Multiple shocks generated by the 2024 May 14 coronal mass ejection

This study characterises a series of typeII radio bursts associated with a CME that occurred on 14 May, focusing on the coronal conditions during the event and identifying the likely location of the shocks where the radio bursts are generated. The CME was tracked using a combination of white light and extreme ultraviolet observations of the solar corona taken by three instruments: GOES-SUVI, two coronagraphs of the SOHO-LASCO, together with ground-based radio observations between 10-240MHz from I-LOFAR. The radial distances of the radio sources were examined using a series of density models, with both PFSS and MHD models used to examine the coronal plasma conditions. Four typeII bursts were identified in the I$-$LOFAR radio dynamic spectrum over $\sim$15minutes, exhibiting features such as band splitting, herringbones, and fragmentation. The shocks were found to have speeds ranging between $\sim$443$-$2075km s$^{-1}$, with drift rates of $\sim-$361 to -78kHzs$^{-1}$. The shocks were found to have a $M_A \approx$ 3.21$-$3.57. indicating that they were super-Alfvénic. The first typeII burst was triggered $\sim$18minutes after the CME launch, with each burst appearing to have been generated at a different height in the corona. Analysis of the derived kinematics and modelling results suggests that the typeII bursts were likely produced at the shoulders of the CME near the flanks, where open magnetic field lines and relatively low Alfvén speeds facilitated shock formation. This multi-instrument study shows that multiple type II bursts from a single CME originated at different coronal heights, with modelling indicating their generation near the CME flanks, where low Alfvén speeds and open magnetic field lines facilitated shock formation.

💡 Research Summary

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The paper presents a comprehensive multi‑instrument analysis of a fast coronal mass ejection (CME) that erupted on 14 May 2024 and the four associated type II radio bursts observed with the Irish Low‑Frequency Array (I‑LOFAR). The CME was tracked from the low corona (∼1.2 R☉) to the outer corona (∼30 R☉) using GOES‑SUVI EUV images and SOHO‑LASCO C2/C3 white‑light coronagraphs. By extracting height–time profiles along 13 radial slits spanning the CME’s upper and lower edges, the authors derived detailed kinematics: speeds increase from ∼100–917 km s⁻¹ in the EUV field of view to 761–1742 km s⁻¹ in LASCO C2, with accelerations ranging from –550 to +714 m s⁻² in the low corona and a general deceleration at larger heights.

I‑LOFAR observed the event continuously between 10:02 UT and 19:07 UT, with a focus on the 17:30–17:48 UT interval that contains the four short‑lived type II bursts. The dynamic spectra, recorded at 2 ms cadence, reveal fundamental and harmonic lanes, clear band splitting, herringbone structures, and fragmentation. The authors manually identified start/end points for each lane, placed 30 evenly spaced time markers, and fitted the resulting frequency–time points using a four‑fold Newkirk density model to convert frequencies to radial heights. Linear regression of height versus time yields shock speeds of 443–2075 km s⁻¹ and drift rates of –361 to –78 kHz s⁻¹. An independent calculation based on the classic drift‑rate formula gives consistent results, confirming the reliability of the height‑time method.

To locate the radio sources, five empirical density models (Allen, Newkirk, Saito, Leblanc, Mann) were tested against the EUV‑derived CME front heights. The authors also employed Potential Field Source Surface (PFSS) extrapolations and a data‑driven magnetohydrodynamic (MHD) model to map the coronal magnetic field and Alfvén speed distribution. Both models show that the CME’s flanks (the “shoulders”) intersect regions of open magnetic field lines and relatively low Alfvén speeds (≈200–350 km s⁻¹). Consequently, the calculated Alfvén Mach numbers for the shocks are 3.2–3.6, indicating strongly super‑Alfvénic conditions.

Crucially, each type II burst appears to originate at a distinct coronal height, ranging roughly from 1.3 R☉ to 2.5 R☉, and the first burst occurs about 18 minutes after the CME launch. The authors argue that the multiple bursts are generated not at the CME nose but at the flanks, where the expanding CME encounters favorable plasma conditions for shock formation. This interpretation is supported by the observed variation in shock speed and acceleration across the slits, the spatial correlation with low‑Alfvén‑speed corridors in the PFSS/MHD maps, and the presence of open field lines that facilitate electron escape and herringbone generation.

The study demonstrates that a single CME can produce several type II bursts, each tied to a different segment of the shock front. This challenges the conventional view that type II emission is confined to the CME nose and underscores the importance of high‑time‑resolution radio spectroscopy combined with EUV and white‑light imaging for diagnosing shock dynamics. The findings have broader implications for space‑weather forecasting: recognizing flank‑generated shocks improves our ability to predict the timing and intensity of solar energetic particle events and geomagnetic disturbances. The authors conclude that continuous, multi‑wavelength observations are essential for refining coronal density and magnetic field models, ultimately leading to more accurate forecasts of CME‑driven space‑weather impacts.