Prototype tracking system for observing the Sun at decimeter and meter wavelengths

Abstract

The outer atmosphere of the Sun is observed to be a very hot plasma medium. Since the frequency of electromagnetic wave emission from a plasma medium depends on the electron number density, high frequency radio waves originate from the denser region as compared to low radio frequencies. The electron density as determined from white-light and radio observations of the solar corona clearly indicates that it gradually decreases from the inner to the outer corona and high frequency radio waves originate from the inner corona whereas the low frequencies emanate from the outer regions. Multi-frequency observations of various coronal radio transients show different types of events originate at different heights (or altitudes) in the corona. So, based on the height of emission, events are classified into centimeter, decimeter, meter type, etc. Although many belong to the aforementioned individual classes, some of them extend from centimeter to meter, and so on. But, present understanding of the latter and the intricate connections between different events that take place in the centimeter, decimeter wave band and those observed at meter wavelength band is very limited because it was not studied extensively. Therefore, observing the integrated radio flux output from the solar corona at many wavelengths simultaneously in the decimeter - meter wave band gives insights on the origin, evolution of different kinds of bursts and their association with other forms of solar activities. The Radio field station of Indian Institute of Astrophysics located at Gauribidanur carries out astronomical observations at low radio frequencies. The log periodic dipole antennas are the primary receiving elements. These are stationary and mounted vertically such that the maximum reception is towards zenith. To observe the sources at different declinations, the maximum response of the antenna is tilted electronically by means of DSBs (Delay Shifter Boards). But since the hour angle of a celestial source changes continuously from the raising time to setting time the use of DSBs would be very limited. This limits the observation duration and the gain of the system also does not remain constant throughout an observation. To overcome such problems, a prototype tracking system was proposed. Therefore, the objective of this project work is to develop a prototype tracking antenna cum correlation spectrograph system for observing the Sun in the 200 – 600 MHz frequency range (i.e. 1.5m to 0.5m wavelength range). The latter range corresponds to a heliocentric distance range of approximately 1.002R0 to 1.25R0, where, R0 is the radius of the solar photosphere. This project report covers the designing of log periodic dipole antennas, configuring commercially available rotors for tracking, designing of analog receiver system which can operate in the 200 – 600 MHz frequency range and an FPGA based digital spectrograph to obtain spectra with high temporal and frequency resolution, etc. Also, the report describes the preliminary attempts to develop an in-house prototype tracking system using off-the shelf miniature motors, gear system, encoders, etc. to study the feasibility of regularizing the tracking mode of observations at the Gauribidanur observatory which can partly supplement the solar radio observations that are being obtained at meter wavelengths regularly. The tracking and control system should be RFI free.