Abstract:
The radio telescopes that are presently in operation specifically to study the Sun are
limited. We have: (1) the Nobeyama radioheliograph in Japan operating at 17 and 34
GHz; (2) the Nan¸cay radioheliograph in France which observes in the frequency band 150-
450 MHz; and (3) the Gauribidanur radioheliograph (30-150 MHz) operated by Indian
Institute of Astrophysics at Gauribidanur Radio Observatory near Bangalore. Since the
electron density (Ne) and hence the plasma frequency (fp) decreases radially outward in
the solar atmosphere, the radio emission at different frequencies originate from different
heights. For example, 30-150 MHz emission is generally emitted over the height range of
≈ 1 − 2 solar radii above the photosphere. The above frequency range is well suited for
observations of enhanced radio emission (both thermal and non-thermal) from the solar
corona. Presently, it is difficult to observe the white light emission from the solar corona
in the above height range even with space coronagraphs because of various practical
difficulties. Radio observations play a useful role to understand the different coronal
phenomenon along with observations from space at different wavelengths. To have a better
understanding, it is essential to have polarization information in addition to imaging and
spectral observations. The existing observations with the Gauribidanur radioheliograph
(GRAPH) and Gauribidanur Low-frequency Solar Spectrograph (GLOSS) are limited
to two-dimensional imaging (Stokes I) and one-dimensional spectral mode (Stokes I),
respectively.
The recently commissioned one-dimensional Gauribidanur Radio Interference Polarimeter
(GRIP) is in operation in both Stokes I and V modes. As part of building the array, I have
been involved in design and fabrication of antennas, analog and digital receiver systems.
I also carried out different test measurements like characterization of antennas, phase
equalization of coaxial cables, radiation pattern measurements, RF network cabling in the field/receiver room and other interferometric measurements. The GRIP consists of
40 frequency independent antennas (log periodic dipoles, LPDs) mounted independently
in 0o and 90o orientation (with respect to the terrestrial north) for the measurement of
polarized radio emission from the solar corona over the frequency range 30-150 MHz.
LPDs were used because of the broad frequency coverage. The observations are limited to
the circularly polarized radio emission (Stokes V) that arises due to propagation effects
and also due to the non-thermal emission mechanisms which occurs frequently in the solar
corona. Any linearly polarized emission (Stokes Q and Stokes U) that is generated at the
source region cannot be observed over the typical observing bandwidths (kHz/MHz) used
in radio astronomy, since the same are depolarized due to Faraday rotation in the solar
corona and Earth’s ionosphere. I was involved in the design/development and fabrication
of the associated front-end analog electronics, back-end digital receiver system, and the
data acquisition/observational/data analysis software.
Weak non-thermal radio activity are often noticed in the solar atmosphere. But their
energy budget and the strength of the associated magnetic field were not established.
Observations with the GRIP indicate that the estimated energy of the frequently observed
type I radio bursts are ≈ 1021 erg. These are the weakest energy releases reported in the
solar atmosphere as on date and they belong to the category of picoflares which is one
of the suggested mechanisms to heat the solar corona. We also estimated the associated
magnetic field and it is in the range ≈ 200 − 1000 mG.
At present, measurements of coronal magnetic fields in white light are not possible due to
various practical difficulties. By making use of indirect radio methods, the magnetic field
strengths of corona is possible to estimate. Using the polarization observation of the group
of type III radio bursts the magnetic field strength of the solar corona was estimated in
different approaches. Firstly, the limb events whose degree of circular polarization (dcp)
is less than 15% was selected. It is known that such events are due to the harmonic
emissions which give the clue to estimate the magnetic field. On the other hand using quasi-periodicity of the mentioned events, the magnetic field was estimated at 1.3 R⊙
(80 MHz). These estimated results are consistent with the existing empirical models and
other reported observations.
Type IVm radio bursts occurs in association with coronal mass ejections (CMEs) and the
source moves in the solar atmosphere with approximate speed of CME. Using GRIP, a
CME associated moving type IV burst was observed at 40 and 80 MHz. A weak circularly
polarized radio emission was observed in both the frequencies. Observationally it was
identified that origin of type IVm burst was due to the gyrosynchrotron emission. Hence
by knowing the dcp and emission mechanism the associated magnetic field strength was
estimated at height of ≈ 1.9 and 2.2 R⊙.
Occultation studies of the Crab nebula by the solar corona was carried out in the polarimetric
mode. We expected to observe circularly polarized radiation, since there could
be possible density inhomogeneities (with associated magnetic field) in the line of sight.
But we did not observe any such emission. It is possible that the polarized signal, if
any were present, was too weak to detect. However the observations lead to scattering
studies at different heights in the lower heliosphere. Observationally it was known that
the flux density of the Crab nebula decreases during its ingress and regains during egress.
The flux density vanished when the Crab nebula was close to solar disk, (in mid June)
≈ 5 R⊙. Based on the occultation observations in years 2011 and 2013, level of turbulence
(C2N
) at different height in the corona was estimated. It was found that the estimates
of C2N
(r) were higher compared to the similar reports from VLBI observations. Using
the estimated C2Nvalues the angular broadening measurements was carried out and the
results are consistent with earlier empirical equations.
We designed and fabricated a cross-polarized log periodic dipole (CLPD) antenna system
to study the circularly polarized radio emission from the solar atmosphere. The performance
study of the CLPD was carried out and found that for commercially available
CLPDs are having the isolation > −20 dB which are not suitable to have a precise polar ization measurements. Our designed CLPD has the isolation as low as < −40 dB. Using a
narrow bandwidth (≈ 65− 85 MHz) CLPD, two element interferometric polarimeter was
constructed and observed the circularly polarized radio emissions from the solar corona.
By designing the broad band CLPD with improved isolation, it can be used in building
the imaging instruments to have the polarized two-dimensional maps of the solar corona.
Also they can be used as feeds for dish antennas.