Multiple shocks generated by the 2024 May 14 coronal mass ejection

TL;DR

The study analyzes a May 14, 2024 CME that produced four closely spaced type II radio bursts, integrating EUV/white-light imaging from SUVI/LASCO with I-LOFAR radio spectra and coronal modeling (PFSS and MAS FORWARD). It shows that the shocks responsible for the type II bursts are super-Alfvénic, with speeds up to $\\sim 2075$ km s$^{-1}$ and Mach numbers around $M_A \\approx 3.21$–$3.57$, and that each burst likely originated at different heights near the CME flanks where open field lines and low Alfvén speeds facilitate shock formation. The analysis highlights the challenges of pinpointing radio source locations without imaging, but supports a scenario where the bursts are produced near CME shoulders in regions of reduced $V_A$ and complex magnetic topology, validated by density-model comparisons (notably $4\times$Newkirk) and PFSS/FORWARD maps. The work emphasizes the value of multi-wavelength data and coronal modeling for constraining shock dynamics in CME events and underscores the need for radio imaging to localize type II burst sources precisely.

Abstract

This study characterises a series of type~II 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 10-240~MHz 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 type~II bursts were identified in the I$-$LOFAR radio dynamic spectrum over $\sim$15~minutes, exhibiting features such as band splitting, herringbones, and fragmentation. The shocks were found to have speeds ranging between $\sim$443$-$2075~km s$^{-1}$, with drift rates of $\sim-$361 to -78~kHz~s$^{-1}$. The shocks were found to have a $M_A \approx$ 3.21$-$3.57. indicating that they were super-Alfvénic. The first type~II burst was triggered $\sim$18~minutes 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 type~II 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.

Figures (13)

• Figure 1: Running$-$ratio images showing the progression of the CME at the eastern limb from SUVI (top panel), LASCO C2 (middle panel), and LASCO C3 (bottom panel). The arc shape is used to estimate the CME expansion speed and angular width. The black dots represent the upper and lower slits, while the red dashes are the triangle base, which denotes the CME's width.
• Figure 2: Top panel: Running ratio images show the EUV wave as observed by SUVI and LASCO C2 and C3, with the slits defined as position angles. Bottom panel: Temporal evolution of the CME angular width and the triangle's base, which is determined by the CME’s widest sector in Fig. \ref{['fig:euv_panels']}'s frames.
• Figure 3: Distributions of the EUV wave's speed (top row) and acceleration (bottom row) along the radial slits in Fig. \ref{['fig:runratio_slits']} in SUVI (left column), LASCO C2 (middle column) and C3 (right column). The legend boxes represent the statistics for each parameter.
• Figure 4: Height$-$time profile for the coronal wave from SUVI along the slits shown in Fig. \ref{['fig:runratio_slits']} and the first type II burst from I$-$LOFAR with different electron density models with a scaling factor from 1 to 4
• Figure 5: I$-$LOFAR dynamic spectrum from 17:30 UT to 17:48 UT, covering 20 to 240 MHz, showing emission associated with the second CME. Boxes 1$-$4 highlight four type II bursts. The last letter in the labels (F or H) indicates whether the emission is fundamental or harmonic. If the label has two letters, it signifies multilane emission or band splitting, with the first letter (L, M, or U) referring to the lower, middle, or upper band, respectively. Each burst exhibits unique spectral features, including band$-$splitting, herringbones, spectral gaps, and fine structures.