Relative Strengths of Fundamental and Harmonic Emissions of Solar Radio Type II Bursts

TL;DR

This study investigates the varying relative strengths of fundamental and harmonic emissions in solar radio type II bursts, revealing that emission dominance depends on source location, viewing angle, and coronal density gradients.

Contribution

It provides the first comprehensive analysis of how heliographic longitude and coronal conditions influence F and H emission intensities in solar radio bursts.

Findings

H emissions are stronger for sources beyond ±75° longitude.

F emissions dominate for sources within ±75° longitude.

Refraction and viewing angle affect emission intensity ratios.

Abstract

Solar radio type II bursts are slow-drifting bursts that exhibit various distinct features such as Fundamental (F) and Harmonic (H) emissions, band-splitting, and discrete fine structures in the dynamic spectra. Observationally, it has been found that in some cases the F emission is stronger than the H emission, and vice versa. The reason for such behavior has not been thoroughly investigated. To investigate this, we studied 58 meter wave (20-500 MHz) type II solar radio bursts showing both F and H emissions, observed during the period from 13 June 2010 to 25 December 2024, using data obtained with the Compound Astronomical Low frequency Low cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers at different locations and Gauribidanur LOw-frequency Solar Spectrograph (GLOSS). We examined the intensity ratios of the H ($I_H$) and F ($I_F$) emissions and analyzed their variation with heliographic longitude. We found that 14 out of 19 bursts originating from heliographic longitudes beyond $\pm75^\circ$ exhibited an $I_H/I_F$ ratio greater than unity. In contrast, 32 out of 39 bursts originating from longitudes within $\pm75^\circ$ showed a intensity ratio less than unity. From these results, we conclude that the relative strength of the F and H emissions can be influenced by refraction due to density gradient in the solar corona, directivity and viewing angle of the bursts.