The frequency ratio and time delay of solar radio emissions with fundamental and harmonic components

TL;DR

This study uses high-resolution imaging spectroscopy to analyze solar radio bursts, revealing that delays in fundamental emissions caused by radio-wave scattering explain observed frequency ratios differing from 2.

Contribution

It provides the first quantitative estimates of fundamental emission delays due to radio-wave propagation effects at multiple frequencies.

Findings

Fundamental emission delays are primarily caused by anisotropic radio-wave scattering.

Simulations match observed delays with realistic anisotropy and density fluctuation levels.

Delay times of 1-2 seconds are consistent with source size and duration observations.

Abstract

Solar radio bursts generated through the plasma emission mechanism produce radiation near the local plasma frequency (fundamental emission) and double the plasma frequency (harmonic). While the theoretical ratio of these two frequencies is close to 2, simultaneous observations give ratios ranging from 1.6 to 2, suggesting either a ratio different from 2, a delay of the fundamental emission, or both. To address this long-standing question, we conducted high frequency, high time resolution imaging spectroscopy of type III and type J bursts with fine structures for both the fundamental and harmonic components with LOFAR between 30 and 80 MHz. The short-lived and narrow frequency-band fine structures observed simultaneously at fundamental and harmonic frequencies give a frequency ratio of 1.66 and 1.73, similar to previous observations. However, frequency-time cross-correlations suggest a frequency ratio of 1.99 and 1.95 with a time delay between the F and H emissions of 1.00 and 1.67 s, respectively for each event. Hence, simultaneous frequency ratio measurements different from 2 are caused by the delay of the fundamental emission. Among the processes causing fundamental emission delays, anisotropic radio-wave scattering is dominant. Moreover, the levels of anisotropy and density fluctuations reproducing the delay of fundamental emissions are consistent with those required to simulate the source size and duration of fundamental emissions. Using these simulations we are able to, for the first time, provide quantitative estimates of the delay time of the fundamental emissions caused by radio-wave propagation effects at multiple frequencies, which can be used in future studies.