Solar Radio Observations At Low Frequencies With High Spectral And Temporal Resolution

Abstract

Ground-based radio observations of the solar corona at low frequencies (< 100 MHz) provide useful information in the height range ≈ 1−2 R☉ above the photosphere which is presently difficult to observe at other regions in the electromagnetic spectrum. Observations in the above height range are crucial since it is widely believed that some of the transient Sun-induced disturbances in the near-Earth space have their origin there. The other advantage is that the radio emission associated with transient solar activities are primarily non-thermal in nature, and hence more intense at low frequencies. Radio spectral observations in the corresponding frequency range are more worthwhile in this regard since they provide information over the entire range of heliocentric distance. Note that the radio emission at different frequencies originate at different distances from the Sun due to the inherent decrease in the electron density with increasing distance and the characteristics of radio wave propagation in an ionized medium like the solar atmosphere. So radio emission associated with any propagating disturbance in the solar atmosphere can be observed as a distinct feature in a time-frequency waterfall plot of the dynamic spectrum. This thesis describes the attempts made in this regard to carry out spectral observations at low frequencies down to the local ionospheric cut-off with high spectral and temporal resolutions at the Gauribidanur Radio Observatory under the auspices of the Indian Institute of Astrophysics. A low-frequency single antenna spectrograph system for observing solar transient radio emissions was configured to operate over the frequency range 15−85 MHz with the commercially available analog spectrum analyzer as back-end. A detailed description on the design of low-frequency antennas and filters is presented. The details of various experimental tests and inferences of the analog front-end components is also described. Using the new low-frequency antenna set-up and the existing instruments at the GRO we reported the radio imaging, spectral, and polarimeter observations of a type II radio burst associated with the solar coronal mass ejection (CMEs). Simultaneous ground-based, low-frequency « 100 MHz) radio imaging, spectral, and polarization observations of type II bursts are quite rare. From the radio spectral and imaging observations in tandem with the coronagraph observations we reinforce the close spatiotemporal association of CMEs and type II radio bursts which has been one of the long standing debates in solar radiophysics. We also had the opportunity to estimate coronal magnetic field strength ahead of and behind a CME, particularly from the dcp of the associated harmonic type II radio burst, with proper justifications. Though observations of circularly polarized harmonic type II bursts have been reported earlier, estimates of the coronal magnetic field from the observed circular polarization are not available. In this respect this was a unique set of observation with a combination of ground-based radio « 100 MHz) and space-based coronagraph observations. Another rare variant of the type II solar radio emission is the multiple type II radio burst phenomenon in which two or more type II bursts occur in quick succession. The occurrence of two successive Type-II bursts (referred to as Type-II doublets) in particular have been attributed to either two successive flares or two successive CMEs or a flare and a CME. We have reported radio spectral and polarimeter observations of two successive split-band Type-II bursts within a time interval of one minute at low frequencies ( < 100 MHz). Interestingly, both bursts exhibit Fundamental Harmonic as well as split-band structure. Type-II doublets with the above combined characteristics have rarely been reported. Our analysis of the data indicates that the first and the second Type-II bursts in the present case were likely due to MHD shocks generated by the near-simultaneous interaction of two different regions of the aforementioned CME with a preceding CME and a pre-existing coronal streamer. In the case of a conventional analog spectrum analyzer, at any give time data is obtained only at a single frequency since it is a swee-tuned instrument. The temporal resolution of such an instrument is determined by the sweep-time and the frequncy range of operation. The number of frequency channels over the operating frequency range is often fixed and hence results in poor frequency resolution for larger bandwidths. Note that an improvement in the temporal resolution could help us to understand the quasi-periodicity in the solar radio bursts and the related estimates of the coronal magnetic field strength. Higher spectral resolution can be used to identify and remove the RFI in frequency space while an improvement in the amplitude resolution leads to an enhancement of dynamic range in the measurements. In view of the above, we commissioned a new digital spectro-corrlator instrument using commercially available digitizer, for regular solar obsevations over the frequency range 15 - 30 MHz. The details of the instrument and the observational results obtained with it are presented in detail. From the encouraging results obtained with the above narrow-band spectrograph system, we configured a wide-band full digital back-end for the new spectrograph array that was recently commissioned at the GRO. An high-speed ADC-FPGA based data recorder was tested and characterised to be used as digital back-end receiver for the spectrograph array. Preliminary observations were carried out with both the digital and analog back-end for the spectrograph array to compare and contrast the results. The characterization of the ADC and the details of the FPGA board are presented in detail. The details on the data acquisition and pre-processing of the spectral data is also described. Quasi-continuum radio emissions of duration 10 - 60 min that occur in close association with CMEs in the solar atmosphere are termed as moving type IV bursts. The latter is found to accompany only about 5% of the CMEs. This provides a unique opportunity to study and infer the characteristics of CMEs viz. electron density, magnetic field strength etc. by simultaneously observing both the phenomena. We have reported the near-Sun (r ≤ 2 R☉) radio heliograph, radio spectrograph, and radio polarimeter observations of a type IVm burst that was co-spatial with the leading edge (LE) of a white light CME. To our knowledge, concurrent observations of the above type have rarely been reported. A complete analysis was performed to infer the electron density of the CME and the associated coronal magnetic field strength.