Highlights of Solar Radio Physics 5:
High dynamic range solar radio images
by combining visibilities from
the Giant Meterwave Radio Telescope and the Nancay Radioheliograph
We present first results from an ongoing program of combining data from the
NRH and the
Giant Meterwave Radio
Telescope GMRT, to produce snapshot images of the Sun at
The data processing includes a superposition of complex visibilities from
NRH and GMRT, a Fourier Transform and a cleaning multi-scale
algorithm. We show results from a simulation and an observation of a complex
noise storm at 327 MHz on August 27, 2002. This illustrates the capacity of
the method to produce high dynamic range snapshot images of a complex Sun,
and shows that composite images are by far better than images from either
gives direct access to points in the Fourier Transform plane
plane) of the brightness distribution. The
coverage is related to the set of available baselines in the antenna array.
Images are subsequenty obtained through a Fourier Transform, usually with a
Imaging variable coronal radio sources is difficult. Rotational synthesis
imaging is excluded, and the uv coverage is usually poor. This reduces
the dynamic range in the images. Now there are several instances in physics
of solar corona where both high resolution and high dynamic range are
essential : structure of non-thermal radio bursts, CMEs, propagation of
radio waves in the turbulent corona, etc.
We combine here data from the 576 baselines (up to 3 km) of NRH and data
from the 435 baselines of the GMRT (up to 27 km), to obtain solar radio
images with unprecedented field, resolution and fidelity.
The composite instrument
uv coverages from NRH and GMRT are complementary.
NRH provides a dense and homogeneous uv coverage near the
origin of the uv plane (up to about 1000 λ by regular steps
of 50 λ at 327 MHz, giving a good description of large scale structures
in a field of one degree, but with a low resolution of 3 arc minutes.
GMRT uv coverage is less dense for small baselines,
which leads to an "aliasing" of large structures, but extends to baselines
of 20000 λ, allowing a resolution down to 20 arc seconds. The
principle is to combine NRH and GMRT uv
coverages, after a suitable intercalibration of both instruments (which
can be complex). The figure on the right gives an example of the resulting
uv coverage. The axes of the image are graded in multiples of the
Results from simulations
For investigating the possibilities of the composite instrument, we made
simulations with several models of the Sun. One model is presented here
in the image on the left. The uv coverage of GMRT is
sparse, especially for large baselines. This produces artefacts on the
image obtained directly by fft, rendering necessary the use of a cleaning
procedure. The standard
procedure performs poorly when sources exhibit a variety of scales
(Wakker and Schwarz, 1991) and we used here a specific multi-scale
CLEAN version. Futhermore, for a complex source as shown here, the cleaning
procedure itself produces artefacts and GMRT baselines beyond
~10000 λ must be excluded. The obtained composite image is shown
on the right. The various spatial scales are properly rendered and the
resolution is better than with NRH alone by a factor ~3.
Results from observations
On August 27, 2002, there was a strong noise storm beyond the West limb and two
weaker noise storms near the center of the disk. A group of type III bursts
occurred near the weak noise storms at 09:04:04 UT. The images below show
measurements at 327 MHz. The larger sizes of the type III bursts near the
center of the disk are obvious. The size of the intense West limb storm is
~50 arc seconds. This is comparable to the sizes reported by
Zlobec et al.
(Solar Phys. 1992, 141, 165-180) from VLA observations, but the dynamic
range on the images here (maximum/rms artefacts) is far better (~300 instead
We conclude that combining data from NRH and GMRT is
possible. Composite images have unprecedented resolution and dynamic range.
This opens a wide field of studies on solar coronal phenomena.
details, see Mercier, C., Subramanian, P., Kerdraon, A., Pick, M.,
Ananthakrishnan, S., and Janardhan, P., 2006, Astronomy and Astrophysics, 447,
December 7, 2006
Claude Mercier (),
with thanks to S. Pohjolainen
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