Development of a broadband radio spectropolarimeter for solar observations

Abstract

Energetic emission from the Sun can be divided into two broad categories, viz. gradual and impulsive. It is inferred from white-light observations that the first group is associated with mass ejections from the solar corona (i.e., coronal mass ejections or CMEs) whereas the second one is more likely related to flares. Near simultaneous spectral observations at low radio frequencies show that above white-light transients are associated with different types of radio bursts (viz. Type I, II, III, etc.). Since the latter are the signatures of shock waves, particle acceleration, etc. in the outer solar atmosphere, one can use radio spectral observations to study the properties of the coronal medium, the above transients, etc. over different phases of the solar cycle. Since the solar corona is a plasma medium which is at a temperature of about a million Kelvin, there is a significant amount of radiation at radio frequencies. Because of the gradual fall of density with increasing distance from the center of the Sun, the frequency of radio radiation also follows the same trend. The observed radio emission from these bursts is best detected in the frequency range of 440 –40 MHz. These frequencies are of particular interest as they typically correspond to a radial height of ≈ 1.1 to 2 R

(density model dependent and R

is the photospheric radius) in the Solar atmosphere. This height range mostly remains as unexplored till date. Therefore, building a broadband low-frequency radio spectrograph is crucial; in this regard, we recently developed and commissioned a Low-frequency Solar Spectrograph at the Gauribidanur observatory called the Gauribidanur LOw-frequency Solar Spectrograph (GLOSS). This thesis work includes the design, and development of a broadband antenna (Log –Periodic Dipole Antenna) which is the primary or the front-end receiver of the GLOSS. Various tests such as tuning of the antenna (matching the antenna impedance throughout the band), characterization of front-end amplifiers, coaxial cables, delay systems used are discussed elaborately. Also, programming the data

acquisition modules, testing and calibration methodology are described. The coronal magnetic field plays a very crucial role in triggering the aforementioned coronal transients; therefore, it becomes necessary to estimate the strength of the associated coronal magnetic field (B) towards predicting the location and time of onset of such events in the context of space weather. The sudden, unpredictable changes in the latter can lead to ground base communication disruption, long-duration power outages, space-borne system failures, etc. There are only a few direct measurements of B available so far, that too only over the inner corona (r ≤ 1.2 R

). Faraday rotation

measurements are used in the outer corona (r > 3 R

). Similar estimates are not

available in the middle corona (1.2 R

< r < 3 R

) due to a variety of reasons. Towards addressing this, a new spectro-polarimeter called the Gauribidanur RAdio Spectro-Polarimeter (GRASP) has been newly installed at the Gauribidanur observatory. Making use of the newly developed LPDAs (40 –440 MHz) a traditional adding interferometer has been set-up with two identical antennas orthogonal to each other. The signals from them are combined using a quadrature power combiner in order to detect the circular polarization (classical method). In this way, the weakly polarized radio emission from the solar corona are detected over a broad frequency regime, i.e., 40 –400 MHz, which will cover almost the middle corona. The description of the front-end, back-end receiver systems, data acquisition modules, calibration methodology is presented. Using the above facilities, viz. GLOSS and GRASP new results are obtained. For example, the salient features of various types of radio bursts and their emission mechanism, estimation of the coronal magnetic field B, etc. The radiation mechanism of slowly drifting radio bursts called moving Type IV (or Type IVm) bursts was studied carefully, and we believe that our suggestions would highly be useful to contemporary researchers. Also, systematic statistical studies were carried out on Type III, Type IV & Type V by supplementing the data obtained with on-board instruments like i) the Solar

& Heliospheric Observatory (SOHO), Large Angle and Spectrometric Coronagraph Experiment (LASCO), ii) CME positional information from Solar TErrestrial RElations Observatory (STEREO), iii) Soft X-ray flux from Geostationary Operational Environmental Satellite system (GOES) and iv) Soft X-ray imaging from Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Estimation of magnetic field strengths at the location of Type II radio bursts is made using appropriate density values. Therefore, the results will be less biased to the selection of density model. By combining radio imaging observations from Gauribidanur RAdioheliograPH (GRAPH) and white-light imaging observations with LASCO-C2 one can understand the CME associated non-thermal emission in a better manner is highlighted. The spectral class and polarization strength determination using GLOSS and GRASP may extensively be used to study various kinds of coronal radio emission in detail is suggested finally.