ChJAA -- 2002, Vol.2, No.1

2002, Vol.2, No.1 (February 20, 2002) Contents

Stochastic Particle Acceleration in Blazar Jets Jian-Cheng Wang ................................................................ Abstract \| \| PDF file (140 KB) \| PS file *.gz (63 KB)	p.1
Properties of BL Lac Objects with Redshift $\leq$ 0.2 Ben-Zhong Dai, Guang-Zhong Xie and Ze-Jun Jiang ............................... Abstract \| \| PDF file (182 KB) \| PS file *.gz (166 KB)	p.8
Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution Jin-Liang Hou,Rui-Xiang Chang and Li Chen Abstract \| \| PDF file (316 KB) \| PS file *.gz (599 KB)	p.17
A Study of the Energy Sources of Herbig-Haro Objects Jing-Wen Wu, Yue-Fang Wu, Jun-Zhi Wang and Kai Cai ............................. Abstract \| \| PDF file (235 KB) \| PS file *.gz (381 KB)	p.33
Photometry of $\delta$ Sct and Related Stars: Results of AD Ari, IP Vir and YZ Boo Ai-Ying Zhou ................................................................... Abstract \| \| PDF file (303 KB) \| PS file *.gz (560 KB)	p.43
A Possible Pumping Mechanism for Interstellar Class II 107 GHz Methanol Masers Han-Ping Liu and Jin Sun ..................................................... Abstract \| \| PDF file (141 KB) \| PS file *.gz ( 81 KB)	p.51
Photometric Observations of the $\delta$ Scuti Star UV Trianguli and Its Evolutionary Status Zong-Li Liu, Ai-Ying Zhou and E. Rodriguez .................................... Abstract \| \| PDF file (277 KB) \| PS file *.gz (378 KB)	p.59
Can Asymmetry of Solar Activity be Extended into Extended Cycle? Ke-Jun Li, Jing-Xiu Wang, Hong-Fei Liang and Xiao-Ma Gu ...................... Abstract \| \| PDF file (195 KB) \| PS file *.gz (253 KB)	p.66
Key Properties of Solar Chromospheric Line Formation Process Zhong-Quan Qu and Zhi Xu ...................................................... Abstract \| \| PDF file (315 KB) \| PS file *.gz (475 KB)	p.71
Solar Flare Activity and Variability of Electric Current Helicity Kim Jik Su, Hong-Qi Zhang, Kim Jin Song, Kim Kum Sok and Xing-Ming Bao ....... Abstract \| \| PDF file (321 KB) \| PS file *.gz (474 KB)	p.81
Spectral Analysis of the Flare of 1998 November 11 based on the Multi-cloud Model Xiao-Ma Gu and Ming-De Ding .................................................. Abstract \| \| PDF file (405 KB) \| PS file *.gz (606 KB)	p.92
Abstract
Stochastic Particle Acceleration in Blazar Jets Jian-Cheng Wang E-mail: jcwang@public.km.yn.cn Abstract The bulk kinetic energy of jets can be dissipated via generating turbulent plasma waves. We examine stochastic particle acceleration in blazar jets to explain the emissions of all blazars. We show that acceleration of electrons by plasma turbulence waves with a spectrum $W(k) \sim k^{-4/3}$ produces a nonthermal population of relativistic electrons whose peak frequency of synchrotron emission can fit the observational trends in the spectral energy distribution of all blazars. The plasma nonlinear processes responsible for the formation of turbulent spectrum are investigated. Increases in the interaction time of turbulent waves can produce a flatter spectrum leading to efficient particle acceleration. Key words: stars: acceleration of particles --- blazars: theory --- plasma: turbulence --- radiation mechanism: nonthermal PDF file (140 KB) \| gzipped PS file (63KB) \| Back to Contents
Properties of BL Lac Objects with Redshift 0.2 Ben-Zhong Dai, Guang-Zhong Xie and Ze-Jun Jiang E-mail: bzdai@public.km.yn.cn Abstract We present a large sample which includes 82 BL Lac objects with redshifts below 0.2 from recent literature. We find strong correlations in both flux and luminosity between the radio (5 GHz) and optical bands (5500 Å). The correlations in other bands are very weak. Five TeV BL Lacs and two suspect sources are found to have similar properties as high-frequency-peaked BL Lacs (HBLs). Our results suggest that both the radio and optical emissions are from the same radiation mechanism in the SSC model. The TeV BL Lac candidates should be HBLs or HBL-like objects with small redshifts. Key words: galaxies: active --- BL Lacertae objects: general --- gamma-ray: observations --- radiation mechanisms: non-thermal PDF file (182KB) \| gzipped PS file (166 KB) \| Back to Contents
Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution Jin-Liang Hou,Rui-Xiang Chang and Li Chen E-mail: hjlyx@center.shao.ac.cn Abstract We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099 Key words: open clusters: abundances --- Galaxy: formation --- Galaxy: evolution PDF file (316 KB) \| gzipped PS file (599 KB) \| Back to alt="$\pm$"> Contents
A Study of the Energy Sources of Herbig-Haro Objects Jing-Wen Wu, Yue-Fang Wu, Jun-Zhi Wang and Kai Cai E-mail: ywu@bac.pku.edu.cn Abstract We make a statistical study of the energy sources of high-velocity phenomena, Herbig-Haro (HH) objects. IRAS counterparts of HH objects are identified. Their colors, brightness, geometric relation to the HH objects and SED are analysed. The sources are found to be concentrated in a band-shaped region in the IRAS color-color diagram. We suggest an explanation of thick surrounding material for this distribution. We propose a new method for identifying the energy sources based on color and brightness. This method is applied to more than 200 HH objects whose energy sources are still unknown. Finally, a group of very young stellar object candidates which have similar properties to the HH energy sources is picked out. Their large-scale distribution is discussed. Key words: star: formation --- ISM: Herbig-Haro objects --- stars: pre-main-sequence PDF file (235 KB) \| gzipped PS file (381 KB) \| Back to Contents
Photometry of $\delta$ Sct and Related Stars: Results of AD Ari, IP Vir and YZ Boo Ai-Ying Zhou E-mail: aiying@bao.ac.cn Abstract Photometric results of three $\delta$ Scuti stars, AD Ari, IP Vir and YZ Boo from new observations obtained at the Xinglong Station of the Beijing Astronomical Observatory(BAO) during 2000 and 2001 are reported. We present here the preliminary analyses. Detailed studies of these stars based on additional data will be published separately. Key words: stars: variables: $\delta$ Scuti --- stars: individual: AD Ari, IP Vir, YZ Boo --- techniques: photometric PDf file (303 KB) \| gzipped PS file (560 KB) \| Back to Contents
A Possible Pumping Mechanism for Interstellar Class II 107 GHz Methanol Masers Han-Ping Liu and Jin Sun E-mail: gaozm@email.bnu.edu.cn Abstract It is recognized that the interstellar methanol-107 GHz masers and OH-4.765 GHz masers towards Class II sources are associated with each other and coexist towards ultracompact HII regions. Therefore we suggest a new pumping mechanism -- methanol masers without population inversion. It can explain the formation of 107 GHz methanol masers, with the 4.765 GHz OH masers acting as a driving coherent microwave field. It is argued that this mechanism is compatible with the astronomical conditions. Key words: H II regions --- masers --- radiation mechanisms: non-thermal --- ISM: molecules --- line: formation PDF file (141 KB) \| gzipped PS file ( 81 KB)\| Back to Contents
Photometric Observations of the $\delta$ Scuti Star UV Trianguli and Its Evolutionary Status Zong-Li Liu, Ai-Ying Zhou and E. Rodriguez E-mail: lzl@bao.ac.cn Abstract UV Tri was observed photometrically from 1999 to 2000 at the Xinglong Station of the National Astronomical Observatories, Chinese Academy of Sciences, and was also observed with Strömgren uvby H $\betta$ filter system at the Sierra Nevada Observatory (Spain) in 2000. From period analyses of the data, three pulsation frequencies, 9.3299, 10.8483 and 3.6035 c d^-1 were obtained. We derived color indices: b-y = 0.215, m₁ = 0.169, c₁ = 0.783, and $\betaa$ = 2.775. With these indices and some calibrations, we obtain: M_v =2.44, M_bol = 2.27, log L/ $L_{\odot}$ = 0.99, and log T_eff = 3.875. Evolutionary sequences of stellar models with 1.002.00 solar masses, at steps of 0.05 $\M_{\odot}$ , are computed. Each sequence consists of 220 evolutionary intervals. From a comparison between the observed and calculated physical parameters we conclude that UV Tri is in an early evolutionary phase before the turn-off point. Key words: $\delta$ Scuti --- stars: oscillations --- stars: individual: UV Tri PDF file (277 KB) \| gzipped PS file (378 KB) \| Back to Contents
Can Asymmetry of Solar Activity be Extended into Extended Cycle? Ke-Jun Li, Jing-Xiu Wang, Hong-Fei Liang and Xiao-Ma Gu E-mail: ssg@public.km.yn.cn Abstract With the use of the Royal Greenwich Observatory data set of sunspot groups, an attempt is made to examine the north-south asymmetry of solar activity in the ``extended'' solar cycles. It is inferred that the asymmetry established for individual solar cycles does not extend to the ``extended'' cycles. Key words: Sun: sunspot --- Sun: active cycle --- Sun: activity PDF file (195 KB) \| gzipped PS file (253 KB) \| Back to Contents
Key Properties of Solar Chromospheric Line Formation Process Zhong-Quan Qu and Zhi Xu E-mail: zqqu@cosmos.ynao.ac.cn Abstract The distribution or wavelength-dependence of the formation regions of frequently used solar lines, H $\alpha$ , H $\beta$ , Ca II H and Ca II 8542, in quiet Sun, faint and bright flares is explored in the unpolarized case. We stress four aspects characterising the property of line formation process: 1) width of line formation core; 2) line formation region; 3) influence of the temperature minimum region; and 4) wavelength ranges within which one can obtain pure chromospheric and photospheric filtergrams. It is shown that the above four aspects depend strongly on the atmospheric physical condition and the lines used. The formation regions of all the wavelength points within a line may be continuously distributed over one depth domain or discretely distributed because of no contribution coming from the temperature minimum region, an important domain in the solar atmosphere that determines the distribution pattern of escape photons. On the other hand, the formation region of one wavelength point may cover only one height range or spread over two domains which are separated again by the temperature minimum region. Different lines may form in different regions in the quiet Sun. However, these line formation regions become closer in solar flaring regions. Finally, though the stratification of line-of-sight velocity can alter the position of the line formation core within the line band and result in the asymmetry of the line formation core about the shifted line center, it can only lead to negligible changes in the line formation region or the line formation core width. All these results can be instructive to solar filtering observations. Key words: line: formation --- radiative transfer --- Sun: chromosphere PDF file (315 KB) \| gzipped PS file (475 KB) \| Back to Contents
Solar Flare Activity and Variability of Electric Current Helicity Kim Jik Su, Hong-Qi Zhang, Kim Jin Song, Kim Kum Sok and Xing-Ming Bao E-mail: hzhang@class1.bao.ac.cn Abstract Recent study using Huairou vector magnetograph data shows that during flare activity, the current helicity exhibits rapid and substantial variations and, in some cases, a recovering phase. We considered various representative the magnetic configurations. First, interacting twisted magnetic flux tubes are analyzed separately for positive, negative and mixed-sign helicity regions. The results show that the helicity during flares decreases in positive-sign, and increases in negative-sign regions. Then, flaring arcade also shows that the magnitude of the helicity decreases after flares. We also investigated current sheets formed by sheared magnetic field and showed that he current helicity (with either positive and negative signs) vanishes after reconnection. The emergence of twisted flux tubes which can trigger flares may be another source of flare-associated variability of current helicity. We demonstrate the relevance of current helicity to the description of flare activity by comparing its variation with that of shear angle in the active region AR 6891. Key words: Sun: flare --- Sun: current helicity --- Sun: magnetic shear PDF file (321 KB) \| gzipped PS file (474 KB) \| Back to Contents
Spectral Analysis of the Flare of 1998 November 11 based on the Multi-cloud Model Xiao-Ma Gu and Ming-De Ding E-mail: ssg@public.km.yn.cn Abstract We have obtained 2D spectra of H $\alpha$ , and CaII $\ilambda$ 8542Å for the flare of 1998 November 11, and derived its 2D velocity field and integrated intensity field. The velocity distribution shows that the red-shift and blue-shift velocities lie respectively in the northern and southern parts of the flare and that the maximum velocity seems to be located in two footpoints of the flare loop system. The integrated intensity distribution shows that the CaII $\ilambda$ 8542Å line is formed at a lower height than the H $\alpha$ line. we used `multi-cloud model' (MCM) to obtain four parameters for the two lines (Doppler width, $\Delta \lambda_D$ , Doppler shift, $\Delta \lambda_0$ , line source function, S, and optical depth at the line center, $\tau_0$ ). We also estimated the column number densities of hydrogen at the second level, N₂, and of the ionized calcium at the third level,N₃, as well as the kinetic temperature, T_c. The wide H $\alpha$ profile at the loop top may be explained by an overlapping of two or more elementary profiles. It is shown that the uncertainty in calibration does not affect the derived Doppler shift and line broadening, only the source function and optical depth. Key words: Sun: activity --- Sun: flares --- methods: data analysis PDF file (405 KB) \| gzipped PS file (606 KB) \| Back to Contents

Chinese Journal of Astronomy and Astrophysics: Online Edition

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Copyright 2001--2007 All rights reserved. The National Astronomical Observatories of
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2002, Vol.2, No.1 (February 20, 2002) Contents

Stochastic Particle Acceleration in Blazar Jets Jian-Cheng Wang ................................................................ Abstract \| \| PDF file (140 KB) \| PS file *.gz (63 KB)	p.1
Properties of BL Lac Objects with Redshift $\leq$ 0.2 Ben-Zhong Dai, Guang-Zhong Xie and Ze-Jun Jiang ............................... Abstract \| \| PDF file (182 KB) \| PS file *.gz (166 KB)	p.8
Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution Jin-Liang Hou,Rui-Xiang Chang and Li Chen Abstract \| \| PDF file (316 KB) \| PS file *.gz (599 KB)	p.17
A Study of the Energy Sources of Herbig-Haro Objects Jing-Wen Wu, Yue-Fang Wu, Jun-Zhi Wang and Kai Cai ............................. Abstract \| \| PDF file (235 KB) \| PS file *.gz (381 KB)	p.33
Photometry of $\delta$ Sct and Related Stars: Results of AD Ari, IP Vir and YZ Boo Ai-Ying Zhou ................................................................... Abstract \| \| PDF file (303 KB) \| PS file *.gz (560 KB)	p.43
A Possible Pumping Mechanism for Interstellar Class II 107 GHz Methanol Masers Han-Ping Liu and Jin Sun ..................................................... Abstract \| \| PDF file (141 KB) \| PS file *.gz ( 81 KB)	p.51
Photometric Observations of the $\delta$ Scuti Star UV Trianguli and Its Evolutionary Status Zong-Li Liu, Ai-Ying Zhou and E. Rodriguez .................................... Abstract \| \| PDF file (277 KB) \| PS file *.gz (378 KB)	p.59
Can Asymmetry of Solar Activity be Extended into Extended Cycle? Ke-Jun Li, Jing-Xiu Wang, Hong-Fei Liang and Xiao-Ma Gu ...................... Abstract \| \| PDF file (195 KB) \| PS file *.gz (253 KB)	p.66
Key Properties of Solar Chromospheric Line Formation Process Zhong-Quan Qu and Zhi Xu ...................................................... Abstract \| \| PDF file (315 KB) \| PS file *.gz (475 KB)	p.71
Solar Flare Activity and Variability of Electric Current Helicity Kim Jik Su, Hong-Qi Zhang, Kim Jin Song, Kim Kum Sok and Xing-Ming Bao ....... Abstract \| \| PDF file (321 KB) \| PS file *.gz (474 KB)	p.81
Spectral Analysis of the Flare of 1998 November 11 based on the Multi-cloud Model Xiao-Ma Gu and Ming-De Ding .................................................. Abstract \| \| PDF file (405 KB) \| PS file *.gz (606 KB)	p.92
Abstract
Stochastic Particle Acceleration in Blazar Jets Jian-Cheng Wang E-mail: jcwang@public.km.yn.cn Abstract The bulk kinetic energy of jets can be dissipated via generating turbulent plasma waves. We examine stochastic particle acceleration in blazar jets to explain the emissions of all blazars. We show that acceleration of electrons by plasma turbulence waves with a spectrum $W(k) \sim k^{-4/3}$ produces a nonthermal population of relativistic electrons whose peak frequency of synchrotron emission can fit the observational trends in the spectral energy distribution of all blazars. The plasma nonlinear processes responsible for the formation of turbulent spectrum are investigated. Increases in the interaction time of turbulent waves can produce a flatter spectrum leading to efficient particle acceleration. Key words: stars: acceleration of particles --- blazars: theory --- plasma: turbulence --- radiation mechanism: nonthermal PDF file (140 KB) \| gzipped PS file (63KB) \| Back to Contents
Properties of BL Lac Objects with Redshift 0.2 Ben-Zhong Dai, Guang-Zhong Xie and Ze-Jun Jiang E-mail: bzdai@public.km.yn.cn Abstract We present a large sample which includes 82 BL Lac objects with redshifts below 0.2 from recent literature. We find strong correlations in both flux and luminosity between the radio (5 GHz) and optical bands (5500 Å). The correlations in other bands are very weak. Five TeV BL Lacs and two suspect sources are found to have similar properties as high-frequency-peaked BL Lacs (HBLs). Our results suggest that both the radio and optical emissions are from the same radiation mechanism in the SSC model. The TeV BL Lac candidates should be HBLs or HBL-like objects with small redshifts. Key words: galaxies: active --- BL Lacertae objects: general --- gamma-ray: observations --- radiation mechanisms: non-thermal PDF file (182KB) \| gzipped PS file (166 KB) \| Back to Contents
Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution Jin-Liang Hou,Rui-Xiang Chang and Li Chen E-mail: hjlyx@center.shao.ac.cn Abstract We compile a new sample of 89 open clusters with ages, distances and metallicities available. We derive a radial iron gradient of about -0.099 Key words: open clusters: abundances --- Galaxy: formation --- Galaxy: evolution PDF file (316 KB) \| gzipped PS file (599 KB) \| Back to alt="$\pm$"> Contents
A Study of the Energy Sources of Herbig-Haro Objects Jing-Wen Wu, Yue-Fang Wu, Jun-Zhi Wang and Kai Cai E-mail: ywu@bac.pku.edu.cn Abstract We make a statistical study of the energy sources of high-velocity phenomena, Herbig-Haro (HH) objects. IRAS counterparts of HH objects are identified. Their colors, brightness, geometric relation to the HH objects and SED are analysed. The sources are found to be concentrated in a band-shaped region in the IRAS color-color diagram. We suggest an explanation of thick surrounding material for this distribution. We propose a new method for identifying the energy sources based on color and brightness. This method is applied to more than 200 HH objects whose energy sources are still unknown. Finally, a group of very young stellar object candidates which have similar properties to the HH energy sources is picked out. Their large-scale distribution is discussed. Key words: star: formation --- ISM: Herbig-Haro objects --- stars: pre-main-sequence PDF file (235 KB) \| gzipped PS file (381 KB) \| Back to Contents
Photometry of $\delta$ Sct and Related Stars: Results of AD Ari, IP Vir and YZ Boo Ai-Ying Zhou E-mail: aiying@bao.ac.cn Abstract Photometric results of three $\delta$ Scuti stars, AD Ari, IP Vir and YZ Boo from new observations obtained at the Xinglong Station of the Beijing Astronomical Observatory(BAO) during 2000 and 2001 are reported. We present here the preliminary analyses. Detailed studies of these stars based on additional data will be published separately. Key words: stars: variables: $\delta$ Scuti --- stars: individual: AD Ari, IP Vir, YZ Boo --- techniques: photometric PDf file (303 KB) \| gzipped PS file (560 KB) \| Back to Contents
A Possible Pumping Mechanism for Interstellar Class II 107 GHz Methanol Masers Han-Ping Liu and Jin Sun E-mail: gaozm@email.bnu.edu.cn Abstract It is recognized that the interstellar methanol-107 GHz masers and OH-4.765 GHz masers towards Class II sources are associated with each other and coexist towards ultracompact HII regions. Therefore we suggest a new pumping mechanism -- methanol masers without population inversion. It can explain the formation of 107 GHz methanol masers, with the 4.765 GHz OH masers acting as a driving coherent microwave field. It is argued that this mechanism is compatible with the astronomical conditions. Key words: H II regions --- masers --- radiation mechanisms: non-thermal --- ISM: molecules --- line: formation PDF file (141 KB) \| gzipped PS file ( 81 KB)\| Back to Contents
Photometric Observations of the $\delta$ Scuti Star UV Trianguli and Its Evolutionary Status Zong-Li Liu, Ai-Ying Zhou and E. Rodriguez E-mail: lzl@bao.ac.cn Abstract UV Tri was observed photometrically from 1999 to 2000 at the Xinglong Station of the National Astronomical Observatories, Chinese Academy of Sciences, and was also observed with Strömgren uvby H $\betta$ filter system at the Sierra Nevada Observatory (Spain) in 2000. From period analyses of the data, three pulsation frequencies, 9.3299, 10.8483 and 3.6035 c d^-1 were obtained. We derived color indices: b-y = 0.215, m₁ = 0.169, c₁ = 0.783, and $\betaa$ = 2.775. With these indices and some calibrations, we obtain: M_v =2.44, M_bol = 2.27, log L/ $L_{\odot}$ = 0.99, and log T_eff = 3.875. Evolutionary sequences of stellar models with 1.002.00 solar masses, at steps of 0.05 $\M_{\odot}$ , are computed. Each sequence consists of 220 evolutionary intervals. From a comparison between the observed and calculated physical parameters we conclude that UV Tri is in an early evolutionary phase before the turn-off point. Key words: $\delta$ Scuti --- stars: oscillations --- stars: individual: UV Tri PDF file (277 KB) \| gzipped PS file (378 KB) \| Back to Contents
Can Asymmetry of Solar Activity be Extended into Extended Cycle? Ke-Jun Li, Jing-Xiu Wang, Hong-Fei Liang and Xiao-Ma Gu E-mail: ssg@public.km.yn.cn Abstract With the use of the Royal Greenwich Observatory data set of sunspot groups, an attempt is made to examine the north-south asymmetry of solar activity in the ``extended'' solar cycles. It is inferred that the asymmetry established for individual solar cycles does not extend to the ``extended'' cycles. Key words: Sun: sunspot --- Sun: active cycle --- Sun: activity PDF file (195 KB) \| gzipped PS file (253 KB) \| Back to Contents
Key Properties of Solar Chromospheric Line Formation Process Zhong-Quan Qu and Zhi Xu E-mail: zqqu@cosmos.ynao.ac.cn Abstract The distribution or wavelength-dependence of the formation regions of frequently used solar lines, H $\alpha$ , H $\beta$ , Ca II H and Ca II 8542, in quiet Sun, faint and bright flares is explored in the unpolarized case. We stress four aspects characterising the property of line formation process: 1) width of line formation core; 2) line formation region; 3) influence of the temperature minimum region; and 4) wavelength ranges within which one can obtain pure chromospheric and photospheric filtergrams. It is shown that the above four aspects depend strongly on the atmospheric physical condition and the lines used. The formation regions of all the wavelength points within a line may be continuously distributed over one depth domain or discretely distributed because of no contribution coming from the temperature minimum region, an important domain in the solar atmosphere that determines the distribution pattern of escape photons. On the other hand, the formation region of one wavelength point may cover only one height range or spread over two domains which are separated again by the temperature minimum region. Different lines may form in different regions in the quiet Sun. However, these line formation regions become closer in solar flaring regions. Finally, though the stratification of line-of-sight velocity can alter the position of the line formation core within the line band and result in the asymmetry of the line formation core about the shifted line center, it can only lead to negligible changes in the line formation region or the line formation core width. All these results can be instructive to solar filtering observations. Key words: line: formation --- radiative transfer --- Sun: chromosphere PDF file (315 KB) \| gzipped PS file (475 KB) \| Back to Contents
Solar Flare Activity and Variability of Electric Current Helicity Kim Jik Su, Hong-Qi Zhang, Kim Jin Song, Kim Kum Sok and Xing-Ming Bao E-mail: hzhang@class1.bao.ac.cn Abstract Recent study using Huairou vector magnetograph data shows that during flare activity, the current helicity exhibits rapid and substantial variations and, in some cases, a recovering phase. We considered various representative the magnetic configurations. First, interacting twisted magnetic flux tubes are analyzed separately for positive, negative and mixed-sign helicity regions. The results show that the helicity during flares decreases in positive-sign, and increases in negative-sign regions. Then, flaring arcade also shows that the magnitude of the helicity decreases after flares. We also investigated current sheets formed by sheared magnetic field and showed that he current helicity (with either positive and negative signs) vanishes after reconnection. The emergence of twisted flux tubes which can trigger flares may be another source of flare-associated variability of current helicity. We demonstrate the relevance of current helicity to the description of flare activity by comparing its variation with that of shear angle in the active region AR 6891. Key words: Sun: flare --- Sun: current helicity --- Sun: magnetic shear PDF file (321 KB) \| gzipped PS file (474 KB) \| Back to Contents
Spectral Analysis of the Flare of 1998 November 11 based on the Multi-cloud Model Xiao-Ma Gu and Ming-De Ding E-mail: ssg@public.km.yn.cn Abstract We have obtained 2D spectra of H $\alpha$ , and CaII $\ilambda$ 8542Å for the flare of 1998 November 11, and derived its 2D velocity field and integrated intensity field. The velocity distribution shows that the red-shift and blue-shift velocities lie respectively in the northern and southern parts of the flare and that the maximum velocity seems to be located in two footpoints of the flare loop system. The integrated intensity distribution shows that the CaII $\ilambda$ 8542Å line is formed at a lower height than the H $\alpha$ line. we used `multi-cloud model' (MCM) to obtain four parameters for the two lines (Doppler width, $\Delta \lambda_D$ , Doppler shift, $\Delta \lambda_0$ , line source function, S, and optical depth at the line center, $\tau_0$ ). We also estimated the column number densities of hydrogen at the second level, N₂, and of the ionized calcium at the third level,N₃, as well as the kinetic temperature, T_c. The wide H $\alpha$ profile at the loop top may be explained by an overlapping of two or more elementary profiles. It is shown that the uncertainty in calibration does not affect the derived Doppler shift and line broadening, only the source function and optical depth. Key words: Sun: activity --- Sun: flares --- methods: data analysis PDF file (405 KB) \| gzipped PS file (606 KB) \| Back to Contents