A Digital Auto Correlation Spectrometer For Solar Burst Obsevations At Decameter Wavelengths
Chellasamy, E Ebenezer
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This thesis reports the design, construction, and the initial results obtained from a digital spectrograph for observations of radio burst emission from the Sun. One of the distinct advantages of radio spectral studies of the Sun during events such as solar flares, coronal mass ejections (CMEs), etc. is that it gives a straight forward estimate of the speed of the outflowing material into the outer atmosphere of the Sun and subsequently the interplanetary medium. It is well known that in the solar atmosphere, the electron density and consequently the plasma frequency gradually decreases with increasing distance from the center. Therefore in a time-frequency plane, the intensity of the associated radio emission should generally drift from high to low frequencies with time. From this, and from a knowledge of the height of the successive plasma levels in the solar atmosphere, one can deduce the speed of the outward moving disturbance. In this respect, a study of the radio bursts at decameter wavelengths is important since such radiation originates from the corona at heights ≥1.3 R⊙ (1 R⊙ = 6.96 x 10° km = radius of the Sun) from the center of the Sun, which are otherwise accessible only through the use of white light coronagraphs atop high altitude mountains and onboard space vehicles. The primary units of the instrument are (i) zero-crossing detector (ZCD), (ii) sampler and (iii) correlator. The function of the ZCD is to digitize the input signal waveform. In the present case, we use a one-bit quantizer, i.e., its output is either a '1' or '0' depending on whether the input is above or below the zero level, respectively. The digitized signals are then sampled at the Nyquist rate using the sampler. The output of the sampler is then passed through a set of shift registers, and finally fed to the correlator. The latter measures the correlation between the signals at its input as a function of the delay (introduced by the shift registers) between them. After successful completion of several static/dynamic tests in the laboratory, the system was moved to the Gauribidanur radio observatory (Latitude: 13°36’12”; Longitude:77°27’07”), and is presently used along with the existing radiohe-liograph to derive both the spectral and spatial information of the various radio emitting transient sources on the Sun, respectively. The R..F. signal (40-150 MHz) from one of the antenna groups of the ra-dioheliograph forms the input to the spectrograph. The signal from the field goes through a series of amplification and mixing operations to bring it down to an I.F. of 10 MHz with a bandwidth of 1 MHz, and then fed to the spectrograph. The present frequency range of the spectrograph is 40 MHz, and the data corresponding to the above band is obtained in steps of 1 MHz by switching the local oscillator through a GPIB interface, after each integration period. Each 1 MHz data is then Fourier tranformed to get its corresponding power spectrum. Successive data sets are then arranged in order according to the frequency of the local oscillator signal to get the spectrum corresponding to the entire 40 MHz band. The initial results obtained with our instrument are also presented.