbright kernels a1 and a2 of BBSO, respectively at the two footpoints of this loop; the second EIT loop connected another two H_αkernels b1 and b2 and radio sources at 7.8 and 8.2 GHz of OVSA. The maximum phase of microwave bursts was evidently later than that of hard X-ray bursts and H_αkernels a1 and a2, but consistent with that of H_αkernels b1 and b2. Moreover, the flare may be triggered by the interaction of the two flaring loops, which is suggested by the cross-correlation of radio, optical, and X-ray light curves of a common quasi-periodic oscillation in the rising phase, as well as two peaks at about 7 and 9 GHz of the microwave spectra at the peak times of the oscillation, while the bi-directional time delays at two reversal frequencies respectively at 7.8 and 9.4 GHz (similar to the peak frequencies of the microwave spectra) may indicate two reconnection sites at different coronal levels. The microwave and hard X-ray footpoint sources located in different EUV and optical loops may be explained by different magnetic field strength and the pitch angle distribution of nonthermal electrons in these two loops.     

Spectral Analysis of a Thermal Flare on 27 October 1993

A thermal subflare occurring on 27 October 1993 was observed during a multi-wavelength campaign with the Yohkoh spacecraft and the Multi-channel Subtractive Double-Pass spectrograph (MSDP) at Pic du Midi. The various instruments provided us with 2-D H spectra, X-ray spectra and X-ray images. A non-LTE computation (including chromospheric condensation) was carried out to determine the dynamic parameters of the flaring chromosphere. By combining these results with the Yohkoh data, we illustrate that momentum is balanced between the upflowing plasma and the downflowing cool plasma during the flare. This result is consistent with the evaporation model.     

Companion Event and Precursor of the X17 Flare on 28 October 2003

A major two-ribbon X17 flare occurred on 28 October 2003, starting at 11:01 UT in active region NOAA 10486. This flare was accompanied by the eruption of a filament and by one of the fastest halo coronal mass ejections registered during the October–November 2003 strong activity period. We focus on the analysis of magnetic field (SOHO/MDI), chromospheric (NainiTal observatory and TRACE), and coronal (TRACE) data obtained before and during the 28 October event. By combining our data analysis with a model of the coronal magnetic field, we concentrate on the study of two events starting before the main flare. One of these events, evident in TRACE images around one hour prior to the main flare, involves a localized magnetic reconnection process associated with the presence of a coronal magnetic null point. This event extends as long as the major flare and we conclude that it is independent from it. A second event, visible in Hα and TRACE images, simultaneous with the previous one, involves a large-scale quadrupolar reconnection process that contributes to decrease the magnetic field tension in the overlaying field configuration; this allows the filament to erupt in a way similar to that proposed by the breakout model, but with magnetic reconnection occurring at Quasi-Separatrix Layers (QSLs) rather than at a magnetic null point. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

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.     

Images of Gradual Millimeter Emission and Multi-Wavelength Observations of the 17 august 1994 Solar Flare

We present a comprehensive analysis of the 17 August 1994 flare, the first flare imaged at millimeter (86 GHz) wavelengths. The temporal evolution of this flare displays a prominent impulsive peak shortly after 01:02 UT, observed in hard X-rays and at microwave frequencies, followed by a gradual decay phase. The gradual phase was also detected at 86 GHz. Soft X-ray images show a compact emitting region (20), which is resolved into two sources: a footpoint and a loop top source. Nonthermal emissions at microwave and hard X-ray wavelengths are analyzed and the accelerated electron spectrum is calculated. This energy spectrum derived from the microwave and hard X-ray observations suggests that these emissions were created by the same electron population. The millimeter emission during the gradual phase is thermal bremsstrahlung originating mostly from the top of the flaring loop. The soft X-rays and the millimeter flux density from the footpoint source are only consistent with the presence of a multi-temperature plasma at the footpoint.     

2002年7月23日X射线耀斑的RHESSI和TRACE观测物理图像

通过对比分析TRACE195A和RHESSI的X射线及Ha的图像，发现2002年7月23日的X4.8级耀斑的主体是一个典型的双带耀斑，它的Ha及TRACE195A的图像都显现出明显的双带耀斑特征，TRACE195A的像还呈现一个环拱结构，其环拱的足点与TRACE的亮带并不重合，但TRACE的亮带与X射线像的位置对应很好．在耀斑极大时刻附近，大于38keV的X射线像呈现一个低的环跨在TRACE的双带上，X射线环的顶点及两足点尤其明显，在这个低的环上方，还存在一个在低能量段明显的X射线日冕源，对该耀斑的空间结构及演化特征作了描述，还简单地讨论了一个可能的理论解释．     

A Statistical Study of Microwave Flare Morphologies

This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints.     

Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare

Solar Physics - Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer...     

From Gigahertz to Millihertz: A Multiwavelength Study of the Acoustically Active 14 August 2004 M7.4 Solar Flare

We carried out an electromagnetic acoustic analysis of the solar flare of 14 August 2004 in active region AR10656 from the radio to the hard X-ray spectrum. The flare was a GOES soft X-ray class M7.4 and produced a detectable sun quake, confirming earlier inferences that relatively low energy flares may be able to generate sun quakes. We introduce the hypothesis that the seismicity of the active region is closely related to the heights of coronal magnetic loops that conduct high-energy particles from the flare. In the case of relatively short magnetic loops, chromospheric evaporation populates the loop interior with ionised gas relatively rapidly, expediting the scattering of remaining trapped high-energy electrons into the magnetic loss cone and their rapid precipitation into the chromosphere. This increases both the intensity and suddenness of the chromospheric heating, satisfying the basic conditions for an acoustic emission that penetrates into the solar interior.     

Highlights of the Flare Build-up Study

Years of preparation within the framework of the Flare Build-up Study culminated with intensive observations of solar flares during the Solar Maximum Year (1979–1981). Scientists operating several spacecraft and roughly 70 ground-based observatories participated in an internationally coordinated effort to observe flares with higher spatial, spectral, and temporal resolution over a wider range of wavelengths than heretofore. The FBS stimulated important advances in theories of magnetic reconnection and the growth of plasma instabilities under preflare circumstances. A series of international FBS workshops facilitated data exchanges and collaborative studies for interpreting and synthesizing the wealth of new information about flares. The FBS ended officially at the Symposium on Synopsis of the Solar Maximum Analysis held 2–5 July, 1986 at the COSPAR meeting in Toulouse, France. Here we summarize highlights of its progress towards an understanding of the storage and release of preflare energy.     

Simulation of heavy ion spectra for the solar event of October 5, 2002

The results of numerical simulation of stochastic acceleration of the heavy ions (³He, ⁴He, ¹⁶O, and ⁵⁶Fe) are presented for the impulsive solar event of October 5, 2002. The energy spectra of the aforementioned particles have peculiarities (depressions) in the low-energy region (≤1 MeV nucleon⁻¹). Coulomb losses in the flare plasma and adiabatic losses during interplanetary propagation are considered in the study as possible causes of such peculiarities.     

2003年10月22日太阳活动区AR0484内日浪事件的研究

利用色球Hα、TRACE／WL、SOHO／EITEuV单色像观测资料及SOHO／MDI光球磁场观测资料，对2003年10月22日太阳活动区AR0484内发生的日浪事件进行了研究．发现：(1)在Ha线心观测上，日浪包含有亮、暗2个分量，这2个分量先后出现而且并不共空间．日浪的亮分量与UV和EUV波段上观测到的喷发具有较好的同时性和共空间性．(2)日浪喷发物质沿着EUV环运动。(3)在光球层，日浪足根处的黑子和磁场有明显的变化．这些观测结果支持日浪的磁重联模型。     

A Revisit of the Masuda Flare

We revisit the flare that occurred on 13 January 1992, which is now universally termed the “Masuda flare”. The new analysis is motivated not just by its uniqueness despite the increasing number of coronal observations in hard X-rays, but also by the improvement of Yohkoh hard X-ray image processing, which was achieved after the intensive investigations on this celebrated event. Using an uncertainty analysis, we show that the hard X-ray coronal source is located closer to the soft X-ray loop by about 5000 km (or 7 arcsec) in the re-calibrated Hard X-ray Telescope (HXT) images than in the original ones. Specifically, the centroid of the M1-band (23 – 33 keV) coronal source is above the maximum brightness of the Soft X-ray Telescope (SXT) loop by 5000±1000 km (9600 km in the original data) and above the apex of the SXT loop represented by the 30% brightness contour by 2000±1000 km (∼ 7000 km in the original data). The change is obviously significant, because most coronal sources are above the thermal loop by less than 6 arcsec. We suggest that this change may account for the discrepancy in the literature, i.e., the spectrum of the coronal emission was reported to be extremely hard below ∼ 20 keV in the pre-calibration investigations, whereas it was reported to be considerably softer in the literature after the re-calibration done by Sato, Kosugi, and Makishima (Pub. Astron. Soc. Japan 51, 127, 1999). Still, the coronal spectrum is flatter at lower energies than at higher energies, due to the lack of a similar, co-spatial source in the L-band (14 – 23 keV), for which a convincing explanation is absent.