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

TL;DR

This study addresses why the fundamental (F) and harmonic (H) components of solar radio type II bursts show different relative strengths. It analyzes 58 meter-wavelength bursts observed with the e-CALLISTO network and GLOSS, quantifying the harmonic-to-fundamental intensity ratio $I_H/I_F$ across epochs and correlating it with heliographic longitude and CME properties. The main finding is a systematic longitude dependence: $I_H/I_F$ tends to exceed unity for activity far from disk center ($|L|>75^{\circ}$) and fall below unity near disk center ($|L|\le75^{\circ}$), with broader CMEs often accompanying stronger H emission. The results implicate coronal density-gradient refraction, source directivity, and viewing angle as key factors shaping F and H intensities, and they motivate larger datasets to confirm and refine this picture for robust solar radio burst modeling.

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.

Figures (2)

• Figure 2: Location of the active regions associated with type II bursts. The blue diamonds indicate type II bursts that originated from and are associated with active regions whose longitudes are $<75^\circ$ and have $I_H/I_F < 1$. The red stars indicate type II bursts associated with active regions whose longitudes are $>75^\circ$ and have $I_H/I_F > 1$. The green diamonds indicate those that violate the conditions mentioned above.
• Figure : (a) Harmonic is stronger ($I_H/I_F > 1$)