Multi-frequency observations of the outer solar corona with the Gauribidanur radioheliograph. Ph. D. Thesis R. Ramesh [Ph.D Thesis]

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1998

A new radioheliograph dedicated to observations of the outer corona of the
Sun has been built by the Indian Institute of Astrophysics at the Gauribidanur
radio observatory and is in operation since April 1997. This instrument
(hereafter referred to as the Gauribidanur radioheliograph, GRH) is capable
of producing two-dimensional pictures of the solar corona in the frequency
range 40 - 150 MHz (a height range of R:i 0.1 - 0.6 R8 above the photosphere).
At present this is the only instrument of its kind in the world in the
above frequency range for regular observations of the Sun.
The GRH is a T-shaped array with a long E-W arm and a short South
arm. It works on the well known principles of synthesis imaging where the
outputs from the individual antenna groups are taken separately to the receiver
building and fed to a receiver system which measures the spatial coherence
between the received signals. Since the GRH has 32 antenna groups
(16 E-W + 16 South), a 1.024 channel digital receiver is used to correlate the
signals received by the different antenna groups. Included in this thesis is a
description of the receiver system and its various sub-units. The details of
the Walsh switching scheme and its effectiveness in removing the contribution
from unwanted signals are described. The tests carried out to measure
the cross-talk between the input signals to the correlator, DC offset in the
A / D convertors, effects of the res ideal errors in the correlator system on the
measured visibilities in the u-v plane a.re discussed in detail.
One of the biggest challenges in the synthesis imaging of any radio source is
the removal of instrumental propagation errors from the observed quantities.
There are several techniques in use at present which derive the correction
terms frorn the observed quantities themselves. Some of the practical and
technical problems in applying the existing techniques to solar observations with arrays like the GRR and the details of the scheme that we have developed
are given. The successful use of our scheme in practice is illustrated
by comparing the maps obtained with the GRR with those made at other
wavelengths as well as using other techniques. A quantitative estimate of the
possible errors in the present scheme is also carried out.
The estimation of electron density at various heights in the ambient corona
as well as above the active regions is generally done using density models
derived from the white light data obtained during solar eclipses and using
coronographs. From radio observations, it is possible to calculate the altitude
of the emitting regions and the density, in an independant way, if the source
under observation shows regular movement across the solar disk. Using the
one-dimensional observations carried out with the E-W arm of the GRR at
51 and 77 MHz, the altitudes of the plasma levels at these frequencies are
derived.
Two-dimensional maps obtained with the GRR on quiet days when no
non-thermal emission (burst activity) was seen in our records, were used
to study the thermal emission from the undisturbed corona. The effects of
coronal streamers, holes, and scattering by small-scale density in homogeneities
in the corona OIl the observed quiet Sun emission and the low values of
the observed brightness temperature are discussed.
An understanding of coronal mass ejections (CMEs) and its origin is one
of active areas of research in the field of solar physics. An attempt has been
made to identify the pre-event signatures of the CMEs from the radio data
observed with the GRIT. In one case study, the mass of the coronal material
that was associated with a halo CME has been estimated from the global
restructuring of the corona in the aftermath of the event.