- Tmt India
International Heliophysical Year (IHY) is a United Nations program spread over the years 2007 and 2008. The thrust area is to study the Sun, Earth's magnetosphere and the interplanetary medium as a coupled system. One of the major components of IHY is the Public Outreach Education Program through which the knowledge on Heliospheric Physics would be disseminated to the general public and more importantly, to inspire the next generation of space scientists who are still in schools and universities. Under this aegis, the Director, Indian Institute of Astrophysics, has initiated several activities to popularize Solar Physics. Recently on the National Science day (28-2-07), several schools and college students visited IIA. The following simple exhibit became an instant hit with them.
A two-element radio interferometer for observations of Sun and other strong sidereal radio sources
As a part of the International Heliophysical Year (IHY) and the institute’s Public Outreach Programme (POP), it is proposed to provide hands-on astronomical observing experience to interested science and engineering graduate student community in the country by donating radio antenna and receiver system to their institutions. The students will be trained to: (i) carry out observations of radio emission from Sun and other strong cosmic radio sources with the above set up, and (ii) develop software for deriving quantitative information from the data acquired.
A prototype system has been designed and is presently in operation at the Gauribidanur radio observatory. The radio frequency (R.F.) signal reception setup is a simple radio interferometer. It consists of two half-wave dipole antennas (tuned to receive R.F signal at 170 MHz) separated by ~ 10 m. The R.F. signal incident on the antennas are amplified independently in a broad band amplifier (Gain, G ~ 28 dB) and then transmitted to the receiver room via coaxial cables of length about 50 m. The signal attenuation in the cable is about 10 dB at 170 MHz. In the receiver room, the signal from each antenna is first passed through a band pass filter of center frequency (fc) = 170 MHz and bandwidth (∆f) = 6 MHz, to minimize the contribution from interfering signals at other frequencies. The insertion loss in the filter is about 3 dB at 170 MHz. The filtered signal is then mixed with a local oscillator (L.O.) signal of frequency 180.7 MHz for down converting the R.F. signal to an intermediate frequency (I.F.) of 10.7 MHz, which facilitates further processing. The output of the mixer is amplified by about 28 dB (to ensure the minimum peak-to-peak level of ~ 100 mV for the input signal to the correlator) and passed through a band pass filter with fc = 10.7 MHz and ∆f = 1 MHz, again to minimize the contribution from spurious signals at other frequencies. The combined loss in the mixer and filter is about 8 dB. The output of the filter is finally fed to the correlator. The latter has also provision for integrating the correlated data before passing it on to the computer for storage.
The following figures show the block diagram of the antenna + receiver system and observations of the Sun with the prototype system on August 7, 2007. In addition to the regular interference fringes due to emission from the background ‘undisturbed’ solar corona, one can also notice intense radio burst emission (around 12 IST) associated with transient activity on the Sun.
1. Antenna (Half-wave dipole tuned to 170 MHz)
2. R.F. amplifier (Gain = 28 dB)
3. R.F. band pass filter (fc = 170 MHz; ∆f = 6 MHz)
4. Mixer (for down converting the R.F. signal)
5. I.F. amplifier (Gain = 28 dB)
6. I.F. band pass filter (fc = 10.7 MHz; ∆f = 1 MHz)
7. Digital correlator
8. Data acquisition system + computer
9. Local oscillator (180.7 MHz, Amplitude = 7 dBm)
10. R.F.coaxial cable
A Spectrum is a distribution of a physical quantity like the light intensity with respect to another physical quantity like the wavelength. A familiar example is a rainbow of colors, dispersion of light by a prism etc. In these examples we find a continuous spectrum, as the color is spread over a wide range of wavelengths. If we observe the ordinary tube light in our houses but restrict the entry of light by placing a slit before the dispersing optical element (a prism or a grating), we can observe bright lines superimposed on the continuous spectrum that is characteristic of the source of light. The first solar spectrum was observed by Fraunhofer and was named after him. It consists of dark lines superimposed on a bright continuum of colors. So the dark lines correspond to the elements present in the atmosphere of the Sun. The dark lines arise due to the relatively cool gas in the solar atmosphere absorbing the light that comes from a greater depth inside the Sun.
|A - (band)||O2||7594-7621|
|B - (band)||O2||6867 - 6884|
|a - (band)||O2||6276 - 6287|
|D - 1, 2||Na||5896 & 5890|
|b - 1, 2||Mg||5184 & 5173|
|G||Fe & Ca||4308|
Since the spectral lines are the fingerprints of the elements present, one can see from the figure and the table that Hydrogen, Calcium, Iron, Oxygen, Sodium and magnesium are some of the elements present in the Sun. The spectrum reveals other information like the temperature, the pressure and the velocity of the gases.
Simple spectroscopes can be constructed to view the spectrum of the Sun. One can find numerous designs in the published literature and in the internet. One such design was used to make a simple box type spectroscope in IIA for educating the school students who visited IIA on the Science Day. A diagram showing the complete construction of the box type spectroscope is shown in the figure.
The box can be constructed out of a thick card board or an aluminium sheet. One slit (width nearly 0.2-0.5mm) is made in one of the faces of the box through which the light is sent. It could be either sunlight or fluorescent light or light from a sodium lamp. Inside the box is a commercially available CD. The light gets diffracted by the CD (the CD acts as a reflection grating) which is kept at an angle of nearly 60 degrees and the spectrum can be viewed through the other view port opening on another side of the box. This is one of those do-it-yourself science experiments which anyone can enjoy doing. Sources like common tube lights in the house or a sodium lamp in the street give bright lines at specific locations superimposed on a continuous background of colors.