On the limitations of using radiometric bursts as diagnostic tools for interplanetary coronal mass ejections by J. Kandekar et al. – Community of European Solar Radio Astronomers

Our Sun is constantly active and much of its activity, such as solar flares and coronal mass ejections, is driven by its magnetic fields. However, this magnetic field cannot be directly estimated; It is necessary to employ various observation 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 localized regions (for example, using band splitting in type II explosions; Smerd and Sheridan 1974), this study links those first signals from the Sun with in situ measurements of the magnetic field at 1 AU. This study can help predict how solar flares affect Earth’s environment. We used long-term data from solar cycles 23 and 24 to determine whether these radio signals could provide information not only about what is happening near the Sun but also about what might be coming toward Earth, particularly coronal mass ejections associated with interplanetary coronal mass ejections (ICME).

This study identified 88 type II radiometric bursts linked to interplanetary CMEs (ICMEs) from solar cycles 23 and 24 (1997-2019), using CME data from the CDAW catalog (Yashiro et al. 2004) and ICME data from Richardson and Reed (2010). Radio observations were obtained primarily from the RSTN network, with complementary data from eCallisto and HiRAS (Kondo et al. 1995). Of these, 31 events showed split-band characteristics, analyzed using dynamic spectra cleaned of interference and interpreted using the Newkirk quadruple density model (Newkirk 1961). The magnetic fields were estimated using the method mentioned in Vrsnak et al. (2002) and compared to in situ measurements at 1 AU from OMNI data via CDAWeb.

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

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

Conclusions

The study analyzed 31 CME-ICME events using type II split-band radio bursts to estimate magnetic fields in the mid-corona. Statistical analysis of the radio parameters did not show a consistent linear correlation between the magnetic fields near the Sun and those measured at 1 AU. This suggests that radiometric data are not reliable for predicting the strength of magnetic fields near Earth, probably due to the complex evolution of CMEs through the heliosphere. Limitations include the lack of intermediate data between ~2 R☉ and 1 AU, which future missions such as PSP and SolO can address. Velocity correlations continue to hold promise 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 Lettersvol. 697, art. No. L9, 2025. doi:10.1051/0004-6361/202553735.

References

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

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

White, SM 2004, Library of Astrophysics and Space Sciences, 314, 89

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