Radio polarization studies of the solar corona at low frequencies

Abstract

The radio telescopes that are presently in operation specifically to study the Sun are limited. We have: (1) the Nobeyama radioheliograph in Japan operating at 17 and 34 GHz; (2) the Nan¸cay radioheliograph in France which observes in the frequency band 150- 450 MHz; and (3) the Gauribidanur radioheliograph (30-150 MHz) operated by Indian Institute of Astrophysics at Gauribidanur Radio Observatory near Bangalore. Since the electron density (Ne) and hence the plasma frequency (fp) decreases radially outward in the solar atmosphere, the radio emission at different frequencies originate from different heights. For example, 30-150 MHz emission is generally emitted over the height range of ≈ 1 − 2 solar radii above the photosphere. The above frequency range is well suited for observations of enhanced radio emission (both thermal and non-thermal) from the solar corona. Presently, it is difficult to observe the white light emission from the solar corona in the above height range even with space coronagraphs because of various practical difficulties. Radio observations play a useful role to understand the different coronal phenomenon along with observations from space at different wavelengths. To have a better understanding, it is essential to have polarization information in addition to imaging and spectral observations. The existing observations with the Gauribidanur radioheliograph (GRAPH) and Gauribidanur Low-frequency Solar Spectrograph (GLOSS) are limited to two-dimensional imaging (Stokes I) and one-dimensional spectral mode (Stokes I), respectively. The recently commissioned one-dimensional Gauribidanur Radio Interference Polarimeter (GRIP) is in operation in both Stokes I and V modes. As part of building the array, I have been involved in design and fabrication of antennas, analog and digital receiver systems. I also carried out different test measurements like characterization of antennas, phase equalization of coaxial cables, radiation pattern measurements, RF network cabling in the field/receiver room and other interferometric measurements. The GRIP consists of 40 frequency independent antennas (log periodic dipoles, LPDs) mounted independently in 0o and 90o orientation (with respect to the terrestrial north) for the measurement of polarized radio emission from the solar corona over the frequency range 30-150 MHz. LPDs were used because of the broad frequency coverage. The observations are limited to the circularly polarized radio emission (Stokes V) that arises due to propagation effects and also due to the non-thermal emission mechanisms which occurs frequently in the solar corona. Any linearly polarized emission (Stokes Q and Stokes U) that is generated at the source region cannot be observed over the typical observing bandwidths (kHz/MHz) used in radio astronomy, since the same are depolarized due to Faraday rotation in the solar corona and Earth’s ionosphere. I was involved in the design/development and fabrication of the associated front-end analog electronics, back-end digital receiver system, and the data acquisition/observational/data analysis software. Weak non-thermal radio activity are often noticed in the solar atmosphere. But their energy budget and the strength of the associated magnetic field were not established. Observations with the GRIP indicate that the estimated energy of the frequently observed type I radio bursts are ≈ 1021 erg. These are the weakest energy releases reported in the solar atmosphere as on date and they belong to the category of picoflares which is one of the suggested mechanisms to heat the solar corona. We also estimated the associated magnetic field and it is in the range ≈ 200 − 1000 mG. At present, measurements of coronal magnetic fields in white light are not possible due to various practical difficulties. By making use of indirect radio methods, the magnetic field strengths of corona is possible to estimate. Using the polarization observation of the group of type III radio bursts the magnetic field strength of the solar corona was estimated in different approaches. Firstly, the limb events whose degree of circular polarization (dcp) is less than 15% was selected. It is known that such events are due to the harmonic emissions which give the clue to estimate the magnetic field. On the other hand using quasi-periodicity of the mentioned events, the magnetic field was estimated at 1.3 R⊙ (80 MHz). These estimated results are consistent with the existing empirical models and other reported observations. Type IVm radio bursts occurs in association with coronal mass ejections (CMEs) and the source moves in the solar atmosphere with approximate speed of CME. Using GRIP, a CME associated moving type IV burst was observed at 40 and 80 MHz. A weak circularly polarized radio emission was observed in both the frequencies. Observationally it was identified that origin of type IVm burst was due to the gyrosynchrotron emission. Hence by knowing the dcp and emission mechanism the associated magnetic field strength was estimated at height of ≈ 1.9 and 2.2 R⊙. Occultation studies of the Crab nebula by the solar corona was carried out in the polarimetric mode. We expected to observe circularly polarized radiation, since there could be possible density inhomogeneities (with associated magnetic field) in the line of sight. But we did not observe any such emission. It is possible that the polarized signal, if any were present, was too weak to detect. However the observations lead to scattering studies at different heights in the lower heliosphere. Observationally it was known that the flux density of the Crab nebula decreases during its ingress and regains during egress. The flux density vanished when the Crab nebula was close to solar disk, (in mid June) ≈ 5 R⊙. Based on the occultation observations in years 2011 and 2013, level of turbulence (C2N ) at different height in the corona was estimated. It was found that the estimates of C2N (r) were higher compared to the similar reports from VLBI observations. Using the estimated C2Nvalues the angular broadening measurements was carried out and the results are consistent with earlier empirical equations. We designed and fabricated a cross-polarized log periodic dipole (CLPD) antenna system to study the circularly polarized radio emission from the solar atmosphere. The performance study of the CLPD was carried out and found that for commercially available CLPDs are having the isolation > −20 dB which are not suitable to have a precise polar ization measurements. Our designed CLPD has the isolation as low as < −40 dB. Using a narrow bandwidth (≈ 65− 85 MHz) CLPD, two element interferometric polarimeter was constructed and observed the circularly polarized radio emissions from the solar corona. By designing the broad band CLPD with improved isolation, it can be used in building the imaging instruments to have the polarized two-dimensional maps of the solar corona. Also they can be used as feeds for dish antennas.