Our Sun is constantly active, and much of its activity, like Solar Flares and Coronal Mass Ejections, is driven by its Magnetic Fields. However, one cannot directly estimate this magnetic field; one needs to employ various observational and radio techniques to estimate these magnetic fields (White 2004; Gopalswamy 2006). While radio-based techniques have been used to explore coronal magnetic fields in localised regions (e.g., using band-splitting in type II bursts; Smerd & Sheridan 1974), this study links those early signals from the Sun to in situ magnetic field measurements at 1 AU. This study can help in predicting how solar eruptions affect the Earth’s environment. We utilised long-term data from solar cycles 23 and 24 to determine whether these radio signals could provide insights not only into what is happening near the Sun but also into what might be coming toward Earth, particularly the Coronal Mass Ejections associated with interplanetary coronal mass ejections (ICMEs).

This study identified 88 metric type II radio bursts linked to interplanetary CMEs (ICMEs) from solar cycles 23 and 24 (1997–2019), using CME data from the CDAW catalogue (Yashiro et al. 2004) and ICME data from Richardson & Cane (2010). Radio observations were primarily obtained from the RSTN network, with supplementary data from eCallisto and HiRAS (Kondo et al. 1995). Of these, 31 events showed split-band features, analysed using dynamic spectra cleaned for interference and interpreted using the four-fold Newkirk density model (Newkirk 1961). Magnetic fields were estimated using the method mentioned in Vršnak et al. (2002) and compared with in situ measurements at 1 AU from OMNI data via CDAWeb.

Figure 1: The left panel features radio data from the Learmonth Observatory, capturing a type II solar burst that began at 08:24 UTC on July 6, 2006. The fundamental and harmonic  bands are highlighted using the white-dashed lines. On the right, LASCO C2 images taken at 09:06 and 10:06 UTC show the CME’s distinct three-part structure: core, cavity, and leading edge. This CME commenced at 08:54 UTC.

The 6 July 2006 type II burst case (See Fig.1) revealed a CME speed of 911 km/s, a shock speed near 369 km/s, and an ICME speed of 380 km/s (Richardson & Cane 2010). Coronal magnetic fields were estimated at 0.45 G near 1.4 R⊙ using split-band radio data and the Newkirk model (Kumari et al. 2019). Upstream magnetic fields at 1 AU were about 4.5 × 10⁻⁵ G, consistent with OMNI measurements. A study of 31 split-band events showed magnetic fields from 0.04 to 4.59 G between 1.1–2.5 R⊙, with a weak correlation (r = 0.14) between near-Sun and near-Earth magnetic fields. Shock heights and uncertainties were derived using Newkirk density models and CME propagation assumptions.

Conclusions

The study analysed 31 CME-ICME events using split-band type II radio bursts to estimate magnetic fields in the middle corona. Statistical analysis of radio parameters showed no consistent linear correlation between near-Sun magnetic fields and those measured at 1 AU. This suggests metric radio data are unreliable for predicting magnetic field strengths near Earth, likely due to complex CME evolution through the heliosphere. Limitations include a lack of intermediate data between ~2 R☉ and 1 AU, which future missions like PSP and SolO may address. Speed correlations remain promising for predictive insights.

Based on the recent study by Kandekar, J. and Kumari, A., “On the limitations of using metric radio bursts as diagnostic tools for interplanetary coronal mass ejections”, Astronomy and Astrophysics Letters, vol. 697, Art. no. L9, 2025. doi:10.1051/0004-6361/202553735.

References

Kumari, A., Ramesh, R., Kathiravan, C., Wang, T. J., & Gopalswamy, N. 2019, ApJ, 881, 24

Vršnak, B., Magdaleni´c, J., Aurass, H., & Mann, G. 2002, A&A, 396, 673

White, S. M. 2004, Astrophysics and Space Science Library, 314, 89

Yashiro, S., Gopalswamy, N., Michalek, G., et al. 2004, J. Geophys. Res. Space Phys., 109, A07105