In the 1970s, the Raman Research Institute (RRI) in collaboration with Indian Institute of Astrophysics (IIA) decided to set up a radio telescope at Gauribidanur, a rural area about 100 km from Bangalore (77.5º 26’07” East longitude and 13.5º 36’12” North latitude) mostly free from urban electronic noise. The T-shaped telescope consist of two arrays of 1000 dipoles, one 1.38 km long along the east-west direction and the other 450 m long in the south direction from the centre of the E-W array. The maximum effective collecting area is about 20,000 square metres.
The instrument was used to study the radio emission from the centre of our galaxy and other radio sources including the Sun, at the frequency of 34.5 MHz. Imaging the radio sky at low frequencies (in decametric wavelengths) is a relatively unexplored region of the electromagnetic spectrum. The array has been used for studies of supernova remnants, ionized hydrogen (when the electron in the hydrogen atom is removed near hot stars) regions in outer space and pulsars. Over 30 pulsars have been studied and observations are continued.
Location of the Observatory
In the late 1990s, IIA scientists set up an array exclusively to study the radio emission from the undisturbed Sun as well as solar bursts at the Gauribidanur radio observatory. This radio telescope, called Gauribidanur Radio heliograph, is a T-shaped array with dimensions of 1.28 km in the East-West direction and 0.45 km in the South direction. The resolution of this telescope is about 5 arc minutes. This is similar to the angle subtended by a football at a distance of about 200 meters. One can make radio pictures of the Sun using this radio telescope and the pictures will be as if taken with a 100 pixel camera on the Sun. The radio heliograph—an array of 192 dipoles—has been in operation since 1997.
When we look at the Sun through a safe eyeglass, we are actually seeing its optical form. During a total solar eclipse we can see the corona, the outer area of the Sun, visible as the Moon hides the optical Sun. The corona has 10 times the radius of the Sun. One need not wait for the eclipse to study the corona using radio waves. In fact, the region which we can all the radio Sun is much larger than the optical image. The size of the visible surface of the Sun can be given as a measurement of the angle subtended by it as seen from the Earth, some 150 million km away. It is calculated that this size works out to 32 arc minutes. But the size of the radio sun is 60 to 80 arc minutes at wavelengths of tens of meters.
Observations of the radio Sun have yielded several interesting features. It is known that the temperature of the corona is about a million degrees Kelvin, even though for unexplained reasons, the temperature down below in the chromosphere is lower. It has been further observed that the electron density surrounding the Sun decreases with distance from the surface and the electron density is related to the frequency of the radio waves emanating from the Sun. Thus higher frequencies (e.g. 100 MHz) would correspond to lower regions of the Sun, the base of the corona, while the lower frequencies (e.g. 30 MHz) would come out from areas of less electron density farther away from the surface. Thus different frequencies correspond to different heights in the corona. Thus by studying the radio waves from the Sun at many radio frequencies, we can determine the characteristics of the Sun like temperature, density etc at different heights.
Everyday, all though the year, the electronics in the radio heliograph is switched on to observe the Sun for six hours from 9.30 a.m to 3.30 p.m. IST. The arrays can be electronically adjusted to change the angle of observation. Four frequencies are chosen from those allotted for radio astronomy in the radio band of 40 to 150 MHz and observations are made sequentially. The array is called meridian transit instrument with limited tracking capability. In a meridian transit, a radio source crosses the meridian circle—an imaginary semicircle joining North and South poles. Observations at the meridian are important, since the effects of ionosphere like refraction on the signals are minimal (when the source is above one’s head).
The band of frequencies corresponds to about 1.3 to 0.8 million km above the visible surface of the Sun. One solar surface is about 700,000 km from the centre of the Sun. The observing frequencies are usually set in interference-free bands, monitored over long periods. The frequencies chosen are unique, as they given information on the solar corona at the height of 0.2-0.8 R above the solar surface. It is difficult to probe this area by ground or space-based coronagraphs.
While observing the Sun, the array is calibrated with a chosen star as the standard candle. The time interval between two successive passages of a star called a sidereal day, is shorter than a solar day of 24 hours by about 4 minutes. Observers go by a sidereal clock for positioning their telescopes towards a particular star.
As the radio waves from the Sun changes its intensity the spectrum that results from such observation is dynamic. It can show a steady curve for quite sometime but can change in a matter of seconds, depending on the solar activity. The array is in fact geared to detect these changes, which at times indicate solar flares and coronal mass ejections that typically blow out a billion tones of the solar atmosphere into space at am incredible speed of 1 to 7 million km an hour!
While the events on the Sun can be recorded at radio wavelengths after about 8 minutes (the time the electromagnetic waves travel from the Sun to the Earth at the speed of light), the actual arrival of the charged particles in the vicinity of the Earth may take one or two days depending on the speed of such ejections from the Sun. Thus there is time for warning those that are likely to be affected: satellite operators, astronauts and even regulators of power grids. For example, prior to a CME event of 23 October 1997, an increase in the radio intensity near the Sun was observed at Gauribidanur. Such enhancements in the radio band at meter wavelengths can be used as precursors of CMEs.
As a place south of India nearer the equator would be a better location to view the centre of our galaxy in radio waves, a helical antenna was constructed on the island of Mauritius. It is a joint project of IIA, RRI and the University of Mauritius. Inaugurated in 1992, it works on 150 MHz.
While the solar wind, the steady stream of charged particles from the Sun is deflected by the Earth’s magnetic field, sometimes they go through the polar regions to enter the Earth’s atmosphere and cause curtains of light called aurora. But the coronal mass ejections are much more powerful and are able to penetrate the magnetic shield and can harm satellites and astronauts.
The Gauribidanur array, the only one of its kind in the world for solar radio observations at low frequencies, would complement the space probes including NASA’s STEREO which has recently given the first 3D images of the Sun. in these days of multi-wavelength observation of celestial objects including the Sun, the observations based on low radio frequencies, not easily possible in the electronically dense countries, has a special role. It is proposed to put the result of the observations online soon so that any interested person or institution can pick it up.
With a view to increasing the resolution of the array, it has been decided to increase its size to 3 km x 3 km and get one arc minute resolution. It is proposed to increase it further to 10 km and obtain 5 seconds of arc in another ten years. IIA has also developed an east-west one-dimensional solar radio polarimeter for measuring the strength of the magnetic field in the corona at about 0.2 to 0.8 R (0.8 to 1.3 million km above the visible surface to the Sun). This is similar to the height range covered by the radio heliograph.
A radio spectrograph receiver (50-1000 MHz) is in operation at the Gauribidanur radio observatory under the UN Basic Science-initiated program in collaboration with the Institute of Astronomy in Switzerland. This instrument is used to study the radio signatures of solar flares.
India is assisting Brazil in the establishment of a unique radio telescope complex, the only one of its kind in South America. The facility will consist of 38 parabolic dishes of 4 m in diameter, each with a maximum baseline of 2 km. It is a decimeter (equal to 0.1 m) radio telescope for observation of the Sun and various galactic and extragalactic radio sources. To start with, the complex has five parabolic dish antennas, each of 4 m diameter in the east-west direction. In the second phase there will be 38 dishes, for which IIA will construct the back-end systems. The solar radio group of IIA has contributed to the configuration of the array, software for data calibration and image synthesis.
There has been plenty of science based on the apparently simple dipoles. The speed of the coronal mass ejection from the Sun is one such fascinating topic under study. Another uses the radio bursts as tracers for the seismology of the corona. These are currently hot topics in understanding the space weather. In an interesting joint effort, the arrays at Gauribidanur, Ootacamund and GMRT near Pune have simultaneously observed transient phenomena like emission from pulsars at different frequencies.