The radio bursts from the outer corona G. Thejappa [Ph.D Thesis]

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1987

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.