Design of digital receivers for low frequency radio astronomy

Abstract

Low frequency radio observations of the solar corona provide valuable information on the heliocentric distance range ≈ 1.1R - 3R , which is relatively difficult to observe from other wavelengths. These observations are crucial as it is believed that events of importance from the point of view of solar-terrestrial relations originate from the above height range. These events are transient in nature and are accompanied by a sudden

non-thermal energy release (bursts) at radio frequencies. Due to their non- thermal nature, these bursts are more intense at low frequencies and thus

can be observed with a better contrast. A primary emission mechanism for these bursts are plasma emission, which is created due to acceleration of electrons in the ambient coronal plasma by shocks associated with transient emission. The nature of the shock, the fluctuations in electron density and other plasma properties can be inferred by studying the temporal, spectral, and spatial characteristics of the radio bursts. This thesis describes design and development of new instrumentation for performing high resolution observations of the solar corona at low frequencies (≤ 400 MHz). To begin with, a Low frequency Long-baseline Interferometer (LBI) experiment was conceived to study small scale angular structures (< 1 0 )

and the role of scattering induced due to density turbulence in the so- lar corona. The antennas, analog, and Field Programmable Gate Array

(FPGA) based digital system were developed in house. Using this sys- tems, observations were carried out at a frequency of 37 MHz with a

band-width of ≈ 2 MHz. In LBI observations, it is not always possible to have a real-time link between the individual receiver stations. This calls for external synchronization between both the stations, which was achieved

using Commercially-Off-The-Shelf (COTS) Global Positioning System Dis- ciplined Oscillator (GPSDO). The complete LBI system design and charac- terization is described.

The aforementioned system was used for observations in two main cam- paigns. First, over an 8 km baseline, with an angular resolution of ≈ 1

0 at 37

MHz, Solar Noise Storms and type III bursts, frequently associated active- regions were observed. These observations, for the first time, showed that

structures as small as 10

in the solar corona, whence radio emission at 37

MHz typically originates. Further, these structures were found to be asso- ciated with weak energy releases (≈ 1018±2

ergs 1). This gave us confidence to repeat the same experiment over a longer baseline during the second

campaign. For this, a group of the Gauribidanur Radio Interferometric Po- larimeter (GRIP) and the Indian Mesospheric-Stratospheric-Tropospheric

(MST) Radar were used as the “arms” of the interferometer. The effective resolution of the interferometer was ≈ 3

00. In this observation campaign, correlated emission of a type III radio burst with frequency fine structure was observed. Analysis indicated that the source region of this burst at 53 MHz was ≈ 1000 .

A FPGA based spectrometer was developed to work in the frequency range of 40 - 80 MHz, with high spectral and temporal resolutions (22 kHz and 5 ms). The objective of developing this instrument was to study fine

structures in time and frequency associated with solar radio bursts. Dur- ing a two month observing period with this instrument, ≈ 20 type III radio

bursts with fine structures in frequency (type IIIb bursts) were observed.

With the high spectral and temporal resolution provided by the instru- ment, the individual spectral fine structures, called “stria” were resolved

in both the domains. The center frequency and bandwidth information of the striae was used to study the electron density inhomogeneities in the solar corona in the heliocentric distance range of ≈ 1.6R - 2.2R , by

the way of density modulation index ( δNe Ne ). It was found that the density inhomogeneities followed a one dimensional Kolmogorov-like turbulence

spectrum. The derived Kolmogorov Power-Law index was used to esti- mate the amplitude of turbulent fluctuations, C

2 n (R) in the aforementioned distance range. The uniqueness of this work is that this is the first instance where ground-based high resolution observations have been used to derive the nature of density fluctuations in the near-sun atmosphere. Indirect measurements of the nature of magnetic fields in the solar 11 erg 10−7 Watt Seconds (Ws)

corona can be obtained from radio polarimeter measurements. Since emis- sion at different frequencies originate from different radial distances, a

spectro-polarimeter would enable measurements of magnetic fields as a function of the heliocentric distance. With this as a motivation, a FPGA based Spectropolarimeter was implemented on a Reconfigurable Open Architecture Computing Hardware (ROACH) Board with a four channel ADC as a Spectropolarimeter Backend for the GRIP array. Based on the

results from this experiment, a new Long-Baseline Polarimeter array op- erating in the frequency range of 15 MHz - 85 MHz was commissioned to

observe the polarization signatures of coronal transients, free of the quiet- Sun background. An upgraded version of the initial Spectropolarimeter

implementation was used as the backend of the new array. Using this, a So- lar noise storm was observed in the aforementioned range of frequencies.

A statistics of the type I, type III, the low and high frequency background continuum was obtained. It is found that type I storm bursts transition to type III at a frequency of ≈ 60 MHz. The spectral index of type I and type III bursts are found to be ≈ +3.5 and ≈ -2.7, respectively. The measured degree of circular polarization was ≈ 90 % for type I bursts and ≈ 25 % for type III bursts. The differences in polarization is explained by interpreting the bursts using magneto-ionic o- and e-modes. A unified scenario of the observation is visualized and described. Solar Radio bursts are sometimes found to occur upto frequencies of

400 MHz. The radiation at such high frequencies originate from coronal re- gions close to the Sun. In order to observe such transient emission and their

radial evolution, a wide-band spectropolarimeter was implemented using ROACH and a 2 channel 1 GHz ADC as a backend to the Gauribidanur Radio Spectropolarimeter (GRASP) antenna system. The system design and characterization is outlined. Preliminary science observations of the Sun is analyzed and the salient features of the result is discussed.