An International Peer Reviewed Research Journal

Vol 17, No. 3 &4

AJP

ISSN : 0971 - 3093

Vol  17,  No. 3 &4, July-December, 2008

 Asian Journal of Physics                                                                                                 Vol. 17, No.3 &4 (2008) 319-345


Genesis of solar flares and associated phenomena


Rajmal Jain1

Physical Research Laboratory, Navrangpura, Ahmedabad – 380 009, India

Lokesh Bharti

Max Planck Institut for Solar System Research, Katlenburg-Lindau, 37191 Germany

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In this review we discuss the concept of the solar flare and associated phenomena. We began with very small-scale solar flare activity such as umbral dots occurring inside the sunspot’s umbra. We present recent evidences for the umbral dots to be the result of magnetoconvection in the photosphere. We also discuss recent discoveries in contrast to previous studies of light-bridges, and their context to flare associated phenomena. The opposite polarity of the light-bridge with respect to its parent sunspot was demonstrated and thereby how low-altitude reconnection causes mass ejection associated with flare/ Ellerman bombs is proposed. We define elaborately the concept of the flare in general and “confined” and “eruptive” classes of flares in particular. The various flare associated phenomena such as surges, spray and veil emission, flare loops have been reviewed with current status on these topics. These subjects have been changing with revolution in observational capabilities. In last two decades YOHKOH, SOHO, TRACE, RHESSI and SOXS, and more recently Hinode missions have changed the earlier concepts significantly, which have, however, enabled us to improve our understanding of this phenomenon on one side, as well as to improve our theories on the other side. The particle acceleration has been reviewed considering as a focal theme subject. The subject of microflares and nanoflares has been always a center of attraction among solar physicists since Parker [1] proposed them as potential candidates for coronal heating, which, however, does not convincingly appear the case, and we made full efforts to describe this phenomena in greater detail. We end this overview with a detailed discussion on how flare/ CME models and theories have changed over time. In this limited review we may not be able to cite the literature comprehensively, but we do try to give both early and modern references wherever possible.

Total Refs : 158

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Asian Journal of Physics                                                                                                  Vol. 17, No.3 &4 (2008) 347-362


Solar flare associated magnetic changes in solar active regions


Jingxiu Wang

National Astronomical Observatories, Chinese Academy of Science, Beijing 100012, China

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Solar flare associated magnetic changes in solar active regions is a long-lasting issue in solar physics. The relevant observational studies have been done in three inter-connected directions: (1) the pre-flare magnetic configuration and evolution, (2) the rapid magnetic changes in the course of flares, and (3) the flare-induced signals in polarization measurements. The importance of these studies is to examine our physical understanding on the flare phenomenon which takes place in a wide range of astrophysical subjects. Although it is well established that flares come from the magnetic energy release in strong magnetic fields of the Sun and stars, current theoretical models are still far from being confronting the detailed observations. In this article we have tried to review some key advances in the relevant studies and suggest possible directions for the future.

Toatal Refs : 83

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Asian Journal of Physics                                                                                              Vol. 17, Nos. 3 & 4  (2008) 363-382


Solar flares and associated phenomena

 

Louise K Harra

UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK.

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A solar flare was first observed in 1859, and since that time scientists have been driven to understand and ultimately predict the most energetic phenomena in the solar system. Flares usually occur in regions with strong magnetic fields located in sunspots, and generally speaking this follows an activity cycle of approximately 11 years. Flares are often related to coronal mass ejections and have an impact on the space weather that surrounds the planets. In this review I will describe the latest understanding on solar flares derived through new simulations and from observations from a wide range of space missions including TRACE, SOHO, RHESSI and the recently launched Hinode and STEREO missions.

Total Refs : 100

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 383-410


High-temperature emission from solar flares

 

Kenneth J. H. Phillips

UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, United Kingdom

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Some of  the most significant advances made with spacecraft instruments observing high-temperature emission from solar flares are reviewed. Solar flares have a basic geometry consisting of a loop or loop system. Model loops include conduction and radiation cooling mechanisms, and correctly predict some of the features, though the nature of bright loop-top sources is not explained: they may be associated with hard X-ray, nonthermal sources above the loop or loop system. Dielectronic satellites of Li-like Fe ions remain the best means of determining the temperature (~ 20 MK) of the hottest part of flares. A superhot component (up to ~ 40 MK) is indicated by the presence of H-like Fe ion line emission. Densities have been determined from line ratio techniques to be as high as 1019 m–3 at flare peak, but more generally 1018 m–3, and 1017 m–3 during flare decay. Mass motions are observed at the flare onset; there is more evidence that upflows result from chromospheric evaporation at flare footpoints. Nonthermal electrons have been detected spectroscopically. Line ratios may eventually give a useful means of determining the lower limit of nonthermal electron distributions at the flare impulsive stage. Finally, the importance of the spectral region 0.12 - 0.16 nm (7.5 - 10 keV) is emphasized. Fe XXV lines with 1s2-1snp (n > 2) transitions  with Fe XXIV satellites offer the means of determining the temperature of the hottest part of the flare and for detecting nonthermal excitation. Micro-calorimeter observation of this region could reveal much physics for large flares and for non-solar sources.

Total Refs : 51

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 411-421


Looptop impulsive hard X-ray source in solar flares observed with Yohkoh/HXT


Satoshi Masuda

Solar-Terrestrial Environment Laboratory, Nagoya University

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan

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A new type of coronal hard X-ray source was first reported in 1994. That hard X-ray source was located above the soft X-ray loop and shows an impulsive behavior similar to those of the footpoint sources. Many solar physicists have tried to understand this source.  However, still there remain many problems.  In this paper, the researches related to this source observed with Yohkoh/HXT are reviewed.

Total Refs: 25

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 423-444


New aspects on particle acceleration in solar flares from RHESSI observations

 Markus J. Aschwanden

Lockheed Martin Advanced TechnologyCenter, Solar and Astrophysics Laboratory,

Bldg.252, Org. ADBS, 3251 HanoverSt., Palo Alto,CA 94304,USA

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In this review we highlight a number of recent RHESSI observations that are directly relevant to the study of particle acceleration processes in solar flares. Many observations confirm our basic standard models of acceleration in various types of coronal magnetic reconnection regions, but reveal a number of unexpected features that either require more detailed magnetic, hydrodynamic, and kinetic modeling or rethinking in terms of alternative models.

Total Refs : 67

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 445-478


Magnetic reconnection and topological trigger in physics of large solar flares


 Boris V. Somov

Astronomical Institute and Faculty of Physics, Moscow State University,

Moscow 119992, Russian Federation

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Solar flares are accessible to a broad variety of observational methods to see and investigate the magnetic reconnection phenomenon in high-temperature strongly-magnetized plasma of the solar corona.  An analysis of the topological peculiarities of magnetic field in active regions shows that the topological trigger effect is necessary to allow for in order to construct models for large eruptive flares. The topological trigger is not a resistive instability which leads to a change of the topology of the field configuration from pre- to post reconnection state. On the contrary, the topological trigger is a quick change of the global topology, which dictates the fast reconnection of collisional or collisionless nature. The current state of the art and development potential of the theory of collisionless reconnection in the strong magnetic fields related to large flares are brieûy reviewed. Particle acceleration is considered in collapsing magnetic traps created by reconnection. In order to explain the formation of coronal X-ray sources, the Fermi acceleration and betatron mechanism are simultaneously taken into account analytically in a collisionless approximation. Finally, the emphasis is on urgent unsolved problems of solar flare physics.

Total Refs : 70

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 479-488


Magnetic transport on the solar atmosphere by turbulent ambipolar diffusion


V. Krishan1,2,3 and S. Masuda3

1Indian Institute of Astrophysics, Bangalore 560034, India

2Raman Research Institute, Bangalore 560080, India

3Solar-Terrestrial Environment Laboratory, Nagoya University,Nagoya, Aichi 464-8601, Japan

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The lower solar atmosphere consists of partially ionized turbulent plasmas harboring velocity field, magnetic field and current density fluctuations. The correlations amongst these small scale fluctuations give rise to large scale flows and magnetic fields which decisively affect all transport processes. The three fluid system consisting of electrons, ions and neutral particles supports nonideal effects such as the Hall effect and the ambipolar diffusion. Here, we study magnetic transport by ambipolar diffusion and compare the characteristic timescales of the laminar and the turbulent ambipolar diffusion processes. As expected from a turbulent transport process, the time scale of the turbulent ambipolar diffusion is found to be smaller by orders of magnitude as compared with the laminar ambipolar diffusion. The nonlinearity of the laminar ambipolar diffusion creates magnetic structures with sharp gradients which are amenable to processes such as magnetic reconnection and energy release there from for heating and flaring of the solar plasma.

Total Refs : 15

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 489-508


Effects of solar flares on the interplanetary medium and terrestrial environment


H. Chandra, Hari Om Vats and Som Sharma

Physical Research Laboratory, Ahmedabad 380009, India

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Entire heliosphere including our planet Earth. Solar flares are one of the most energetic events in the whole solar system. The effects of solar flares are very complex on the interplanetary medium and terrestrial environment. There are direct and indirect effects on our environment. Some of these are almost instantaneous, whereas others are delayed. This article reviews many of these effects both on statistical basis as well as on a few case studies.

Total Refs : 89

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 509-532


X-ray spectroscopy of solar flares


Rajmal Jain

Physical Research Laboratory, Navrangpura, Ahmedabad – 380 009, India 

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In this review article we firstly describe the X-ray emission from the solar flares and the focal science aspects that require the X-ray spectroscopy to improve understanding of the solar flares. We present briefly the X-ray spectroscopy carried out with scintillation detectors mounted onboard SMM and YOHKOH in last century, and then with next generation solid state detectors onboard  RHESSI and SOXS missions in this decade. We consider the X-ray high resolution spectroscopy as important diagnostic tool to understand the solar flares mechanism, and therefore we present in particular the instrumentation details, and results from Si and CZT detectors of SOXS mission in this article, which may enable youngsters to plan next generation space-borne experiments with superb resolution using advance technology. Total Refs : 52

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Asian Journal of Physics                                                                                               Vol. 17, Nos. 3 & 4 (2008) 533-553


Seismic signatures of solar flares


S. C. Tripathy

National Solar Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA

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The detection of helioseismic signature of flares has opened up new possibilities for comprehending the seismic activity of the Sun and flare dynamics. In this review, I focus on the current state of our knowledge relating solar flares to oscillations of the Sun. 

Total Refs : 70

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