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07603nam a2200265Ia 4500 |
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IN-BaIIA |
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20211108141601.0 |
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211028s9999 xx 000 0 eng d |
| 040 ## - CATALOGING SOURCE |
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| Transcribing agency |
IIA Library |
| 080 ## - UNIVERSAL DECIMAL CLASSIFICATION NUMBER |
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| Universal Decimal Classification number |
043:52 |
| Item number |
THE |
| 100 ## - MAIN ENTRY--PERSONAL NAME |
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| Personal name |
Thejappa, G. |
| 9 (RLIN) |
15973 |
| Relator term |
Author |
| 245 #4 - TITLE STATEMENT |
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| Title |
The radio bursts from the outer corona |
| Statement of responsibility, etc. |
G. Thejappa |
| Medium |
[Ph.D Thesis] |
| 260 ## - PUBLICATION, DISTRIBUTION, ETC. |
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| Place of publication, distribution, etc. |
Bangalore |
| Name of publisher, distributor, etc. |
Indian Institute of Astrophysics |
| Date of publication, distribution, etc. |
1988 |
| 300 ## - PHYSICAL DESCRIPTION |
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| Extent |
178p. |
| 502 ## - DISSERTATION NOTE |
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| Degree type |
Doctor of Philosophy |
| Name of granting institution |
Indian Institute of Astrophysics, Bangalore |
| Year degree granted |
1987 |
| 520 ## - SUMMARY, ETC. |
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| Summary, etc. |
Over the whole electromagnetic spectrum of the Sun<br/>the metre wavelength band (I to 10m) is unique. Shorter wavelengths,<br/>from '"( -rays to microwaves, come mostly from regions<br/>containing dense matter associated with the Visible Sun as we<br/>know it - the Photosphere and Chromosphere - and longer wavelengths<br/>come mainly from interplanetary space. Metre waves<br/>alone are generated in the tenuous Plasma known as the solar<br/>corona (ordinarily visible only at the time of a total eclipse),<br/>and they reveal a spectacular range of phenomena undreamt<br/>of before their discovery. It is here many types of Solar radio<br/>bursts are generated. Generally these radio bursts are excited<br/>by two types of disturbances: (l) Electron beams and (2) Collisionless<br/>shock waves. The Physics of the beam - plasma system<br/>is more or less understood both theoretically as well as experimentally,<br/>whereas the detailed study of the collisionless shock waves<br/>has been started recently by insitu experiments and subsequent<br/>theoretical advances. We study theoretically the acceleration<br/>of electrons, excitation of low as well as high frequency turbulence<br/>in the shock fronts and the subsequent radio emission processes.<br/>We apply this analysis to various burst phenomena such as Type<br/>I, Type II, absorption bursts, etc.<br/>In the first chapter, we have broadly indicated the difference<br/>between inner and outer corona and classified the types of disturbances<br/>that perturb the corona into two classes: (1) electron beams and (2) collisionless shock waves. The interrelation between<br/>these two types of disturbances is also discussed. We then<br/>give a preview of the results presented in the thesis.<br/>In the second chapter, we study the type I burst phenomena.<br/>Assuming that weak shocks driven by newly emerging magnetic<br/>flux are responsible for type I radio bursts, we derive the dispersion<br/>relation for ion sound waves generated in a collisionless shock<br/>propagating perpendicular to the magnetic field. Usirg quasilinear<br/>analysis the energy density of the ion-sound turbulence is estimated<br/>and compared with the lower hybrid turbulence generated under<br/>similar conditions. We finally show that ion-sound turbuleoce<br/>is a better candidate for the generation of type I radio bursts<br/>in the solar corona. Since type I chains are the most direct<br/>evidence for the weak shocks responsible for their generation,<br/>measurements of the frequency drift rate and the bandwidth<br/>of these chains, can be used to estimate the upstream velocity<br/>and the density jump across the shock. The Alfven velocity<br/>and hence the coronal magnetic field can be calculated by substituting<br/>the above values in the modified Rankine - Hugoniot relation.<br/>Therefore we devise a method to estimate the radial dependeoce<br/>of the coronal magnetic field above mild active regions using<br/>the Type I chain data. These results are compared with the<br/>existing estimates and found to be in good agreement.<br/>In chapter III, we describe the Radio telescope which<br/>was used to study the fine structure of the solar radio emission at decameter wavele~ths. The main fine structures observed<br/>using the above telescope are : (1) the peculiar time profiles<br/>of type III storm bursts and (2) absorption bursts. It has been<br/>shown that the peculiar time structures in type III profiles are<br/>not due to random superposition of bursts with varyi~ amplitudes.<br/>It has also been shown that they are not manifestations of fundamental<br/>- harmonic pairs. Some of these profiles can be due<br/>to superposition of bursts caused by ordered electron beams<br/>ejected with a constant time delay at the base of the corona.<br/>Regardi~ the absorption bursts, it has been shown that the ionsound<br/>turbulence, generated by a laterally moving shock wave<br/>can act as an absorber of the decametric continuum radiation<br/>travelling radially outward, converting it into longitudinal Langmuir<br/>turbulence by three wave interactions. The duration of the absorption,<br/>the depth and the bandwidth are explained selfconsistently<br/>in this model.<br/>The most enigma tic type of decametric radio bursts are<br/>the drift pair bursts. The data on drift pair bursts obtained<br/>using the swept frequency spectrograph at Nancay, France, have<br/>been analysed in chapter IV. We have detected for the first<br/>time features like drfit pair chains and vertical drift pair bursts.<br/>The drift pairs and their associated phenomena like chains and<br/>vertical bursts can be interpreted if one assumes that the double<br/>plasma resonance layer, where the radiation is proposed to<br/>be generated is different at different instants of time so that<br/>one gets a slope in the frequency time plane. If one assumes considerable fluctuations in some microscopic parameters<br/>such as density and magnetic field, it is possible to have drift<br/>of all types. A steep variation in the magnetic field is derived<br/>assuming that the density is not affected by DP activity, in<br/>the case of vertical DPs.<br/>In chapter V, it is proposed that the majority of shock<br/>waves responsible for the generation of type II radio bursts<br/>are supercritical. It is also proposed that the reflected ions<br/>behave like a beam in the foot and the ramp and like a ring<br/>in the downstream, i.e., just behind the overshoot. These are<br/>described by drifted Maxwellian and Dory-Guest-Harris distributions<br/>respectively. The ion beams are unstable and can drive the<br/>low frequency waves, whose frequency lies between the electron<br/>and ion cyclotron frequencies. These waves are absorbed by<br/>the ambient electrons, leading to the formation of electron<br/>"tails", in upstream as well as in downstream. On entering<br/>the cold background these hot electrons, in turn, drive the high<br/>frequency Langmuir oscillations to high level energy densities<br/>10-5 - 10-4 in the upstream as well as in the downstream.<br/>The conversion of plasma waves into electromagnetic waves<br/>is caused by the induced scattering of plasma waves off ions<br/>or by merging of two Langmuir waves. The brightness temperatures<br/>in the lower and upper bands depend on the number densities<br/>in the accelera ted beams. Since the number density of<br/>the electron beams in the downstream is less than that of upstream,<br/>the U band iis fainter than L band as experimentally observed thus explaining naturally the band splitting in Type II bursts<br/>and the difference in brightness of the two bands. The role<br/>of nonlinear processes is also studied.<br/>In chapter VI we briefly summarize the main conclusions<br/>of the thesis. We also briefly mention the importance of our<br/>results. The future observations and theoretical work to be<br/>done in these lines are also suggested. |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Astronomy and Astrophysics Thesaurus |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Ph.D Thesis |
| 9 (RLIN) |
1344 |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Radio Astronomy |
| 9 (RLIN) |
2849 |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Radio Bursts |
| 9 (RLIN) |
7726 |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Solar Corona |
| 700 ## - ADDED ENTRY--PERSONAL NAME |
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| Personal name |
Ch. V. Sastry and M. N. Anandaram |
| Relator term |
Supervisor |
| 9 (RLIN) |
48847 |
| 856 ## - ELECTRONIC LOCATION AND ACCESS |
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| Uniform Resource Identifier |
<a href="http://prints.iiap.res.in/handle/2248/95">http://prints.iiap.res.in/handle/2248/95</a> |
| Link text |
Click Here to Access eThesis |
| 942 ## - ADDED ENTRY ELEMENTS (KOHA) |
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| Koha item type |
Thesis & Dissertations |
| Source of classification or shelving scheme |
Universal Decimal Classification |
