| Abstract : | Estimation of solar coronal magnetic field strength (B) is of profound interest as it plays a major role in the dynamics of the solar corona like development of coronal structures, coronal activities, background corona, etc. The B estimates in heliocentric distance range, r ≈ 1.2 – 3 Ro is limited as the field strengths are in the order of few Gauss, where Ro is Photosphere radius. Also, the B estimates especially in the quiet coronal regions at these ‘r’ are rare. In the recent past, researchers have pointed out the possibility to estimate the B using thermal radio emission from the corona using magneto-ionic theory. They suggest that the B makes the plasma medium anisotropic and thereby splits the original randomly polarized thermal radio emission into two orthogonally circularly polarized components i.e., the ordinary and extraordinary modes. These two components are differently absorbed by the magnetized plasma due to the difference in absorption coefficients and the directions of the wave propagation. So, the resultant radiation will have the sense of circular polarization of the dominant component and detecting this degree of circular polarization (DCP) gives an idea about the B which caused it. Using Ray-tracing techniques and Haselgrove equations, the variation of DCP for various radio frequencies, coronal electron densities and magnetic fields are investigated. The simulation results indicate that the DCP is low for higher radio frequencies and increases as we observe at lower radio frequencies (DCP is significant at meter wavelengths and above radiation).
In view of the above, to locate and detect the circularly polarized radio emission from solar corona, the Gauribidanur RAdioheliograPH (GRAPH) array is being augmented. Present GRAPH is a T-shaped radio interferometric array that can produce 2D total intensity images in 35-85 MHz frequency range (corresponds to coronal observations at r ≈ 1.2-2.0 Ro). |
