<p>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 (<InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math> <msub> <mi>I</mi> <mi mathvariant="normal">H</mi> </msub> </math></EquationSource> <EquationSource Format="TEX">$I_{\mathrm{H}}$</EquationSource> </InlineEquation>) and F (<InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math> <msub> <mi>I</mi> <mi mathvariant="normal">F</mi> </msub> </math></EquationSource> <EquationSource Format="TEX">$I_{\mathrm{F}}$</EquationSource> </InlineEquation>) emissions and analyzed their variation with heliographic longitude. We found that 14 out of 19 bursts originating from heliographic longitudes beyond <InlineEquation ID="IEq3"> <EquationSource Format="MATHML"><math> <mo>±</mo> <msup> <mn>75</mn> <mo>∘</mo> </msup> </math></EquationSource> <EquationSource Format="TEX">$\pm 75^{\circ }$</EquationSource> </InlineEquation> exhibited an <InlineEquation ID="IEq4"> <EquationSource Format="MATHML"><math> <msub> <mi>I</mi> <mi mathvariant="normal">H</mi> </msub> <mo stretchy="false">/</mo> <msub> <mi>I</mi> <mi mathvariant="normal">F</mi> </msub> </math></EquationSource> <EquationSource Format="TEX">$I_{\mathrm{H}}/I_{\mathrm{F}}$</EquationSource> </InlineEquation> ratio greater than unity. In contrast, 32 out of 39 bursts originating from longitudes within <InlineEquation ID="IEq5"> <EquationSource Format="MATHML"><math> <mo>±</mo> <msup> <mn>75</mn> <mo>∘</mo> </msup> </math></EquationSource> <EquationSource Format="TEX">$\pm 75^{\circ }$</EquationSource> </InlineEquation> 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.</p>

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

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发表日期：

2025年11月26日

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Rishikesh G. Jha,
K. Sasikumar Raja,
R. Ramesh,
C. Kathiravan,
Christian Monstein

摘要

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 ( $I_{H}$ $I_{\mathrm{H}}$ ) and F ( $I_{F}$ $I_{\mathrm{F}}$ ) emissions and analyzed their variation with heliographic longitude. We found that 14 out of 19 bursts originating from heliographic longitudes beyond $\pm 75^{\circ}$ $\pm 75^{\circ }$ exhibited an $I_{H} / I_{F}$ $I_{\mathrm{H}}/I_{\mathrm{F}}$ ratio greater than unity. In contrast, 32 out of 39 bursts originating from longitudes within $\pm 75^{\circ}$ $\pm 75^{\circ }$ 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.