The radio bursts from the outer corona [Ph.D Thesis]
G. Thejappa
- Bangalore Indian Institute of Astrophysics 1988
- 178p.
Over the whole electromagnetic spectrum of the Sun the metre wavelength band (I to 10m) is unique. Shorter wavelengths, from '"( -rays to microwaves, come mostly from regions containing dense matter associated with the Visible Sun as we know it - the Photosphere and Chromosphere - and longer wavelengths come mainly from interplanetary space. Metre waves alone are generated in the tenuous Plasma known as the solar corona (ordinarily visible only at the time of a total eclipse), and they reveal a spectacular range of phenomena undreamt of before their discovery. It is here many types of Solar radio bursts are generated. Generally these radio bursts are excited by two types of disturbances: (l) Electron beams and (2) Collisionless shock waves. The Physics of the beam - plasma system is more or less understood both theoretically as well as experimentally, whereas the detailed study of the collisionless shock waves has been started recently by insitu experiments and subsequent theoretical advances. We study theoretically the acceleration of electrons, excitation of low as well as high frequency turbulence in the shock fronts and the subsequent radio emission processes. We apply this analysis to various burst phenomena such as Type I, Type II, absorption bursts, etc. In the first chapter, we have broadly indicated the difference between inner and outer corona and classified the types of disturbances that perturb the corona into two classes: (1) electron beams and (2) collisionless shock waves. The interrelation between these two types of disturbances is also discussed. We then give a preview of the results presented in the thesis. In the second chapter, we study the type I burst phenomena. Assuming that weak shocks driven by newly emerging magnetic flux are responsible for type I radio bursts, we derive the dispersion relation for ion sound waves generated in a collisionless shock propagating perpendicular to the magnetic field. Usirg quasilinear analysis the energy density of the ion-sound turbulence is estimated and compared with the lower hybrid turbulence generated under similar conditions. We finally show that ion-sound turbuleoce is a better candidate for the generation of type I radio bursts in the solar corona. Since type I chains are the most direct evidence for the weak shocks responsible for their generation, measurements of the frequency drift rate and the bandwidth of these chains, can be used to estimate the upstream velocity and the density jump across the shock. The Alfven velocity and hence the coronal magnetic field can be calculated by substituting the above values in the modified Rankine - Hugoniot relation. Therefore we devise a method to estimate the radial dependeoce of the coronal magnetic field above mild active regions using the Type I chain data. These results are compared with the existing estimates and found to be in good agreement. In chapter III, we describe the Radio telescope which was used to study the fine structure of the solar radio emission at decameter wavele~ths. The main fine structures observed using the above telescope are : (1) the peculiar time profiles of type III storm bursts and (2) absorption bursts. It has been shown that the peculiar time structures in type III profiles are not due to random superposition of bursts with varyi~ amplitudes. It has also been shown that they are not manifestations of fundamental - harmonic pairs. Some of these profiles can be due to superposition of bursts caused by ordered electron beams ejected with a constant time delay at the base of the corona. Regardi~ the absorption bursts, it has been shown that the ionsound turbulence, generated by a laterally moving shock wave can act as an absorber of the decametric continuum radiation travelling radially outward, converting it into longitudinal Langmuir turbulence by three wave interactions. The duration of the absorption, the depth and the bandwidth are explained selfconsistently in this model. The most enigma tic type of decametric radio bursts are the drift pair bursts. The data on drift pair bursts obtained using the swept frequency spectrograph at Nancay, France, have been analysed in chapter IV. We have detected for the first time features like drfit pair chains and vertical drift pair bursts. The drift pairs and their associated phenomena like chains and vertical bursts can be interpreted if one assumes that the double plasma resonance layer, where the radiation is proposed to be generated is different at different instants of time so that one gets a slope in the frequency time plane. If one assumes considerable fluctuations in some microscopic parameters such as density and magnetic field, it is possible to have drift of all types. A steep variation in the magnetic field is derived assuming that the density is not affected by DP activity, in the case of vertical DPs. In chapter V, it is proposed that the majority of shock waves responsible for the generation of type II radio bursts are supercritical. It is also proposed that the reflected ions behave like a beam in the foot and the ramp and like a ring in the downstream, i.e., just behind the overshoot. These are described by drifted Maxwellian and Dory-Guest-Harris distributions respectively. The ion beams are unstable and can drive the low frequency waves, whose frequency lies between the electron and ion cyclotron frequencies. These waves are absorbed by the ambient electrons, leading to the formation of electron "tails", in upstream as well as in downstream. On entering the cold background these hot electrons, in turn, drive the high frequency Langmuir oscillations to high level energy densities 10-5 - 10-4 in the upstream as well as in the downstream. The conversion of plasma waves into electromagnetic waves is caused by the induced scattering of plasma waves off ions or by merging of two Langmuir waves. The brightness temperatures in the lower and upper bands depend on the number densities in the accelera ted beams. Since the number density of the electron beams in the downstream is less than that of upstream, the U band iis fainter than L band as experimentally observed thus explaining naturally the band splitting in Type II bursts and the difference in brightness of the two bands. The role of nonlinear processes is also studied. In chapter VI we briefly summarize the main conclusions of the thesis. We also briefly mention the importance of our results. The future observations and theoretical work to be done in these lines are also suggested.
Astronomy and Astrophysics Thesaurus Ph.D Thesis Radio Astronomy Radio Bursts Solar Corona