Researchers at the Indian Institute of Astrophysics have resolved a long-standing puzzle about the relative strengths of fundamental and harmonic components in type II solar radio bursts. The study examined radio emissions produced by coronal shocks triggered by solar flares and coronal mass ejections and aimed to explain occasions when the harmonic appears stronger than the fundamental. Type II bursts are slow-drifting radio emissions that typically move outward through the solar corona at about 1000 km/s.

The team analysed data from global Compound Astronomical Low Frequency Low Cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers and used observations from the Gauribidanur Low Frequency Solar Spectrograph (GLOSS) at Gauribidanur Radio Observatory to trace the origin and characteristics of 58 type II bursts. The combined dataset provided diverse viewing angles and detailed local spectral measurements. Analysis concentrated on how propagation and refractive effects in the corona affect the visibility of fundamental and harmonic components.

The study found that events originating from active regions with heliographic longitudes greater than 75° tend to show stronger harmonic emissions, whereas events nearer the solar disk centre with longitudes less than 75° exhibit stronger fundamental emissions. The pattern was attributed to refractive effects, directivity and viewing angle in the solar corona, which can render the fundamental weaker or missing at Earth for limb events. Harmonic emission was reported to have a broader cone angle and so can be detected from far-side sources.

The findings improve understanding of how shocks generate radio waves and how those waves travel through the corona and could aid prediction of space weather and related effects on Earth. The researchers indicated that the large CALLISTO dataset is amenable to further study and that machine learning techniques could be applied to probe the data more deeply. The work was published in Solar Physics and involved collaborators from the Indian Institute of Astrophysics and IRSOL in Switzerland.