**Executive Summary**
Researchers from the Indian Institute of Astrophysics (IIA) have resolved a long-standing mystery regarding the relative strengths of fundamental and harmonic radio emissions in solar coronal shocks. The study, published in the journal *Solar Physics*, identifies heliographic longitude and refractive effects as the primary factors influencing how these emissions reach Earth. This breakthrough improves the understanding of solar radio wave generation and enhances the accuracy of weather forecasting through the analysis of type II solar radio bursts.
**Key Points / Main Content**
**Characteristics of Type II Solar Radio Bursts**
* Type II bursts are slow-drifting radio emissions triggered by solar flares or coronal mass ejections.
* These bursts typically drift through the radio waveband at speeds of approximately 1000 km/s.
* They are identified by specific plasma parameters, drift rates, and spectral indices, moving from high to low frequencies as shocks travel outward.
* The emissions consist of two parts: the fundamental (base note) and the harmonic (overtone).
**The Research Findings**
* While theory suggests fundamental emissions should be stronger, observations often show stronger harmonic emissions.
* The study finds that events originating from solar longitudes greater than 75° (away from the disk center) exhibit stronger harmonic emissions.
* Events originating closer to the solar disk center (longitudes less than 75°) show stronger fundamental emissions.
* The discrepancy is caused by refractive effects in the solar corona, directivity, and the viewing angle; fundamental emissions from the edges of the sun often cannot reach Earth, whereas harmonics have broader cone angles that allow them to remain detectable.
**Methodology and Future Work**
* The team analyzed 58 type II solar radio bursts using data from the Compound Astronomical Low Frequency Low Cost Instrument for Spectroscopy and Transportable Observatory (CALLISTO) network.
* Primary data was also sourced from the Gauribidanur Low Frequency Solar Spectrograph (GLOSS) operated by the IIA.
* Researchers intend to adopt machine learning techniques to further analyze the vast amounts of data collected by global spectrometers.
**Impact Analysis**
**Scientists and Astronomers**
**Impact**
They now have a corrected understanding of why radio burst strengths vary, resolving a conflict between theoretical expectations and observational data.
**Action Required**
Researchers should incorporate these findings into solar atmospheric models and transition toward using machine learning for processing large-scale spectroscopic data.
**Weather Forecasters**
**Impact**
The study provides better insights into how solar shocks generate radio waves and travel through the corona, which is critical for space weather predictions.
**Action Required**
Forecasting agencies should utilize this improved understanding of type II bursts to refine their weather prediction models.
**Indian Institute of Astrophysics (IIA) / Department of Science and Technology (DST)**
**Impact**
The successful study validates the utility of autonomous Indian research institutions and specific facilities like the Gauribidanur Radio Observatory.
**Action Required**
Continue to support and operate the Gauribidanur Low Frequency Solar Spectrograph (GLOSS) and international collaborations involving the CALLISTO network.
Key Entities Referenced
Indian Institute of Astrophysics (IIA): An autonomous institution under the Department of Science and Technology that led the research into solar coronal shocks and radio emissions.
Department of Science and Technology (DST): The government department that oversees the Indian Institute of Astrophysics and the scientific research highlighted in the report.
CALLISTO: The Compound Astronomical Low Frequency Low Cost Instrument for Spectroscopy and Transportable Observatory, a global network of instruments used to analyze solar radio bursts.
Gauribidanur Radio Observatory: The facility operated by IIA where data was obtained using the Gauribidanur Low Frequency Solar Spectrograph (GLOSS) for the study.
Ministry of Science & Technology: The central ministry responsible for overseeing the research and scientific advancements in solar weather forecasting.
Ministry of Science & Technology
Probes into long-standing solar mystery can help
predict weather forecasting
Posted On: 27 MAR 2026 3:07PM by PIB Delhi
Researchers delving into solar coronal shocks triggered by solar flares have cracked a long-standing
puzzle about the reason behind the strange variation in the relative strengths of radio waves called
fundamental (base note of the wave) and harmonic emissions (overtone) for different bursts.
The study improves scientists’ understanding of how solar shocks generate radio waves and how those
waves travel through the corona and can help predict weather forecasting.
Solar coronal shocks triggered by solar flares or coronal mass ejections produce radio emission of a
particular kind, known as type II solar radio bursts. These bursts, also called slow-drifting bursts, typically
drift at speeds of about 1000 km/s and are detected in the radio waveband.
Type II solar bursts are identified based on characteristics such as drift rate, spectral index and various
other plasma parameters and they slowly drift from high to low radio frequencies as the shock moves
outward.
Type II bursts usually appear in two parts -- fundamental emission and harmonic emission. Theoretically,
the fundamental emission is expected to be stronger than the harmonic. However, observations show that
in some cases the harmonic can be stronger than the fundamental.
A team led by astronomers from the Indian Institute of Astrophysics (IIA), an autonomous institution of
the Department of Science and Technology (DST), addressed the question that intrigued scientists -- why
the relative strengths of the fundamental and harmonic emissions vary for different bursts.
They analyzed data from Compound Astronomical Low Frequency Low Cost Instrument for Spectroscopy
and Transportable Observatory (CALLISTO) instruments located around the globe to investigate the
puzzle by tracing the origin and characteristics of 58 type II solar radio bursts. This provided new insights
into the mystery.
For the process, they utilised the data obtained using Gauribidanur Low Frequency Solar Spectrograph
(GLOSS) from Gauribidanur Radio Observatory, operated by IIA.
“Our study finds that events originating from active regions located at heliographic, or solar, longitudes
greater than 75° tend to show stronger harmonic emissions. In contrast, events originating closer to the
solar disk center (heliographic longitudes less than 75°) exhibit stronger fundamental emissions. Such
behaviour is attributed to the refractive effects in the solar corona, directivity and viewing angle of the
solar radio emissions. Because of these effects, events associated with active regions greater than 75°,
cannot reach the Earth and thus appears either missing or weaker. However, harmonics have broader cone
angles and thus stronger emission can reach the Earth”, said. K. Sasikumar Raja, the P.I. of the study.Fig: Top: a Type II burst on 26/10/2023 where the fundamental is stronger than the harmonic, and
bottom: a burst on 16/7/2024 where the opposite can be seen.
“CALLISTO and other spectrometers around the globe have collected huge amounts of data. Such data
plays a tremendous role in understanding various puzzles like the ones mentioned. We would like to adopt
machine learning techniques to probe such data further” said Rishikesh G. Jha, the first author and a
student at IIA.
This work was published in the journal Solar Physics, authored by Rishikesh G. Jha, K. Sasikumar Raja,
R. Ramesh, and C. Kathiravan from the Indian Institute of Astrophysics, along with Christian Monstein
from Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Università della Svizzera italiana, Locarno,
Switzerland.
*****
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